Fertilizer Management: Fertilizer Efficiency

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MAXIMIZING FERTILIZER USE EFFICIENCY
I Assessment of nutrient use efficiency

Three basic questions must be answered in all assessments of fertilizer use efficiency:

• How much of the nutrients applied are taken up by the crop?
• How much additional yield is obtained for each additional unit of nutrient uptake?
• To what extent can the crop benefit from the nutrients not recovered by the crop during the period of assessment?

There are five indices that can be used to assess nutrient use efficiency.

Partial factor productivity (PFP)

PFP answers the question: How much yield is produced for each kg of fertilizer nutrient (FN) applied?

PFP_FN = kg bunch kg^-1 fertilizer nutrient (FN) applied:
PFP_FN = BY+FN / FN (1) 
where BY_+FN is the bunch yield (kg ha^-1) and FN is the amount of fertilizer nutrient applied (kg ha^-1).

Because BY at a given level of FN represents the sum of yield without fertilizer inputs (BY_0FN ) plus the increase in yield from applied fertilizer (ΔBY_+FN),

PFP_FN = (BY_{0 FN} + ΔBY_+FN) / FN (2)

or

PFP_FN = (BY_{0 FN} / FN) + (ΔBY_+FN / FN) (3)

and by substitution with equation (5):

PFP_FNFN = (BY_{0 FN} / FN) + AE_FN (4)
where AE_+FN is the agronomic efficiency of applied fertilizer nutrients (see below).

Equation 4 shows that PFP_FN can be increased by increasing the uptake and use of indigenous soil-N resources (measured as BY_0FN) and increasing the efficiency of applied fertilizer nutrient use (AE_FN).

Agronomic efficiency (AE)

AE answers the question: How much additional yield is produced for each kg of fertilizer nutrient (FN) applied?

AE_FN = kg bunch yield increase kg^-1 FN applied (often-used synonym: nutrient use efficiency):
AE_FN = (BY_+FN – BY_{0 FN}) / FN (5)
where BY_+FN is the bunch yield in a treatment with fertilizer nutrient application; BY_{0 FN} is the bunch yield in a treatment without fertilizer nutrient (FN) application; and FN is the amount of fertilizer nutrient applied, all in kg ha^-1.

AE_FN represents the product of the efficiency of nutrient recovery from applied nutrient sources (= recovery efficiency, RE_FN) and the efficiency with which the plant uses each unit of nutrient acquired (= physiological efficiency, PE_FN):

AE_FN = PE_FN x RE_FN (6)

Both RE_FN and PE_FN thus contribute to AE_FN, and each can be improved by crop and soil management practices, including general crop
management practices and those specific to nutrient management, e.g. a more balanced N:P:K ratio or improved splitting and timing of nutrient applications (see Table 2 and 3).

Because AE_FN = PE_FN x RE_FN, it is necessary to quantify the relative contribution of each component to explain measured differences in agronomic efficiency that result from different nutrient or crop management strategies.

Recovery efficiency (RE)

RE answers the question: How much of the nutrient applied was recovered and taken up by the crop?

RE_FN = kg fertilizer nutrient taken up kg^-1 fertilizer nutrient applied:
RE_FN = (UN_+FN – UN_{0 FN}) / FN (7)
where UN_+FN is the total palm uptake of fertilizer nutrient measured in aboveground biomass in plots that receive applied fertilizer nutrient at the rate of FN (kg ha^-1); and UN_{0 FN} is the total nutrient uptake without the addition of fertilizer nutrient.

RE_FN is obtained by the ‘nutrient difference’ method based on measured differences in plant nutrient uptake in treatment plots with and without applied nutrient (Equation 7). Recovery efficiency of applied nutrient is estimated more accurately when two treatments with a small
difference in the application rate are compared:

RE_FN = (UN_FN2 – UN_FN1) / (FN_FN2 – FN_FN1) (8)
where RE_FN is the recovery efficiency (kg nutrient uptake kg^-1 nutrient applied); UN is the total nutrient uptake in bunches, fronds and trunk (kg ha^-1); and FN is the amount of fertilizer nutrient added (kg ha^-1) in two different nutrient treatments (FN2 and FN1) e.g. FN2 receiving a larger nutrient rate than FN1.

RE_FN is affected by agronomic practises and rainfall (Table 2)

Physiological efficiency (PE)

PE answers the question: How much additional yield do I produce for each additional kg of nutrient uptake?

PE_FN = kg bunch yield increase kg^-1 fertilizer FN taken up:
PE_FN = (BY_+FN – BY_{0 FN}) / (UN_+FN – UN_{0 FN}) (9)
where BY_+FN is the bunch yield in a treatment with fertilizer nutrient (FN) application (kg ha^-1); BY_{0 FN} is the bunch yield in a treatment without fertilizer nutrient (FN) application; and UN is the total uptake of fertilizer nutrient (kg ha^-1) in the two treatments.

PE_FN represents the ability of a plant to transform a given amount of acquired fertilizer nutrient into economic yield (oil or bunches) and largely depends on genotypic characteristics such as the bunch index and internal nutrient use efficiency, which is also affected by general crop and nutrient management (Table 2).

Internal efficiency (IE)

IE answers the question: How much yield is produced per kg fertilizer nutrient (FN) taken up from both fertilizer and indigenous (soil) nutrient sources?

IE_FN = kg bunch kg^-1 FN taken up:
IE_FN = BY / UN (10)
where BY is the bunch yield (kg ha^-1), and UN is the total uptake of fertilizer nutrient (kg ha^-1).

This definition of IE_FN includes FN taken up from indigenous and fertilizer sources. IE_FN largely depends on genotype, harvest index, interactions with other nutrients and other factors that affect flowering and bunch formation.

II Implementation of nutrient use efficiency assessment in oil palm fertilizer experiments

In annual crops, destructive sampling methods can be used to measure nutrient uptake in fertilized and unfertilized plots in each crop season and fertilizer nutrient use efficiency can then be calculated by difference (Dobermann and Fairhurst, 2002). The relative ease with which this can be carried out explains why in grain crops, measurement of nutrient use efficiency is standard practice when analyzing data from field fertilizer experiments. Destructive sampling cannot be used in oil palm fertilizer experiments, however, because it is costly and precludes the possibility of further measurements in the experiment. For this reason, Fairhurst (1996) and Fairhurst (1999) devised a nondestructive approach to measure nutrient uptake, based on standard methods for estimating above ground biomass production in trunk, leaf, bunches (Corley et al., 1971, Appendix 6) combined with tissue analysis. Nutrient uptake is calculated from the nutrient concentration and the amount of biomass produced (kg ha^-1 yr^-1) respectively in the trunk, leaves, and bunches, and nutrient use efficiency is measured by comparing nutrient uptake in different treatments in fertilizer experiments.

Differences in nutrient use efficiency between plantations, blocks, single palms or fertilizer sources are explained by a range of factors (Table 2). The goal of a good field management is to maximize uptake by identifying possible limiting factors and implementing remidial measures.

These methods were used to assess nutrient use efficiency in six fertilizer trials at Bah Lias Research Station (BLRS) (Prabowo et al., 2002). Preliminary results from one year of measurements indicate recovery efficiencies of 19–36% (N), 7–29% (P), 29–70% (K) and 10–60% (Mg) (Table 1). Large differences in RE were measured for different fertilizer sources of P and Mg fertilizer and RE was much greater when these nutrients were supplied in soluble forms respectively as TSP and kieserite (Table 1).

Table 1. Recovery of nutrients from mineral fertilizers in five fertilizer experiments in North
Sumatra, Indonesia (after Prabowo et al., 2002).

In almost all cases, RE was greater for each nutrient when other nutrients were supplied in non-limiting amounts. RE was smaller in Trial 231 where high rainfall resulted in large fertilizer nutrient losses in surface water runoff and eroded soil (Prabowo et al., 2002). In Trial 231 RE was >100% for K where yield was less than 23 t ha^-1. This suggests that palms were able to use soil indigenous K more efficiently after K deficiency had been corrected.

The separation of AE into its components of RE and PE provides the means to identify problems in fertilizer response experiments. For example it may be possible to achieve large values for RE but low values for PE result in low values for AE. Field management factors can be separated into those affecting RE and PE (Table 2). For example, RE may be large in a fertilizer treatment but a low value for PE is caused by inter palm competition and the genetic characteristics of the planting material.

Table 2. Examples of factors affecting and physiological efficiency (PE) and recovery
efficiency (RE) of fertilizer nutrients in oil palm.

Table 3. Effect of fertilizer placement on bunch yield in Malaysia.

Reference
Goh K.J., Rolf Härdter and Thomas F. (2003) Fertilizing for maximum return. In: Thomas Fairhurst and Rolf Hardter (eds). Oil palm: Management for large and sustainable yields. Potash & Phosphate Institute and International Potash Institute: 279-306

Note: The full list of references quoted in this article is available from the above paper.

Fertilizer Management: Computation

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Nutrient balance approach

The methods to estimate the fertilizer rates, which have been described so far, are all empirical and therefore, should be used within the same environments where they have been developed. This limitation is partially overcome by methods which are based on the principles of plant nutrition. One of these methods is called INFERS (Kee et al ., 1994) which follows the nutrient balance approach and plant nutrient demand. These are the foundations of modern plant nutrition in the field, and recently have been advanced for dealing with soil nutrient depletion in African agriculture in general (Smaling et al ., 1999; Corley and Tinker, 2003). Although a number of past papers have discussed nutrient balance approach (Hew and Ng, 1968; Ng, 1977), only the INFERS model has been described briefly by Kee et al . (1994) and Corley and Tinker (2003) to illustrate the approach for oil palm.

The nutrient balance approach specifically attempts to balance the nutrient demand with the nutrient supply. In the oil palm agro-ecosystem, the components of nutrient demand are plant nutrient uptake for growth and production, nutrient losses through soil processes such as runoff and leaching (environmental losses) and nutrient immobilization (Figure 1). The components of nutrient supply are precipitation, pruned fronds, applied by-products such as empty fruit bunches. Any shortfall between nutrient supply and demand is met by fertilizer input. Ng (1977) considered the major variables in the nutrient balance sheet to be soil nutrient supply to the oil palm and plant nutrient demand.

Note: POME denotes palm oil mill effluent while EFB denotes empty fruit bunches

Figure 1: Nutrient cycles for nitrogen in oil palm plantations

Plant nutrient demand is the requirement for essential elements by a growing plant (Corley and Tinker, 2003). It can be separated into two processes: growth demand and deficiency demand (Tinker and Nye, 2000). The underlying theory of these two “demands” is quoted verbatim from Corley and Tinker (2003) as follows:

Nutrient amount (content) in palm, N = XW and uptake rate = = 

where N is the total nutrient in the palm, W is the mass, X is the fractional content of the nutrient and t is time. The first term in the uptake rate represents the growth demand because the nutrient percentage remains constant as the plant grows at a rate . However, during the correction of a nutrient deficiency, the second term applies, as the weight is a constant with varying nutrient concentration. In fact, both processes probably occur at the same time. Without the differentials and ignoring change in structure of plant material, a simple approximation for the uptake is:

X ₂ (W ₂ – W ₁) + W ₁ (X ₂ – X ₁) = X ₁ (W ₂ – W ₁) + W ₂ (X ₂ – X ₁) = X ₂ W ₂ – X ₁ W ₁

for times t1 and t2 and the meaning of the terms remains the same.

The main components of growth demand in the oil palm are nutrients immobilized in palm tissue by growth and nutrients exported in the FFB. The major components of deficiency demand are increase in palm nutrient content to correct nutrient deficiency and increase in soil nutrients. Changing the present state in these four components to the optimum level and maintaining the optimum state are the central tenets of INFERS model. That is, these four components, FFB yield, growth (palm size), nutrient concentration in palm (usually the leaf nutrient concentration in Frond 17 is used as an indicator) and soil nutrient concentration, form the targets in INFERS. Since these targets differ according to palm age, environment and economic situation, the palm nutrient requirements will also vary. Coupled with different fertilizer use efficiency, the fertilizer rates required for each field will change accordingly. This is indeed the essence of site-specific fertilizer recommendations. A brief description of INFERS module for computing fertilizer rates using N as an example is provided below. The detailed structure of INFERS is provided by Kee et al. (1994) and Corley and Tinker (2003) while the research which supports the model has been well described by Corley and Tinker (2003).

Since INFERS is based on the principle of plant demand and nutrient supply, the four targets to be achieved or maintained must be set correctly. The first target is usually based on the site yield potential using a model called ASYP (Kee et al., 1999). The growth rate is based on the increasing dry weight of Frond 17 as determined from its dimension (Corley et al., 1971) with palm age. It should be noted that the growth rate of oil palm and the maximum frond dry weight depend on the environment. This information is freely available from many experiments conducted on oil palm in Malaysia. The target for the leaf nutrient concentration in Frond 17 may be based on single nutrient critical levels for different environment and palm age or TLC method as described earlier. Since four targets are used in the model, the computed fertilizer rates are less sensitive to changes in leaf nutrient concentration compared to the earlier methods discussed above. The target for soil nutrient contents depends on the soil nutrient classification table (Table 1) or user’s preference for nutrient buildup, maintenance or depletion although INFERS does not in principle aim to deplete soil nutrients.

Table 1: Classification of soil nutrient status for oil palm

The main nutrient demand in the oil palm agroecosystem is probably by the plant. The plant nutrient demand can be separated into four components: canopy, trunk, root and FFB. The equations to calculate the palm N demand are shown below. The figures in subscript, 1 and 2, denote time 1 (present state) and time 2 (a year later).

1. Nutrient demand of the canopy

Canopy N growth demand (g N/palm) = 0.155* (Pinnae N (%)₁) (Frond17 dry weight (g)₂– Frond17 dry weight (g)₁)

Canopy N deficiency demand (g N/palm) = (0.155 * (Frond17 dry weight (g)₂) – 236.817)* (Pinnae N (%)₂ – Pinnae N (%)₁)

where Frond 17 dry weight is measured using the non-destructive method of Corley et al . (1971) and Pinnae N is obtained from the standard leaf nutrient analysis adopted by the oil palm industry in Malaysia (Foster, 2003).

2. Nutrient demand of the trunk

Trunk N growth demand (g N/palm) = 0.01 * Trunk N concentration (%)₁(Trunk dry weight (g)₂ – Trunk dry weight (g)₁)

Trunk N deficiency demand (g N/palm) = 0.01 * Trunk dry weight (g)₂(Trunk N concentration (%)₂ – Trunk N concentration (%)₁)

The trunk N concentration (%) is estimated by the linear-plateau model as follows:

• Trunk N concentration (%) = 1.369 – 0.117 (age (yr)) for palm <= 8.5 years old
• Trunk N concentration (%) = 0.351 for palm > 8.5 years old

The trunk dry weight is estimated by the equations proposed by Corley and Bruere (1981) as follows:

• Trunk volume (cm³) = ? x d²x h /4
  where d = trunk diameter (cm), usually measured at 1m above the ground
  h = trunk height (cm), usually measured to Frond 41
• Trunk density (g/cm³) = 0.083 + 0.0076 (age (yr))
• Trunk dry weight (g) = Trunk volume x Trunk density

The above equations indicate that for palm above 8.5 years old, a constant value for growth demand of trunk may be used since height increment, diameter and N concentration in the trunk are constants and increase in trunk density is relatively small. Also, there is no deficiency demand due to constant trunk N concentration.

3. Nutrient demand of the roots

The N concentration in the roots of oil palm is relatively constant across palm age and soil types at about 0.39 %. Thus, oil palm roots are assumed to have no deficiency demand.

The growth demand of the oil palm roots is calculated using an empirical equation based on root:shoot ratio as follows:

Root:shoot ratio = 1.92 (Palm age (yr))^-1.11

The difference in root weights between year 1 and year 2 is multiplied by the constant root N concentration to give the root N demand. It should be noted that the above equation to compute the root weight is based on palms with relatively good nutrition. It is known that root:shoot ratio tends to be higher for palms in poor nutritional state.

4. Nutrient demand of the FFB

At present, it is assumed that the N concentration of FFB is not affected by palm age or nutrition, and remains constant at 3.195 g N per kg FFB. Therefore, there is only growth demand by the production of FFB as follows:

FFB N growth demand (g N/palm) = FFB (kg)₂ x 3.195

The soil nutrient demand generally involves two soil processes; soil nutrient build-up and soil nutrient losses. Soil nutrient build-up may be necessary if the soil nutrient status is low or where the soil activity ratio indicates nutrient imbalance as discussed earlier. The soil nutrient losses in the oil palm agroecosystem mainly arise from erosion, runoff and leaching. Corley and Tinker (2003) consider these losses as environmental losses or demand. The erosion and runoff losses can be estimated using the model suggested by Morgan et al. (1984) and leaching losses by Burn’s model (Burns, 1974). Although these sub-models are built into INFERS model, they require many state variables and parameters, and therefore are beyond the scope of this paper. In general, soil N losses through the above processes should not exceed 10 % if the fertilizer is properly applied and correctly timed. N volatilization losses from urea or urea based fertilizers can be considered as part of soil N demand but they are usually taken into account after computing the final fertilizer rate assuming no losses initially. That is, if one expects volatilization losses to be about 30 %, then the final N fertilizer rate is adjusted 30 % upwards.

The major nutrient supply in the oil palm agroecosystem is shown in Figure 3. INFERS assumes that nutrient supply from the atmospheric and rainfall deposition is small and no decrease in soil or plant nutrient content is expected unless done on purpose. For example, it is sometimes necessary to deplete, say soil exchangeable Ca and Mg which may be too high and causing poor K uptake as in ultrabasic soils or the palms on peat soils have too high N and too low K, by the appropriate fertilizer withdrawal. Similarly, the residual value of large dressings of phosphate rock and ground magnesium limestone (Goh et al., 1999b) can be up to three years’ demand and these nutrients can probably be omitted in such cases (Corley and Tinker, 2003). The nutrient supply from by-products such as empty fruit bunches (EFB) and palm oil mill effluent (POME) is well known and can be easily accounted for.

The computations of nutrient balance are subject to errors as in all mathematical and statistical models, and depend on reasonable or achievable targets. Thus, to prevent over manuring, INFERS has set a maximum N uptake rate of 1180 g per palm per year as measured under good environmental conditions.

The conversion of nutrient requirement of oil palm to fertilizer equivalent depends on the expected fertilizer efficiency at the site. Since fertilizer efficiency varies across sites, it is ideal that fertilizer response trials on similar soil types are available in the vicinity. In general, the N fertilizer efficiency in Malaysia varies from 30 to 70 %. This wide range in fertilizer efficiency is due to the very different environments where they were measured e.g. fertile coastal clays to infertile Malacca series soils. In reality, the average fertilizer efficiency over three years or more within a site is relatively similar. Therefore, the fertilizer efficiency at a site may be estimated from past fertilizer history and nutrient uptake rate as a first approximation as described step-by-step below.

1. Figure 2 shows a hypothetical response curve of nutrient uptake to fertilizer input. It generally follows a modified Mitscherlich equation or a linear-plateau model. Under an ideal situation, we should know three points:

• Point A: Nutrient uptake without fertilizer input i.e. soil nutrient supply
• Point C: Targeted nutrient uptake at the correct fertilizer rate
• Point B: Average last two to three years nutrient uptake at applied fertilizer rates

Point A and point C are usually unknown from past historical data although point A can be estimated using Foster’s soil based system as discussed earlier. However, point B and the targeted nutrient uptake line are known.

Figure 2: A hypothetical response curve of N nutrient uptake to N fertilizer input and a method to predict the N fertilizer rate for the following year

2. Point B can be calculated based on the model described earlier using the actual yield, dry weight and nutrient concentration in Frond No. 17.

3. The targeted nutrient uptake is calculated based on the targeted yield (site yield potential), dry weight and nutrient concentration in Frond No. 17 for the site.

4. We can then draw a tangent passing through point B to the targeted nutrient uptake line. The point where it cuts (point D) gives the estimated fertilizer rate. This generally underestimates the fertilizer requirement due to higher environmental demand (Corley and Tinker, 2003) with increasing fertilizer rate. We have not fully addressed this issue although a 10% higher rate for N and K appears satisfactory.

5. Another problem which has not been solved is the known fact that fertilizer use efficiency (FUE) declines with increasing fertilizer rate. It generally follows a declining exponential model, FUE = exp(-kF), where F is the fertilizer rate (kg/palm/yr) and k is a constant. This constant is mainly affected by fertilizer sources and environment.

6. This method avoids the necessity to estimate the fertilizer use efficiency and soil nutrient supply directly. However, it is highly dependant on a reasonable starting value (point B) and the targets to avoid over fertilization.

7. A reasonable point B can be obtained if one follows the six tools available to monitor palm health, and changes in soil nutrients and fertilizer use efficiency as listed below:

• Leaf nutrient status
• Soil nutrient status
• Nutrient deficiency symptoms
• Vegetative growth rate and canopy sizes (Classification)
• Yield (site yield potential)
• Fertilizer efficiency

An example showing the computation of N fertilizer rate (kg AC/palm/year) using INFERS model for the low N scenario as provided in the earlier illustrations of fertilizer recommendation systems is given below. The required variables measured in 1993 and 1994, and targets for 1995 are given in Table 2 and the calculated nutrient uptake and fertilizer rate are shown in Table 3. For simplicity, it is assumed that the soil N status is satisfactory and therefore, soil N demand is equaled to zero.

Table 2: Measurements made on oil palm planted in 1979 on Batang (lateritic) Family soil to demonstrate INFERS model

Table 3: Computed N uptake and N fertilizer rate based on variables in Table 16 using INFERS model

The calculated N fertilizer rate is similar to that of Foster’s system but it is the only known fertilizer recommendation system for oil palm that accounts for both deficiency ad growth demands explicitly. It also avoids the problem of dilution or concentration effect of leaf nutrient due to changing canopy sizes. The relatively low N fertilizer rate in the present example is due to the relatively high soil N supply as shown by the past historical data. In general, higher N rate is recommended to account for the decline in fertilizer use efficiency with increasing fertilizer rate due to higher N environmental losses if the first approximation method is used as discussed above. This implies that the model tends to underestimate the fertilizer requirements of oil palm when the initial fertilizer rates are far below the optimum rates. However, the error gets smaller as the recommended fertilizer rates move towards the optimum rates and from experience, the model outputs converge within 3 years under the worst scenario.

Fertilizer Management: Nutrient Losses

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Nutrient losses
Land-clearing and preparation
Runoff and topsoil erosion
Leaching

Nutrient losses 

Soil indigenous and fertilizer nutrients that are not taken up by the palm or adsorbed onto soil particles are dissolved and lost through surface runoff, volatilization, denitrification, or leaching. Adsorbed nutrients may also be lost in eroded soil and sediments.

Losses are more pronounced at particular phases in the life of an oil palm plantation. The potential for nutrient losses is probably greatest immediately after land-clearing when the soil surface is exposed to erosion and uncontrolled surface runoff losses before legume cover plants (LCP) have been established. Losses can be great when large amounts of nutrients such as potassium (K) are released when the standing biomass (e.g. fronds, trunks) is burned during plantation development.

The other period when the risk of nutrient losses is high occurs when ground vegetation is sparse due to poor light penetration through the closed oil palm canopy (Breure, this volume). At canopy closure, the LCP dies off and a large amount of nitrogen (N) is released from the decomposing LCP biomass. Unless palm growth is vigorous, losses of mineralized N due to leaching are likely to be large.

Nutrient losses are more pronounced in areas of the plantation where steep topography and inadequate soil conservation measures result in erosion and uncontrolled surface wash. Clearly, a proper assessment must take account of these temporal and spatial aspects of the potential for nutrient losses.

Leaching losses are more prevalent in coarse-textured soils in high rainfall areas where large fertilizer application rates are required but fertilizer recovery efficiency is poor. Nitrogen may be lost to the atmosphere due to volatilization but, as we shall see, N fertilizers differ in their susceptibility to volatilization losses, which are also affected by the field conditions when the fertilizers are applied.

Land-clearing and preparation 

Oil palm is planted on a variety of land types:

• Logged primary or secondary forest land.
• Land abandoned to alang-alang (Imperata cylindrica) after a period under slash-andburn agriculture.
• Land replanted from plantation crops (e.g. rubber, cocoa, oil palm).

Proper land-clearing techniques are required in order to conserve indigenous soil nutrient supplies and the nutrients returned to the soil in the cleared biomass (Gillbanks, this volume). In the past, when tropical rainforest was converted to oil palm, the felled trees and the organic residues of the previous vegetation were often burned to make field operations (i.e. lining, planting, drainage) easier and thus reduce labor costs, but large amounts of N and sulfur (S) were lost to the atmosphere in the process.

In an experiment to measure the effects of burning biomass on soil properties in Benin, West Africa on acid sand, soil chemical properties and yield were measured at 20 years (Trial A) and 10 years (Trial B) after planting (Sly and Tinker, 1962). In Trial A, there was a significant increase in soil exchangeable K in the burnt treatment (Table 1) but in Trial B, exchangeable Ca and Na, organic carbon and total N were larger in the unburnt treatment (Table 1).

In the first four years of production, yield was larger in the burnt treatment (Trial A) but there were no significant differences in yield when averaged over 11 years of production. The authors concluded that the burning of felled forest under typical Nigerian conditions was not detrimental to later growth and yield of oil palms, and has definite practical advantages in implementing field work (Sly and Tinker, 1962).

Foong (1984) measured soil chemical properties of a virgin Munchong (Typic Hapludox) soil at intervals after land-clearing and LCP establishment (Table 2). Six months after land-clearing, there was a discernible increase in soil pH due to the liming effect of the ash from the burnt vegetation. This seemed to be a temporary effect, as soil pH decreased to 3.9–4.0 thereafter. Total phosphorus (P) content also decreased after land-clearing but was increased substantially after planting LCP, due to the application of phosphate rock during the establishment of legume cover plants (LCP). Organic carbon (C) and total N also decreased at first, but were replenished by the LCP at 62 months after land-clearing when the LCP was shaded out by the oil palm canopy. There were small changes in soil available P and exchangeable K over the period monitored, but there was an increase in soil Ca and Mg. Thus, with proper LCP establishment, soil chemical properties could be maintained or even improved at this site during the first five years after planting (YAP).

In recent years, burning has been prohibited by legislation in Malaysia and Indonesia in response to concerns about environmental pollution and zero-burn land-clearing techniques were developed (Mohd. Hashim et al., 1993). Zero-burn replanting techniques may contribute to improved soil physical and chemical properties because the large quantity of biomass and nutrients contained in palm trunks and fronds is conserved and returned to the soil (Goh and Härdter, this volume; Redshaw, this volume). Felled trunks and fronds should be chipped and spread over the soil surface to provide mulch, reduce localized nutrient build-up, and minimize potential leaching losses.

Although zero-burn replanting techniques are currently the norm in the oil palm industry, it should be pointed out that pest control measures may be exacerbated due to an increase in the population of Oryctes beetles and rats. Thus, whilst zero burn land clearing results in reduced smoke emissions and improved soil properties it amy also result in an increase in pesticide use.

It should be remembered that the cost of replenishing soil fertility is almost always larger than the cost of implementing proper land clearing, land preparation and soil erosion control techniques that contribute to the conservation of indigenous nutrient supplies. Mechanical clearing and burning can result in increased surface runoff, topsoil erosion, leaching, N-volatilization, and P-sorption (von Uexküll, 1986). Soil damage during site preparation may be so severe that LCP establishment is greatly impaired, and this must be avoided.

Runoff and topsoil erosion 

Surface runoff water is the amount of water contained in rainfall and runoff received from higher elevations that does not infiltrate the soil. Runoff is greater where the soil structure has been damaged due to compaction, which causes a reduction in the soil water infiltration rate. In a study in West Sumatra on 10-yearold oil palms, significant spatial variability was found when soil water infiltration rates in the soil beneath the palm circle, path and frond stack were compared. Infiltration rate increased in the order path < circle < frond stack. The larger infiltration rate in the frond stack was attributed to the effect of pruned fronds on soil structure. The smaller infiltration rate in the circle and path was related to soil compaction due to wheelbarrow and human traffic (Fairhurst, 1996).

In a simulation study of in-field transport of fruit bunches and fertilizers, Tan and Ooi (2002) showed that infiltration rate in the mechanization path could be reduced to zero after 24 runs by a 2.3 t mini-tractor grabber carrying a 1 t load. Thus, the use of low ground pressure vehicles for infield transport is strongly advocated to reduce soil compaction.

Soil erosion occurs when soil cover is poor and particles of soil are detached by raindrops and carried offsite. Preventive strategies include the installation of erosion bunds (on slightly sloping land), palm platforms (on sloping land), and contour terraces (on steeply sloping land) (Gillbanks, this volume). Nutrient loss due to erosion are greater on steep slopes are where rainfall intensity is greater, but losses can be reduced by improving soil cover and installing soil conservation structures (Kee and Chew, 1996). It is therefore very important to practice selective weeding in mature oil palm plantations to preserve groundcover and reduce the amount of nutrients lost in surface runoff and eroded soil. When properly arranged in the inter-rows, pruned fronds are an important means to reduce run-off and erosion and thus should not be removed from the field for other purposes (Redshaw, this volume).

The amount of nutrients lost due to runoff and topsoil erosion may be large (Maene et al., 1979) (Table 3) and are usually greater than losses due to leaching. Losses of N and boron (B) in runoff water were greater than 10% of the amount applied as fertilizer, but losses were smaller for the nutrients K, magnesium (Mg) and P (Table 3). This was probably due to the greater solubility of N and B fertilizers and the adsorption of K, Mg and P on soil complex. Losses from surface runoff were larger in the uncovered soil in the harvest path, compared to the interrows, where pruned fronds provide soil cover (Table 3) and improve soil structure and the rate of water infiltration (Fairhurst, 1996).

Other studies indicate that the amount of fertilizer nutrients lost due to surface runoff could be related to the amount and intensity of rainfall immediately after fertilizer application. Kee and Chew (1996) found that N concentrations in runoff water collected after the first rain event following fertilizer application in the wet month of October were 89 mg kg^-1 for Rate 1 at 65 kg N ha^-1 and 135mg kg^-1 for Rate 2 at 130 kg N ha^-1, compared to 4 mg kg^-1 in the control plot.

During the dry period when there was no rain for five days after fertilizer application, however, the N concentrations in the runoff water collected after the first rain event were much lower at 30 mg kg^-1 (Rate 2), and <5 mg kg^-1 (Rate 1 and the control plot). The drier soil surface appeared to result in an increase in the infiltration rate and thus a greater proportion of applied fertilizer was washed into the soil. Similar trends were observed for P, K and Mg fertilizers.

Phosphorus is more likely to be lost due to sheet erosion as it is less soluble than other nutrients and is held strongly on soil particles (particularly in highly weathered inland and upland soils). Sheet erosion also results in the loss of organic matter that forms an important part of the cation exchange capacity in highly weathered tropical soils. Steeply sloping inland and upland soils are more vulnerable to sheet erosion, and thus the effect of erosion on soil fertility in these soils is more pronounced. The subsoil in highly weathered soils is characterized by low cation exchange capacity (CEC) and the presence of small concentrations of plant available K, P, and Mg. The subsoil is thus a less-favorable environment for root growth and root activity, particularly if the concentrations of Al³⁺, H⁺ and Mn²⁺ are large due to low soil pH. For these reasons, the concentration of oil palm feeder roots is greatest in the upper 30 cm of soil (Ng, et al., on botany, this volume). Cover plants are very difficult to establish on areas affected by sheet erosion, and usually soil P must first be replenished before a full LCP canopy can be established. The importance of LCP in soil conservation is illustrated by Ling et al. (1974) in an experiment in Malaysia where runoff and soil loss decreased from 22% under bare soil conditions to 1% where soil surface was covered with LCP (Figure 1).

To summarize, measures to minimize nutrient losses due to surface runoff and soil erosion include the following:

• Maintain adequate groundcover by selective weeding, so that harvesting is not obstructed and competition from weeds is minimized,
• Implement contour planting with properly designed terraces and platforms on steep land,
• Align cut fronds along the contour,
• Mulch with empty fruit bunches,
• Avoid fertilizer application when heavy rainfall is likely to occur, and
• Install contour soil bunds.

Leaching 

Leaching losses occur when nutrients are dissolved into the drainage water as it percolates through the soil profile. Leaching is particularly problematic on coarse-textured soils in the humid tropics where rainfall exceeds evapo-transpiration. Other factors that affect nutrient losses by leaching include soil pore size, rainfall intensity, the initial water content of the soil, and the amount and timing of fertilizer application. The cations Ca²⁺, Mg²⁺, and K⁺ and the anions NO₃ ^– and Cl^– are most prone to leaching (Foong, 1993) (Table 4). Leaching losses are generally larger in older palms, probably because larger amounts of fertilizer have been applied.

In a catchment study in the same plantation where Foong (1993) conducted an experiment on leaching losses, the exceptionally large Mg losses were attributed to the excessive application of kieserite and the application of N and K fertilizers that displaced Mg from cation exchange sites into the soil solution. Losses of P were very small, due to its comparative immobility in the soil (Chang et al., 1994).

In a study on nutrient leaching on Orlu and Algba series (Rhodic Paleudult) soils in Nigeria, Omoti et al. (1983) distinguished between nutrients originating from the soil indigenous supply and nutrients added in mineral fertilizers by using fertilized and unfertilized lysimeters installed 60 cm below the soil surface. Losses of NH₄-N and K were small in young palms in the absence of fertilizer, but for all nutrients, leaching losses from fertilizer were smaller in the older palms compared with the young palms (Table 5).

This outcome is to be expected since the older palms have better root system to absorb applied and indigenous soil nutrients, a larger demand for nutrients, and a higher transpiration rate with a consequent lower water balance for leaching.

Losses of NO₃-N, K and SO₄-S all were greater in the unfertilized soil in older palms compared with young palms, presumably because of nutrient accumulation in the soils due to fertilization prior to the measurements (Table 5).

Calcium was leached in the greatest quantity followed by Cl, SO₄-S, and Mg. As could be expected, losses of NO₃-N were greater than of NH₄-N. It appears that Ca was the main carrier for the anions in these soils, probably due to the relatively low K rate applied during the experiment.

Clearly, there is a wide difference between nutrients in terms of their susceptibility to leaching. Nutrient losses may be large, particularly where organic matter status of the soil is low, in coarse-textured soils, and in areas with high rainfall.

To summarize, measures to minimize nutrient losses due to leaching include the following:

• Implement balanced nutrition (nutrients supplied according to crop demand).
• Split large application rates into a number of smaller doses (particularly for N, K, and Mg).
• Spread fertilizers evenly to maximize contact with the root system.
• Avoid fertilizer application during periods of heavy rainfall (by using statistical techniques or expert systems to predict the occurrence of dry periods).
• Apply empty bunches and cut fronds to increase soil organic status and cation exchange capacity.
• Increase the soil pH through liming to increase soil cation exchange capacity in variable charge soils

Reference
Goh K.J., Rolf Härdter and Thomas F. (2003) Fertilizing for maximum return. In: Thomas Fairhurst and Rolf Hardter (eds). Oil palm: Management for large and sustainable yields. Potash & Phosphate Institute and International Potash Institute: 279-306

Note: The full list of references quoted in this article is available from the above paper.

Fertilizer Management: Application

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Once the amount required and source of each fertilizer nutrient has been determined (Foster, 2003), a strategy for the placement, and frequency and timing of application must be considered.

Strategies for the placement of fertilizer
Frequency and timing of fertilizer application
Timing of fertilizer application

Strategies for the placement of fertilizer 

It is axiomatic that fertilizers should be placed where they can most readily be absorbed by feeding roots of the crop. The proportion of the soil volume exploited by the oil palm increases with palm age (Ng et al ., 1968; Ruer, 1967) but the rate of expansion depended on soil type (Tan, 1976). Palms absorbed labeled 32P applied over 30 m from the point of application, even when the palms were separated by a 65 cm deep trench (Zaharah et al ., 1989). Physical disturbance of the soil in the path inter-row due to mechanized fruit collection also affected root growth in this zone (Mokhtaruddin et al ., 1992) and the quantity of roots was increased by more than 20% following sub-soiling of compacted palm inter-rows (Caliman et al ., 1990b).

Based on cursory investigations in the field, it is sometimes asserted that there are generally more active feeder roots in the soil beneath the frond stack compared with soil from beneath the weeded circle. In a detailed study in West Sumatra on palms 10 YAP, however, no difference was found in feeder root length density between these two zones but root length density was smaller in soil beneath the harvesting path, where the soil was more compacted than the other two zones due to frequent wheelbarrow traffic (Fairhurst, 1996) (Figure 1). In the soil beneath the area where fertilizer had been applied, root length density was greater, suggesting that roots proliferate where the concentration of nutrients is greatest (Figure 2). Other workers reported the positive tropism of oil palm roots towards areas with better water and nutrient supply, with a greater concentration of roots in soil beneath the frond stack in the palm inter line (Bachy, 1964; Tailliez, 1971), and at the edge of palm circles where there had been an accumulation of organic debris (Purvis, 1956). The quantity of roots in soil beneath the harvesting path was reported to be small (Hartley, 1977).

Figure 1. Contour map showing root length density (RLD) in a transect between three palms across the harvest path and frond stack interrows in a field of palms in West Sumatra 10 years after field planting (Fairhurst, 1996).

Figure 2. Root length density of primary, secondary, tertiary and quaternary roots in the circle facing the front stack (Circle S) and harvest path (Circle P), and frond stack in a field of palms in West Sumatra 10 years after field planting (Fairhurst, 1996). [Bars represent standard error of the means, n=7)

Fertilizer application rates may be very large, particularly when the rate is calculated based on the area of soil over which the fertilizer is applied. Palm circles occupy only 20% of the soil surface area under oil palm and thus, for example, 1.5 kg palm^-1 urea applied over the weeded circle is equivalent to an application of 1,000 kg ha^-1.

From an agronomic point of view the application of fertilizers over the weeded circle would, at first, appear to be unsatisfactory because

• the root system in mature palms extends far beyond the boundary of the weeded circle (Ng, et al ., on botany, this volume),
• the soil beneath the circle may have insufficient cation exchange capacity to store the large amount K and Mg applied but not immediately taken up by the palm, resulting in increased leaching losses,
• the application of large amounts of a particular cation (e.g. K) may result in the displacement and leaching of another cation (e.g. Ca), and
• the application of large quantities of urea and sulfate of ammonia may cause soil acidification (and a consequent reduction in cation exchange capacity in variable charge soils).

Some arguments can be made in favor of fertilizer placement over the frond stack:

The infiltration rate in soil beneath the frond stack is more rapid, however, and this may result in greater losses of K and Mg fertilizers due to leaching. Since the water infiltration rate in the soil in the weeded circle is often reduced due to compaction, however, fertilizers applied over the weeded circle may be washed out and distributed over the surrounding area. Clearly, the selection of a suitable placement strategy must take into account the nature of the fertilizer material, the particular nutrient applied and the age of the palms.

There are three reasons why there was, in the past, a tendency to apply fertilizers over the circle:

We will now review some past experiments that investigated the effect of fertilizer placement on nutrient use efficiency. Fertilizer placement studies have generally produced inconclusive results despite large yield responses to fertilizer in a number of experiments (Table 1). In fertilizer experiments carried out in Malaysia, yield was larger when P was applied in the harvest path avenue compared to the frond stack and circle, and when K was applied in the frond stack compared to the circle (Foster and Dolmat, 1986). In contrast, Teoh and Chew (1985) and Yeow et al ., (1982) found no difference in yield between different placement strategies. Of particular interest is the increased response to fertilizer in experiments carried out in Malaysia when palm fronds were broadcast over the inter-rows compared to the placement of fronds in alternate palm rows, and when fertilizer was applied together with an application of 3.5 t ha^-1 empty bunches (Chan et al ., 1993). To summarize, fertilizer application over clean weeded palm circles, over the outside edge of the weeded circle, or over the frond stack gave similar yield responses in mature oil palms planted on coastal soils, NPK fertilizer could be applied in alternate avenues in the oil palm plantations without reducing efficiency.

Table 1 . Effect of fertilizer placement on bunch yield in Malaysia.

Foster and Tayeb (1986) measured the effect of different fertilizer placement strategies on yield of palms 7-9 and 10-11 YAP (Figure 3). Very similar results were obtained for both age groups:

Goh et al . (1996) measured K uptake indirectly in an experiment with palms 16 YAP on a Rengam Series soil (Typic Paleudult). Two 1-m² plots were marked within each microsite, i.e. palm circle, interrow, frond stack and harvest path. At each micro site, one plot was isolated by a trench (0.3 m wide x 0.9 m deep) and K uptake was estimated from total K contents in the 1-m² plots by difference. The plots were allowed to settle for a year before K fertilizer treatment (500 kg K ha^-1) was applied. In the fertilized plots, K uptake was greatest in the palm circle, followed by the inter-row, frond stack and harvest path, where uptake was probably affected by soil compaction (Table 2). In unfertilized plots, K uptake was greatest in the palm circle where the concentration of exchangeable K (0.22 cmol kg^-1) was the smallest of the areas sampled.

Figure 3a. Effects of different fertilizer N placement strategies on bunch yield in oil palm at 7-9 and 10-11 years after field planting (Foster and Tayeb, 1986).

Figure 3c. Effects of different fertilizer K placement strategies on bunch yield in oil palm at 7-9 and 10-11 years after field planting (Foster and Tayeb, 1986).

Table 2 . Effect of frequency of fertilizer application on oil palm yield in Malaysia.

In addition to nutrients supplied in fertilizer, small quantities of nutrients may be added in rainfall. Annual rainfall of 2,000 mm in Malaysia contained about 5 kg K ha^-1 yr^-1 but a substantial amount of K was leached from the canopy resulting in the addition of 36 kg ha^-1 yr^-1 to the soil in through-fall (Goh et al ., 1994).

One reason for the inconclusive results in past investigations on the effect of fertilizer placement is that gradients in root distribution may already have been established at the start of each experiment. Thus, when treatments to compare broadcast fertilizer with application in weeded circles are installed in a field of palms where root gradients are already pronounced, nutrient uptake is likely to be less efficient in areas of the field that have not received fertilizer or pruned fronds in the past, such as the harvest path, and where root development is poor. Ideally experiments on fertilizer placement should be established in fields of young palms so that both uptake efficiency and the effect of nutrients on root development are taken into account.

Broadcasting fertilizers over the entire soil surface under mature palms has also been advocated because it results in an overall buildup of soil fertility (and probably more uniform root distribution), avoids excessive nutrient buildup (and acidification) in the palm circle, and reduces leaching losses of K and Mg in the palm circle. Clearly, fertilizer placement is not an issue in plantations that have changed to mechanical fertilizer application due a shortage of labor for manual application. Fertilizer use efficiency may increase where fertilizers are broadcast due to more even root distribution.

Fertilizer placement strategies for mature palms must take into account the characteristics of each fertilizer, oil palm root development and palm age (Table 3). Placement strategies should also be adjusted to take into account soil properties, weed management (some companies prefer bareground conditions or sparse vegetation favoring fertilizer application in the palm circle), and rainfall distribution.

It is recommended that bunch ash is applied around the weeded circle to palms 4 -7 YAP, and outside the weeded circle in palms >7 YAP .

Table 3. Recommendations for fertilizer placement by manual application for oil palm.

Frequency and timing of fertilizer application  

Hew and Ng (1968) showed that uptake efficiency was increased with more frequent applications of fertilizer and designed a schedule for fertilizer application according to tree age and fertilizer source.

The frequency of fertilizer application is constrained by

• the time it takes to apply a single application of fertilizer in a management unit,
• the number of fertilizers that must be applied in a year, and
• the requirement for a period of two months without fertilizer application prior to leaf sampling.

Thus, there is potential for ten fertilizer ‘applications in a year assuming one application can be completed within a month in a single management unit of 1,000 ha. The most suitable frequency for fertilizer application depends on:

• the nutrient’s susceptibility to leaching,
• the soil’s capacity for nutrient retention, and
• local patterns of rainfall distribution and intensity.

Because NO₃^– produced from the mineralization of N-fertilizer is highly susceptible to leaching, more frequent applications may be required for N fertilizers than for P fertilizers, which are comparatively immobile in the soil. Frequency of K and Mg application should be related to soil clay content and mineralogy, and the soil’s cation exchange capacity.

On a sandy soil in Malaysia, the yield response to P, applied as rock phosphate was greater when applied annually compared to once in four years, but frequency of application had no effect on leaf P content (Foong and Sofi, 1995) (Table 4). Larger yields were obtained when N, P, and K were applied three times a year compared to once a year on a Rengam soil (sandy clay texture) with small cation exchange capacity (<10 cmol kg^-1) (Foster and tayeb, 1986) but on Serdang (silty clay loam texture) and Munchong (clay texture) soils with a small cation exchange capacity there was no advantage from increased frequency of application of NK fertilizer, provided fertilizers were applied during periods of low rainfall (Teoh and Chew, 1985) (Table 4). Results from other fertilizer frequency experiments on mature oil palms are more equivocal (Chan et al ., 1993; Chan et al ., 1994) (Table 4). The general trends showed that N, K, and NK fertilizers could be applied once a year for optimum yield, while the less-soluble phosphate rock could be applied in alternate years. It should be noted, however, that these experiments used soluble fertilizers on heavy textured sandy-clay to heavy-clay soils and may not be applicable to light-textured soils.

Although humid tropical climates with annual rainfall of 2000 – 2,500 mm imply the loss of large amounts of nutrients through leaching, the large evaporative demand of oil palms suggests that leaching losses may in fact be small (Chang and Chow, 1985). Nutrients lost by leaching represented between 2-5 % of the nutrient content of fertilizers applied to a clay loam soil in a lysimeter planted with oil palms and legume cover crop where annual rainfall was 1,800-3,000 mm. Losses were different for each nutrient, increasing in the order P<N=K<Mg, and the largest losses occurred during periods when monthly rainfall exceeded 200 mm (Foong, 1993). In contrast, on an acid sand soil in Nigeria where annual rainfall was 2,000 mm, 34, 18, 172, and 60 % respectively of the fertilizer N, K, Ca, and Mg were leached from the soil in an experiment in which lysimeters were installed 150 cm below the palm circle. These two experiments illustrate the larger amounts of nutrients, which may be lost through leaching on coarse textured sandy soils (probably with small cation exchange capacity) compared to clay soils.

Table 4. Estimated K uptake by oil palm from different soil zones on a Rengam Series (Typic Paleudult) soil in Malaysia (Goh et al ., 1996).

To summarize, whilst there is no empirical proof that increasing the frequency of application always increases uptake efficiency, it is common practice to apply N and K fertilizers 2-3 times per year to reduce the risk of nutrient losses, and kieserite and rock phosphate once per year. Application frequency is usually increased in very young palms where, for practical reasons, the use of compound and mixed fertilizers (mixtures) supplemented with straight fertilizers is common. Fertilizers are spread much more evenly with mechanical application when compared with manual application and it may be possible to decrease the frequency and increase the application rate at each dose without adversely affecting uptake efficiency.

It is clear that applying very large amounts of fertilizer to any crop at one time may result in large losses due to leaching, surface runoff and erosion. The planter must therefore attempt to synchronize the supply of mineral fertilizer nutrients with palm demand. Unlike annual crops, the demand for nutrients in oil palm is continuous and in the end, the optimal frequency is a compromise between meeting nutrient demand, and supplying these nutrients without incurring excessive labor costs or organizational difficulties.

Timing of fertilizer application  

Very little has been published on the effect of the timing of fertilizer application on fertilizer use efficiency (Teoh and Chew, 1980). Runoff losses, however, can exceed 45% of rainfall during months with high rainfall (November-December). Unlike other crops, where fertilizer application must be timed according to particular phases of vegetative and generative growth, the oil palm produces bunches throughout the year and thus requires a continuous supply of nutrients. The importance of timing is thus mainly related to the use of N fertilizers that are susceptible to loss by volatilization (Thompson, 2003). It may be possible to improve the timing of N fertilizer application by taking into account rainfall patterns and distribution and for this purpose each plantation should install a rain gauge (mm month^-1) and a pluviometer (rainfall distribution during each day). To optimize recovery efficiency of N from urea, applications should always be followed by light rain and urea should never be applied to dry soil.

To summarize, fertilizer application should be avoided during months with a high probability of rainfall exceeding 250 mm month^-1 and months with >15 rain-days. Losses of soluble P, K, and Mg fertilizers from runoff are smaller if applied in dry months (<100 mm month^-1) in Malaysia.

Details on placement, frequency and timing of fertilizer application can be found under deficiency page.

Apply fertilizer in terrace area manually. Fertilizer brought in by buffalo (Photo taken by GKJ)	Applying fertilizer manually (Photo taken by GKJ)
Well spread out fertilizer mixture (Photos taken by GKJ)
Using buffalo to deliver fertilizers in terraced areas (Photos taken by GKJ)
Aerial application (Photo – courtesy of Chung GF)	Mechanical spreader to apply fertilizer (Photo taken by OLH)

Reference
Goh K.J., Rolf Härdter and Thomas F. (2003) Fertilizing for maximum return. In: Thomas Fairhurst and Rolf Hardter (eds). Oil palm: Management for large and sustainable yields. Potash & Phosphate Institute and International Potash Institute: 279-306

Note: The full list of references quoted in this article is available from the above paper.

Fertilizer Management: Sources

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A wide range of fertilizer products is available on the market. The choice of fertilizer depends on the following factors:

• Nutrients required.
• Availability of fertilizers.
• Physical and chemical properties (nutrient concentration, availability) of fertilizers.
• Cost ($ kg^-1 N, P, K, Mg, B, and Cu).
• Soil characteristics (pH, clay content and type, texture).
• Terrain (e.g. flat, sloping, hilly).
• Palm age and condition.
• Climate.
• Availability of labor.

In general, the water soluble fertilizers are used for immature palms, correction of nutrient deficiencies, and aerial application. Water insoluble fertilizers (e.g. dolomite, rock phosphate) are used on acid soils to provide a sustained slow release of nutrients, to counter the acidifying effect of urea and SOA, and to build up soil fertility. The common fertilizers used in oil palm are listed by Goh and Härdter (2003) and a comprehensive account is given by Chew et al. (1994). Also, the common sources of fertilizers for each nutrient are described under Deficiency section.

Reference
Goh K.J., Rolf Härdter and Thomas F. (2003) Fertilizing for maximum return. In: Thomas Fairhurst and Rolf Hardter (eds). Oil palm: Management for large and sustainable yields. Potash & Phosphate Institute and International Potash Institute: 279-306

Note: The full list of references quoted in this article is available from the above paper.

Fertilizer Management: Toxicity

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Aluminum toxicity is widespread in the humid tropics where soils are acid and highly weathered. Low pH results in the accumulation of soluble Al in the soil that reduces root growth and impairs root function. Aluminum toxicity is thus manifested in the appearance of Mg and K deficiency, particularly in young palms but can be corrected by applying rock phosphate or dolomite at planting to ameliorate soil pH. Magnesium fertilizers have also been shown to reduce the effect of Al toxicity on plant growth for a variety of crops (Grimme and Härdter, 1991).
Nickel toxicity can be a difficult problem in soils derived from ultrabasic or ‘serpentine’ rocks. Palms affected by Ni toxicity exhibit narrow, fish-net like chlorotic patterns on younger leaves, and growth can be severely retarded. Nickel toxicity is difficult to correct and, whilst mulching with EFB (where available), liming, and additional K fertilizer may help to alleviate the symptoms, it is not recommended to plant oil palm on ultrabasic soils. Some sources of poor quality rock phosphate are also known to contain a large concentration of Ni, and these fertilizer materials should not be used.
Micronutrient toxicities are rare under humid tropical conditions and are usually only found after excessive application of micronutrient fertilizers. Great care must be taken when choosing the application rate for micronutrient fertilizers since the difference between deficiency and toxicity is often quite small. Copper toxicities may develop as a result of excessive and careless fertilizer application, the latter especially from excessive use of Cu-containing fungicides.
Copper toxicity symptoms first appear as small, oval or round, light brown spots on the leaf surface. The spots have depressed centers and may coalesce into extensive necrotic areas with yellow margins on the leaf surface.
Boron toxicity symptoms start on the younger leaves where interveinal chlorotic streaks appear on leaf tips. Chlorosis is rapidly followed by necrosis, developing from the distal to proximal end of leaves. Boron toxicity may be corrected by the application of N fertilizers, which precipitate B and improve palm growth by diluting the concentration of B in palm tissue.
Boron toxicity
The application of molybdenum to palms as foliar spray resulted in a decrease in yield in young palms on an acid sand soil in Nigeria (Ataga et al ., 1982). Bunch yield was also reduced when palms were treated with a foliar spray of manganese. In both cases, however, no leaf symptoms were reported.
Excessive application of soluble fertilizer may lead to fertilizer scorch to the oil palm canopy	Agronomic team (Zulkifli, GHH, AS, HR) examining hyperacidity problem of oil palm on deep peat
The hyperacidity disorder can be found on palms planted on excessively drained acid sulfate and peat soils. Typical symptoms include the gradual necrosis and desiccation of leaflets on lower fronds, but the spear and young leaves are not affected. Liming with dolomite or BA is not very effective and hyperacidity can be better prevented by maintaining the water-table just above the jarosite layer or 30 cm from the surface, whichever is lower.
Hyperacidity	Hyperacidity – damage to rachis
(Photos taken by GKJ)
Reference Goh K.J. and Rolf Härdter (2003) General oil palm nutrition. In: Thomas Fairhurst and Rolf Hardter (eds). Oil palm: Management for large and sustainable yields. Potash & Phosphate Institute and International Potash Institute: 191-230
Note: The full list of references quoted in this article is available from the above paper.

Fertilizer Management: Principles

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Objectives of fertiliser management
Computation of fertiliser requirement
Balanced fertilisation
Potential nutrient losses and environmental concerns
Economics of fertiliser recommendations
Additional agronomic principles for young palms

Objectives of fertiliser management 

i) The objectives of fertiliser management in oil palm used to be straightforward as follows:

• To supply each palm with adequate nutrients in balanced proportion to ensure healthy vegetative growth and optimum economic FFB yields.
• To apply the fertilisers in the prescribed manner over the areas of the estate that are likely to result in the most efficient uptake of nutrients.
• To integrate the use of mineral fertilisers and palm residues.

ii) However, the following conditions make achieving the objectives a challenge nowadays:

• Shortage of reliable and skilled workers, and high turn-over in work force.
• Environmental concerns which are related to over-fertilisation, land degradation, and pollution from heavy metals e.g. cobalt and eutrophication by P.
• Expansion of oil palm into areas with little information on the soil properties, climate etc which are necessary for good fertiliser management e.g. the cultivation of oil palms on ultrabasic soils.
• Managing larger manuring blocks which can result in over generalisation. In fact, this approach goes against the current trend of site-specific fertiliser management and precision agriculture.
• Rising fertiliser prices which increase production costs.
• Planting oil palm in countries where lack of clear law and order or understanding them can be a yield-limiting factor e.g. Indonesia and southern Phillipines.

iii) Therefore, the agronomic principles of an effective fertiliser management should take all the above into account and balance the above needs and objectives with the resources in the estates. The key steps are:

• Determine the growth and yield targets.
• Assess the nutrient requirements to attain the above and prevent the occurrence of nutrient deficiency.
• Assess the management level and resources of the estate.
• Ascertain the most efficient and cost effective fertilisers and applications of fertilisers to meet the nutrient requirements.
• Compute the economics of the recommendations and expected results.
• Monitor the outcome including the economic returns.
• Decide on further action required and repeat the steps if necessary.

iv) Most of these steps should be covered by other lectures in this course but for completeness and comprehensibility of our lecture, we shall briefly discuss them.

Computation of fertiliser requirement 

i) There are several methods commonly used for the formulation of fertiliser recommendations. These include:

• Critical leaf and/or soil nutrient level method
• Optimum nutrient ratio method
• Yield response function method and
• Nutrient balance method

ii) In actual practice, derivation of fertiliser rates does not rely exclusively on any one method. An integrated approach, which combines the above methods, is usually adopted and AAR is one of its proponents.

iii) Primarily, the nutrient balance method is employed first to compute the nutrient requirements of oil palm in a manuring block. This approach assumes that the oil palm agroecosystem has definite components of nutrient removal (demand) from the system and nutrient return (supply) to the system (Figure 1). The components of nutrient demand are:

• Growth
• Yield
• Nutrient losses through leaching, run-off and erosion
• Nutrient removed by pest damage and
• Nutrient non-availability and antagonisms.

The components of nutrient supply are:

• Nutrient returns from the palms, e.g. pruned fronds
• Nutrient returns from leguminous covers
• Rainfall
• Soil
• Fertilisers

The basic principle is then to estimate the total demand of the palm and match it with the nutrient supply by the oil palm agroecosystem excluding the fertiliser component. The shortfall between the nutrient demand and supply, which is also called gross nutrient requirements, should be met by fertilisers.

iv) A number of studies have been made to quantify the various components of nutrient demand and supply in the oil palm agroecosystem.

v) The two largest components of nutrient demand are Growth and Yield. They are also the first key steps in an effective fertiliser management scheme as outlined earlier. Thus, it is essential that the agronomist estimates the growth rate and yield trend of a manuring block right from the start. A typical example of the growth rate of oil palm using leaf area as the criterion is shown in Figure 2. Coupled with the leaf nutrient concentrations, the agronomist will be able to estimate the nutrient requirements necessary to attain the expected growth. Similarly, the yield profiles in different regions of Malaysia as illustrated in Figure 3 will provide a clue on the nutrient removal per year from the manuring block which should be replaced by fertiliser inputs.

vi) On the nutrient supply side, available data suggests that atmospheric returns are probably insignificant. However, pruned fronds can provide substantial nutrients to the palms to the tune of 36% for N and 27% for K on poor inland soils in Peninsular Malaysia. In mature oil palm areas, the last component of nutrient supply is soils. Unfortunately, most Malaysian soils including those from Sabah are inherently poor in nutrients particularly N and P (Table 1). Therefore, most of the nutrients required by the palms have to come from fertilisers, usually in mineral forms.

• An example of the computation of nutrient balance and fertiliser requirements to sustain 30 t/ha/yr in a mature oil palm field is shown in Table 2. It is assumed that the oil palm is in a steady state and grown on a soil with poor fertility. Under steady state condition, the canopy size remains constant and therefore, the nutrient requirements for canopy growth should be met by the nutrients recycled from the pruned fronds. The final analysis shows that the annual fertilisers needed for each palm to satisfy the gross nutrient requirements totalled 10.75 kg and comprise 4.22 kg Ammonium chloride, 0.97 kg Jordan phosphate rock, 3.59 kg Muriate of Potash and 1.97 kg Kieserite.

• While the nutrient balance approach provides the gross nutrient requirement, it does not work out the fertiliser requirements directly. We need information from fertiliser trials to enlighten us on the optimum fertiliser rates and the yield responses. In Sabah, the oil palms respond mainly to N fertiliser followed by K and P fertilisers (Table 3). The response to N generally exceeds 15 % except on Lumisir Family soil. The latter might be attributed to its high inherent soil fertility status as indicated by the yields in the control plots (no fertiliser). K responses are mainly lower than those experienced in Peninsular Malaysia. Again, this can be explained by the relatively high soil exchangeable K status as shown in Table 1. These results strongly imply that the agronomist must know and understand the soil properties in the manuring blocks, not just the soil names, to draw up proper and effective fertiliser recommendations to the estates.

• We can also predict the fertiliser efficiency in each trial by plotting the gross nutrient requirements against the fertiliser rates as shown in Figure 4 while Table 4 shows the fertiliser efficiencies in some coastal and inland soils in Peninsular Malaysia. The highest K fertiliser efficiency was in Munchong series soil at 83%. This was due to the poor soil K reserve and good yield response to K fertilization. The lowest fertiliser K efficiency was found in Briah series soil at 19% due to high fertiliser rates and soil K status. In general, fertiliser efficiency is affected by the gross nutrient requirement, imbalanced nutrition, fertiliser rates, soil fertility and nutrient losses.

• Collating and assimilating the data from fertiliser trials conducted worldwide have enhanced the confidence of the agronomists to extrapolate the results to other sites with similar conditions and combining them with nutrient balance computation, leaf analysis and soil fertility status to produce the fertiliser recommendations.

Balanced fertilisation 

• High fertiliser rates alone will not always provide optimum economic returns: a balanced fertiliser program is also essential as illustrated in Table 5. Nitrogen increased yield by 49% in the presence of high K rate. Similarly, there was a 25% yield response to K when high N rate was applied. Both N and K also had beneficial effect on the vegetative dry matter production.

• Apart from the above, application of K fertiliser will decrease oil to bunch ratio in the absence of N fertiliser (Table 6). However, with sufficient N level, K fertiliser generally increased the oil to bunch ratio to similar level compared to the control.

• Positive interactions of K fertiliser with other agronomic practices such as mulching, frequency of application and frond placement have been reported to increase yield between 4% and 14%.
• While capitalising on synergistic effects will improve yield and fertiliser efficiency, avoidance of antagonistic effects is also necessary to maximise fertiliser use. For example, high K rates have been shown to depress Mg and B uptakes and might decrease yield.

Potential nutrient losses and environmental concerns 

The recommended fertilisers should be applied in a manner that they are absorbed by the palms at maximum efficiency. This is best done by minimising fertiliser losses in the plantation, which is even more important now in view of the current economic woes. It should also minimise environmental problems if any.

Nutrients may be lost by surface run-off, leaching through the soil profile, nutrients fixation, volatilisation and immobilisation by ground covers in young oil palm. An understanding of these nutrient loss mechanisms is essential to alleviate them and improve fertiliser efficiency.

i) Surface run-off

• On average 11% of N, 3% of P, 5% of K, 6% of Mg and 5% of Ca applied can be lost in surface run-off alone (Table 7). These results were obtained during a low rainfall year with only 1426 mm on a 9% slope. The most susceptible areas for run-off tend to occur in the harvester’s path and along the oil palm rows where the soils are more compacted and the ground vegetation is generally sparse.

• More recent data obtained by AAR also indicate that the mean run-off losses as percentage of the nutrient applied are within the following ranges: 5-8% N, 10-15% K, 4-6% Mg and less than 2 % for P (Table 8). These results show that soluble nutrients such as N, K and Mg are more susceptible to run-off losses. We further found that nutrient losses via surface run-off are highly dependent on the rainfall pattern at the time of fertiliser application, particularly during the first few rains after application and the antecedent moisture status of the soil. Other equally important factors, which might affect run-off, are the canopy cover, rainfall intensity and quantity, soil characteristics and slope.

ii) Leaching losses

• Leaching losses during the first four years of oil palm growth (as % of total nutrient applied) have been found to be about 17% N, 10% K and 70% Mg. Losses are substantially reduced to about 3% N, 3% K and 12% Mg when the palms are fully matured (Table 9). The main reasons for the high leaching losses during the early stage of palm growth are probably poor palm canopy cover, less extensive root system and ground covers are generally not well established especially during the first year after planting.

iii) P Fixation

• Losses due to fixation by the soil involve mainly phosphate fertilisers. The P fixing capacities of some of the common Malaysian soils are shown in Table 10. The amount of P ‘fixed’ ranged from 208 mg to 1172 mg per kg soil and is related to its clay mineralogy. Although soils with high P fixing capacity improve P dissolution of phosphate rock, they also decrease the soil solution P (intensity), which is required for plant uptake. The general approach is to use less reactive phosphate rock and concentrated application of fertiliser through high rate and banding for these soils.

iv) Volatilisation losses

• Volatilisation losses are only significant when urea is surface applied, usually over the compacted weeded palm circles. High volatilisation losses in the oil palm field occurred at high rates of fertilization and on light texture soils as shown in Table 11.
• To increase the efficiency of urea, it should preferably be buried in the ground. However this practice is only suited to small-scale cultivation and unlikely to be practical and economical on a large plantation. Correct timing provides a more suitable means to improve the efficiency of applied urea. For example, volatilisation loss is reduced if urea is applied when moderate rains are expected so that the fertiliser may be washed into the soil.

v) Immobilisation by ground cover in young oil palm

• Weed growth is strongest in high light conditions in immature plantation. The young palms without extensive root systems are less able to compete for nutrients at this stage, which reduce their nutrient uptake and growth (Table 12). One point of interest is that the total N immobilised by the ground covers commonly exceeded run-off losses and immobilisation by young oil palms.
• With respect to interrow vegetation management, spraying out the competitive weeds in the interrow vegetation at immaturity and maturity on Selangor series soil (fertile soil) gave the highest oil palm yields after 4 and 6 ½ years respectively. On the other hand, over spraying could lead to bare ground conditions which might cause higher leaching losses, reduce soil moisture and result in poorer soil structure. This in turn may lower FFB yield.

Economics of fertiliser recommendations 

• The plantation industry is a business proposition and as such, the economic value of a fertiliser is important. This is because the application of fertiliser necessarily increases the cost of production, which has to be at least offset by an increase in yield in order to be profitable.
• Owing to the delay in the effect of fertiliser on yield, the additional return from the increased yield may be realised in full only after 8 months or even a few years. Furthermore, the magnitude of yield response may vary considerably and the economic comparisons of fertilisers should be based on a discounted cash flow or a similar scheme over the specified period.
• An example of the economic computation of two sources of fertiliser is provided in Table 13. We choose kieserite versus ground magnesium limestone (GML) to illustrate the point that knowing the agronomic efficiency of a fertiliser as obtained from fertiliser trials is insufficient to recommend its application. Table 13 shows that the agronomic efficiency of GML based on substitution rate was only 74% as effective as kieserite. However, GML was only one-third the price of kieserite at the time of writing. This favoured GML with the consequent relative economic efficiency reaching 2.5. This meant that GML was 1.5 times more efficient compared to kieserite in economic terms.
• Using the above approach, an expensive fertiliser may be more economical to use if its agronomic efficiency far outweighs its price ratio compared to its competitors.
• Although the above computation is a standard in economics, of late there are counter arguments which suggest that the selection of a fertiliser should be based on its agronomic efficiency instead of economic efficiency. This contrasting proposition stems from the fact that commodity prices are usually unpredictable and therefore, the economic efficiency can vary substantially. Such view is probably a fallacy since decision-making processes in agriculture, like all businesses, are always done in the face of uncertainty, be it prices or weather etc. Moreover, the use of tender fertiliser prices will allay or negate part of the problems. In plantation agriculture, profit considerations are given the highest priority and therefore, the economic efficiency will always take the centre stage.

Additional agronomic principles for young palms 

The strategy in young palms, apart from the above, should be:

• To minimise nutrient requirements by maximising returns from the biomass of the previous crops e.g. rubber, cocoa or oil palm by the shredding and no-burn techniques currently practised in many plantations.
• To promote growth of very good leguminous covers with high P and Mg applications and subsequent large nutrient return including N fixed.

Such an approach would reduce fertiliser requirements of the young palms substantially and improve growth and yields, thereby leading to extensive benefits all round.

Reference
Goh, K.J., Teo, C.B., Chew, P.S. and Chiu, S. B. (1999) Fertiliser management in oil palm: Agronomic principles and field practices. In: Fertiliser management for oil palm plantations, 20-21, September 1999, ISP North-east Branch, Sandakan, Malaysia: 44 pp

Note: The full list of references quoted in this article is available from the above paper.

Oil Palm: Fertilizer Management

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Introduction

• Many papers have been written to highlight the importance of fertilisers for oil palm. The main premise is that healthy palms will produce optimum FFB (fresh fruit bunch) yield, which is the primary commodity of most plantations.
• Oil palm is unrivalled in its ability to convert solar energy into dry matter and vegetable (palm) oil. However, this process requires a large amount of nutrients which must be supplied by the soil or fertilisers.
• Unfortunately, most soils grown with oil palms have low soil fertility and therefore, mineral fertilisers are usually necessary to achieve and sustain good palm nutritional status and large yields.
• In fact, fertilisers alone constitute about 24% of the total production cost of oil palm in Malaysia. The present economic slowdown has caused the Malaysian Ringgit to depreciate against the US dollar with the consequent rise in most fertiliser prices. This has increased the production cost of oil palm by as much as 13%.
• One of the best means to reduce production cost is to sustain maximum yield at any one site. The maximum yield is usually close to the optimum yield because of the high indirect costs in oil palm management. However, the optimum yield is subject to the vagaries of commodity prices and therefore, difficult to predict, let alone sustain. Hence, we advocate the approach to maximise and maintain the highest yield possible at any one site, which is also known as site yield potential.
• The above is one of the central tenets of plantation management because it enables the highest revenue to be attained at the lowest possible cost for an assured best profit. This will help to enhance the attractiveness of the oil palm industry.
• In fact, the ability of the oil palm industry to compete with others is highly essential if we are to attract reliable and skilled workers and reduce the high turn-over of work force. This is vital towards the long-term sustainability of oil palm plantations.
• The above points show that the benefits of sound fertiliser management for oil palm go beyond preventing nutrient deficiency and maintaining healthy palms, which have long been recognised by the industry. Therefore, it is not surprising that the Malaysian oil palm industry has invested millions of dollars in research and development on fertiliser use since the 1920’s when oil palm was first commercially grown.
• These notes discuss the main issues of fertiliser management in oil palm in the face of the changing scenarios in plantation management. It covers the following:
  1. Agronomic principles in fertiliser management
  2. Deficiency symptoms and correction
  3. Toxicity symptoms
  4. Sources of fertilizer
  5. Methods of fertilizer application
  6. Computation of optimal fertilizer rates
  7. Pathway of soil and fertilizer nutrient losses
  8. Fertilizer efficiency
  9. Future works & Research in oil palm agronomy & work on precision agriculture
  10. Current Challenges in oil palm plantations

Reference
Goh, K.J., Teo, C.B., Chew, P.S. and Chiu, S. B. (1999) Fertiliser management in oil palm: Agronomic principles and field practices. In: Fertiliser management for oil palm plantations, 20-21, September 1999, ISP North-east Branch, Sandakan, Malaysia: 44 pp

Leaching Losses: Conclusion

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The downward movement of N fertilizer was mainly in the form of NH_4-N with very little NO_3-N because the source of fertilizer was NH₄Cl. This study also indicated that the rate of nitrification was probably slow during the monsoon period. The concentration of N and K in the soil solution decreased with soil depth being highest at 30 cm from the soil surface followed by 60 and 120 cm. The N leaching losses of the applied N fertilizer during the monsoon period in Sabah, North Borneo were 1.0 and 1.6% for treatments, N1P2K0 and N1P2K1, respectively. Higher K leaching losses were obtained at 5.3 and 2.4% for N0P2K1 and N1P2K1, respectively. The groundwater quality under mature oil palms did not exceed the contamination level set by WHO when N and K fertilizers were applied at their optimum rates for oil palm. However, there was a possibility of pollution of groundwater quality when excessive N fertilizer was applied which was mainly in the form of NH₄ ⁺ ion. The concentration of NO₃ ^– ion in the groundwater was below 0.5 mg L^-1 even when excessive N fertilizer was applied due to low nitrification rate.

Leaching Losses: Discussion

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Downward movement of N and K: The concentrations of inorganic N in the soil solutions throughout the vertical soil profile were mainly dominated by NH₄⁺ ion rather than NO₃^– because the source of N fertilizer was ammonium chloride. Significant increased in NO_3-N in the soil solution was only observed 75 days after fertilizer application. This implied that nitrification was relatively slow in the soil during the monsoon period. This might be attributed to the high NH₄⁺ concentration which inhibits the activity of nitrifiers in the soils and the low soil organic matter which reduces the population of nitrifiers[40].

The concentrations of N and K in the soil solution decreased with soil depth being highest at 30 cm from the soil surface followed by 60 and 120 cm. Similar findings were reported by Schroth et al.[32]. This nutrient profile might be partially explained by dilution effect as the solubilized N and K fertilizers seeped downward with the surplus soil moisture from the high rainfall during the monsoon period. Besides this, nutrient uptake by palm roots will remove some of these ions resulting in lower nutrient concentrations in the soil solution with deeper soil depth. Furthermore,
Kee et al.[36] reported that roots reduced the movement of exchangeable K down the soil profile. This implied that soil solution K⁺ as well as NH₄⁺ also reduced as ions moved downward. At all three depths, higher concentration of inorganic N was obtained at N1P2K1 compared with N1P2K0. This could be to K enhancing the downward movement of NH₄⁺ in the soil solution[37].

We also found higher K concentration in the soil solution when N was applied (N1P2K1 versus N0P2K1). This could be due to the displacement of K⁺ from the soil colloidal surfaces to soil solution by NH₄⁺ from the nitrogen fertilizer. Moreover, vegetative uptake of NH₄⁺ will increase the production of H⁺ in the soil[38] which then displaced K⁺[15] into the soil solution.

Forty five days after fertilizer application, the NH₄⁺ concentration in the top 30 cm was significantly lower by about 33% (Fig. 4). Although part of the NH₄⁺ disappearance can be accounted for by palm uptake, probably a larger amount had moved beyond its depth as indicated by the increased NH₄ ⁺ concentrations in the lower soil depths. By 105 days, almost all the applied NH₄⁺ had disappeared from the top 30 cm of the soil profile. The rate of decline in NH₄⁺ concentration in the soil solution was about 1.3 mg L^-1. Thus, the interval of applying N fertilizer during the monsoon period should be between 90 and 105 days to avoid excess NH₄⁺ in the soil solution. The disappearance of total inorganic N in the top 30 cm was even more rapid when both N and K fertilizers were applied (Fig. 9). The rate of decline in K⁺ concentration was about 1.4 mg L^-1 and virtually all the applied K disappeared at about the same time as NH₄⁺ ion since both nutrients are likely to move down the profile together[36].

N and K leaching losses: The amount of N and K in the leachate obtained at 120 cm depth are considered as leaching losses since most oil palm roots are found within the top 60 cm of the soils[7,39]. The quantity of N and K leaching losses in this study were a function of the volume of water in the soil, fertilizer treatment and rate of nutrient uptake by the palm roots. The overall leaching losses of inorganic N were 1.0 and 1.6% of the applied fertilizer for N1P2K0 and N1P2K1, respectively. This conforms with the findings of Chang and Zakaria[9] and Foong[10]. These authors ascribed the low N leaching losses under mature oil palms to the high uptake of both soil moisture and N to sustain productivity. The N leaching loss was higher in the presence of K fertilizer due to the displacement of NH₄⁺ ion by K⁺ ion as discussed earlier.

The K leaching losses were higher than N at 5.3 and 2.4% for N0P2K1 and N1P2K1, respectively. These results were agreeable with those of Foong[10] where K leaching rate was higher than N in the higher weathered tropical soils. Unlike N, the K leaching losses were lower in the presence of N fertilizer. This might be indirectly related to better K uptake by the palms in well balanced fertilizer treatment resulting in better productivity (Table 1) and thus, higher K off-take via the fresh fruit bunches which contain large amount of K[7]. The consequent is lower K+ concentration in the soil solution.

Groundwater quality: The groundwater quality was only affected by the NH₄⁺ where its concentration went beyond the WHO[30] limit of 0.5 mg L^-1 when N fertilizer was applied at twice the optimum rate for oil palms. This was mainly contributed by the large amount of unabsorbed N from the soluble N fertilizer which was still present in the soils during the monsoon period. The concentration of NO₃^–N in the groundwater was very low at 0.5 mg L^-1 even at the highest N fertilizer rate tested which agreed with our contention that nitrification rate was low in this soil. The NO₃^–N concentration was far below the maximum limit set by WHO[30], which was 10 mg L^-1. Most of the N from the fertilizer that reached the groundwater in the monitoring well was mainly dominated by NH₄ ⁺ rather than NO₃⁺ which corresponded well with the composition of inorganic N in the soil solution as discussed earlier.

The applications of K fertilizer increased the K mean concentrations of groundwater to between 4.28 and 9.54 mg L^-1 which were below the WHO[30] limit of 12 mg L^-1. The higher K concentration in the groundwater in the absence of N (N0P2K1) compared with N1P2K1 might be partially explained by its higher leaching losses due to poorer K uptake by the palm. Nevertheless, in certain days during the monsoon period, the K concentration in the well exceeded 12 mgL^-1 but it was only for a short period and only occurred when excessive N (N2) was applied. Furthermore, the K rate in this study was above the optimum rate for oil palm to ensure its sufficiency for full expression of yield responses to N and P.