The acidulation effects of nitrogenous fertilizers on selected chemical and microbiological properties of soil

Abstract

The acidulation effects of the prolonged use of nitrogenous fertilizers on selected chemical and microbiological properties of a silt loam soil (Typic Ochraqualf) were assessed.

Soil samples were collected from experimental plots initially established in 1971 that were annually treated with ten N fertilizer treatments and planted to corn. Residual soil acidity found to be associated with 9 annual applications of the N fertilizers was of the order: (NH₄)₂ SO₄ > NH₄C1 > anhydrous NH₃ (at 300 kg N/ha) = NH₄NO₃ > urea = ureaform = anhydrous NH₃ (at 150 kg N/ha) > Ca(NO₃)₂ NaNO₃. Extractable P, Fe, Mn and Al followed a pattern of increasing availability with a decrease in soil pH. Exchangeable Ca and Mg levels were significantly higher in those treatments that did not show a significant decrease in soil pH over the 9‐year duration.

Significant differences by N treatment among selected microbial groups were also determined. The annual mean counts of soil fungi were significantly higher for the most and least acidifying treatments, namely ammonium sulfate and sodium nitrate. However, the mean counts of soil bacteria and actinomycetes were non‐significant for any of the fertilizer treatments although the counts of both microbial groups declined with the addition of N. Significant differences in the mean counts of nitrifying and denitrifying bacteria were obtained with the ammonium sulfate treatment showing the greatest reduction in MPN counts. Microbial diversity (H') indices were also determined and correlated with soil pH. A quadratic relationship was determined for phyleal diversity (the total population of bacteria, actinomycetes and fungi) with soil pH, while a linear function best described physiological diversity (the nitrifying and denitrifying bacteria) versus soil pH.