The Effect of N Fertilizer Source on Grain Yield, N Uptake, Soil pH, and Lime Requirement in No-Till Corn

The increasing use of no-till crop production with urea based N fertilizers in the Northeast makes it imperative that we obtain a more quantitative understanding of both the magnitude of apparent NH₃ losses from nonincorporated urea fertilizers in the field and of the factors that contribute to those losses. A 4-year field study was conducted on a Murrill silt loam (Typic Hapludult) to compare the yields and N uptake by no-till corn (Zea mays L.) receiving five rates of broadcast, unincorporated N (0 to 202 kg/ha) as NH₄NO₃, urea, urea-NH₄NO₃ solution (UAN), or (NH₄)₂SO₄. The effects of these sources on the soil pH of the 0 to 2.5 cm and 0 to 17 cm layers and on the lime requirement (LR) in the 0 to 17 an layer were also measured. In 2 of the 4 years, either urea, UAN, or both produced signifcantly lower yields and N uptake than the non-urea sources at the higher (101 and 202 kg/ha) N rates. Apparent N volatilization losses ranged from 0 to 35%. Comparing the results from this experiment with those from other field experiments reported in the literature where the effect of rainfall after N application on apparent N volatilization losses could be measured showed that the following observations applied to all: (1) there was insignificant NH, volatilization loss from unincorporated urea fertilizers if at least 10 mm of rain fell within 48 hours after fertilizer application; (2) if 10 mm or more rain fell 3 days after the urea was applied, volatilization losses were slight (<10%); (3) if 3 to 5 mm of rain fell within 5 days, or 7 to 9 mm within 9 days, volatilization losses could be moderate (10 to 30%); and (4) if no rain fell within 6 days, the loss could be substantial (>30%). The pH in the surface 2.5 an of soil in the plots receiving 202 kg/ha/year of N as NH₄NO₃ urea, or UAN for 5 years was approximately 5.7 or one unit below that in the check. In the plots receiving this rate of (NH₄)₂SO₄, the soil pH in the 0 to 2.5 cm layer was 4.7. As expected, the 101 kg N/ha/year rate produced half was much lowering of pH as the 202 kg/ha rate. Lime requirement measurements in the surface 17 cm of soil receiving 202 kg N/ha/year for 5 years indicated that approximately 67% of the acidity theoretically produced in the nitrification of NH₄⁺ remained to be neutralized with the less acidifying sources (NH₄O₃, urea, and UAN) and that approximately 100% of that from (NH₄)₂SO₄ required neutralization.