Long-term use of urea vs. anhydrous ammonia for N fertilization of a Dark Brown loam: I. Soil acidification and nutrient availability
The decline of indigenous soil fertility on the pra1r1es has necessitated higher nitrogen fertilizer inputs into cropping systems. Our objectives were to quantify the acidifying effects of urea and anhydrous ammonia nitrogen injected at 0.1 m depth annually at rates of 45, 90, and 180 kg ha-1 in a Dark Brown loam at Scott. Soil acidity increased with increasing rates of nitrogen fertilization and anhydrous ammonia was more acidifying than urea. After 9 years of anhydrous ammonia application at 180 kg ha-1, soil pHCaCl2 at injection depth had dropped to 4.2 compared to 5.2 in the unfertilized check treatment. This acidification was associated with a depletion of exchangeable base cations (1307 kg calcium ha-1, 307 kg magnesium ha-1) and a concomitant increase in exchangeable acidity in the top 0.3 m. The base depletion and the increase in exchangeable manganese were correlated with the amount of nitrate leached into 0. 3 to 1.5 m depth (r = 0.66 and 0.79***). The soil solution was more acidified 6 days after fertilizer injection than 3 days before and 26 days after injection. With increasing acidity, the solution concentration of potentially phytotoxic manganese increased from 0.2 to 29 mg l-1 and nutrient ratios changed significantly, e.g., the molar K/NH4-ratio decreased from 5.4 to 0.1. Our data suggest that large losses of nitrogen beyond rooting depth and to atmosphere take place where high rates of fertilizer nitrogen are used.
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