Soil fertility decline: definitions and assessment

In permanent agricultural systems, soil fertility is maintained through applications of manure, other organic materials, inorganic fertilizers, lime, the inclusion of legumes in the cropping systems, or a combination of these. In many parts of the world the availability, use, and profitability of inorganic fertilizers have been low whereas there has been an intensification of land-use and an expansion of crop cultivation onto marginal soils. As a result, soil fertility has declined and it is perceived to be widespread, particularly in sub-Saharan Africa.(1-3(1), (2), (3)) Soil fertility decline is considered as an important cause for low productivity of many soils.(4), (5) It has not received the same amount of research attention as soil erosion; possibly as soil fertility decline is less visible and less spectacular, and more difficult to assess. Assessing soil fertility decline is difficult because most soil chemical properties either change very slowly or have large seasonal fluctuations; in both cases, it requires long-term research commitment. There are several other confounding factors that make assessment of soil fertility decline complicated (e.g., spatial and temporal variation, soil analytical methods), and, for those reasons, other techniques have been used to estimate the rates and changes in soil fertility decline. The methods to assess soil fertility decline are described in this entry.