SOIL ACIDIFICATION FROM LONG-TERM USE OF ANHYDROUS AMMONIA AND UREA

Acidity generated by N fertilizers depends on factors such as the composition of the fertilizer, climatic and soil conditions, and the crops grown. Our objective was to quantify the acidifying effects of urea and anhydrous NH3 when used as fertilizers for cereal production in Saskatchewan, Canada. The fertilizers were injected annually (at 10-cm depth) into a medium-textured, moderately acid (pH approximate to 5.5) Typic Haploboroll, at rates of 0, 45, 90, and 180 kg N ha(-1) for 9 yr. Soil acidity increased as N application rate increased, with anhydrous NH3 causing greater acidification than urea. Although pH values as low as 4.3 were recorded in soil treated with anhydrous NH3, KCl-exchangeable acidity remained low. The major effect of acidification was a depletion of exchangeable Ca and Mg. The solubility of Mn (but not Al) increased substantially as pH decreased, with solution concentrations of almost 30 mg Mn L(-1) being recorded 6 d after injection of NH3. Acidity generated by anhydrous NH3 compared well with values predicted assuming that all of the applied NH3 was oxidized to NO3- (with the production of 1 mot H+ mol(-1) of N) and that these protons were partly neutralized by OH- released when NOS was taken up and assimilated by plants. Acidification due to export of bases in grain was insignificant because wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) remove only a slight excess of cations over anions. Urea failed to realize its full acidification potential because of an apparent loss of urea-N from the soil by NH3 volatilization.