Alum sludge derived from a municipal wastewater plant was used as a soil amendment in a greenhouse study with barley (Hordeum vulgare) as the test crop. Treatment variables included the soil pH (4.5, 5.1 and 6.5), the amount of Al in the sludge (control = 30 mg Al-T/g; alum sludges = 38 and 52 mg Al-T/g), and the sludge application rate (100 and 270 kg N-T/ha). Soil amendment with the two alum sludges reduced soil pH, increased Al3+ activity in the soil solution, and reduced barley growth over the 6-week experiment. Barley growth decreased as the Al3+ activity in the sludged soil solution increased, but for a given (Al3+) the phytotoxicity of Al was markedly pH dependent. For example, at a pH of 5.0 +/- 0.1 an Al3+ activity of 0.5 mu M was sufficient to inhibit plant growth by about 50; this IC50 value increased five-fold to about 2.5 mu M when the soil pH was 4.5 +/- 0.1. This decrease in the toxicity of Al3+ with acidification was explained in terms of a competitive interaction between the H+-ion and Al3+ at the root surface. Stepwise multiple regression allowed the prediction of aerial leaf biomass from soil pH and sludge application rate. References: 52
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