Management and crop residue influence soil aggregate stability

Soil management is one of the most important factors influencing the structure of soils. The interaction of management (including tillage and crop rotation history) with soil biochemistry, soil aggregation, and soil humus composition was determined in a native prairie and a producer field situation in 1997. A comparison of a native prairie and an adjacent conventional corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation on the same soil type found that the Webster soil (fine-loamy, mixed, superactive, mesic Typic Endoaquoll) after soybean (Cs plant) was lower in monosaccharide content and protein content as determined by ion chromatography, and lower in phenolic acid content than the Webster soil after corn (C-4 plant) or in native prairie as determined by gas chromatography. A wet, nested sieve aggregate stability measurement determined that the prairie soil had a higher mean aggregate size (1.85 mm) when compared with the soil in the presence of decomposing corn (1.0 mm) or soybean (0.34 mm) residues. Mean aggregate size was found to be correlated with soil monosaccharide content (r = 0.75), total soil protein content (r = 0.995***), total soil phenolic acid content (r = 0.997***), and alkaline extractable humic substance content (r = 0.98**). Alkaline extractable humic substances were correlated with the phenolic acid content of the humic substances (r = 0.996***). The results suggest that the decrease in soil stability after soybean growth was due to a decrease in the content of soil humic substances caused by the substantially lower phenolic adds content (humic acid precursors) in the soybean residue.