Soil Acidification from Long-Term Use of Nitrogen Fertilizers on Winter Wheat

Although N fertilizers are not acidic, their inputs to soil are acid forming. As a result of the long-term use of N fertilizers, soils in the Great Plains are becoming more acidic and this acidity may become a yield-limiting factor. In 1970, long-term plots were initiated to compare sources (anhydrous NH3, NH4NO3, urea, and S-coated urea), application rates (34, 68, 136, and 272 kg N ha−1), and an untreated check (0 N) on wheat (Triticum aestivum L.) grain yield, soil pH, exchangeable base cations, and Al saturation. For the soil properties evaluated, significant differences among the different N sources did not exist after 30 annual applications of N fertilizer. The long-term N fertilization significantly reduced soil pH in the surface soil layer (0–15 cm), especially at the higher application levels. Soil pH decreased with time and was significantly related to the amount of total N applied for each N source. Nitrogen fertilization with each N source significantly increased exchangeable Al and Al saturation (Alsat) but decreased exchangeable base cations (Ca2+ and Mg2+). Both exchangeable Al and Alsat increased with increasing N rate and were inversely related to soil pH. Despite decreased soil pH levels to <5.0 as early as 1980 in the experiment, significant reductions of wheat yield did not occur until 1995. Reductions in yield occurring between 1995 and 2002 coincided with the greatest change in soil pH occurring during the same time period.