Intervention(s)
We study the impacts of mechanistic explanations on beliefs about fertilizers and adoption choices among smallholder tomato farmers in Eastern Uganda. Mechanistic explanations are explanations which break down a system or process into the causal interactions among its parts. In the context of this study, these explanations include descriptions of fertilizer nutrients, their roles in plant growth, and the processes through which they move through the soil and the plant. We recruit 900 smallholder farmers in Bugisu sub-region, Uganda. Fieldwork conducted by the authors found that farmers in this region have little understanding of how fertilizers work, rarely experiment with novel combinations on their own, and rely on outside recommendations that may or may not fit their own context or be based in agronomic evidence. We design a set of treatments to test whether mechanistic explanations can address these challenges. One-half of the farmers will receive a placebo training that introduces an agronomist-recommended fertilizer recipe and demonstrates it yield, but provides no further explanations. The other half will in addition receive mechanistic explanations about fertilizers and the mechanistic rationale behind the demonstrated recipe.
In the first group, the control group, farmers attend a demonstration of a specific agronomist-recommended fertilizer recipe for grey soil. The training will inform the farmers of the types, timings, and amounts of fertilizer used, and the yields that were generated. Farmers will be able to see for themselves the condition of the fruiting plants, and ask the facilitator questions about the fertilizer recipe. The training will also cover methods of fertilizer application, such as how fertilizer should be inserted into planting holes or dispersed around the plant stem as a top dressing. Importantly, this control training will not inform farmers about the macronutrients that are contained in fertilizers, nor the functions of these nutrients and the processes through which they affect plant growth. The control training corresponds in content to what is typically seen in a standard extension or commercial demo.
The treatment group, will receive the control training as well as two additional modules. One module describes which macronutrients are contained in synthetic fertilizers, and the macronutrient composition of common fertilizers on the market. The second describes how nutrients affect plant growth, and the processes by which they move and interact in the soil and the plant. This module will describe different soil archetypes, including how soil texture and components affect the ability to retain nutrients, the nutrients that are naturally available in different soils, and mechanisms through which nutrients are retained or lost (e.g., nitrogen leaching, potassium binding). The module also also describes the roles of each of the primary macronutrients in plant growth. These roles are then given mechanistic explanations: for example, phosphorus is needed early in the planting season because the plant can recycle this nutrient many times throughout the season. This training, if leveraged by the farmer, can generate beliefs about the production function through deductive reasoning. It can do this ex-ante. For example, a farmer who learns about clay’s tendency to contain and retain potassium can deduce ex ante (without experimentation) that a soil with high clay content needs little potassium supplementation. A farmer who understands mechanisms can also generalize more effectively from observed experiments (ex-post learning). If the above farmer observes a high return to a certain amount of potassium supplementation on a high-clay soil, he can extrapolate that the marginal return will be even higher on a gray soil that is low in clays.