Integrated Soil Fertility Management 


Soil fertility depends on the soil’s origin (alluvial soils, soils derived from different types of parent rock, etc.), its texture, structure and organic-matter content, and farmer’s soil-fertility management in the past. Dark soils usually have high organic-matter content. Red soils, characteristic of a large part of Sub-Saharan Africa, are in general acid and poor in organic matter.


Soils differ in their capacity to store nutrients available to plants. Clayey soils have a good capacity to store nutrients and to release them gradually to the plant roots, because they are composed of very fine particles. Sandy soils have a very limited capacity to store nutrients. Thus, the application of large quantities of fertilizer to sandy soils is not advised, because significant losses can be expected.


Organic-matter content is also important. A low organic-matter content has a harmful effect on soil structure and increases erosion risks. Soils with high organic-matter content have a higher water-holding capacity and are an important source of nutrients for the plants. Nutrients easily available to plants are localized either in the soil solution, or are adsorbed (concentrated in the topsoil) by the well-decomposed soil organic matter or the available clay particles. The outside surface of this well-decomposed soil organic matter and of available clay particles form the exchange complex. Nutrients adsorbed by this exchange complex form a reserve of nutrients easily available to plants. This stock is gradually recharged by soil nutrient reserves that are less accessible to the plant.


An increase in the organic-matter content cannot be reached within one or two cropping seasons, it is a long-term project. Improving soil texture requires large investments and is in practice impossible.


Soil-fertility management is crucial for maintaining or increasing yields and incomes of the majority of farmers in Sub-Saharan Africa. How they manage that fertility not only determine the yield of the present season, but it can also have a significant impact on future yields.


The soil’s nutrient stock can be enriched or depleted through farmer management practices. For instance, the incorporation of crop residues in the soil and fertilizer application increase nutrients in the soil, while harvesting removes nutrients. This means that there are nutrient cycles in the soil that can be balanced or unbalanced. Without human intervention, these nutrient cycles are generally balanced. But such a situation is exceptional. Human intervention, e.g. introduction of mineral fertilizer or increase of cropping intensity (e.g. two crops per year on the same field instead of one crop) requires changes in soil-fertility management to avoid unbalanced nutrient cycles. Such instability may result in decreasing fertility and lower yields in the long run.


In Sub-Saharan Africa, farmers are often obliged to mine their soils: they remove more nutrients than they return. In the uplands, farmers’ practice of burning vegetation to facilitate soil preparation for cultivation decreases the carbon ratio in the soil and may result in leaching of significant amounts of other nutrients with the first rains. Farmers are also often obliged to use the same plot for several years without any fertilizer input. With each harvest, nutrients leave the field without being replaced. In order to increase soil fertility, farmers used to leave the plot fallow for some time. Unfortunately, this practice is increasingly being abandoned in Sub-Saharan Africa, where farmers are obliged to intensify their land use, often without compensating for nutrient losses. Without compensation, excessive nutrient exportation from the field may result in an unbalanced nutrition situation in the long term. Lowland soils are in general more robust and fertile than upland soils, but poor soil-fertility management of lowlands may also lead to nutrient-deficiency symptoms in the long term.


It has been estimated that during the last 30 years, an average of 22 kg nitrogen, 2.5 kg phosphorus and 15 kg potassium per hectare has been lost each year on 200 million of hectares of cultivated land in Sub-Saharan Africa (not including South Africa). This negative balance has a depleting effect on soil fertility and results in yield decreases.


Extract from PLAR-IRM: Technical Manual (Wopereis et al., 2009)