Two easy entry points for soil fertility management in East Africa’s highlands
Distance from the homestead and soil fertility classes are clear entry points through which to target soil fertility management technologies. This is according to an article published in the Soil Science Society of America Journal by, including others, Keith Shepherd and Ric Coe of the World Agroforestry Centre. The article also states that soil fertility often decreases with increasing distance from homesteads, and farmers reinforce this by allocating nutrient resources to fields they view as most fertile.
Spatial variability in soil fertility is often observed in the smallholdings of sub-Saharan Africa. Referred to as soil fertility gradients (SFG), they result both from natural variability in soil types and the differential allocation of nutrient and labour resources to various fields by farmers. When nutrient resources or labour are limited, farmers tend to allocate them to the fields and gardens around their homesteads, leading to, typically, lush crops around the homestead and poor crops in fields further away.
How crops respond to applied fertilizers depends on how responsive the soil is. Nitrogen-fixing legumes—often proposed as a key component of integrated soil fertility management (ISFM) strategies—tend to perform weakly in the poorest fields of a farm. This kind of information is vital when farmers design ISFM strategies, especially in light of current investments in soil fertility being planned for sub-Saharan Africa.
Apart from agroecological potential, the major determinants of soil heterogeneity are: varying intensity of land use and management; sparse or dense human population; extensive or intensive livestock production systems; and varying market orientations. The study aimed to understand the effect and interactions of these determinants—from regions to households. Working with smallholder production systems to boost the productivity and sustainability of forestry and agroforestry, reverse land degradation and increase incomes is a key focus of the CGIAR’s Collaborative Research Programmes on Trees, Forests and Agroforestry and Water, Land and Ecosystems—of which the World Agroforestry Centre is a key partner.
Patterns of soil heterogeneity were examined in agricultural systems located in medium- to high-potential agroecological zones of East Africa, which encompass a wide variability in soil types, topography, farm sizes, market access, and farming practices. 250 farms were studied in six study sites from two districts in central Kenya, two in western Kenya, and two in eastern Uganda. Socioeconomic and management aspects were analysed. Farm areas were studied as fields—basically land units with homogeneous management. Heterogeneity in soil fertility was assessed as variability between fields of an individual farm, disregarding variability within fields.
At all sites, fields perceived to be poor were generally located at a relative distance of 0.6 with respect to the homestead (51–135 m). A number of other significant findings, implications and recommendations emerge from the research:
- Spatial heterogeneity of soil fertility in smallholder agricultural landscapes of East Africa was large, with some distinct patterns across sites, but not all such patterns could be generalized.
- Variability within farms was significant, indicating the need for research and recommendations focused at farm scale to reveal specific differences between fields and more easily identify soil fertility niches.
- Differences in resource endowment and farming practices add an extra layer of complexity and should be used to delineate recommendation domains for ISFM, prior to niche-based technology targeting.
- Blanket recommendations for soil management technologies are not suitable, as they do not take the layers of complexity within farms into account.
- Fields and soils that respond to interventions should be differentiated from fields that are nonresponsive due to severe degradation. The latter need to be rehabilitated before soil fertility technologies can be applied.
- Fields categorized according to distance from the homestead, and fields falling in different soil fertility classes as perceived by farmers, are already being managed differently by farmers and constitute clear entry points through which to target soil fertility recommendations.
Tittonell, P; Muriuki, A; Klapwijk, C J; Shepherd, K D; Coe, R; Vanlauwe, B. 2013. Soil heterogeneity and soil fertility gradients in smallholder agricultural systems of the East African highlands. Soil Science Society of America.