| Autor: |
Fox, R. H., Roth, G. W., Iversen, K. V., Piekielek, W. P. |
| Zdroj: |
Agronomy Journal; November 1989, Vol. 81 Issue: 6 p971-974, 4p |
| Abstrakt: |
Mounting concern about NO3pollution 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 maysL.) NO3concentrations 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 NO3concentration was not an accurate predictor of soil N availability nor of the probability that a site would respond to N fertilizer. The NO3concentration 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−1. No sites responded to N fertilizer when the soil NO3‐N concentration was greater than 25 mg kg−1. Soil NO3concentration and soil N‐supplying capability (NSC) at soil NO3levels 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. |
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