Relationship between Late Spring Soil Nitrate Concentrations and Corn Yields in New York
- S. D. Klausner**,
- W. S. Reid and
- D. R. Bouldin
Proper N management is important to optimizing profit and minimizing N loss. The efficiency of N use may be improved in humid regions with a reliable soil test to guide fertilizer N recommendations. Recently, a presidedress nitrate soil test (PSNT) has shown promise as a means of quantifying the size of the potential mineralizable organic N pool in soil. This study was conducted to determine if the PSNT could be used for corn (Zea mays L.) production to identify N responsive and non-responsive sites, predict the economic optimum fertilizer N rate, and improve the current fertilizer N recommendation procedure. The PSNT gave a useful and defined critical level of 21 ppm nitrate-N, delineating N responsive from nonresponsive sites. The success rate for making the correct decision about N responsiveness was 84%, indicating a good relationship between the PSNT and the ability of the soil to supply N. The PSNT was less precise in determining the economic optimum fertilizer N requirement (R1 = 0.50) for sites below the critical level. This probably was due to the inability of the PSNT to quantify organic N mineralization. The standard N recommendation, based on estimates of N availability from organic sources, was a slightly better predictor of the fertilizer N requirement (R2 = 0.67) than the PSNT. There is a need to further improve the methodology used for formulating fertilizer N recommendations.
Proper N management is important to optimizing profit and minimizing N loss. The efficiency of N use may be improved in humid regions with a reliable soil test to guide fertilizer N recommendations. This study was conducted to determine if a presidedress nitrate soil test (PSNT) for corn could be used to identify N responsive and nonresponsive fields, predict the economic optimum fertilizer N rate, and improve the current fertilizer N recommendation procedure.
Traditional fertilizer N recommendations are based on N credits from soil organic matter, crop residues, animal manures, and residual N carryover. The precision of the fertilizer recommendation is often variable owing to the difficulty in estimating N mineralization rates and N losses. Recently, a PSNT procedure was developed to measure the amount of soil nitrate in the surface 12 in. of soil when the corn is 6 to 12 in. tall. The PSNT has shown to be reasonably well correlated with the ability of the soil to supply N. Improvements in the fertilizer N recommendation through the use of the PSNT seem apparent.
The study was conducted at 78 locations across New York from 1986 through 1991. Locations were selected to typify crop rotational sequences and N management practices. Multiple rates of fertilizer N were applied at 29 of the locations for the purpose of calculating economic optimum fertilizer N rates. The PSNT was correlated with relative yield at all sites. The relationship between the economic optimum fertilizer N rate, PSNT value, and the standard fertilizer N recommendation was compared.
Was the PSNT able to identify N responsive and nonresponsive corn fields with a reasonable degree of accuracy?
The PSNT correctly identified N responsive and nonresponsive sites with a high degree of accuracy. The general relationship between PSNT and relative corn yield is shown in Fig. 1. The vertical line defines the critical level that approximates the PSNT value that divides N responsive from nonresponsive sites. The critical level and acceptable relative yield (horizontal line) were developed by a graphical method where the intersection of the two lines was placed to minimize the total number of observations in the upper left quadrant (overestimating the fertilizer N needed) and the lower right quadrant (underestimating the fertilizer N needed). The success rate for correctly identifying responsive and nonresponsive sites was 84% (100%-[11% + 5% for the upper left and lower right quadrants, respectively]).
Did the PSNT improve the standard fertilizer N recommendation procedure?
The PSNT did not compare as well as the standard N recommendation with respect to predicting the economic optimum fertilizer N rate. The coefficient of determination (R2) for the strength of the relationship between the economic optimum fertilizer N rate and the PSNT and standard N recommendation was 0.50 and 0.67, respectively. The reason for the slightly better performance of the standard N recommendation compared with the PSNT probably is associated with the fact that the former is based on a long term research initiative to develop estimates of N availability from organic N sources, while refinements in the PSNT are still in their infancy.
The PSNT offers some excellent guidance for managing N. The major advantage of the soil test is that it can be used with a relatively high degree of confidence to determine the need (not the amount) for side-dress N. The PSNT was less precise in determining the economic optimum fertilizer N rate for sites below the critical level. The accuracy of the fertilizer N recommendation may be improved with the combined use of the PSNT to determine the need for sidedress N and the standard N recommendation to determine the rate of application. Research should continue to improve the relationship between both the PSNT and the standard N recommendation; and the economic optimum fertilizer N rate.
Copyright © 1993. . Copyright © 1993 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, 5585 Guilford Rd., Madison, WI 53711 USA