Phosphate, Fe and Mn uptake of N₂ fixing red clover and ryegrass from an Oxisol as affected by P and model humic substances application. 1. Plant parameters and soil solution composition

Abstract

Abstract The effect of humic substances on P‐availability in soil is still debated. Therefore, the effect of model humic substances synthesized from hydroquinone on P, Fe, and Al solubility in a strong P fixing Oxisol and on P acquisition by red clover and ryegrass was investigated. After 4 months of incubation, P concentration of soil solution had increased by a factor of > 10 at the highest humic level (50 g humics kg −1 soil), accompanied by a similar increase in Fe and Al concentrations. Soil samples with 0, 10, 30, 50 g humics kg −1 soil were planted with red clover and ryegrass. Red clover showed a small increase of shoot yield and a moderate increase of P uptake after humics addition. High humics levels increased slightly Fe concentration in the shoots but strongly that of Mn leading to Mn toxicity. Ryegrass showed a strong increase in shoot yield after humics addition of about 150 % at the highest humics level compared to the control without humics. At each humic level, P application (100 mg kg −1 soil) had no effect on P uptake of red clover and a small effect on P uptake by ryegrass. The relatively small effect of humics and P application on shoot yield of clover compared to grass can be explained by chemical P mobilization of red clover via exudation of citrate (about 12 μmol citrate g −1 soil). This agrees with the finding that P solubility increased in the soil under red clover but not under ryegrass from the first to the second harvest, indicating that red clover mobilized P.