REDUCTION OF POTASSIUM FIXATION BY 2 HUMIC-ACID FRACTIONS IN VERMICULITIC SOILS

The capacity of soil organic matter (SOM) to reduce Ii fixation was in vestigated in calcareous vermiculitic soils. Humic acid fractions were extracted with the use of a modification of the standard NaOH extract ion procedure. The mobile humic acid pool (MHA) was extracted without initial decalcification. Addition of MHA to the vermiculitic soils red uced K fixation and resulted in greater total extractable K and highly labile K, as well as greater plant K uptake. The Ca humate pool (CaHA ) was extracted after decalcification, and it had little effect on K a vailability. An increase in extractable and highly labile Ii occurred within 4 h of MHA addition and thus appears to result from a rapid exc hange reaction and not from mineral dissolution; the latter was confir med in separate experiments with organic acids. This rapid effect coul d be reproduced by addition of dodecylammonium chloride. We speculate that the enriched amino acid content and increased solubility of MHA, compared with the more humified CaHA, contribute to the greater influe nce of MHA on K equilibria. The less soluble, more condensed CaHA is m ore likely to precipitate with Ca2+, The capacity of solubilized MHA t o hold K+ against soil adsorption was also apparent when soil was sepa rated from MHA by a dialysis membrane. These results provide further e vidence that young N-rich SOM fractions such as MHA play an important role in governing K availability in soils with high K fixation potenti al.