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 NO3-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.