The effect of NH₄ fertilizer on soil acidification is evident in the data from long-term studies from Rothamsted Experimental Station in the U.K. (Figure 9.2; Johnston et al., 1986). Surface soil pH decreased by up to 1.6 pH units with the application of (NH₄)₂SO₄ fertilizer, whereas the pH remained about the same with nitrate fertilizer (NaNO₃). Soil acidification rate increased with the increasing rate of (NH₄)₂SO₄ as the surface soil pH decreased 0.9, 1.2, and 1.6 pH units with the annual addition of 48, 96, and 145 kg N ha⁻¹, respectively. Soil pH has been reported to decrease by 1 pH unit after growing legume pasture continuously for more than 30 years in Australia (Bolan and Hedley, 2003). The application of S fertilizer also leads to soil acidification, with the elemental S fertilizer oxidizing to produce sulfuric acid, similar to the oxidation of sulfides as described in Section 9.3.5.

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FIGURE 9.2. The effects of different rates of ammonium and nitrate fertilizers over 100 years on the surface soil (0–23 cm) pH. From the Park Grass Experiment, Rothamsted in the U.K. The fertilizer doses given in the figure were applied annually

drawn using data from Johnston et al., 1986.

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