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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

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

^a Sustainable Agricultural Systems Lab., Animal and Natural Resources Inst., Bldg. 001, Rm. 201, BARC-West, 10300 Baltimore Ave., Beltsville, MD 20705
^b Dep. of Crop and Soil Environmental Sciences, Smyth Hall, Virginia Tech, Blacksburg, VA 24061

^* Corresponding author (John.spargo{at}ars.usda.gov).

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.

Abbreviations: EONR, economically optimum nitrogen rate • EOY, economically optimum yield • ISNT, Illinois soil nitrogen test • N_fF, nitrogen fertilizer application factor • PPNT, preplant nitrate test • PSNT, presidedress nitrate test • RY, relative yield • SOM, soil organic matter • _N0, estimated yield without fertilizer N

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