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Site‐specific N management (SSNM) has been suggested as one means of further increasing the efficiency with which N fertilizers are used and reducing environmental impact. Field studies to evaluate the potential for SSNM to reduce NO 3 ‐N leaching from irrigated maize ( Zea mays L.) were conducted from 1994 to 1997. Uniform management (UM) was compared with a SSNM strategy (variable rate technology, VRT) based on an existing N recommendation algorithm for maize using grid sampled soil organic matter and root zone soil residual NO 3 ‐N. A third treatment (reduced variable rate technology, RVRT) evaluated the potential for a reduced rate of N to adequately supply crop N demand when combined with variable rate application. Averaged across all site‐years, there was no significant difference in the total amount of N applied, 142 kg N ha −1 with UM, 141 kg N ha −1 with VRT. Treatment mean grain yields ranged from 4.5 to 13.9 Mg ha −1 and were influenced relatively little by treatment, with VRT yield significantly reduced compared with UM in two site‐years, and UM yield significantly reduced compared with VRT in one site‐year. Treatment mean soil residual NO 3 ‐N in the 0.9‐m root zone ranged from 2.7 to 14.0 mg kg −1 , and was low (6 mg kg −1 ), there were no significant differences between UM and VRT treatments, while RVRT treatment reduced residual NO 3 ‐N for three site‐years. We conclude that the spatial application of the existing recommendation algorithm developed for uniform application may be inappropriate, at least for these sites, and that unique recommendation equations for major soils and climatic regions may be necessary to achieve substantial increases in N‐use efficiency. This study also suggests that improved recommendation algorithms may often need to be combined with methods (such as remote sensing) to detect crop N status at early, critical growth stages followed by carefully timed, spatially adjusted supplemental fertilization to achieve optimum N‐use efficiency.
Ferguson et al. (Fri,) studied this question.