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.

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Zaharah A.R. (1979). Phosphate adsorption by some Malaysian soils. Pertanika 2(2):78-83.

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

Changes in Soil Properties: Conclusions

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This study indicates that soil pH, organic C and total N tended to decrease with time in the oil palm agro-ecosystem. However, the decline in soil pH was slight whereas those related to organic C and total N corresponded to the period where the oil palm biomass was burnt or partially burnt at replanting. The large increases in soil organic C with the current norm of zero burnt replanting technique were favourable in regards to sustainability and land degradation.

There were large positive changes in soil P and exchangeable K, which might be attributed to the applications of higher rates of phosphate rocks and K fertilizer especially from the 1990s following the results of fertilizer response trials. However, excessive build-up of soil nutrients on the highly weathered tropical soils of Ultisols should be avoided due to their generally low nutrient holding capacity, which may increase the risk of pollution. On the other hand, nutrient depletion should also be prevented as they commonly lead to lower production in the long-term. Thus, close monitoring of the changes in soil nutrients in the oil palm estates is essential.

This study shows explicitly that the soil fertility status of the Ultisols under oil palm in Johor was enhanced through sound fertilizer management practices and zero burnt replanting technique.

Changes in Soil Properties: Results and Discussion

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The changes, Δ, to pH, organic C, total N, total P, Bray-2 P, exchangeable K and exchangeable Mg were discussed from the following aspects:

• Site: The different sites where sampling was carried out i.e. in IR and PC
• Depth: Comparison of rates of change in soil nutrients between the two depths i.e. 0 – 15 cm and 15 – 45 cm
• Initial value: The initial value, x ₁
• Time / period: The soil sampling data were grouped into 3 main periods i.e. 1982 and earlier (coinciding with burning as the main land clearing method), 1983 to 1994 (where partial burning was generally carried out) and post 1994 (where zero burning was the main land clearing method). The lag time for all 3 periods are not the same.

Site, Depth and Initial Value

Table 2 summarizes the sample size for the data used as well as the median initial value of each parameter for both sites (IR and PC) and depths (0 – 15 cm and 15 – 45 cm) while Table 3 summarizes the % change of each parameter.

Table 2 : Sample size and median initial value of each soil chemical property (parameter)

Soil pH

Most crops grow best at a certain range of soil pH as soil acidity affects plant growth in many ways. However, the oil palm is tolerant to high acidity and is able to grow well under a broad range of soil pH (Goh and Chew, 1995a), from 4.0 to 5.5. Hyperacidity symptoms could be observed when soil pH is below 3.5 as the low pH is deleterious to normal root growth and function (Poon, 1983).

In general, there was a slight decline in soil pH of less than 3.2 % for both micro-sites and depths (Table 3). The declines in soil pH might be mainly attributed to nitrification process, which oxidizes NH₄ + to NO₃^– and releasing H⁺ ions as follows:

Therefore, N sources from applied fertilizers, organic manure and legumes containing or forming NH₄⁺ increases soil acidity.

The change in soil pH (Table 3) was more pronounced in the IR (pH lower by 3.2%) compared with the PC and this could have been attributed to the higher initial value of pH in the IR (Table 2), which was more susceptible to change due to the sigmoidal pH buffering curve. Moreover, the lower exchangeable bases in the IR (Table 2) would reduce its buffering capacity against the H⁺ produced from the nitrification process resulting in a larger decline in soil pH.

The lower initial value of pH in the PC (pH = 4.26) could be due to the prolonged concentrated application of N fertilizers in its limited space compared with the IR (Kee et al ., 1995). Most experimental evidence also supports the view that the electrical potential gradient maintained by the H⁺ efflux pump provides the driving force for active uptake of K⁺ across the plasma membrane (Haynes, 1990). Therefore, with more rooting activity for K uptake in the PC (Kee et al ., 1995), higher extrusion of H⁺ would occur resulting in low soil pH. The same would also be expected to occur to Ca₂⁺ and Mg₂⁺ uptake. The NO₃^– is also leachable to lower depths along with the basic ions of K⁺, Ca₂⁺ and Mg₂⁺. As these bases were removed and replaced by more H⁺, soils at lower depths (2^nd depth of 15 – 45 cm) became slightly more acidic, which was evident from the results in Table 3.

This acidifying effect has also been reported in established oil palm plantations (Chew and Pushparajah, 1996), whereby fertilizer applications have been carried out as a routine. From our data, the magnitude of decline in pH also appeared to be larger with a longer lag time in sampling (Figures 2a and 2b) i.e. % drop in pH greater when the two samples were taken at longer interval. Kee et al . (1995) working on Musang series soil (Typic Paleudult) showed that the surface soil pH after 7 years of applying fertilizers at the highest rate within a circle of about 2 m radius around the palm base declined markedly to 3.8 compared with control plots of 4.2, a drop of almost 10% compared with the approximately 3% obtained via our data (extrapolated for 7 years from Figure 2b). All in all, the drop in pH although noted, was generally small and probably negligible to the oil palm, which is tolerant to such conditions.

Nevertheless, we should still take necessary measures to prevent soil pH from going lower as it is usually used as a criterion of land degradation. Some measures to minimize the reduction of soil pH include the following:

• Broadcasting of fertilizers instead of applying them in a band or concentrating them only to the PC
• Use of ammonium nitrate or urea will have lower acidifying effect. However, urea application should be restricted to clayey soils and properly timed to reduce volatilization losses whereas ammonium nitrate should be avoided in sandy soils due to its higher leaching potential
• Application of alkaline fertilizers e.g. magnesium carbonate and empty fruit bunches
• Maintain proper water-table over acid sulphate layers in acid sulphate soils to prevent or reduce the oxidation of sulphite that results in sulphuric acid build-up

Soil Organic Carbon

Managing the soil organic carbon (organic C) would go hand in hand with the management of soil organic matter (SOM) and in oil palm plantations, soil fertility is mainly determined by the management of SOM including soil biological activity, soil nutrient management etc. Soil fertility management on the other hand is a keystone of sustainable agriculture. As the oil palm plantations have many sources of SOM, most of them are produced daily by the living oil palm itself such as pruned fronds, male inflorescences, shed leaf bases and roots as well as palm bio-products of empty fruit bunches (EFB), palm oil mill effluent (POME), decanter sludge, shell and fibre, natural vegetation and legumes, efficient management of SOM and the soil organic C would then ensure soil fertility, hence sustainability (Figure 3).

Soil organic C decreased at both depths (0-15 cm and 15-45 cm) with the lower depth dropping at a greater magnitude of 15.76% (Table 3). Although this decrease in soil organic C was substantial, it was localized and may not represent a decline in organic carbon on an agro-ecosystem basis as exemplified by a simplified oil palm “Carbon Cycle” (Table 4). The carbon supply from the oil palm ecosystem was computed from data of pruned fronds only without considering the following where data are unavailable:

• How much is lost as CO₂
• How much is the net return from other palm organic sources e.g. palm bio-products (EFB, POME etc.), male inflorescence, frond buds and root turnover
• How much is the contribution of ground vegetation

Table 4 : Simplified oil palm “Carbon Cycle”

Source 1: Henson (1999)
Note 2: Weighted age from data of Table 3

The 9.7 % and 15.8% reduction in organic C resulted in a total of 710 kg/ha/yr loss in organic C. Nevertheless, annually more than 4000 kg/ha (Table 4) of carbon can be potentially replenished by the palm’s pruned fronds alone. However, no data is currently available to determine the conversion efficiency of pruned fronds to organic C in oil palm agro-ecosystem in the long-term.

Chew and Pushparajah (1996) reported that organic C increase was not noted in oil palms due mainly to the uneven redistribution of recycled fronds, which could also explain the reduction of organic C obtained by our data set. In general, the IR, where soil sampling is carried out are kept relatively free of pruned fronds and hence no addition of organic matter to this micro-site would occur except from the low density of light vegetation kept by cultural practices, if any. Therefore, the practice of stacking pruned fronds in neat, narrow rows should probably be changed to wider placement covering as much of the IR as possible in view of the above. Establishing vigorous shade tolerant perennial leguminous cover crops such as Mucuna bracteata may offer another option to build up the SOM.

Total N

The Ultisols in Johor have low total N contents with a median value of 0.12 % in the top soil and 0.09 % in the lower depth (Table 2). Total N decreased in both soil depths in tandem with their soil organic C. This implies that most of the total N in the soils is in the organic form. The declines in total N contents contradicted the work of Chew and Pushparajah (1996) and Ling et al . (1979) who reported a lack of changes in soil N under oil palm cultivation.

The drop in total N in the first soil depth was not as accentuated as the second depth (Table 3). This might be ascribed to the higher rooting activity and root turn-over of oil palm in the upper soil depth, which may contribute substantially to the organic C balance (Henson, 1999). Similarly, the higher biological activity of the topsoil and potential addition of organic matter from the oil palm such as pruned fronds, EFB and POME, and the leguminous cover crops during the palm’s immaturity phase might have mitigated the decrease in soil organic C in the first depth and therefore, total N content also compared with the lower depth (Table 3). Nonetheless, the approaches to enhance SOM as suggested earlier are also appropriate to sustain total N content of the soils.

Total and Bray-2 P

The Ultisols in Johor were also generally marginal in P status (Table 2). Phosphate rocks have been used to supplement the P requirement of the oil palm and positive P responses have been obtained (Goh and Chew, 1995b). Despite the regular applications of phosphate rocks in the oil palm estates, total P and Bray-2 P posted increases in the IR only with slight declines in the PC (Table 3). This might be attributed to the policy of applying phosphate rocks in the interrow of mature palms and onto the legumes in immature palms. These results were agreeable with Hartemink (2003) who detected much higher soil P levels in soils under perennial crops compared with natural forest and Ling et al . (1979) who found a build-up of P even when the fields were burned for planting.

The higher increase in Bray-2 P compared with total P following phosphate rock applications corresponded well to the findings of Zaharah (1979). Bray 2-P extracted substantial amount of phosphate rocks apart from the dissolved P, which accentuated the P increase in particular when the initial P values were low (Table 2).

Build-up of both total and Bray-2P was also noted in the upper layer due to surface applications of P fertilizers, the low dissolution of phosphate rocks and slow migration of dissolved P into the lower soil depth due to P fixation or adsorption by sesquioxide. Thus, most of the dissolved P accumulated in the top 30 cm of tropical soils (Zaharah, 1979).

Exchangeable Kand Mg

Most Ultisols also have low exchangeable cations of K and Mg and those in Johor are no exception (Table 2). Even if complete recycling of EFB and POME produced from the FFB yield is to be carried out, the nutrients required by the palms for high yield levels are still higher than the nutrient supply from the oil palm agro-ecosystem (Ng et al ., 1999). Therefore, they need to be augmented by inorganic fertilizers to maintain the yield levels.

This resulted in positive changes to exchangeable K in the IR and both soil depths (Table 3). Slight increase in exchangeable K was also reported by Ling et al . (1979) when improvement in the legume coverage was obtained. Kee et al ., (1995) also noted a four-fold increase in exchangeable K, which were evident to a depth of 60 cm. The build-up of exchangeable K in the IR compared with the PC (Table 3) could be due to the applications of K fertilizers in the former micro-site for mature palms in particular with the wider use of fertilizer spreaders. In addition, pruned fronds and EFB, both containing high amounts of K are also normally applied in the IR.

Exchangeable Mg was noted to decrease for both micro-sites and depths (Table 3) due mainly to the generally high rates of NH₄⁺ fertilizer and KCl fertilizer used, which were known to cause the soil Mg to leach down due to the ionic exchange between the above cations at the soil complex. The magnitude of drop in exchangeable Mg in the PC although not statistically significant, was lower compared with the IR. This might be ascribed to the practice of applying Mg fertilizer, Kieserite, in the former micro-site. The negative soil Mg balance should be arrested with closer monitoring of the soil Mg status and the applications of ground magnesium limestone and/or kieserite where appropriate.

Time / Period

The data were also summarized to coincide approximately with the 3 different practices of land preparation for oil palm planting i.e. burning, which was generally the case in 1982 and earlier, partial burning (1983 to 1993) and the zero burn technique (McCulloch, 1982; Hashim et al ., 1993) which is the most common method from 1994 onwards. We shall discuss the two extreme periods only i.e. the burnt and zero burnt to avoid complication in the interpretation since the partial burnt period might include some fields that were fully burnt or zero burnt.

Burning of the organic palm biomass at replanting had little effect on the long-term soil pH (Table 5) although short-term increase in soil pH had been reported (Ling et al ., 1979), which was attributed to the addition of exchangeable bases in the ash. These two results probably indicated that the ash effect on soil pH was temporary and easily nullified by the applications of acidifying fertilizers to the palms.

With the zero burn technique, soil organic C improved in the IR and in both depths (Table 5). Sly and Tinker (1962) also concurred that slight depression of organic C would be obtained if burning was carried out during land clearing. Similarly, Ling et al . (1979) reported reduction of soil organic C by burning in a jungle clearing although slight increase in the topsoil organic C was noted twelve months after the land clearing. Khalidet al . (2000) also found that different replanting policies of partial burning, stacking and pulverizing resulted in varied increases in soil organic C with the latter two methods yielding the highest increases.

Table 5 : % change for each soil chemical property (parameter) based on median for different periods

In general all the major soil nutrients of total N, total P, Bray-2 P and exchangeable Mg improved during the zero burnt period. Soil exchangeable K on the other hand appeared to yield better results when burning was carried out. This was agreeable with the observations made by Sly and Tinker (1962) that burning depressed nitrogen and decreased soil magnesium and calcium but gave larger exchangeable K. The increase in total N during the zero burnt period corresponded with the build up in soil organic C, which implied a large return of N from the previous palm biomass. On the other hand, the large positive changes in Bray 2-P and exchangeable K in the PC could be attributed to the increased P and K fertilizer rates to the palms following the higher yield responses to these nutrients (Goh et al ., 2000). The build-up of soil exchangeable Mg with zero burnt replanting technique will require further work to ascertain.

Extreme Soil Nutrient Changes

Although the median values of soil nutrient changes in the Ultisols under oil palm in Johor were favourable in the light of sustainability, land degradation and the environment, there were extreme values for all the soil chemical properties (Appendices 1c and 1d). These extreme values could be outliers due to sampling or analytical errors but nevertheless it is still prudent to scrutinize them because excessive soil nutrient depletion (negative values) and build-up (positive values) are both detrimental to the oil palm agro-ecosystem. Long-term nutrient depletion is known to cause yield declines in many agricultural crops (Hartemink, 2003) whereas excessive build-up of N and P can cause ground water pollution and eutrophication, respectively. In general, the data showed that build-up of soil nutrients was dominant compared with soil nutrient depletion in the Ultisols under oil palm in Johor (Appendices 1 c and 1 d).

Changes in Soil Properties: Materials and Methods

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The data used in this paper were extracted from long-term soil analysis results of samples collected from 21 estates from 1967 to 2000. The estates were located from northern to southern Johor along the central belt (Figure 1). They covered over 37,000 ha, of which data from approximately 13,000 ha were used. Only analysis data of Ultisols from Johor were reported in this paper and they encompassed 26 soil series, which were mostly developed over igneous rocks, sedimentary rocks and riverine alluvium. Examples of soils in the study area would include Rengam series (deep coarse sandy clay soil derived from granite), Bungor series (deep fine sandy clay soil derived from sandstone/shale) and Holyrood series (deep coarse sandy clay loam soil derived from sub-recent alluvium).

Sampling Method

36 soil-sampling sites were randomly located in a representative area of each field. At each sampling site, soil samples were collected from two micro-sites of palm circles (PC) and interrows (IR) using a 2.5 cm diameter screw auger. At each micro-site, soil samples from two depths of 0 – 15 cm and 15 – 45 cm were extracted. This gave four distinct soil samples (two micro-sites x two depths) for each field, which were each bulked from the soil samples taken from the 36 sampling sites.

Chemical Analysis

Each soil sample was analyzed for some or all of the soil chemical properties listed in Table 1. We also restricted the data set to soil samples where they were collected and analyzed at the same time for two different soil depths and/or sites in each field. Thus, the number of samples available varied among the soil chemical properties (Table 1).

Soil pH was determined in the supernatent suspension of 1:2.5 soil: water. Organic carbon was analysed by the Walkey-Black procedure while total N was extracted using micro-Kjeldahl method. Total P was extracted using 6M HCl whereas available P followed Bray No. 2 procedure. Exchangeable K, Mg and Ca were extracted by leaching the soil sample with 1 M ammonium acetate solution at pH 7.0. K was then determined using flame photometer while Mg and Ca were determined using atomic absorption spectrophotometer. Details of the above analytical methods can be found in Moris and Mohinder (1980).

Table 1: Number of soil samples analyzed for each soil chemical property

Soil Nutrient Changes

Quantifying the rates of change in soil chemical properties for different soils and climate is necessary to reckon if any, the soil fertility decline as very few studies have been conducted in which these rates were calculated (Hartemink, 2003). In this paper, the rate of change was calculated for each soil chemical property as follows (Hartemink, 2003):

where Δ = the rate of change in %
x ₁ = the initial value of the variable at sampling time t ₁
x ₂ = the final value of the variable at sampling time t ₂

The soil samples where x ₁ and x ₂ were determined came from the same field. The period between the soil samplings t ₂– t ₁varied from one to 13 years.

Statistical Analysis

The main descriptive statistics such as mean, median, minimum, maximum, standard deviation, standard error of mean were calculated to examine the means and distributions of the initial value (x ₁) and change (%) for each soil chemical property (Appendices 1a to 1d).

Since the rate of change (%) for each soil chemical property was not normally distributed, non-parametric statistics and median were used to analyze the data. Wilcoxon’s signed rank test (Steel and Torrie, 1981) was used to investigate the differences in the rate of change (%) of each soil chemical property between the two sites of IR and PC, and between the two soil depths (0-15 cm vs. 15-45 cm).

In the Wilcoxon’s signed rank test, the sample size (Table 2) did not equal to half the number of soil samples collected (Table 1). This was because in calculating the rate of change (%), we took two soil samples (x ₁ and x ₂) that were collected from the same field but at different sampling periods (t₁ and t₂). This implied that if a field was sampled twice at two different sampling periods, then the sample size equaled to one. On the other hand, if a field was sampled three times, then the sample size equaled to three, i.e. the rates of change (%) were calculated fromx ₁ and x ₂, x ₂ and x ₃ as well as x ₁ and x ₃. The same applies for other soil samples taken at different periods from the same fields i.e. four sampling periods would yield a sample size of four.

Changes in Soil Properties: Introduction

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The oil palm requires large amount of nutrients to sustain its growth and production so that high yield levels of 30 tons/ha/yr (Ng et al ., 1999) or more could be achieved and maintained. This is mainly due to the low soil fertility in most Malaysian soils (Law and Tan, 1973) in particular the Ultisols of Johor (Ng, 1969). High fertilizer rates are also essential to prevent negative soil nutrient balance and in many instances, to build-up the soil fertility to ensure sufficient nutrients are present in the soil solution for maximum uptake by the roots (Goh and Chew, 1995a).

Traditionally, soil samples are taken on a regular basis for nutrient analysis and are commonly used in the diagnosis of fertilizer requirements in oil palms, monitoring soil fertility and ensuring that fertilizers recommended have been applied. Thus, large long-term data set on soil nutrients are available from most big plantation houses. Despite this, changes in soil nutrients under oil palm as influenced by agro-management practices have not been reported. This is particularly important as the issues of soil nutrient changes have since gone beyond their traditional uses and are nowadays frequently regarded as one of the most important measures of sustainability and impact on the environment. Tinker (1993) pointed out that for an agriculture crop to be sustainable, one of the criteria should include preserving the resource base on which it rests upon whereas Hartemink (2003) has also argued that a drop in world food production might be attributed to the decline of soil nutrients. Soil nutrient changes with regards to the impact on the environment would normally be scrutinized from the point of land degradation and potential pollution, which should be avoided in order to be sustainable.

This paper studies the soil nutrient changes of pH, organic carbon, total nitrogen, total and Bray-2 phosphate and exchangeable potassium and magnesium in different micro-sites (palm circles and interrows) and depths under oil palms. The effects of time and the different replanting practices from previously on soil nutrient changes were also investigated.