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:

  • Soil P fixation is reduced due to the effect of organic residues on soil properties.
  • There may be a greater proportion of fine feeder roots (tertiary and quaternary roots) in soil beneath the frond stack.
  • Surface wash of fertilizers may be reduced by the protective layer of pruned fronds lying on the soil surface.

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:

  • First, some of the N supplied in fertilizers applied over the inter-row will be taken up by ground cover vegetation and lost when slashed ground vegetation decomposes on the soil surface,
  • Second, N volatilization losses are greater when urea is applied over decomposing organic debris where urease activity is greater, and
  • Third, it is much easier for the manager to verify that fertilizers have actually been applied and spread properly when they are applied over the weeded 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:

  • With one application of N per year, yield was greater when N fertilizer was applied over the weeded circle, but when N was supplied in three applications, there was no difference between the placement strategies.
  • Phosphorus was most effective when broadcast over the avenue, while K was most effective when broadcast over frond stack (Figure 3).

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.