RELATIONSHIPS BETWEEN AGGREGATION AND IRON-OXIDES IN TERRA ROSSA SOILS FROM SOUTHERN ITALY

Thirteen samples from six Terra Rossa profiles of southern Italy were studied to determine the influence of Fe oxides in aggregation. Removal of all the Fe oxides by the citrate-bicarbonate-dithionite treatment resulted, in all samples, in a decrease in the proportion of the silt plus sand fraction and an increase in the clay fraction. The change in the particle size distribution (PSD) was highly correlated with the total Fe oxide content and with the hematite content but not so well correlated with the goethite content. When the samples were treated with acid NH4-oxalate to remove the poorly crystalline Fe oxides, there was a decrease in the proportion of the sand fraction and, in general, an increase in the proportion of silt and little or no increase in the proportion of clay; the change in PSD was better correlated with the ratio oxalate-extractable Si/oxalate-extractable Fe than with any other single property. This indicates that silica may contribute to aggregation, probably by forming bridges between Fe oxides and other soil particles.