SOIL FERTILITY RESEARCH IN SUB-SAHARAN AFRICA - NEW DIMENSIONS, NEW CHALLENGES

As population pressure keeps increasing, productive land in sub-Sahara n Africa (SSA) is becoming increasingly scarce, while soil fertility r esearch in SSA is shifting its focus away from production per se towar ds sustainable production. The vital role of spatial and temporal scal es and their heterogeneity is highlighted in this paper. Spatial varia tion of soil nutrient stocks is shown for agro-ecological zones and so il classification units in East and West Africa, but also at the farm level and lower levels. Whereas researchers have traditionally focusse d much on macrovariability, SSA farmers tend to cherish and maintain m icrovariability. Meanwhile, soil nutrient stocks are not static entiti es. On the contrary, studies in different parts of Africa, and at diff erent spatial scales, show that they are being depleted at alarming ra tes as nutrients are annually taken away in crops or lost in processes , such as leaching and erosion which far exceed the nutrient inputs th rough fertilizers, deposition, and biological fixation. This shows how the absence of spatial and temporal variation in models based on soil fertility variables strongly diminishes their predictive value, and t he role attributed to them in supporting land-use planning and farm ho usehold decision-making. An attempt should be made to quantify and mon itor the different nutrient flows that play a role on farms and at hig her levels of aggregation. It is argued that improved soil fertility c an be achieved through integrated nutrient management (INM), as this s cientific field is presently replacing traditional fertilizer research . Calculating and monitoring nutrient flows and quantifying the impact of INM systems on soil fertility, and hence on agricultural productio n and its sustainability, is a high priority issue. The shift of the r esearch focus to INM systems implies a broader mandate and greater res ponsibility for soil and plant analytical laboratories in SSA.