A Soil Test for Nitrogen Availability to Corn

A soil test for N availability to corn ( Zea mays L.), NO ‐ 3 ‐ to 30cm when plants are 15 to 30 cm tall, was evaluated on a wide range of soils at 33 locations in Vermont and one in New York state. The NO ‐ 3 ‐N present at the time of sampling is a result of an integration of all the soil and weather factors that have influenced the availability of N prior to the time for sidedress fertilizer application. There was a typical response relationship between soil test NO ‐ 3 ‐N and corn silage yield on plots that only received limited amounts of starter N fertilizer at planting. The Cate‐Nelson procedure for NO ‐ 3 ‐N vs relative yield (yield without sidedress N/yield with sidedress N) indicated that there was a greater probability of response to N fertilizer when NO ‐ 3 ‐N was less than 36 kg ha −1 than at higher soil test values. The currently used N fertilizer recommendation procedure is based on estimates derived from cropping history, yield goal, soil type, and manure management information provided by farmers. Compared with the current method of estimating N fertilizer needs, recommendations based on the NO ‐ 3 ‐N soil test call for much less N fertilizer application and are more accurate in separating sites with varying response probabilities. As a routine laboratory procedure, the proposed test has numerous advantages over other N availability procedures. However, special procedures are needed, including rapid soil treatment after sampling (to eliminate N transformations), sampling the soil at a different time than for the usual soil test, and a rapid sample turnaround time in the laboratory.