PHOSPHATE, FE AND MN UPTAKE OF N-2 FIXING RED-CLOVER AND RYEGRASS FROM AN OXISOL AS AFFECTED BY P AND MODEL HUMIC SUBSTANCES APPLICATION .1. PLANT-PARAMETERS AND SOIL SOLUTION COMPOSITION

The effect of humic substances on P-availability soil is still debated . Therefore the effect of model humic substances synthesized from hydr o-quinone be 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 b y a factor of >10 at the highest humic level (50 g humic kg(-1) soil) accompanied be a similar increase in Fe and Al concentrations. Soil sa mples with 0, 10, 30, 50 g humics kg(-1) soil were planted with red cl over and ryegrass. Red clover showed a small increase of shoot yield a nd a moderate increase of P uptake after humics addition. High humics levels increased slightly. Fe concentrations in the shoots but strongl y that of Mn leading to Mn toxicity Ryegrass showed a strong increase in shoot yield after humics addition of about 150% at the highest humi cs 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 cl over and a small effect on P uptake by ryegrass. The relatively small effect of humics and P application on shoot yield of clover compared t o grass can be explained be chemical P mobilization of red clover via exudation of citrate (about 12 mu mol citrate g(-1) soil). This agrees with the findings that P solubility increased in the soil under red c lover but not under ryegrass from the first to the second harvest, ind icating that red clover mobilized P.