Management of soil organic matter in the farming systems of the low land humid tropics of West Africa: a review

Abstract

Soil organic matter is the key to successful and sustained productivity of soils of the tropics. This is because soil organic matter positively affects structure, aggregation, porosity, microbial activity, pore size distribution and water retention capacity of the soil. Furthermore, soil organic matter is the major nutrient storage site for the low-activity-clay soils of the tropics and so affect nutrient retention capacity, availability and mobility of macro- and micro-nutrients. It increases the water use efficiency, and therefore attenuate runoff and erosion and consequently the productivity of the soil. The low land humid tropics is characterised by high temperature, high relative humidity, high rainfall intensity and high microbial activity which all encourage rapid mineralisation, depletion and erosion of organic matter leading to soil deterioration. Many cultural practices and operations encourage rapid depletion of soil organic matter while others are associated with soil organic matter build up. Practices like crop rotation, multiple cropping, mulching, alley cropping, following and farm yard manuring, encourage soil organic matter accretion. However, the effectiveness of these practices in increasing soil organic matter depends on: 1. (a) amount and frequency of residue application; 2. (b) the nature and C:N ratio of the mulching material, or manure; 3. (c) rainfall amounts, intensity and distribution, soil moisture and clay contents. Land clearing with heavy machinery is associated with removal of biomass from the field, while conventional ploughing and harrowing lead to soil organic matter depletion. These practices should be discouraged or modified to reduce their negative effects on soil organic matter. Effects of green manuring and burning, on soil organic matter are questionable. Research is needed on processes and pathways of crop residue decomposition to provide clues to possible interventions aimed at management of soil organic matter. Work is also needed to improve the efficiency of agro forestry systems in accreting soil organic matter in soils of the low land humid tropics.