Effects of Soil Acidification on Chemical and Mineralogical Properties of a Limed Soil

Two soil acidulents were compared for their effects on soil chemical and mineralogical properties. Aluminum sulfate [Al₂(SO₄)₃ · 18 H₂O] powder was applied at rates of 0.13, 0.26, 0.52, and 1.05 kg m⁻³ (7.5, 15.0, 30.0, and 60.0 g per pot, respectively). Elemental S powder was added at rates of 0.03, 0.06, 0.13, and 0.26 kg m⁻³ (1.9, 3.8, 7.5, and 15.0 g per pot, respectively). Sawdust was incorporated into the soil at rates of 0, 20, 40, and 60 m³ m⁻³ (% by volume). The soil used in this experiment was a previously limed sandy loam surface from a Typic Hapludult with an initial pH of 6.2. On a mass basis, approximately eight times as much Al₂(SO₄)₃·18 H₂O as elemental S was needed to cause a two-unit decrease in soil pH. The acidity produced by S maintained low pH longer than Al₂(SO₄)₃·18 H₂O. Extractable levels of Ca and Mg in the highest acidulent treatments were approximately double that of the controls at the conclusion of the experiment. This is a probable response to lime particles from a previous liming being dissolved by acids. Extractable K, because of destruction of illitic clay minerals, was significantly higher in the S treatments than in the Al₂(SO₄)₃ treatments or controls. Extractable Al levels were greater than the controls only at higher rates of both acidulents. Extractable Al levels at a given pH were higher in the Al₂(SO₄)₃-treated soils than in those treated with S. Destruction of clay minerals, including gibbsite, kaolinite, and hydroxy interlayered vermiculite were evident in the highest S treatment, even though both the Ca/Mg and K/NH₄ cation exchange capacity values increased when compared to the controls. Aluminum sulfate had little effect on clay mineral stability when compared to the controls, and may have increased the heat stability of K saturated hydroxy interlayered vermiculite.