Manganese Solubility and Phytotoxicity Affected by Soil Moisture, Oxygen Levels, and Green Manure Additions

Abstract

Manganese (Mn) toxicity is common in tropical acid soils, second only to aluminum toxicity. Changes in soil pH and redox potential (E_h) caused by soil moisture conditions and organic amendments certainly affect Mn solubility and toxicity to plants grown in soils with high Mn reserves. Laboratory incubation and greenhouse experiments were conducted to quantify such effects using a high-Mn Oxisol and a moderate-Mn Mollisol from Hawaii, with soybean (Glycine max L. cv. Kahala) being a test crop. The soils were mixed with a green manure (leucaena leaf) at 0, 10, and 20 g kg⁻¹, and were incubated at 24 ± 1°C at 80% of field water holding capacity—equilibrated with air (FC), or N₂ gas (N₂)—or submerged under 8 cm of water. Soil pH, E_h, and soluble Mn as measured in saturated paste extract, Mehlich-3 and hydroxylamine hydrochloride (NH₂OH) solutions were periodically determined over 56 days of incubation. The submergence and N₂ treatments increased soil pH towards 7.0 and decreased soil E_h from 600 mV to between 300 and 400 mV, indicating a more reducing environment, which seemed to be controlled by the MnO₂-Mn²⁺ and/or FeOOH-Fe²⁺ redox couples. Green manure additions had similar effects, but to a lesser extent. Consequently, soluble soil Mn concentrations increased by 100–1000 fold relative to those of the controls (no manure, at FC). There was a marked decrease in soybean growth and a marked increase in leaf Mn concentration in the treatments of submergence and green manure additions. A 10% reduction in dry matter yield was expected when leaf Mn exceeded 200 mg kg⁻¹, leaf Ca/Mn (weight-to-weight) <80, saturated paste Mn >0.50 mg L⁻¹, Mehlich3 Mn >200 mg kg⁻¹, and NH₂OH Mn >1100 mg kg⁻¹. The effects were most evident at 14–28 days after incubation.