PROLONGED LEACHING OF MINERAL FOREST SOILS WITH DILUTE HCL SOLUTIONS - THE SOLUBILITY OF AL AND SOIL ORGANIC-MATTER

Long-term acidification has been shown to result in a considerable dec rease in the amount of organically bound soil Al and in a gradual decr ease in the solubility of Al. We examined the solubility of soil organ ic matter (SOM) and Al in four acid mineral soils (one Arenosol Ah, tw o Podzol Bh, and one Podzol Bs) as they were leached sequentially usin g a solution containing 0.001 M HCl and 0.01 M KCl. The acid leaching resulted in relative decreases in Al that were 2-6 times greater than for organic C. The organic C and Al dissolved by the acid leaching ori ginated mainly in the pyrophosphate-extractable fraction of the elemen ts. Protonation seems to be a major mechanism in stabilizing the resid ual SOM, as indicated by small changes in effective cation exchange ca pacity with the degree of acid leaching. In the samples of Podzol Bh a nd Arenosol Ah soils the solubility of Al (defined as log(10){Al3+} 1.5pH) in equilibrium suspensions (0.01 M KCl) was closely related to the ratio of pyrophosphate-extractable Al to pyrophosphate-extractable organic C. The Podzol BS Sample probably contained a small amount of a surface-reactive Al(OH)(3) phase, which rapidly became depleted by t he acid leaching.