FIELD EVALUATION OF THE EFFECTS OF A UREASE INHIBITOR AND CROP RESIDUES ON UREA HYDROLYSIS, AMMONIA VOLATILIZATION AND YIELD OF CORN

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Abstract
The effects of a urease inhibitor and crop residues on urea hydrolysis, NH3 volatilization and silage corn (Zea mays L.) yield were assessed on an Ormstown silty clay loam soil under field conditions. Chopped timothy straw (Phleum pratense L.) was spread on the soil surface at 0 and 4600 kg∙ha−1. Urea solutions treated with 0, 0.25, 0.50, 1.0, 2.0 and 3.74 kg∙ha−1 of an urease inhibitor phenylphosphorodiamidate (supplied by The Crop Science Laboratory, Allied Corporation, Solvay, N.Y.), and NH4NO3 solutions with no inhibitor were added as dribble bands to the soil surface at 0 and 75.0 kg N∙ha−1. Urea-N remaining in the soil over a period of 14 days following urea application was measured. The extent of NH3 volatilization was assessed by capturing and measuring NH4–N using plastic cylinders (micro-plots) over a period of 17 days following urea application. Dry matter yield of corn was obtained 114 days after planting. Hydrolysis of urea was relatively rapid and 3–26% of the applied N was hydrolyzed within 2 days following urea application to the soil. The higher rates of inhibitor application (2.0 and 3.74 kg∙ha−’) tended to reduce urea hydrolysis. The inhibitor rate effect increased with time though not always significantly. More than 50% of the added N was still present as urea after 4 days with some exceptions. At day 8, the amount of N that was present as urea ranged from 4 to 28% for the lower rates and from 20 to 37% for the higher rates of inhibitor applied. Almost all the added urea had disappeared by 14 days. Measured losses of NH3 by volatilization within 17 days never exceeded 1% of the fertilizer N. Addition of N resulted in 14% increase in dry matter yield of corn. Nitrogen source, crop residues and urease inhibitor had no appreciable effect on crop yield. Key words: Urea-N transformations, urea fertilizer, NH3 losses, urease activity, straw