Micronutrient dynamics in black oat and maize supplied with dacite rock powder and dairy sludge on a tropical soil

Abstract

Rock powders and organic by-products are increasingly considered as alternative nutrient sources in sustainable cropping systems, yet their effects on micronutrient supply and potential toxicity remain poorly documented. This greenhouse study evaluated the micronutrient dimension of a fertilization strategy based on dacite rock powder and dairy sludge previously shown to improve soil chemistry and macronutrient supply for black oat and maize. A Typic Hapludox soil received ten treatments combining liming, mineral fertilizers, dacite rock powder, and dairy sludge at increasing doses. Black oat and maize were grown sequentially in the same pots for 70 days each. Post-harvest soil samples were analyzed for extractable copper (Cu), manganese (Mn), and zinc (Zn), and leaf and root tissues were analyzed for Cu, Mn, and Zn concentrations. Micronutrient uptake was estimated by combining tissue concentrations with dry matter production. Mixtures of dacite rock powder and dairy sludge increased leaf and root biomass of both crops, with maize leaf dry matter exceeding that of the mineral fertilizer treatment at the highest mixed dose. Soil Cu, Mn, and Zn remained within ranges typical of weathered tropical soils, and leaf concentrations of all three micronutrients stayed within accepted sufficiency ranges, with no indication of toxicity. In maize, mixed treatments corrected a latent Cu deficiency observed in limed control plants, increasing leaf Cu concentrations from values near 2–3 mg kg⁻¹ to approximately 7–9 mg kg⁻¹. Total Cu, Mn, and Zn uptake per plant increased with higher by-product doses, despite decreases in some tissue concentrations at the highest biomass levels, indicating a nutrient dilution effect rather than emerging deficiency. Root Mn concentrations increased more strongly than leaf Mn, especially in maize, suggesting root retention as a tolerance mechanism that limited excessive Mn accumulation in shoots. Overall, the combined use of dacite rock powder and dairy sludge supported macronutrient and micronutrient supply to black oat and maize without creating detectable toxicological constraints under the conditions tested.