Crop growth and water/solute movement are affected by soil properties. Crop growth is affected by soil moisture retention, which relates to soil structure (particle and pore size distribution), which is greatly affected by soil C levels. Soil hydraulic conductivity depends on particle size distribution, porosity, bulk density, and preferential flow paths, which also relates to soil C levels and their effect on soil structure. Uncertainty exists concerning the impact of cover crops with conservation tillage on the total agricultural environment. A study conducted from 2002 – 2005 by the University of Georgia, University of Florida, and USDA-ARS, assessed the effects of cover crops on C/N sequestration and soil physical properties in coastal plain soils. The cropping systems were (1) sunnhemp (Crotalaria juncea L.), crimson clover (Trifolium incarnatum L.), sweet corn (Zea mays L.); (2) sunnhemp, fallow, sweet corn; (3) fallow, crimson clover, sweet corn; (4) fallow, fallow, sweet corn; or (5) fallow, fallow, fallow. Three N rates (0, 75, or 150 kg/ha) were tested on the corn for cropping systems 1, 2, and 3, while N rates of 0, 75, 150, 225, and 300 kg/ha were used for cropping system 4. No N was applied to cropping system 5. There were four replicates of each cropping system/N fertilizer rate treatment. Carbon/nitrogen sequestration was determined from samples of the top 2.5 cm of soil collected quarterly. Soil physical property measurements were made on minimally disturbed cores (7.6 cm height X 7.6 cm diameter) collected from the top 7.6 cm of soil of each plot three times annually using an impact type sampler. Soil physical property measurements on the cores included bulk density (BD), saturated hydraulic conductivity (HC), and soil moisture retention (MR). Results from the study showed cover crop differences in C/N sequestration and soil physical properties. Rotations with sunnhemp had greater total C and N than the other cropping systems. For soil physical properties there were significant differences in BD, HC and MR between soil in the rows and that in the row middles for all treatments. Bulk densities were lower, HC's were greater, and MR's were greater in the rows than in the middles. No differences in soil physical properties were found among N fertilizer rates. Also, cover crops and N fertilizer rate did not affect MR. However, significant differences in BD and HC were found between rotations with sunnhemp as crop 1 and the other rotations. The rotations with sunnhemp as crop 1 had lower BD and greater HC than the other cropping systems, particularly in the soil samples collected within the row. Inclusion of sunnhemp, which produces large quantities of biomass, as a cover crop in rotations on coastal plain soils benefited the soil by increasing C/N sequestration and hydraulic conductivity (greater water infiltration at the surface and less runoff), and decreasing bulk density. The study showed the significant effects that cover crop type in agricultural systems has on organic matter accumulation including positive effects on soil physical properties important to crop production.

Partially Supported from Southern Region SARE Grant No. LS02-014