Effects of NH3 and urea on K distribution in root and rhizosphere of maize

The effects of NH₃ formed by urea hydrolysis on K distribution in maize roots and the rhizosphere were examined by electron probe x-ray microanalysis. Fresh weight of seedlings growing on calcareous soils was decreased by applying 200 ppm N as urea attributable to the inhibition of the development of root hairs and lateral roots. In the U-200 treatment, little K accumulated in the roots but K concentration in the rhizosphere soil increased. Such a pattern does not appear in roots receiving 200 ppm as ammonium sulfate or on calcareous soils with 100 ppm N as urea or with 200 ppm as urea in an acidic clay loam. The results indicate that K efflux is responsible for the growth depression and that K efflux from the high concentration of NH₃ formed when urea is hydrolysed rather than from the NH₄⁺ ion. Applying K fertilizer with urea should alleviate the adverse effects of urea on plant growth on calcareous soils by improving K status of the plant and by the decrease in rhizosphere soil pH which considerably reduces NH₃ concentration. Management designed to limit pH increase during urea hydrolysis should both prevent NH₃ injury and reduced N losses.