Mehlich 3 as an indicator of grain nutrient concentration for five crops in sub-Saharan Africa

Soil testing for available nutrients is an important tool to determine fertilizer rates, however many standard methods test the availability of a single nutrient only. In contrast, Mehlich 3 (M3) is a multi-element test for predicting crop yield responses to the addition of macro and micro-nutrients. However, the M3 test has rarely been validated against crop nutrient concentrations, which limits its application for dietary improvement studies in sub-Saharan Africa. This study used 1,096 paired soil and crop samples of five crop types: maize, rice, sorghum, teff and wheat, covering a broad range of soil types and soil properties in Ethiopia and Malawi (e.g., pH 4.5 - 8.8; Olsen P <1 - 280 ppm). The samples were selected based on “high” or “low” grain nutrient concentrations, and the respective soil available nutrients were measured with M3 and other extraction tests: CaCl2 (P, K, Mg, Mn), Ca(NO3)2 (K and Mg), Olsen P, sequential extraction (S), and DTPA (Mn, Fe and Zn). The primary objective was to test how well the M3 nutrient concentrations corresponded to grain nutrient concentrations. The M3 concentrations followed the trend of the “high” and “low” grain concentrations in nearly all nutrients and crops, and this was statistically significant in teff and wheat for all nutrients. The results were best for macronutrients, and slightly less good for micronutrients, probably partly because the concentration of micronutrients in the selected soil samples was generally quite low. Compared to the other multi-element extractant (CaCl2), the M3 test corresponded better to the grain concentrations of K and Mg, and equally well to Olsen P, sequential extraction (S), and DTPA predictions of P, S, Zn and Fe, respectively. M3 extracted much greater concentrations than the other tests, and this was more pronounced in alkaline soils. Given that the M3 test corresponded well to grain nutrient concentrations across a range of soils and crops in SSA, we conclude it could be regarded as a suitable universal test for many nutrients. We also defined thresholds for M3 values, defining below optimum, optimum and above optimum soil fertility status. These results validate the use of the M3 extractant to assess soil fertility and develop fertilizer recommendations for improved produce quality to enhance diets.