Losses are more pronounced at particular phases in the life of an oil palm plantation. The potential for nutrient losses is probably greatest immediately after land-clearing when the soil surface is exposed to erosion and uncontrolled surface runoff losses before legume cover plants (LCP) have been established. Losses can be great when large amounts of nutrients such as potassium (K) are released when the standing biomass (e.g. fronds, trunks) is burned during plantation development.
The other period when the risk of nutrient losses is high occurs when ground vegetation is sparse due to poor light penetration through the closed oil palm canopy (Breure, this volume). At canopy closure, the LCP dies off and a large amount of nitrogen (N) is released from the decomposing LCP biomass. Unless palm growth is vigorous, losses of mineralized N due to leaching are likely to be large.
Nutrient losses are more pronounced in areas of the plantation where steep topography and inadequate soil conservation measures result in erosion and uncontrolled surface wash. Clearly, a proper assessment must take account of these temporal and spatial aspects of the potential for nutrient losses.
Leaching losses are more prevalent in
coarse-textured soils in high rainfall areas
where large fertilizer application rates are
required but fertilizer recovery efficiency is
poor. Nitrogen may be lost to the atmosphere
due to volatilization but, as we shall see, N
fertilizers differ in their susceptibility to
volatilization losses, which are also affected
by the field conditions when the fertilizers are
Proper land-clearing techniques are required in order to conserve indigenous soil nutrient supplies and the nutrients returned to the soil in the cleared biomass (Gillbanks, this volume). In the past, when tropical rainforest was converted to oil palm, the felled trees and the organic residues of the previous vegetation were often burned to make field operations (i.e. lining, planting, drainage) easier and thus reduce labor costs, but large amounts of N and sulfur (S) were lost to the atmosphere in the process.
In an experiment to measure the effects of burning biomass on soil properties in Benin, West Africa on acid sand, soil chemical properties and yield were measured at 20 years (Trial A) and 10 years (Trial B) after planting (Sly and Tinker, 1962). In Trial A, there was a significant increase in soil exchangeable K in the burnt treatment (Table 1) but in Trial B, exchangeable Ca and Na, organic carbon and total N were larger in the unburnt treatment (Table 1).
In the first four years of production, yield was larger in the burnt treatment (Trial A) but there were no significant differences in yield when averaged over 11 years of production. The authors concluded that the burning of felled forest under typical Nigerian conditions was not detrimental to later growth and yield of oil palms, and has definite practical advantages in implementing field work (Sly and Tinker, 1962).
Foong (1984) measured soil chemical properties of a virgin Munchong (Typic Hapludox) soil at intervals after land-clearing and LCP establishment (Table 2). Six months after land-clearing, there was a discernible increase in soil pH due to the liming effect of the ash from the burnt vegetation. This seemed to be a temporary effect, as soil pH decreased to 3.9–4.0 thereafter. Total phosphorus (P) content also decreased after land-clearing but was increased substantially after planting LCP, due to the application of phosphate rock during the establishment of legume cover plants (LCP). Organic carbon (C) and total N also decreased at first, but were replenished by the LCP at 62 months after land-clearing when the LCP was shaded out by the oil palm canopy. There were small changes in soil available P and exchangeable K over the period monitored, but there was an increase in soil Ca and Mg. Thus, with proper LCP establishment, soil chemical properties could be maintained or even improved at this site during the first five years after planting (YAP).
In recent years, burning has been prohibited by legislation in Malaysia and Indonesia in response to concerns about environmental pollution and zero-burn land-clearing techniques were developed (Mohd. Hashim et al., 1993). Zero-burn replanting techniques may contribute to improved soil physical and chemical properties because the large quantity of biomass and nutrients contained in palm trunks and fronds is conserved and returned to the soil (Goh and Härdter, this volume; Redshaw, this volume). Felled trunks and fronds should be chipped and spread over the soil surface to provide mulch, reduce localized nutrient build-up, and minimize potential leaching losses.
Although zero-burn replanting techniques are currently the norm in the oil palm industry, it should be pointed out that pest control measures may be exacerbated due to an increase in the population of Oryctes beetles and rats. Thus, whilst zero burn land clearing results in reduced smoke emissions and improved soil properties it amy also result in an increase in pesticide use.
It should be remembered that the cost of replenishing soil fertility is almost always larger than the cost of implementing proper land clearing, land preparation and soil erosion control techniques that contribute to the conservation of indigenous nutrient supplies. Mechanical clearing and burning can result in increased surface runoff, topsoil erosion, leaching, N-volatilization, and P-sorption (von Uexküll, 1986). Soil damage during site preparation may be so severe that LCP establishment is greatly impaired, and this must be avoided.
Runoff and topsoil erosion Top
In a simulation study of in-field transport of fruit bunches and fertilizers, Tan and Ooi (2002) showed that infiltration rate in the mechanization path could be reduced to zero after 24 runs by a 2.3 t mini-tractor grabber carrying a 1 t load. Thus, the use of low ground pressure vehicles for infield transport is strongly advocated to reduce soil compaction.
Soil erosion occurs when soil cover is poor and particles of soil are detached by raindrops and carried offsite. Preventive strategies include the installation of erosion bunds (on slightly sloping land), palm platforms (on sloping land), and contour terraces (on steeply sloping land) (Gillbanks, this volume). Nutrient loss due to erosion are greater on steep slopes are where rainfall intensity is greater, but losses can be reduced by improving soil cover and installing soil conservation structures (Kee and Chew, 1996). It is therefore very important to practice selective weeding in mature oil palm plantations to preserve groundcover and reduce the amount of nutrients lost in surface runoff and eroded soil. When properly arranged in the inter-rows, pruned fronds are an important means to reduce run-off and erosion and thus should not be removed from the field for other purposes (Redshaw, this volume).
The amount of nutrients lost due to runoff and topsoil erosion may be large (Maene et al., 1979) (Table 3) and are usually greater than losses due to leaching. Losses of N and boron (B) in runoff water were greater than 10% of the amount applied as fertilizer, but losses were smaller for the nutrients K, magnesium (Mg) and P (Table 3). This was probably due to the greater solubility of N and B fertilizers and the adsorption of K, Mg and P on soil complex. Losses from surface runoff were larger in the uncovered soil in the harvest path, compared to the interrows, where pruned fronds provide soil cover (Table 3) and improve soil structure and the rate of water infiltration (Fairhurst, 1996).
Other studies indicate that the amount of fertilizer nutrients lost due to surface runoff could be related to the amount and intensity of rainfall immediately after fertilizer application. Kee and Chew (1996) found that N concentrations in runoff water collected after the first rain event following fertilizer application in the wet month of October were 89 mg kg-1 for Rate 1 at 65 kg N ha-1 and 135mg kg-1 for Rate 2 at 130 kg N ha-1, compared to 4 mg kg-1 in the control plot.
During the dry period when there was no rain for five days after fertilizer application, however, the N concentrations in the runoff water collected after the first rain event were much lower at 30 mg kg-1 (Rate 2), and <5 mg kg-1 (Rate 1 and the control plot). The drier soil surface appeared to result in an increase in the infiltration rate and thus a greater proportion of applied fertilizer was washed into the soil. Similar trends were observed for P, K and Mg fertilizers.
Phosphorus is more likely to be lost due to sheet erosion as it is less soluble than other nutrients and is held strongly on soil particles (particularly in highly weathered inland and upland soils). Sheet erosion also results in the loss of organic matter that forms an important part of the cation exchange capacity in highly weathered tropical soils. Steeply sloping inland and upland soils are more vulnerable to sheet erosion, and thus the effect of erosion on soil fertility in these soils is more pronounced. The subsoil in highly weathered soils is characterized by low cation exchange capacity (CEC) and the presence of small concentrations of plant available K, P, and Mg. The subsoil is thus a less-favorable environment for root growth and root activity, particularly if the concentrations of Al3+, H+ and Mn2+ are large due to low soil pH. For these reasons, the concentration of oil palm feeder roots is greatest in the upper 30 cm of soil (Ng, et al., on botany, this volume). Cover plants are very difficult to establish on areas affected by sheet erosion, and usually soil P must first be replenished before a full LCP canopy can be established. The importance of LCP in soil conservation is illustrated by Ling et al. (1974) in an experiment in Malaysia where runoff and soil loss decreased from 22% under bare soil conditions to 1% where soil surface was covered with LCP (Figure 1).
To summarize, measures to minimize nutrient losses due to surface runoff and soil erosion include the following:
Leaching losses occur when nutrients are dissolved into the drainage water as it percolates through the soil profile. Leaching is particularly problematic on coarse-textured soils in the humid tropics where rainfall exceeds evapo-transpiration. Other factors that affect nutrient losses by leaching include soil pore size, rainfall intensity, the initial water content of the soil, and the amount and timing of fertilizer application. The cations Ca2+, Mg2+, and K+ and the anions NO3 - and Cl- are most prone to leaching (Foong, 1993) (Table 4). Leaching losses are generally larger in older palms, probably because larger amounts of fertilizer have been applied.
In a catchment study in the same plantation where Foong (1993) conducted an experiment on leaching losses, the exceptionally large Mg losses were attributed to the excessive application of kieserite and the application of N and K fertilizers that displaced Mg from cation exchange sites into the soil solution. Losses of P were very small, due to its comparative immobility in the soil (Chang et al., 1994).
In a study on nutrient leaching on Orlu and Algba series (Rhodic Paleudult) soils in Nigeria, Omoti et al. (1983) distinguished between nutrients originating from the soil indigenous supply and nutrients added in mineral fertilizers by using fertilized and unfertilized lysimeters installed 60 cm below the soil surface. Losses of NH4-N and K were small in young palms in the absence of fertilizer, but for all nutrients, leaching losses from fertilizer were smaller in the older palms compared with the young palms (Table 5).
This outcome is to be expected since the older palms have better root system to absorb applied and indigenous soil nutrients, a larger demand for nutrients, and a higher transpiration rate with a consequent lower water balance for leaching.
Losses of NO3-N, K and SO4-S all were greater in the unfertilized soil in older palms compared with young palms, presumably because of nutrient accumulation in the soils due to fertilization prior to the measurements (Table 5).
Calcium was leached in the greatest quantity followed by Cl, SO4-S, and Mg. As could be expected, losses of NO3-N were greater than of NH4-N. It appears that Ca was the main carrier for the anions in these soils, probably due to the relatively low K rate applied during the experiment.
Clearly, there is a wide difference between nutrients in terms of their susceptibility to leaching. Nutrient losses may be large, particularly where organic matter status of the soil is low, in coarse-textured soils, and in areas with high rainfall.
To summarize, measures to minimize nutrient losses due to leaching include the following: