SOIL SOLUTION CHEMISTRY AND ALFALFA RESPONSE TO CACO3 AND MGCO3 ON ANACIDIC GLEYSOL

Soil acidity is a limiting factor for forage production. Liming is a c ommon agricultural practice for acid soils, yet there is limited infor mation on the effects of soil solution chemistry in response to liming . Soil from the Ap horizon of an Orthic Humic Gleysol was amended with 0, 2.5 or 5.0 g CaCO3 kg(-1) and 2.1 or 4.2 g MgCO3 kg(-1) to determi ne the changes due to liming in soil solution composition before plant ing and after three cuts of alfalfa (Medicago saliva L.). The soil sol ution samples were extracted by immiscible displacement with C2Cl4. Th e low equivalent rate of CaCO3 and MgCO3 decreased the concentrations of Fe from 889 to less than 22 mu M, Mn from 286 to less than 6 mu M, Al from 45 mu M to undetectable level before plant growth. Soil pH, di ssolved organic carbon (DOC), Cu and NH4-N concentrations in the soil solutions extracted after the third cut of alfalfa were increased comp ared with those measured before planting. Concentrations of Ca, Mg, K, Na, Mn, Zn, Fe, Al, NO3-N, SO4 and Si were all decreased after the th ird cut compared with those measured before planting. Step-wise multip le regression analysis indicated that the dry matter (DM) yield of the first cut was positively correlated to NO3-N and negatively correlate d to Mn concentration in the soil solutions (R(2) = 0.65*); whereas t he DM yield of the second and third cuts and of the roots were negativ ely correlated with Mn concentrations (R(2) = 0.75*, 0.63**, and 0.60 *, respectively). The regression analysis supported visual Mn toxicit y, suggesting that Mn toxicity, not Al concentration, was the main lim itation to alfalfa growth in unlimed soil.