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.

Leaching Losses: Result

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Ammonium nitrogen: The mean NH_4-N concentration of N1P2K1 at 33.69 mg L^-1 was significantly higher than N1P2K0 at 8.15 mg L^-1 (Fig. 1). Both treatments had higher NH_4-N concentrations than treatments without N (N0P0K0 and N0P2K1). In the presence of K, NH_4-N concentrations increased 4.1 fold when N fertilizer was applied and 3.5 times in the absence of N application.

Most of the NH_4-N was found in the top 60 cm of the soil profile where majority of the palm roots may be located[7]. However, the mean NH_4-N concentrations decreased significantly with soil depth (Fig. 2). It was 17.89 mg L^-1 at 30 cm depth declining to 12.19 and 6.52 mg L^-1 at soil depths of 60 and 120 cm, respectively. The decline in NH_4-N concentration was more rapid between 30 and 60 cm compared with the lower soil depths.

The changes in NH_4-N concentrations among fertilizer treatments and between soil depths varied significantly across time (Fig. 3 and 4). At day 15, which was the first sampling date after treatments, the NH_4-N concentrations in the soil solution of N0P0K0 and N0P2K1 were 1.99 and 5.48 mg L^-1, respectively (Fig. 3). This indicated that even without N fertilizer, there were NH_4-N ions present in the soil solution probably from the native soil N and decaying palm biomass. These values provided the baseline NH_4-N concentrations in the soils in the experimental site implying poor N fertility. With N treatments, N1P2K0 and N1P2K1, the NH_4-N concentrations at the 15th day were 20.82 and 121.35 mg L^1-, respectively. The NH_4- N concentrations of N1P2K1 treatment decreased sharply between day 15 and day 30 and then more gradually until it reached 1.86 mg L^-1 at day 150. However, the NH_4-N concentrations were statistically similar to the baseline values from day 75 after treatment. The NH_4-N concentrations of N1P2K0 also declined rapidly and reached the baseline value 30 days after treatment. It continued to decrease to 1.60 mg L^-1 at day 150.

Fig. 1: Concentration of NH_4-N in soil solution for each fertilizer treatment

Fig. 2: Concentration of NH_4-N in soil solution across soil vertical profile

Fig. 3: Fertilizer × time effect on concentration of NH_4- N in soil solution (a) for comparing two times at difference level of treatments (b) for comparing two times at the same level of treatments

Fifteen days after treatments, the NH_4-N concentrations in the soil solution were similar at soil depths of 30 and 60 cm (Fig. 4). They were 40.45 and 50.55 mg L^-1, respectively and both concentrations were higher than at 120 cm depth of 21.23 mg L^-1.

Fig. 4: Depth × time effect on concentration of NH_4-N in soil solution (c) for comparing two times at difference level of depths (d) for comparing two times at the same level of depths

The NH_4-N concentrations in all soil depths decreased rapidly and reached similar values after 90 days from treatments. The NH_4-N concentration at day 150 at 30 cm depth was 1.1 mg L^-1, 60 cm depth 0.73 mg L^-1 and 120 cm depth 1.32 mg L^-1 (Fig. 4).

Nitrate nitrogen: The average NO_3-N concentration at 15 days after treatment was low at 0.214 mg L^-1 (Fig. 5). It gradually rose to 0.485 mg L^-1at day 60. A period of relatively dry weather between day 45 and 60 seemed to enhance nitrification resulting in the NO,_3- N concentration increasing significantly to 1.37 mg L^-1 at day 75. However, it declined continuously to 0.141 mg L^-1 at day 150 indicating that the transformation of NH_4-N to NO_3-N was not a major process during the monsoon period. The mean NO_3-N concentrations between 30 and 60 cm soil depth were 0.599 and 0.732 mg L^-1, which were significantly higher than at 120 cm soil depth of 0.266 mg L^-1 (Fig. 6). The proportion of NO_3-N to total inorganic N was in the range of 0.03-0.06 only indicating relatively low nitrification rate.

Total inorganic nitrogen: The total inorganic N was mainly composed of NH_4-N and thus the effects of fertilizer treatments on its concentrations were similar to NH_4-N concentrations as discussed earlier. Briefly, the total inorganic N concentration of N1P2K1 at 35.03 mg L^-1was significantly higher than the other three fertilizer treatments (Fig. 7). Although the total inorganic N concentrations in the first three treatments were statistically insignificant, there was a clear trend showing higher N concentrations in the presence of K (6.09 mg L^-1) and N (8.45 mg L^-1) compared with control (1.69 mg L^-1) as shown in Fig. 7.

Fig. 5: Concentration of NO_3-N in soil solution over time

Fig. 6: Concentration of NO_3-N in soil solution across soil vertical profile

The mean total inorganic N concentration at 30 cm soil depth was 18.71 mg L^-1 decreasing to 12.94 mg L^-1 at 60 cm depth and 6.79 mg L^-1 at 120 cm. Thus, 150 days after fertilizer treatments, a large proportion of inorganic N was still found in the top 60 cm soil depth where most palm roots were present despite the high rainfall during the monsoonal period.

The three factor interaction, fertilizer x depth x time, was significant for total inorganic N concentration. This significant interaction was mainly due to the sharp increase in inorganic N concentration upon the applications of N and K fertilizers, N1P2K1, compared with those without N input. For example, the inorganic N concentrations of N0P0K0 were very low and fluctuated between 0.37-12.11 mg L^-1 at 30 cm soil depth over the 150 days of measurements after treatment (Fig. 9). They were even lower with inorganic N concentrations ranging from 0.39-1.75 and 0.32-1.10 mg L^-1 at 60 and 120 cm depth, respectively. In contrast, the inorganic N concentration of N1P2K1 at 30 cm depth was about 10 fold higher at 127 mg L^-1 just 15 days after application.

Fig. 7: Concentration of total inorganic N in soil solution for each fertilizer treatment

Fig. 8: Concentration of total inorganic N in soil solution across soil vertical profile

In fact, the inorganic N seemed to have move downwards to at least 60 cm depth where its N concentration was even higher at 179 mg L^-1. However, at all soil depths, the inorganic N concentrations of N1P2K1 decreased rapidly where by 105 days after treatments, they were similar to the other treatments except at 120 cm depth. The inorganic N concentrations appeared to hover around 13.5 mg L^-1 between 90 and 135 days after treatment (Fig. 9) before declining to 4.3 mg L^-1 at day 150.

Potassium: Similar to N, the K concentrations in the soil solution decreased with soil depth where the concentrations were 105, 75 and 48 mg L^-1at 30, 60 and 120 cm soil depth respectively (Fig. 10). However, unlike N, the decline in K concentration was much smaller and between 30 and 120 cm, it was only about 2 fold.

Fig. 9: Effect of fertilizer x time x depth on concentration of total inorganic N in soil solution (a) 30 cm depth (b) 60 cm depth (c) 120 cm depth

In the absence of K application such as treatments, N0P0K0 and N1P2K0, the K concentrations were very low ranging from 1.3-10.7 mg L^-1 (Fig. 11). Upon K application, the K concentrations were increased to 237 mg L^-1 after just 15 days in N0P2K1 and 295 mg L^-1 in N1P2K1. The K concentrations for both treatments then decreased rapidly in the next 15 days before a more gradual decline was observed. At day 150, the K concentration of N0P2K1 was 67 mg L^-1 and that of N1P2K1 was 126 mg L^-1. The difference in K concentration at day 150 was about 59 mg L^-1which was similar to the differential at day 15 suggesting that the trend lines over time were parallel for both treatments. It also implied that this differential K concentration was due to the displacement of exchangeable K to the soil solution by NH₄₊ and the“disappearance” of K from the soil solution across time was not affected by N application.

Fig. 10: Concentration of K in soil solution across soil vertical profile

Fig. 11: Fertilizer x time effect on concentration of K in soil solution (k) for comparing two times at different level of treatments (l) for comparing two times at the same level of treatments

Leaching losses at 120 cm soil depth: The amount of leaching losses measured at 120 cm soil depth over 150 days was based on the volume of soil solution and nutrient concentrations for two replicates only to avoid missing values (Table 1). The leaching losses of inorganic N ranged from 0.043-0.589 g m^-2. For treatment, N1P2K1, it translated into a leaching loss of N of only 1.6% of the applied N fertilizer. This was agreeable with the results of Foong[10] for mature palms. Most of the N loss was in the form of NH_4-N since nitrification rate seemed low in the soils. Without the application of K fertilizer, the leaching losses of K were only 0.09 g m^-2 (Table 1). Applications of K fertilizer increased the leaching
losses of K over 150 days to 6.35 and 2.92 g m^-2 for N0P2K1 and N1P2K1, respectively (Table 1). These were equivalent to 5.3 and 2.4% of the applied K fertilizer for the above treatments, respectively. The higher K losses from the applied fertilizer in the absence of N (N0P2K1) might be attributed to the poorer yield compared with N1P2K1 and thus, lower K uptake from the soils by the palms (Table 1).

Table 1:Cumulative leaching losses of N and K fertilizers under mature oil palms as influenced by fertilizer treatments

Groundwater quality: The NH_4-N concentrations in the groundwater of the monitoring wells were similar for N treatments at N0 or N1 rate regardless of K application rate (Fig. 12). They were mainly below the WHO’s maximum admissible limit of drinking water of 0.5 mg L^-1. However, when excessive N rate, which was about twice the optimal N rate for oil palm, was applied, the NH_4-N concentration in the groundwater was increased to 2.7 mg L^-1. This indicated that contamination of the groundwater of monitoring well might occur if large amount of unabsorbed N from soluble N fertilizer was present in the soils during the monsoon period.

The NO_3-N concentrations in the groundwater were also raised by the applications of N fertilizer ranging from 0.07-0.25 mg L^-1 (Fig. 12). However, they were all below the WHO’s maximum admissible limit of drinking water of 10 mg L^-1.

Without K application, the K concentrations in the groundwater were very low at less than 1 mg L^-1 (Fig. 12). The applications of K fertilizer increased the K concentrations of groundwater to between 4.28 and 9.54 mg L^-1. The higher concentration of K in the groundwater in the absence of N (N0P2K1) compared with N1P2K1 might be contributed by the higher leaching losses due to poorer K uptake by the palm as reflected the poorer yield (Table 1). However, only when excessive rate of N was applied (N2P2K1), the concentration of K in the groundwater was increased
which was higher than at N0P2K1 and N1P2K1.

Fig. 12: Effect of fertilizer treatments on groundwater quality

Fig. 13: Rainfall pattern and N and K concentrations in groundwater during the monsoon period (Oct 08-Feb 09) in Sabah at 15 days interval for a period of 150 days

Rainfall pattern and nutrient concentrations in groundwater in monitoring well: The NH_4-N concentrations were relatively similar between the fertilizer treatments in the first 30 days after application (Fig. 13). However, the continuous heavy rains over the next 15 days resulted in a sharp rise in NH_4-N concentration in the groundwater of N2P2K1. It then showed a declining trend in NH_4-N concentration although high rainfall tended to increased it temporary. The NH_4-N concentration in the groundwater of N1P2K1 started to decrease 45 days after fertilizer application whereas in N0P2K1 treatment, it declined to its baseline value after only 30 days. The NH_4-N concentrations of both treatments were similar from day 75 onwards.

The NO_3-N concentrations in the groundwater of all fertilizer treatments were low but despite this, the fluctuations in NO_3-N concentration seemed to correspond with the rainfall pattern particularly for N2P2K1 (Fig. 13). The temporal variation in total inorganic N concentrations was similar to NH_4-N concentrations since the latter ion dominated the fraction of inorganic N (Fig. 13).

The K concentrations in the groundwater of the fertilizer treatments, N2P2K1, N1P2K1 and N0P2K1, were similar 15 days after applications. However, the heavy rains between day 15 and 45 which caused an increase in NH_4-N concentration as discussed above probably also displaced soil K with consequent higher K concentrations in the groundwater of treatments, N2P2K1 and N1P2K1. In treatment N2P2K1, the K concentration declined during the period of 45-60 day when rainfall was low before increasing again probably due to the excess applied K reaching the groundwater. However, the same effect was not seen in treatment, N1P2K1, because there was no excess K⁺ reaching the groundwater. Without N, the K concentration in the groundwater declined continuously until 90 day before it increased again. This implied that it took about 90
days before the applied K which was not absorbed by the palms reached the groundwater during the monsoon period.

Leaching Losses: Materials and Methods

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The experiment was located within an existing long term N, P and K fertilizer response trial on oil palm in Tawau, Sabah, Malaysia. Tenera (Dura × Pisifera) oil palms were planted in the field in 1982 in a triangular pattern with a planting distance of 9.2×9.2×9.2 m. The site was undulating to rolling with slope of 0-6°. The soil type was mapped as Kumansi soil series (Typic Hapludults) or Haplic Acrisols under the FAO classification. The experimental design of the fertilizer response trial comprised a 3x3x2 factorial combination of N, P and K arranged in a Randomized Complete
Block Design (RCBD) with 3 replicates. The plot size was 30 palms. To achieve the objectives of this study, 4 treatments from each replicate were selected as follows: N0P0K0, N0P2K1, N1P2K1 and N1P2K0. The N and K fertilizers were applied in the palm circle, about 1-2 m from the palm stem. Annually, 3 rounds of N and 2 rounds of K were applied. P fertilizer was broadcast evenly in the inter row areas and over the frond heaps once a year. The N source was Ammonium Chloride (AC) which has an N concentration of 25%. The N rates were 0 kg AC palm^-1 year^-1(N0), 3.75 kg AC^-1 palm year^-1 (N1) and 7.5 kg AC palm^-1 year^-1 (N2). The K source was Muriate of Potash (MOP) (K = 49.8%) which was applied at 0 kg MOP palm^-1 year^-1 (K0) and 4.5 kg MOP palm^-1 year^-1 (K1). Jordanian Rock Phosphate (JRP) was used as a source of P with application rates of 4.0 kg JRP palm^-1 year^-1 (P2) and 0 kg JRP palm^-1 year^-1 (P0).

The soil water sampler was a standard vacuum lysimeter (Soil Moisture Equipment Corp., Santa Barbara, Canada; Model: 1900L), which wascommonly used for studying pore liquid sampling from vadose zone in order to measure leaching losses[18,33]. It is also called a suction lysimeter. It comprised a Polyvinyl Chloride (PVC) tube with an external diameter of 4.85 cm and a porous ceramic cup with length of 5.8 cm mounted to one end. To install the lysimeter, a vertical hole was drilled with a soil auger, which had a diameter similar to that of the lysimeter.
To optimise the contact surface between the suction ceramic cup and the soil, a small amount of slurry with the soil material from the auger was made and poured back into the hole before inserting the suction lysimeter. Once the lysimeter was located at the correct depth, the hole was backfilled with the same soil material from the auger. The soil was then stamped firmly around the lysimeter to prevent surface water
from running down the cored hole. In order to study the downward movement of N and K nutrients, the lysimeters were installed at depths of 30, 60 and 120 cm in each treatment.

The monitoring well was constructed based on the principle described by Bouwer[19]. It comprised a PVC pipe with an external diameter of 6.0 cm. Due to soil and geological constraint, monitoring wells were only installed in 6 plots with treatments comprising N0P0K0, N0P2K1, N1P2K1 and N1P2K0, N2P2K1. The water samples from both lysimeter and monitoring well were collected at 15 day intervals starting from the first fertilizer application on 23/9/2008. The samplings were carried out for a period of 5 months from October 2008 to February 2009 (150 days) or a total of 10 samplings, which basically covered the entire monsoon period in Sabah.

All water samples were collected and stored in a narrow mouth polyethylene bottle with cap[34]. In this study, water samples collected from the monitoring well was referred to as groundwater[19,20] and that from the lysimeter at 120 cm depth as leachate[35]. The samples were preserved with 2-4 mL of chloroform per litre of water to retard bacteria decomposition prior to laboratory analysis. Samples were then analysed immediately upon arrival at the laboratory or were refrigerated at 4°C if analysis cannot be carried out on the same day[34]. Samples were filtered and analysed for ammonium-N (NH_4-N) using automated phenate method, nitrate N (NO_3-N) + nitrite N (NO_2-N) and NO_2-N using automated hydrazine reduction method on a Bran and Luebbe AutoAnalyzer 3[34]. Potassium (K) concentration was analysed using flame photometric method with a Sherwood flame photometer[34]. The concentration of total inorganic N was calculated as the sum of N concentration in each form of inorganic N(NH_4-N + NO_3-N + NO_2-N) in the sample. NO_3-N concentration was obtained by subtracting NO_3-N + NO_2-N with NO_2-N.

The experiment was carried out using a split plot design where fertilizer treatments were randomised in the main plots and depths of lysimeter in the sub plots. To test treatment effects on leaching over time, analysis of variance was performed using GenStat statistical software by treating times of sampling as repeated measures. The effect of fertilizer treatments on groundwater was examined by regarding the fertilizer x time as a factorial combination without replication and the higher interaction was used as an error term.

Leaching Losses: Introduction

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Nitrogen and potassium are the two most needed nutrients by oil palms in Malaysia, which are commonly met through fertilizers. When applying fertilizers to the palms, our goal is to maximize nutrient uptake by the palms from fertilizers for optimum growth and yield. Malaysia is located in a hot, humid tropical climate marked by seasonal rainfall with annual amount exceeding 2500 mm. Generally, the climate isinfluenced by the northeast and southwest monsoons. In Sabah, East Malaysia (North Borneo), most of the rains fall during the northeast monsoon (October to
February). During this period, considerable amount of water will be lost through both runoff and deep percolation to beyond the rooting zone. The same processes of water loss may also carry substantial amount of soluble plant nutrients. Schroth et al.[1] concluded from his study that even in well-designed perennial cropping systems, high rainfall and permeable soils which are typical over large areas in the humid tropics, make it unlikely that leaching losses of soluble nutrients can be completely avoided.

Leaching is the translocation of solutes[2] beyond the rooting zone. Some authorities define leaching more stringently as the removal of solutes entirely out of the solum, representing a loss of materials from the soil profile[3] but according to other experts leaching also includes the translocation of solutes within the solum[4]. Saffigna and Philips[5] considered leaching as the downward movement of fertilizer or waste in soil with the drainage water. When solute leached below the rooting zone, it is unavailable for plant uptake and therefore, lost from the soil-plant system. Depending on the amount of water draining out of the rooting zone, the leached solute may simply accumulate at depth in the soil or may pollute the underlying groundwater. Leaching losses, especially of readily soluble forms of N and K have generally been assumed to be substantial
in cropped land in the humid tropics in view of the frequent and intense rain storms[6]. Furthermore, Corley and Tinker[7] surmised that nitrogen and potassium are the elements most at risk to leaching because of the rather weak adsorption of ammonium and potassium ions and nil adsorption of nitrate. Omoti et al.[8] reported an average leaching losses of 11 kg N (34%) and 10 kg K (18%) of the applied NK fertilizers from
both young and mature plantations. Chang and Zakaria[9] in their review showed that the combined losses of N and K from leaching and runoff from catchment with young oil palm to be less than 2 kg N and 8 kg K ha^-1 year^-1. Although Foong[10] found that fertilizer losses in a lysimeter when the palm was 1-4 years old were 17% for N and 10% for K, he confirmed that the losses were low at 2.1% for N and 2.7% for K when the palm was 5-14 years old. He ascribed the much higher leaching losses when the palm was immature to smaller plant nutrient uptake and soil
disturbance during the construction of lysimeter and planting of the palm.

The most commonly used inorganic nitrogen fertilizers in the oil palm plantations are ammonium nitrate, ammonium sulphate, ammonium chloride andurea. Nitrogenous fertilizer may be lost in the form of ammonium (NH₄⁺) or nitrate (NO₃^–). However, studies have shown that different sources of nitrogen fertilizer have different effect on N leaching[11-14]. Also, the fertilizer rates have a strong impact on nutrient leaching in many crops[15-18]. Currently, information on the effect of fertilizer rates on leaching losses of nutrient in the oil palm plantation is still scanty.

Groundwater is that portion of water beneath the surface of the earth that can be collected with wells, tunnels, or drainage galleries, or that flows naturally to the earth’s surface via seeps or springs[19]. Price[20] defined groundwater as water in the saturated zone that is below the water table. Groundwater pollution caused by agricultural activities is a serious problem in many regions of the world. Phillips and Burton[21] expressed concerns that surface-applied fertilizers such as Di-Ammonium Phosphate (DAP) and potassium chloride (KCl) may be contributing to a decline in local groundwater quality under pine tree planted on sandy soil. High rates of N fertilizer used in the production of continuous corn have resulted in excessive nitrate-N leaching in groundwater which frequently exceeded the maximum contamination level of 10 mg L^-1[22]. Babiker et al.[23] investigated the nitrate contamination of groundwater by agrochemical fertilizers in Central Japan using geographical information system and found that the nitrate concentration of groundwater under vegetable fields was significantly higher than those under urban land or paddy fields. The adverse health effects of high nitrate levels in drinking water are well documented[24-26]. The most well known are methemoglobinemia, gastric cancer and non-Hodgkin’s lymphoma. While the usual level of ammonium (NH₄⁺) ion does not pose a direct risk to human health, a high NH₄⁺concentration may suggest the presence of more agricultural contaminants, such as pesticides. Furthermore, in aerobic condition, NH₄⁺ may be transformed to nitrate (NO₃^–) via nitrification[27]. Since groundwater is an indispensable water resources for human consumption especially in developing countries and the fact that eventually contaminants in the groundwater will be discharged into the river or streams which is also a source of drinking water, most authors referred to the drinking water standard guidelines as a baseline to assess the contamination level[27-29]. WHO[30] has set a maximum admissible limit in drinking water for NO_-3N as 10 and NH_-4N as 0.5 mg L^-1. While there is little evidence that K in drinking water is detrimental to human health, an increase in K⁺ concentration in groundwater may lead to a breach of the drinking water limit of K⁺ of 12 mg L^-1[30,31].

Schroth et al.[32] studied the spatial pattern of oil palm root system and concluded that in the absence of roots in the inter-tree spaces between the palms, nitrate would eventually be leached out of the soil profile and into the groundwater. Although various studies on groundwater contamination due to fertilization in different crops have been explored, there is still very little information on the effect of fertilization on groundwater quality in the oil palm plantation especially in Malaysia.

The economic success of the oil palm plantation may be the first concern, but it is now essential to determine the potential impact of fertilization in agriculture on the environment as a way of showing that the plantation company is involved in agricultural practices, which is sustainable, both economically and environmentally. In view of this, a study was carried out to study the downward movement and leaching of N and K nutrients as well as groundwater quality as affected by fertilizers during the monsoon period in Sabah, Malaysia.

Soil Management: Leaching Losses

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Abstract

Problem statement: The oil palms are mainly grown in the humid tropics with high rainfall. Soluble Nitrogen (N) and Potassium (K) fertilizers are commonly required by the oil palm plantations to maximize palm productivity due to the highly weathered soils with low fertility. Thus, leaching losses of N and K nutrients may be unavoidable and these nutrients may move further downward and eventually cause groundwater pollution. This study reports the leaching of N and K nutrients in a mature oil palm field as affected by fertilizer rates and soil depths and its effect on groundwater quality during the monsoon period in Tawau, Sabah.

Approach: The sources of N and K fertilizer were Ammonium Chloride (AC) and Muriate of Potash (MOP), respectively. Soil water samplers were installed at depths of 30, 60 and 120 cm in four fertilizer treatments, namely, N0P0K0 (Control plot, no N and K), N0P2K1 (K1 = 4.5 kg MOP palm^-1 year^-1), N1P2K1 (N1 = 3.75 AC kg palm^-1 year^-1) and N1P2K0. Three replications were used in the experiment. Monitoring wells were installed in the above treatment plots and in another treatment, N2P2K1 (N2 = 7.5 kg AC palm^-1-1 year^-1) to investigate the effect of excessive N rate on groundwater quality. Samplings were done at 15 day intervals for a duration of 150 days from October 2008-February 2009 to cover the entire monsoon period in Sabah. Water samples were analyzed for NH_4-N by automated phenate method, NO_3-N + NO_2-N and NO_2-N by automated hydrazine reduction method on Auto Analyzer 3 and K by flame photometric method using flame photometer. Results: The mean NH_4-N concentration of N1P2K1 at 33.69 mg L^-1 was significantly higher than N1P2K0 at 8.15 mg L^-1. In the presence of K, NH^4-N concentrations increased 4.1 fold when N fertilizer was applied and 3.5 times in the absence of N application. The mean NH_4-N concentration was 17.89 mg L^-1 at 30 cm depth declining to 12.19 and 6.52 mg L^-1 at soil depths of 60 and 120 cm, respectively. The transformation of NH_4-N to NO_3-N was not a major process during the monsoon period. The leaching losses of inorganic N were 1.0 and 1.6% of the applied fertilizer for N1P2K0 and N1P2K1 respectively. For K, the leaching losses were 5.3 and 2.4% for N0P2K1 and N1P2K1 respectively. The concentrations of NH_4-N, NO_3-N and K in groundwater ranged from 0.23-2.7, 0.07-0.25 and 0.63-9.54 mg L^-1, respectively. Conclusion/Recommendations: N and K concentrations in the soil solution decreased with soil depth and their leaching losses were related to rainfall pattern, fertilizer treatment and nutrient uptake by roots. Groundwater quality was not affected by the applications of N and K fertilizers at the optimum rates for mature oil palms.

Reference
Petronella G. A., Mohd, K. Y., Nik, M. M., Goh, K. J. and Gan, H. H. 2009. Effect of N and K Fertilizers on Nutrient Leaching and Groundwater Quality under Mature Oil Palm in Sabah during the Monsoon Period. American Journal of Applied Sciences 6 (10): 1788-1799.

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

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Changes in Soil Properties: Appendix

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Appendix 1a: Descriptive statistics for the initial values (x₁) of soil chemical property at two sites

Appendix 1b: Descriptive statistics for the initial values (x₁) of soil chemical property at two depths

Appendix 1c: Descriptive statistics for the rates of change (%) of soil chemical property at two sites

Appendix 1d: Descriptive statistics for the rates of change (%) of soil chemical property at two depths