The slow reactions between soil and anions: 4. effect of time and temperature of contact between soil and molybdate on the uptake of molybdenum by plants and on the molybdate concentration in the soil solution

Molybdate solutions were mixed with soil and incubated at constant temperatures for up to 224 days. The uptake of molybdate by plants was then measured. Uptake decreased with increasing period of contact. The rate of decrease was greatest when the temperature of incubation was high. It is argued that the decreased uptake was due to slow conversion of the adsorbed molybdate into a more tightly bound form. This process was studied by measuring its effect on the concentration of molybdate in solutions in equilibrium with the soil. Equations were developed to describe the effects of time and temperature on the decrease in solution concentration. As had been observed in analogous work with phosphate, these equations described the effects closely. Neither the rate of change of concentration nor the effects of temperature on the change was related to soil pH. Because high temperatures of incubation accelerated the change to a more tightly bound form, they also resulted in low solution concentration. However, the equilibrium between adsorbed molybdate and solution molybdate was affected differently by temperature, and high temperatures resulted in high solution concentration. Hence the adsorption reaction was exothermic.