An analysis of in‐field soil testing and mapping for improving fertilizer decision‐making in vegetable production in Kenya and Ghana

In-field soil testing and soil mapping can contribute to addressing the challenge of poor soil fertility and limited fertilizer application across sub-Saharan Africa. Semi-quantitative colorimetric methods, such as paper test strips, are frequently employed in soil nutrient assessment across developing countries, especially in South-East Asia. This research investigated the accuracy of nutrient-sensitive paper strips and smartphone, which was re-purposed to act as a reflectometer, to assess soil nitrate-N, and different methods for mapping soil fertility to identify areas of land that are suitable for human waste-derived fertilizers (HWDF) application. The study entailed testing soil samples across 42 different farms in Kenya and Ghana and compared it to laboratory results in-country. It was found that paper strips were capable of assessing available nitrate-N concentration present in the soil within +/- 20 kg ha(-1) of the standard method for 86% of the farms. Paper strips were less effective in Ghana as they had been calibrated for a method that was not used by local laboratories. Paper strips were not effective for HWDF samples, where chemical interferences and concentration of different forms of nitrates were too high, resulting in overestimation of readings and thus negatively affecting any associated nutrient management advice. Soil mapping has the potential to use open-source data to inform farmers through mobile technology. For soil mapping two methods were deployed which includes targeting organic matter deficient areas and stakeholder led mapping, with the latter shown to be more effective in identifying areas for HWDF application.