Effect of initial soil water content and application of water on urea applied to pasture

Abstract

A 15N study with microplots was conducted to determine the effect of initial soil water content and of water application on the recovery of 15N-labelled urea applied at 60 kg N/ha to a paspalum-dominant pasture. A second experiment with the same pasture type investigated the effects of individual urea granules on soil pH and mineral nitrogen (N) after application to a moist soil with and without follow up rain and to wet soil without follow up rain. The 15N balance showed that initial soil water content and 10 mm of simulated rainfall affected the recovery of 15N in the soil/pasture. Fertilizer recovery was lowest (79%) from dry soil (evaporation minus rainfall (ER) 50 mm) without rainfall, but when the initial soil water (ER 25 mm) was higher, the recovery of fertilizer was greater. Simulated rainfall (10 mm) after urea application to the dry soil increased urea recovery to 90%. The recovery of applied 15 N was greater than 90% following the application of the urea to saturated soil (E-R 0 mm) and was comparable to the recommended procedure of irrigation after application. In experiment 2, the initial soil water content and follow up rain (10 mm) were important factors affecting soil pH and mineral N concentrations at urea granule sites after urea was applied to soil. Urea increased soil pH of granule sites to more than 8.5 in moist soil, but with 10 mm of rain or with wet soil, pH only reached 7.6. Similar effects with soil mineral N were also measured. The effects of these changes in pH and mineral N are discussed in relation to recovery of urea applied to pasture soil.