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.