Amelioration of Subsoil Acidity through Surface Application of Organic Manures

Subsoil acidity is a serious constraint to crop production, and is difficult to correct by conventional liming practices. Thus, different approaches to ameliorating acid subsoils are needed. A column leaching study was conducted to compare the effects of chicken manure and sewage sludge amendments, each at 20 g kg−1, with CaSO4 and CaCO3, each at 5.56 cmol (½Ca2+) kg−1, on Al and Ca concentrations in the subsoil of an Ultisol having pH 4.4, 80% Al saturation and 2.78 cmolc kg−1 exchangeable Al. Treatments were incubated moist for 14 d with the top 5 cm soil of 50 cm long soil columns, then leached with 37.8 cm of water in 5 d. The soil columns were then cut into four equal segments for chemical analysis. Results showed that the manures were effective in increasing soil‐solution pH and Ca, and decreasing soil‐solution Al and percentage of Al saturation in the subsoil, particularly below the 15 cm depth. Soil‐solution C, nearly constant through the soil profile, averaged 2.8 mM in the CaSO4 treatment and 7.6 mM in the chicken manure treatment. A subsequent solution experiment on the competitive complexation of Al and Ca with model organic ligands supported the contention that (i) manure‐derived organic molecules facilitated the downward movement of Ca as Ca complexes, and (ii) along their downward path, these Ca complexes reacted with Al, releasing Ca that would become plant‐available, and forming complexed Al that may be nonphytotoxic.