Farmer‐led maize biochar trials: Effect on crop yield and soil nutrients under conservation farming

In extensive farmer‐led trials practicing conservation farming (CF) in three regions of Zambia (Mongu: sandy soils; Kaoma: sandy or loamy sand soils; Mkushi: sandy loam or loamy soils), we studied the effects of biochar made of maize cobs (0, 2, and 6 t ha⁻¹ corresponding to 0, 0.8, and 2.5% per basin) at different fertilizer rates of NPK and urea on crop yield of maize (Zea mays) and groundnuts (Arachis hypogaea). Conservation farming in this case combines minimum tillage (how basins), crop rotation and residue retention. For the first time, the effect of biochar on in situ soil nutrient supply rates [determined by buried Plant Root Simulator (PRS™) exchange resins] was studied, as well as the effects of biochar on elemental composition of maize. Effects of 0–10% (w:w) biochar addition on soil physical and soil chemical properties were determined in the laboratory. At all sites there was a consistent positive response in crop yield upon the addition of biochar. However, due to a great variability between farms there were no significant differences in absolute yields between the treatments. In the sandy soils at Mongu, relative yields (i.e., percentage yield with biochar relative to the same fertilizer rate without biochar) of maize grains and maize stover were significantly increased at recommended fertilizer rates (232 ± 60%) and at half the recommended rate (128 ± 6%), respectively. In addition, biochar significantly increased concentrations of K and P in maize stover. In situ soil nutrient supply rates as measured by PRS™‐probes were highly spatially variable with no consistent effects of the different treatments in the three regions. By contrast, the fraction of plant available water (Vol.‐%) significantly increased upon the addition of biochar in all three soils. The increase caused by 10% biochar addition was of factor 2.5 in Mongu (from 4.5% to 11.2%) and 1.2 in both Kaoma (from 14.7% to 18.2%) and Mkushi (from 18.2% to 22.7%). Cation exchange capacity, pH, and exchangeable K significantly increased upon the addition of 10% (w:w) biochar in all three regions with a subsequent increase in base saturation and decrease of available Al³⁺. Our findings suggest that the addition of biochar in combination with CF might have a positive impact on crop growth and that this positive effect is mainly caused by increases in plant‐available water and decreased available Al.