MANAGING SOILS FOR LONG-TERM PRODUCTIVITY

Meeting the goal of long-term agricultural productivity requires that soil degradation be halted and reversed. Soil fertility decline is a k ey factor in soil degradation and is probably the major cause of decli ning crop yields. There is evidence that the contribution of declining soil fertility to soil degradation has been underestimated. Sensitivi ty to soil degradation is implicit in the assessment of the sustainabi lity of land management practices, with wide recognition of the fact t hat soils vary in their ability to resist change and recover subsequen t to stress. The concept of resilience in relation to sustainability r equires further elaboration and evaluation. In the context of soil deg radation, a decline in soil fertility is primarily interpreted as the depletion of organic matter and plant nutrients. Despite a higher turn over rate of organic matter in the tropics there is no intrinsic diffe rence between the organic matter content of soils from tropical and te mperate regions. The level of organic matter in a soil is closely rela ted to the above and below ground inputs. In the absence of adequate o rganic material inputs and where cultivation is continuous, soil organ ic matter declines progressively. Maintaining the quantity and quality of soil organic matter should be a guiding principle in developing ma nagement practices Soil microbial biomass serves as an important reser voir of nitrogen (N), phosphorus (P) and sulphur (S), and regulates th e cycling of organic matter and nutrients. Because of its high turnove r rate, microbial biomass reacts quickly to changes in management and is a sensitive indicator for monitoring and predicting changes in soil organic matter. Modelling techniques have been reasonably successful in predicting changes in soil organic matter with different organic ma terial inputs, but there is little information from the tropics. Nutri ent depletion through harvested crop components and residue removal, a nd by leaching and soil erosion accentuates the often very low inheren t fertility of many soils in the tropics. An integrated approach invol ving inorganic and organic inputs is required where animal and plant r esidues are returned, as far as practicable. Chemical fertilizers alon e cannot achieve long-term productivity on many soils and organic mate rial inputs are required to maintain soil organic matter levels and cr op productivity. A major research effort is required to develop improv ed strategies for halting and reversing soil degradation if long-term productivity is to be secured.