Influence of rotation cropping and sugarcane production on the clay mineral assemblage

The present study examines the influence of unfertilized crop production on Calcic Haplustept soils in southwestern Iran. Soil profiles from about 100 years of rotation cropping (wheat, corn, barley and vegetables) and those from 40 years of irrigated sugarcane production were compared with the adjacent uncultivated soils. Particle size analysis revealed there was an increase in clay contents of soils from the rotation cropping. There was 66.1% clay in the Ap horizon of rotation cropping soil and 56.8% was in the A horizon of uncultivated soil. On the other hand, the sums of sand and silt fractions were 43.2 and 33.9% for uncultivated and rotation cropping soils, respectively. Therefore, differences between clay contents could be attributed to cultivation and subsequent mineral alteration. Organic matter contents were greater under cultivated soils; consequently the cation exchange capacity under sugarcane (19 cmolc kg− 1) and rotation cropping (30 cmolc kg− 1) were greater than for the uncultivated soils (16 cmolc kg− 1). Under cultivation, both the gypsum and calcium carbonate content of soils were lower, especially in the surface horizon. The lower electrical conductivity of soil solutions further reflected enhanced leaching of the cultivated soils. Depletion of K by the sugarcane is due to the release of nonexchangable K from interlayer positions in micaceous minerals. Expandable minerals were abundant in the Ap horizon of the cultivated soils, unlike in the uncultivated soils, while only small amounts were found in C horizons of cultivated soils. This trend agrees with changes in soil physico-chemical properties, especially for fixed K and increased CEC. TEM showed that the subsurface horizons of the cultivated soils contained more palygorskite than the surface. It can be concluded that intensive cropping and strong irrigation over a long time could have brought about changes from micaceous and palygorskite minerals to expandable minerals.