Submitted to: World Fertilizer Congress of International Scientific Centre of Fertilizers
Publication Type: Proceedings
Publication Acceptance Date: 9/13/1997
Publication Date: N/A
Citation: Interpretive Summary: Nitrogen is most often the limiting nutrient in crop production. Use of nitrogen fertilizers overcomes soil deficiencies, but on average only 30 to 70% of the applied nitrogen is taken up by the target crop. The balance of the applied nitrogen is lost to crop production and may be leached to the groundwater in the nitrate form. If nitrates are leached to groundwater, they become a health hazard. A fertilizer decision aid has been developed which predicts the amount of nitrate nitrogen which will be naturally produced during the early growing season. By applying only the amount of nitrogen fertilizer to soil needed to meet the crop demand, risk of leaching nitrate nitrogen to groundwater is reduced. Also, the use efficiency of applied nitrogen is increased. The decision aid uses temperature, rainfall, soil clay content, soil pH and soil bulk density to predict the relative aeration in the soil. This is then used to predict the amount of nitrate nitrogen which will be produced by soil microbial action. Field tests of the decision aid obtained the same grain yields with, on average, about 10% less applied fertilizer nitrogen. Used over the entire cornbelt, this would suggest a potential improvement of more than $300 million each year.
Technical Abstract: Microbial production of nitrate, the main form of N uptake by corn, is a function of soil aeration. Accurate prediction of naturally produced nitrates aids efficient use of N-fertilizer. A N-fertilizer prediction aid has been developed using three static soil factors, clay content, pH, and organic matter content, and three variable factors, temperature, rainfall, and soil bulk density. Single variable mathematical functions describe soil water content, temperature, and pH effects on nitrate formation. These factors are integrated to provide estimates of daily nitrate production. Trials of the decision aid were conducted on farms in Minnesota. Farmers conducted normal planting practices but left 6 to 12 rows of corn without added N. Soil samples were collected by taxonomic mapping unit at planting time and again just before addition of fertilizer at the 5-leaf growth stage. Amounts of nitrate in the 0-2 ft profiles at the 5-leaf stage of growth are predicted with the aid. Fertilizer-N needed for optimal yields was obtained as: [20 ppm - predicted amount nitrate ppm ]*8 ppm lbs. per acre. The factor, 20 ppm, is a threshold concentration beyond which maize shows little or no response to added fertilizer. Nitrate concentrations within a field strip varied by as much as 100%. The observed change in nitrate between the two samplings ranged from-0.68 to 11.7 ppm with a mean of 2.5 +/- 4.2 ppm compared with the predicted change in nitrate which ranged from 0.5 to 10.7 with a mean of 2.3 +/- 2.1 ppm. Using the decision aid, the same yields were obtained usually with less applied fertilizer N.