Effect of low-rate commercial humic acid on phosphorus availability, micronutrient uptake, and spring wheat yield

A greenhouse study was conducted to determine the effects of low-rate commercial humic acid (HA) on phosphorus (P), iron (Fe), and zinc (Zn) availability and spring wheat yields, in both a calcareous soil and a noncalcareous soil. In Phase I, soluble P concentrations were monitored at 1.9, 3.8, and 5.7 cm from a monoammonium phosphate ( MAP) fertilizer band that had either been coated with one of two HA products at the equivalent of 1.7kg HA ha(-1), a label rate, or left uncoated. Sampling occurred periodically up to 48 d after fertilizer application. In Phase II, uptake of P, Fe, and Zn and grain yield were measured in soils that had been fertilized with 7.5 or 25 kg P ha(-1), either coated with HA or left uncoated. In Phase I, only three significant differences (P = 0.05) out of 66 comparisons were found in soluble P concentrations between HA and control treatments at time points ranging from 4 to 48 d after fertilization. In addition, no significant differences were found in nutrient uptake, shoot biomass, or grain yield between HA and control treatments. These greenhouse results suggest that low commercial HA rates (similar to 1.7 kg HA ha(-1)) may be insufficient to enhance spring wheat growth.