Ridge-Till Corn and Urea Hydrolysis Response to NBPT

Abstract

Ammonia volatilization from surface-applied, urea-based fertilizers can result in reduced fertilizer N availability to the crop, particularly in minimum tillage systems with substantial surface residue. The objective of this study was to evaluate the potential for the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) to minimize ammonia loss and preserve fertilizer N availability of urea and urea-ammonium nitrate (UAN) solution applied to ridge-till, irrigated corn (Zea mays L.). The study was conducted at three sites over 3 yr, 1990 to 1992. The soil for all three site-years was a Crete silt loam (fine, montmorillonitic, mesic Pachic Argiustoll). Urea and UAN solution (28% N) were applied following planting via two methods, broadcast (BR) and surface band (SB), at two N rates, 100 and 200 lb/acre, with and without NBPT at the rate of 1.0 lb/acre. A nonvolatile check of UAN knifed into the soil, with and without NBPT, was applied each year. Grain yield, stover yield, ear leaf N and chlorophyll, basal stalk nitrate, grain N, grain moisture, and stover N were evaluated. A lab incubation study was also conducted to evaluate the effect of subsurface band application of UAN with and without NBPT on the rate of urea hydrolysis and fertilizer N availability to the crop. Nitrogen did not limit yield in 1990. Precipitation soon after fertilization in 1990 and 1991 resulted in little or no benefit from the use of NBPT. Limited precipitation and low humidity for an extended period following fertilization in 1992 resulted in a 56.6 bu/acre increase in yield when NBPT was applied with urea (averaged over rates and application methods), but no yield increase when NBPT was applied with UAN. No differences in yield were observed between BR and SB application methods. Parameters which are indicative of N use efficiency, such as grain N, basal stalk nitrate, leaf N and chlorophyll, responded to source, rate, and application method variables even when grain or stover yield did not. The lab incubation found that urea hydrolysis was reduced when NBPT was added to UAN in a subsurface band, but not slowed to the extent that N availability to the crop would be reduced over the growing season. This study found the yield response of corn to urea or UAN solution containing NBPT to be highly variable, dependent mainly on the climatic conditions following fertilization. The use of NBPT can be an effective management tool to minimize the potential for yield loss due to ammonia volatilization from urea fertilizers applied to high residue conditions in years when climatic conditions are conducive to N loss.