Soil and Tissue Nitrate Tests Compared for Predicting Soil Nitrogen Availability to Corn
1989
Fox, R. H. | Roth, G. W. | Iversen, K. V. | Piekielek, W. P.
Mounting concern about NO₃ pollution of the nation's ground and surface waters make accurate N-fertilizer recommendations increasingly important. A study was initiated in 1984 to determine if presidedress soil or corn stalk (Zea mays L.) NO₃ concentrations could be used to predict soil N availability and the need for sidedress N fertilizer. Nitrate concentration was determined in the surface 30 cm of soil and in the lower 10 cm of corn stalks 22 to 37 d after emergence (corn was in the 5- to 6-leaf stage). Eighty-seven N-response experiments were conducted over a 4-yr period in central and southern Pennsylvania. A number of these sites were on fields that had received high rates of manure in the past. Corn stalk NO₃ concentration was not an accurate predictor of soil N availability nor of the probability that a site would respond to N fertilizer. The NO₃ concentration in the surface 30 cm of soil 4 to 5 wk after emergence was a good indicator of whether a response to sidedress N fertilizer would be attained. The best critical level for separating responsive from nonresponsive sites was 21 mg NO₃-N kg⁻. No sites responded to N fertilizer when the soil NO₃-N concentration was greater than 25 mg kg⁻. Soil NO₃ concentration and soil N-supplying capability (NSC) at soil NO₃ levels below the critical level were not correlated well enough (r² = 0.34) to use the predicted NSC for adjusting N-fertilizer recommendations. As a result, the best use of this test in Pennsylvania appears to be in identification of nonresponsive sites rather than predicting N-fertilizer rates. Use of this test by farmers in Pennsylvania will help minimize the practice of applying “insurance” N when N contributions from manure and legumes in rotations are uncertain. Paper no. 8030 of the Journal Series of the Pennsylvania Agric. Exp. Stn.
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