Evidence Insight: Increasing Maize Yields with Soil Testing and Field Specific Fertilizer Recommendations in Ghana

Though soil testing and the use of field specific recommended fertilizers can enhance productivity and increase profits for small-scale farming families, three barriers to their adoption have proved critical. First, many farmers often cannot afford to buy fertilizers; second, most farmers are illiterate and rely on the traditional guesswork of farming which may not be appropriate for specific farms; thirdly, lack of access to soil testing facilities and tools.
In Ghana, our multi-disciplinary team used our low-cost Asaasepa soil testing kit to pair field-specific recommendations for fertilizer purchase to be applied on the field. Our results show that farmers benefitted significantly in both yields and profits when (1) fertilizer recommendations were calibrated to the needs of individual farms and (2) cost of production drastically reduced when farmers applied only recommended amount of fertilizer.
More than one third of the world’s population depend directly on agriculture for their livelihoods, and the percentage is much higher in low-income countries such as many countries in Africa. Fertilizers could transform yields and profits but small-scale farmers face important barriers to adopting them.
Despite national efforts to convince farmers that applying mineral fertilizer is profitable, fertilizer is only effective when it resolves deficiencies in a farm’s soils. Across Sub-Saharan Africa very few small-scale farmers know which nutrients are deficient in their soils. Though nitrogen deficits are commonplace, nitrogen applications may be more effective when other deficiencies are remedied at the same time.
In Ghana, national recommendations from the various district agriculture directorates can be too spatially coarse when they contain sub-regions with very different soils and fertilizer needs. Government(MoFA) fertilizer recommendations for maize are more than 20 years old and do not include potentially important macronutrients such as Sulfur and Potassium.
Our team of soil scientists and economists conducted a randomized control trial offering soil test recommendations and fertilizer subsidies to small-scale maize farmers in Eastern Region, Ghana. The results provide important insights for increasing maize productivity in regions where limited fertilizer use constrains agricultural growth.
Testing For Higher Yields
This 2021-2022 experiment tested two potential barriers to adopting fertilizers. First, we tested whether site-specific fertilizer recommendations are more appropriate than generic regional recommendations. Second, we examined whether financial constraints limit investments in fertilizer. Across 41 villages, we assigned 300 farmers to the control group and 500 farmers across three treatment groups:
Treatment 1: Recommendations for the appropriate type and amount of fertilizer based on a soil test of the farmer’s main maize plot.
Treatment 2: Vouchers which fund the cost of fertilizer to cover a 0.5-acre maize plot, but can be redeemed for any agricultural input or for cash.
Treatment 3: Plot-specific fertilizer recommendations and the voucher.
Before any treatments, fewer than one percent of farmers in the total sample used any fertilizer. For plot soil tests we used Asaasepa Soil Test Kit, a tool developed by Soil Solutions Ltd, which combines fast field tests with information communications technology (ICT) to provide detailed fertilizer recommendations.

Tests and Subsidies Increased Yields

During the study, drought caused a 30-percent loss in yields, averaging 121 kg/acre relative to baseline across all groups except farmers who received both the Asaasepa Soil Test recommendations and input vouchers. Farmers who received either recommendations or vouchers but not both had nearly the same yield declines as farmers who received neither.

This impact on yields was driven by farmers purchasing fertilizer following the plot-specific recommendations. Of farmers who received both Asaasepa Soil Test recommendations and vouchers, 92 percent purchased fertilizer. Of farmers who only received vouchers, 31 percent purchased mostly urea following the government agric office extension recommendation for the region. Those recommendations did not include Ammonium Sulfate, which our soil test results recommended for 95 percent of farmers in the sample. Nearly all 1,001 soil tests recommended fertilizers not prescribed by the government extension services, including sulfur, the soil nutrient deficiency we found to be most prevalent in the area.

Scaling Higher Productivity

The results suggest that plot-specific fertilizer recommendations could be combined with financial assistance to improve farm productivity and profits. This is particularly true if government recommendations do not match the prevailing limitations of local soils.

Successfully scaling this intervention will depend in part on access to site-specific soil testing. Asaasepa Soil Test Kit is a convenient and fast field-based tool to provide detailed recommendations at about 40 GHS per test ($3), not including the labor cost of a professional to conduct the testing. Extension agents can test multiple farms in just one trip to a village. If an agent can conduct 10 tests in one day, a conservative figure, the total cost for each soil test is still just under 55 GHS ($4.30).

Scaling this intervention could further innovate and improve on our implementation and results. Most farmers complain they do not receive the government subsidy fertilizer so a Community base smaller subsidy could be sufficient if offered immediately after harvest, when farmers have cash, rather than at the beginning of the planting season, when financing can be scarce. The benefits can be substantial. In our study, the profits gained from applying the average recommendations after paying for a soil test and fertilizer is approximately 550 GHS on a one-acre farm, or about 21 days of work at the median daily wage.