The effects of inoculants and nutrients

Women's group growing cowpeas in Ghana. Photo: Ken Giller

At the start of N2Africa, the benefits of phosphorus fertilizer and rhizobium inoculation (on soyabeans in particular) were known. Widespread testing of a range of nutrients for a diversity of legumes confirmed the positive effects of phosphorus and showed that other legumes also benefit from inoculation. The response to other nutrients was less clear-cut.

As part of N2Africa’s Development to Research approach, we carried out numerous trials across the N2Africa countries over the past eight years to test the performance of different inputs (inoculants, fertilizers, manure and lime) on farmers’ fields. Important questions to be answered with these trials were: what the effects are of phosphorus and inoculant on the yields of the various legumes and whether there is evidence that potassium or other nutrients are the limiting factors for particular legumes or locations. Answers to these questions would provide a solid foundation for appropriate recommendations to farmers on how to improve their yields.

Video: Prof. Janet Sprent and others explain the science of inoculation and the production of inoculants. Produced by Taskscape Associates

The trials showed a universally positive response to phosphorus (P). Application of 20 to 30 kg P per ha in fertilizer increased yields in common beans, cowpeas and soyabeans by an average of almost 300 kg per ha. Groundnuts showed a more modest response, but still almost 200 kg per ha.


Potassium (K) is another important element in crop nutrition. For legumes, we only found significant positive effects on common beans in East Africa, with yield increases of about 150 kg per ha. Responses in other regions and in other legumes were not consistent. In specific cases, positive responses to secondary nutrients and micronutrients were observed.

Farmers showing their cowpea crops in Wedza, Zimbabwe. Photo: Ken Giller

We assessed the effects of inoculants in soyabeans and other legumes separately from the nutrient responses. The positive effects of inoculants in soyabeans are relatively well-known and the N2Africa trials confirmed this. Inoculation responses varied substantially between countries and years but inoculation resulted on average in a relatively modest increase of 115 kg of additional grain yield per hectare. This increase of about 10 per cent is in line with results from countries in Latin America, where soyabean inoculation is a common practice. Moreover, given the estimated cost of 10 USD per hectare, it is a cost-effective technology.

Phosphorus and inoculant remain the most reliable inputs for increasing the yield of grain legumes

Apart from soyabeans, N2Africa also evaluated inoculation in trials with chickpeas, cowpeas and common beans. In all three crops, we observed significant yield increases, similar to those achieved for soyabeans. For common beans, results were however somewhat inconsistent, with an overall average response as high 288 kg per ha in Ethiopia compared with negligible responses in the trials Kenya and Rwanda. This could be due to variability in inoculant quality as much as to the effects of local conditions.

Esther Chinedu, N2Africa's Farm Liaison Officer for Nigeria, unloads rhizobial inoculant boxes. Photo: N2Africa

Finally, there were a number of inputs that were not systematically tested across crops and countries, but were generally observed to have yield-enhancing effects where used: organic manure, nitrogen fertilizer and liming. Manure gave fairly consistent yield increases for all legumes and countries. Nitrogen fertilizer was tested with common beans in northern Tanzania and gave a positive effect. Liming enhanced grain yields of common beans in southwestern Uganda. Although all three may offer opportunities for increasing yields, they are not always cost-effective options for smallholders, as the costs and the amounts required to boost yields may be high.

Overall, we can conclude that phosphorus and inoculant remain the most reliable inputs and can be generally recommended for increasing grain legume yields. Other nutrients may also be required, but the yield response and economic benefits still need to be assessed per legume and per location.

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