Effect of Urea on Availability of Micronutrients in Arid Soils

Application of urea can theoretically increase micronutrient availability either through the acidifying effect during its nitrification or through replacement of micronutrients from exchangeable sites on clays by the NH₄⁺ formed after its hydrolysis. However, long-term application of urea is often blamed for deficiency of micronutrients under field conditions. Two experiments were conducted in six soils of arid zone to reason out this apparent contradiction. Nitrification of urea in arid soils showed no effect on micronutrient availability as high buffering capacity of soils prevented any change in soil pH. However, only above a specific concentration (threshold concentration), urea increased the DTPA-extractable micronutrients. Threshold urea concentration ranged from 750 to 2500 µg urea-N g⁻¹ in different soils and increased with clay content. Replacement of micronutrients from exchangeable sites on clays or Fe/Al oxides by the NH₄⁺ formed or chemical reactions of ammonium carbonate and ammonium carbamate formed after urea hydrolysis with minerals of micronutrients in soil may be the reason behind this increase. The DTPA-extractable Mn increased by 26 to 135%, Fe by 23 to 77%, Cu by 31 to 180% and Zn by 21 to 88% in different soils at the threshold concentration. Enhanced micronutrient levels associated with urea addition could be responsible for increased their utilization by wheat plants. Possibility of urea concentration at micro-sites exceeding the threshold level even under normal field application rates of urea has been discussed. We deduce from these results that repeated application of urea and cultivation of crops would gradually deplete the supply of micronutrients from the exchangeable site or from some minerals which would ultimately reflect in their deficiency in soil. Detailed studies are however, required to confirm these observations in future.