Molybdenum availability in Andisols and its effect on biological parameters of soil and red clover (Trifolium pratense L.).

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