LARGE GRANULES, NESTS OR BANDS - METHODS OF INCREASING EFFICIENCY OF FALL-APPLIED UREA FOR SMALL CEREAL-GRAINS IN NORTH-AMERICA

In North America where the climate is cool enough only one crop is gro wn yearly, N fertilizers are sometimes applied in the previous fall ra ther than in the spring for fall- or spring-sown cereal grains. Howeve r, in areas where snow accumulates in winter, fall application of N fe rtilizers is generally inferior to spring application. Substantial nit rification takes place in winter and subsequent N loss occurs primaril y in early spring by denitrification after the snow melt. Immobilizati on of N is also greater with fall- than spring-applied N fertilizers. Nitrogen is more efficiently retained in the soil as NH4 and thus more effectively used by plants if formation of nitrite (NO2) and NO3 is r educed or prevented by inhibiting nitrification. The nitrification is reduced when urea is placed in bands, because of high pH, ammonia conc entration and osmotic pressure in the soil. The rate of nitrification is further reduced when urea is placed in widely-spaced nests (a numbe r of urea prills placed together at a point below the soil surface) or as large urea granules (LUG) by reducing contact between the nitrifyi ng bacteria and the NH4 released upon urea hydrolysis. A further reduc tion in nitrification from LUG can be obtained by addition of chemical nitrification inhibitors (such as dicyandiamide (DCD)) to LUG. The co ncentration of a chemical inhibitor required to suppress nitrification decreases with increasing granule size. The small soil-fertilizer int eraction zone with placement of urea in nests or as LUG also reduces i mmobilization of fertilizer N, especially in soils amended with crop r esidues. The efficiency of fall-applied N is improved greatly by placi ng urea in nests or as LUG for small cereal grains. Yields of spring-s own barley from nests of urea or LUG applied in the fall are close to those obtained with spring-applied urea prills incorporated into the s oil. Delaying urea application until close to freeze-up is also improv ed the efficiency of fall-applied N. This increased effectiveness of u rea nests or LUG is due to slower nitrification, lower N loss over the winter by denitrification, and reduced immobilization of applied N. F all application of LUG containing low rates of DCD slows nitrification , reduces over-winter N loss, and causes further improvement in yield and N uptake of winter wheat compared to urea as LUG alone in experime nts in Ontario; in other experiments in Alberta there is no yield adva ntage from using a nitrification inhibitor with LUG for barley. Placem ent of LUG or nests of urea in soil is an agronomically sound practice for reducing N losses. This practice can eliminate or reduce the amou nt of nitrification inhibitor necessary to improve the efficiency of f all-applied urea where losses of mineral N are a problem. The optimum size of urea nest or LUG, and optimum combination of LUG and an effici ent nitrification inhibitor need to be determined for different crops under different agroclimatic conditions. The soil (texture, CEC, N sta tus), plant (winter or spring crop, crop geometry, crop growth duratio n and cultivar) and climatic (temperature, amount and distribution of precipitation) factors should be taken into account during field evalu ation of LUG. There is a need to conduct region-specific basic researc h to understand mechanisms and magnitudes of N transformations and N l osses in a given ecosystem. Prediction of nitrification from LUG or ur ea nests in various environments is needed. In nitrification inhibitio n studies with LUG and chemical nitrification inhibitors, measurements of nitrifier activity will be useful. Finally, there is need for deve lopment of applicators for mechanical placement of LUG or urea prills in widely-spaced nests in soil.