Soil fertility research in sub‐Saharan Africa: New dimensions, new challenges

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Abstract
As population pressure keeps increasing, productive land in sub‐Saharan Africa (SSA) is becoming increasingly scarce, while soil fertility research in SSA is shifting its focus away from production per se towards sustainable production. The vital role of spatial and temporal scales and their heterogeneity is highlighted in this paper. Spatial variation of soil nutrient stocks is shown for agro‐ecological zones and soil classification units in East and West Africa, but also at the farm level and lower levels. Whereas researchers have traditionally focussed much on macrovariability, SSA farmers tend to cherish and maintain microvariability. Meanwhile, soil nutrient stocks are not static entities. On the contrary, studies in different parts of Africa, and at different spatial scales, show that they are being depleted at alarming rates as nutrients are annually taken away in crops or lost in processes, such as leaching and erosion which far exceed the nutrient inputs through 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 the role attributed to them in supporting land‐use planning and farm household decision‐making. An attempt should be made to quantify and monitor the different nutrient flows that play a role on farms and at higher levels of aggregation. It is argued that improved soil fertility can be achieved through integrated nutrient management (INM), as this scientific 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 production and its sustainability, is a high priority issue. The shift of the research focus to INM systems implies a broader mandate and greater responsibility for soil and plant analytical laboratories in SSA.