Effectiveness of innovative organic amendments in acid soils depends on their ability to supply P and alleviate Al and Mn toxicity in plants

Soil acidity with high Al3+ and Mn2+ is one of the major constraints to global food production. Lime is effective to increase soil pH, but it is not always readily available and can be expensive. This study aimed to evaluate the effectiveness of organic amendments that may be viable for treating soil acidity. Thirteen organic amendments (including manures, composts, biochars and plant residues) were added (1% soil weight) to two acid soils differing in pH buffer capacity, and Al3+ and Mn2+concentrations. They included a Dermosol with a pHCaCl2 of 4.1 and a Sodosol with pHCaCl2 of 4.0. Four inorganic amendments (lime, dolomite, gypsum, and KH2PO4) were included for comparison. The Al-sensitive wheat ES8 was grown for 49 days. Organic amendments (manures, composts, biosolids and biochars) outperformed or matched the shoot biomass response to lime in both soils. The most effective treatments were poultry litter, poultry-litter-biochar and biosolids, which increased shoot biomass by 128%, 158% and 95% for the Dermosol and by 58%, 43% and 33% for the Sodosol, respectively, compared with the limed controls. Organic amendments increased soil pHCaCl2 by up to 0.32 and 0.62 units and Olsen-P concentration by 16.1 μg g−1 and 30.7 μg g−1 for the Dermosol and Sodosol, respectively. Shoot biomass correlated positively with Olsen-P (R2 = 0.85) but negatively with concentrations of extractable Mn (R2 = 0.62) and Al (R2 = 0.58). Organic amendments were effective ameliorants for soil acidity. Their effectiveness depends on their ability to supply nutrients, primarily phosphorus, and to overcome Al3+ and Mn2+ toxicities.