Digging Deeper: Evaluating the Impact of Customized Fertilizer Recommendations on Smallholder Farmer Investment, Yields, and Learning in Tanzania
Summary
Soil quality and soil nutrient limitations are largely unobservable to farmers. Local soil heterogeneity can be substantial and further impede farmers' learning from each other. Yet, local soil quality and properties are critically important for the profitability and adoption of fertilizers, hybrid seed, and irrigation. Rapid and cost-effective soil testing can provide farmers with information about plot-specific nutrient deficiencies and associated input recommendations.
In partnership with OCP Tanzania, the research team is evaluating the effects of (1) providing plot-level soil testing to a subset of farmers in a village and (2) providing plot-level soil testing to a subset of village farmers, PLUS providing the rest of the farmers in the village a set of village-level soil recommendations. The project uses a cluster randomized controlled trial (RCT) in 270 Tanzanian villages to test the effects on farmer beliefs, investment decisions, yields, and profitability. The project focuses on potato farmers, an important commercial crop in the region.
In all treatment villages, the project will conduct soil tests for a randomly selected group of farmers as well as for farmers who are identified by the local extension as lead farmers - known and respected farmers with at least five years of experience growing potato who also tend to try out new techniques and technologies Farmers whose plots are tested will receive their individual soil test results and associated management recommendations (what quantities of which fertilizers).
The study varies across treatment villages in how information is conveyed to farmers whose plots are not tested. These farmers receive either (a) Tanzania’s national fertilizer recommendations for potato in the region or (b) aggregated soil test results and recommendations based on the samples collected within their own village.
The project generates new evidence on the benefits of soil testing in smallholder farming communities, with particular attention to scale and spillover effects. By comparing farmers in treatment villages who receive individual soil tests to comparable farmers in control villages, the study estimates the direct impact of individual testing. The sampling design also enables estimation of spillover effects by comparing farmers in treatment villages who do not receive individual tests to similar households in control villages.
Heterogeneity analysis will inform future targeting strategies for soil testing, strengthening its social and policy relevance. Analysis of geo-referenced soil test data generated by the study, combined with pre-existing data shared by implementing partners, will characterize variation in soil conditions and limitations within and across villages and support the tailoring of future recommendations. The study also evaluates the effects of providing village-level recommendations that transform an inherently private good (a plot-level soil test) into a quasi-public good (a village-level recommendation derived from aggregated plot-level results).
From a pilot to a full RCT
The project was implemented in two phases. In phase 1, the research team conducted a pilot study to determine whether small-sample soil tests could be reliably aggregated at the village level and to assess operational feasibility. The pilot included stakeholder engagement with OCP Tanzania and the Tanzania Agricultural Research Institute (TARI), farmer focus groups, and analysis of 11,400 existing soil tests. Results confirmed strong differences across villages, but considerable similarity within villages, and produced statistical support for using village-level averages. The pilot also led to refinements in the experimental design, including adjustments to information treatments based on privacy and implementation considerations.
Following the successful pilot, phase 2 consists of a full RCT. The RCT evaluates the causal impact of plot-level soil testing and village-level aggregation of soil test results on farmer beliefs, fertilizer investment decisions, yields, and profitability.
What is OCP School Lab (OSL)?
OCP School Lab (OSL) is an agricultural extension initiative led by OCP Africa (as part of the OCP Nutricrops ecosystem) that delivers mobile soil testing and farmer training directly to rural communities. Through traveling laboratories, the program conducts soil analysis and provides tailored fertilizer recommendations adapted to local crops and soil conditions, complemented by interactive training on good agricultural practices and soil fertility management.
“The OCP Africa School Lab has raised awareness among more than 1 million farmers (as of end 2024) about the importance of soil analysis, promoting the use of scientific tools to enhance agricultural productivity.” OCP Group Sustainability Integrated Report 2024.