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Soil erosion, which is a widespread problem in semiarid areas, may lead to a decline in soil productivity since the finest and most fertile soil particles are those which are generally removed. Our objective was to determine the distribution of soil organic matter, phosphorus, potassium and cation exchange capacity within the fine fractions (< 2 mu m and 2-20 mu m) of the soil. Samples were taken from the top 20 cm of 14 cultivated soils and six forest soils. The organo-mineral size fractions from soil samples were isolated without chemical pretreatment by ultrasonic dispersion in water followed by sedimentation-syphonation. The distribution of organic matter within size fractions varied with land use. The cultivated soils had a greater percentage (on average, about 30%) of total soil C in the < 2 mu m fraction than the soils under natural vegetation (on average, about 18%), in which the total soil C was associated with the 2-20 mu m fraction to a greater extent than in cultivated soils. The distribution of the soil N between the clay and fine silt size fractions followed a similar pattern to that shown by soil C. The C/N ratio became smaller as particle size decreased. The higher C/N ratio obtained for the 2-20 mu m fraction for both forest and cultivated soils suggests the presence of less decomposed organic matter, while the organic matter associated with the < 2 mu m fraction can be considered to be more humified. The cation exchange capacity of whole soil and organo-mineral fractions were closely correlated with their respective C contents. The clay-size fraction had the highest CEC, which was related to its mineralogical composition.

The data confirm that the proportion of soil organic matter depends on the stabilizing capacity of the different size fractions, both the clay and fine silt size fractions playing an important role in semiarid soils. To the detriment of the soil’s organic matter content these fractions are easily eroded in soils under arid and semiarid conditions, which may render them suitable for agricultural purposes. (C) 1999 Elsevier Science B.V. All rights reserved.