COMPARISON OF EXCHANGEABLE Al, EXTRACTABLE Al, AND Al IN SOIL FRACTIONS

Abstract

Previous results indicated that the activity of Al3+ in soil solution, Al saturation of the CEC, and 0.01 M CaCl2-extractable Al were related to plant growth. Exchangeable Al values determined by 1 M KCl and 1 M NH4Cl, extractable Al values determined by 1 M NH4OAc, pH 4.8, 0.01 M CaCl2, 0.5 M CuCl2, 0.33 M LaCl3, and 0.005 M NTA, were compared with each other and to the activity of Al3+ in soil solution as well as Al in soil fractions. Aluminum in fractions was measured to determine the forms solubilized by the extractants. The fractions extracted were exchangeable [M Mg(NO3)2], organic bound, Mn oxide, amorphous Fe and Al oxide, and crystalline Fe and Al oxide. The order of Al extracted was CuCl2 > NH4OAc, pH 4.8 .simeq. LaCl3 .simeq. NTA > CaCl2. One M KCl and NH4Cl, and CuCl2, LaCl3, NTA, and NH4OAc, pH 4.8, gave similar relative Al values, especially for topsoils, and were not correlated with CaCl2-Al values. The exchangeable fraction Al [M Mg(NO3)2] was well correlated with the activity of Al3+ in the soil solution and Al saturation of the CEC. Organic fraction Al was negatively correlated with the activity of Al3+ in the soil solution indicating that organic matter can bind Al lowering its concentration in the soil solution. Aluminum in the crystalline Fe/Al oxide fraction correlated well with that extracted by CuCl2, LaCl3, NTA, and NH4OAc, pH 4.8, leading to the possible conclusion that these extractions solubilize Al from plant-unavailable fractions.