we partnered with Agropecuaria Amozoc, who agreed to offer the tailored high-quality fertilizer packages to farmers as long as we guaranteed a minimum volume of sales.<sup>20</sup> Second, the in-kind grant was intended as a “push” to farmers to experiment with higher quality inputs (fertilizers manufactured by a reputable high-quality firm). Finally and perhaps most importantly, cash grants were simply not possible because organizations such as QFD were not allowed by law to disburse cash grants. We next discuss the three sub-interventions in greater detail.

# **3.1 Soil Analysis and Specificity of Recommendations**

**Soil analysis:** The research team collected soil samples from the registered sub-plot of every study farmer (treatments and control). The samples were analyzed by Fertilab, a very well known and respected soil testing laboratory in Mexico.<sup>21</sup> Online Appendix A provides details on the soil analysis protocol. The soil analysis recorded (for each plot) the soil texture (percentage of sand, silt and clay), its ability to retain and transfer nutrients (pH levels, sand and lime concentrations, saturation points and cationic exchange capacity or CEC) as well as the levels of the primary macronutrients (nitrogen N, phosphorous P and potassium K), secondary macronutrients (calcium, magnesium, and sulfur), selected micronutrients and the level of organic matter in the soil. Figure OA1 in the Online Appendix provides an example of the soil analysis produced by Fertilab.

**Recommendations:** In addition to the soil analysis, Fertilab also provided the nutrient levels and corresponding fertilizer dosages required to produce maize yields of 4.5 metric tonnes per hectare (mt/ha) under normal rain and temperature conditions. Recommendations are based on a heuristic model that assumes that a certain quantity of N, P, K and micronutrients are needed to reach a target yield per hectare.<sup>22</sup> In theory, the target yield should be chosen to maximize farmer profits. The model, however, assumes that yields are roughly linear in inputs and we therefore chose the target of 4.5 mt/ha because it equated the average cost of fertilizers according to the model to the average expenditure in fertilizers by farmers at baseline.

> 20The dealer was able to blend fertilizers on-site. Packages were available for pick-up from the dealer store which was on average 17.2 km (s.d. 6.7) away from the average farmer. All study farmers had access to the dealer.

> 21Fertilab ( `https://www.fertilab.com.mx/` ) was recommended to us by CIMMYT staff.

> 22This assumption is grounded in the Law of the Minimum formulated by J.V. Liebig in the 1850s which suggests that to reach a target yield, a certain quantity of each nutrient is needed (similar to a Leontief production function). The Fertilab model uses this as well as a cost minimization approach given the price of available fertilizers. For example, for N one can use urea, DAP or ammonium sulfate. Taking into account the cost of the different fertilizers and the soil absorption capacity, the model selects the cheapest fertilizer package to meet the nutrient requirements. If the soil is pH negative (alkaline), then it is better to use ammonium sulfate rather than urea, but if the soil is pH positive (acid) then urea is preferred. There are other maize yield models such as CERES and NLEAP but many of the variables and parameters required by these models are unknown for Tlaxcala (and Mexico in general).

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