Phytotoxicity of Salts in Composted Sewage Sludge and Correlation With Sodium Chloride, Calcium Nitrate, and Magnesium Nitrate

The phytotoxicity of salts in composted sewage sludge (CSS) was evaluated. Concentrations of sodium (Na+), chloride (Cl−1), calcium (Ca2+), and magnesium (Mg2+) were present at levels that would induce salt stress in plants. Nutrient imbalances were also found that would adversely affect the use of CSS as a growth medium. To further understand the phytotoxic nature of these salts, sodium chloride (NaCl), calcium nitrate [Ca(NO3)2] and magnesium nitrate [Mg(NO3)2] solutions were used to simulate the composition of salts found in CSS in an investigation of radish (Raphanus sativus L.) seed germination. High concentrations of Ca2+ (92.1 mmol.L−1) and Mg2+ (27.4 mmol.L−1) inhibited seed germination to an equal extent as did Na+ (40.6 mmol.L−1). The lower concentration of Ca2+ (10 mmol.L−1), however, significantly relieved the stress caused by NaCl. These results indicated that the composition and total amount of Na+, Cl−1, Ca2+, and Mg2+ in CSS should be carefully monitored before it is used as a soil amendment or growth medium.