PHYSICAL PROTECTION OF SOIL ORGANIC STUDIED USING ACETYLACETONE EXTRACTION AT VARIOUS INTENSITIES OF ULTRASONIC DISPERSION

Soil structure can protect soil organic matter against microbial attac k by physical protection within stable soil aggregates. We have evalua ted a technique for extracting organic matter from aggregates with dif ferent stabilities. Organic bonded-S, organic bonded-P and some polyva lent metals (Mg, Mn, Ca, Fe, Al, Zn and Cu) were measured in aqueous a cetylacetone extracts of a native and a cultivated soil subjected to d ifferent intensities of ultrasonic dispersion during extraction. Incre asing ultrasonic energy input resulted in increased extraction of orga nic S, organic P and polyvalent metals, until full dispersion was achi eved. Dispersion of the cultivated soil required less energy input tha n dispersion of the native, uncultivated soil. This is consistent with a lower organic matter content resulting in lower aggregate stability . Polyvalent metals are believed to be responsible for the stabilizati on of aggregates by forming clay-polyvalent metal-organic matter compl exes. In our experiment Mn, Ca, Zn and Cu were only related to stabili zation of sand-size aggregates, whereas Al, Fe and Mg were also relate d to the stabilization of microaggregates. Extraction of S was increas ed 2.0 and 1.3 times with dispersion of the native and the cultivated soils, respectively, showing that a major part of soil organic S is in timately associated with microaggregates. For both soils, a residual S pool of insoluble organic matter comprised about 50% of total organic S. It is suggested that this extraction technique effectively divides soil organic S into non-protected, protected and insoluble organic S.