Residual soil nitrate-N (RSN) is a principal determinant used across the northcentral region of the USA to adjust fertilizer N application rates to maize (Zea mays L.). Preplant soil sampling is usually performed to depths as deep as 120 cm to assess RSN. It is important to understand the temporal and spatial (depth) variation in RSN acquisition by maize to improve N fertilization strategies especially given the investment in soil analysis required for variable rate technologies (VRT). Research wasinitiated in 1994 to assess the effects of concentration and depth distribution of RSN as will as soil, crop and N fertilization management impacts on RSN use efficiency. Bromide was used as an RSN tracer and depth injected (at 20, 60 and 80 cm ) in contiguous microplots within a broad range of RSN depth and concentration distributions. Covariance analysis of 72 RSN profiles during a 2-year period resulted in our ability to account for 97% of the variation in grain yield and N uptake by including a description of RSN profile distribution in our analysis of variance. Variation in the depth of maize RSN acquisition was significantly influenced by year and attributed to differences in plant N demand (annual variation) as well as N fertilization and croprotation. These results imply that the use of technologies that assess real-time plant N demand will improve variable rate technologies and most likely improve our ability to increase N use efficiency. Delaying the time of N fertilization also improvesthe acquisition of deep RSN.
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