THE EFFECT OF 40 YEARS OF CONTINUOUS CORN CROPPING ON SOIL ORGANIC-MATTER CHARACTERISTICS

Changes in humus and some of the major extractable components of soil organic matter were examined, following forty years of conventionally tilled continuous corn cropping compared with those of an adjacent unt illed native grassland soil. Results indicate that long-term continuou s cropping caused a significant reduction in the content of alkali ext ractable and water soluble carbon, as well as in the phenolic and chlo roform extractable compounds, while no differences in volatile acids a nd n-hexane extractable substances were found. On the basis of organic C, corn cropping led to a relative enrichement of phenolic compounds, volatile acids and substances extractable by Na4P2O7, but it did not affect the substances extractable by water. Except for the humificatio n ratio (HR), the humification parameters, such as humification degree (HD), the extracted humic and fulvic acids, the C(HA)/C(FA) ratio and the humification index (HI) revealed a higher degree of humification of the organic matter in native grassland than in the continuous corn cropping system. IR spectra of n-hexane, chloroform, alkaline and wate r extracts failed to show significant differences between sites under native grassland and sites under continuous corn.