RELEASE OF POTASSIUM AND MAGNESIUM FROM SOIL FRACTIONS AND ITS KINETICS

The development of integrated soil fertility programs will involve the consideration of mineralogical and taxonomical classifications of soils. The objective of this work was to investigate K and Mg release from silt and clay fractions of four Quebec soils. All soils were characterized by a mixed clay mineralogical composition; feldspars and quartz dominated the silt and vermiculite and hydrous micas the clay fractions. Fifteen sequential extractions with 5 x 10(-4) M citric acid, through a period of 3200 h, and a single extraction with boiling 1.0 M HNO3 were conducted. The kinetics of slowly exchangeable K and Mg release were described using a parabolic diffusion equation. Among particle-size fractions, the largest amounts and rate of release were in the fine silt for K, and in the fine and medium silt for Mg. Citric acid extraction resulted in the dissolution of trioctahedral phyllosilicates from the soil fractions of the two Laurentidian soils and to the transformation of dioctahedral micas to vermiculite in the Appalschian soils. The amounts of K released from the soil fractions in diluted citric acid solutions were significantly related to their amounts of HNO3-extractable forms (r = 0.76, P less-than-or-equal-to 0.01) but not to the structural or total forms. These results extend to soil minerals the results of previous studies on the resistance of pure species to hydrolysis and complexation reactions by citric acid. This study also suggests that information about silt fractions must be included in mineralogical classifications of soils for K and Mg management.