EFFECT OF SOIL MOISTURE AND AIR RELATIVE HUMIDITY ON AMMONIA VOLATILIZATION FROM SURFACE-APPLIED UREA

Abstract

Environmental conditions are important in determining the potential for ammonia volatilization from surface-applied urea. This study evaluated the influence of soil water potential and air relative humidity on ammonia volatilized from surface-applied urea. A laboratory study was conducted to evaluate the importance of moisture on ammonia volatilization from prilled, granular, and solution urea, which were surface-applied to a Captina silt Ioam (Typic Fragiudult). Treatments included air at 25 and 85% relative humidity and soil water potentials of −0.033 and <-1.5 MPa. The temperature was 25 ± 2°C, and the air-flow rate was 8.4 cm2 cm−2 min−1, which was equivalent to 2 volume exchanges min−1. At a constant soil water potential of −0.033 MPa and air at 25% relative humidity, ammonia volatilized from the three urea sources averaged 46.4% after 257 h, and no significant source effect was observed. When an initial water potential of −0.033 MPa was allowed to decrease immediately after urea application, by passing 25% relative humidity air across the soil surface and not replenishing soil water, average ammonia volatilization was reduced to 10.8%. The mechanism for this reduction in ammonia loss was inhibition of urea hydrolysis as the soil dried. Delaying the onset of drying by 120 h resulted in nearly complete urea hydrolysis, and volatilization losses averaged 45.2% of applied N. An initial moisture of <-1.5 MPa subjected to air at 85% relative humidity resulted in ammonia losses at a more variable and generally slower rate than from a moist soil, but ammonia losses reached levels similar to those from an initially moist soil by 290 h. These data indicated that soil water potential, air relative humidity, and time before the soil surface dried were important in determining ammonia volatilization from urea. © Williams & Wilkins 1987. All Rights Reserved.