Illinois Soil Nitrogen Test for Prediction of Fertilizer Nitrogen Needs of Corn in Virginia

Current N needs for corn (Zea mays L.) in Virginia are estimated based on expected yield and are adjusted to account for the estimated amounts of N mineralized from organic sources such as manures, biosolids, and legumes. The dynamic nature of soil N creates significant uncertainty in estimates of N availability from these sources. Accurate quantification of organic N mineralization during the corn growing season should improve the precision of fertilizer N recommendations. The Illinois soil N test (ISNT) has shown promise as a tool for estimating mineralizable soil N and predicting corn fertilizer N needs. Other research has found the ISNT to be poorly related to labile soil N and unrelated to corn response to fertilizer N. The objective of this study was to determine if the ISNT predicts the fertilizer N response of corn grown in a diverse set of Virginia agroecosystems. We conducted 29 on‐farm fertilizer N response trials in major corn‐producing regions of Virginia. The ISNT was significantly related to estimated yield without fertilizer N (R2 = 0.57, P < 0.0001) and relative yield (R2 = 0.64, P < 0.0001). We also found that the ISNT extracted a consistent percentage of total soil N (16.3 ± 0.7%), suggesting that it is a poor indicator of labile N. In fact, total soil N did as well as the ISNT at predicting yield without fertilizer N (R2 = 0.53, P = 0.0002) and relative yield (R2 = 0.64, P < 0.0001). The ISNT offered no more information about corn yield response to fertilizer N than did total soil N content.