Soil Fertility Decline on Agricultural Plantations in the Tropics

Maintaining the soil chemical fertility is a key prerequisite to sustain crop productivity in the tropics. Several studies perceive that soil fertility decline is widely spread in tropical regions and that it is caused by inadequate nutrient replenishment and high losses as compared to natural ecosystems. Although this has been recognised for some decades, there is a need for hard data on soil changes to improve our understanding of agricultural systems and to design sustainable cropping systems. Most studies on soil fertility decline focussed on subsistence farmers and used nutrient balances to investigate agricultural sustainability. In this paper changes in soil chemical properties under different land-use system in the humid and sub-humid tropics are discussed with special attention to agricultural and forestry plantations. There were clear differences in the rate of soil fertility decline between land-use systems. Annual cropping systems and sugar cane showed the largest decline in soil fertility. Soil fertility decline on perennial crop plantations (sisal, cocoa, rubber, oilpalm) was lower, whereas soil changes on forest plantations were variable. Soil organic C declined on average in all land-use systems. The decline was largest in annual cropping systems and lowest in soils under perennial crops. Differences are related to soil management systems (level of nutrient inputs) and crop characteristics (trees vs. annual crops). As plantation agriculture is a major contributor to the income of many tropical countries and provides hundreds of thousands of people with labour and income, it is crucial that the soil resource base of agricultural plantations is sustained and the judicious use of inorganic fertilisers is indispensable.