SOIL MICROAGGREGATION AS INFLUENCED BY UNCHARGED ORGANIC CONDITIONERS

To assess the interaction of water soluble stabilizing agents with soi l particles, soil microaggregation was studied after treatments with t wo uncharged organic conditioners, Polyvinyl alcohols (PVAs) and Dextr ans, of different molecular weight. The size distribution of microaggr egates (diameter <250 mu m) was determined on two soils of low organic matter contents with differing texture by means of a laser light tech nique. PVAs and Dextrans modified the microaggregate size distribution , increasing the proportion of >75 mu m aggregates. The extent of this modification increased with the molecular weight of the conditioners and, for the same molecular weight, decreased with increasing amount o f conditioner used. The aggregating effect of PVAs was stronger on par ticles smaller then 5 mu m, which formed microaggregates in the range 10-100 mu m which were not broken down even by ultrasonication. S.E.M. micrographs of soil aggregates confirmed the results reported above. PVAs and Dextrans appeared to produce a more porus structure with more aggregates of about 100 mu m size, both in the clay soil and, to a le sser extent, in the sandy soil.