Abstract

Greenhouse experiments were conducted to evaluate the effect of different rates of molybdenum (Mo, 0-0.58 mg kg-1) on the yield and quality of Trifolium pratense L. (red clover), nitrogenase, and some biological parameters (microbial biomass, L-asparaginase activity, and L-glutaminase activities) in four Andisols. The effect of lime and phosphorus on Mo availability under laboratory conditions was also evaluated. Preliminary laboratory incubation studies showed that the concentration of available Mo in soil increased significantly with the addition of phosphorus, lime, and Mo but less than that found in greenhouse experiments. In red clover, the critical range of Mo concentration to be considered deficient appeared to be 0.4-0.5 mg Mo kg-1 of dry matter. Dry matter production and red clover protein content increased significantly as Mo concentration increased in the shoots (from 0.5 to 1.0 mg Mo kg-1 dry matter). The nitrogenase activity and the number and weight of the nodules were significantly influenced by the concentration of available Mo in the soil. In general, 50 to 100 μg kg-1 of available Mo were necessary to maintain or increase the biological fertility in soils, as estimated by the measurement of several fertility-linked soil parameters, although exceeding 200 μg Mo kg-1 had a detrimental effect on microbial biomass, L-asparaginase activity, and L-glutaminase activity.