PROPERTIES OF SOIL PARTICLE-SIZE SEPARATES AFTER 40 YEARS OF CONTINUOUS CORN

Changes in chemical and mineralogical characteristics associated with different particle size fractions in soil after 40 years of continuous production of corn by the conventional tillage method (CC) as compare d with those of an adjacent native grassland site (NG) are investigate d. Results indicate that corn cropping in a soil previously supporting native vegetation produces a decline in total and humified organic ma tter, phenolic compounds, enzymatic activities, cation exchange capaci ty (CEC), and hydrosoluble ions, both in the whole soil and in its par ticle-size separates. The largest losses in organic carbon (C) and nit rogen (N) contents of the cultivated soil were observed in the sandy f ractions, the lowest in the silt+clay separates. The humification inde x (HI) indicates a higher degree of humification of the organic matter in NG than in CC samples. For both NG and GC sites the finest fractio n (silt+clay) resulted to be enriched in organic C, total N, humus, ph enolic compounds, enzyme activity, CEC, and hydrosoluble ions with the only exception of mineral N forms and sulphates (SO4). Slight differe nces were observed in the mineralogical composition of NG and CC soils . The sandy fractions of NG showed greater amounts of phyllosilicates while a lower content was found in the silt+clay fraction of CC as a c onsequence of a crumbling of parent rock into small pieces induced by repeated tillage practices.