Alleviating Soil Acidity With Crop Residues

Soil acidity, which is characterized by mainly aluminum (Al) toxicity and often calcium (Ca) deficiency, is a serious constraint for crop production. To separate these two factors and to alleviate the acidity problem using organic materials instead of lime, a greenhouse experiment was conducted on two acid soils of Hawaii. Eight treatments were established: (i) unamended (control); (ii) 4 cmol(c)/kg as Ca(OH)(2); (iii) 4 cmol(c/)kg as MgO; (iv) 4 cmol(c)/kg as CaSO4 center dot 2H(2)O; (v) 10 g/kg ground fresh cowpea (Vigna unguiculata) leaves; (vi) 10 g ground, dried, fresh cowpea leaves ashed at 350 degrees C for 4 h and then added to 1 kg soil; (vii) 10 g/kg ground fresh pineapple (Ananas comosus) crowns; and (viii) ashed pineapple at rate equivalent to (vii). Desmodium intortum cv. greenleaf, which is a tropical forage legume highly sensitive to soil acidity, was used as the test plant. The results indicated that treatments 1 to 4 could distinguish Al toxicity from Ca deficiency in acid soils and that crop residues could be used to alleviate soil acidity; the fresh cowpea amendment was most effective. More specifically, soil pH was raised moderately by crop-residue amendments, yet exchangeable Al was lowered substantially. Furthermore, soil-solution Al was complexed substantially by dissolved oxidizable carbon, particularly by organic acids produced by the crop residues. There was a good correlation between plant growth and the activity of total monomeric hydrolytic species of Al (Sigma{Al}). Significant growth reduction would be expected when Sigma{Al} > 12.0 mu M.