Oilseed radish effects on soil structure and soil water relations

Oilseed radish (Raphanus sativus spp. oleifera) reduces sugarbeet cyst nematode (Heterodera schachtii) populations. Fall-incorporated radish biomass may also increase the yield and quality of subsequently grown sugarbeet (Beta vulgaris L.) by improving soil physical and hydraulic properties. This field study determined radish effects on near-surface soil aggregate stability, water-stable aggregate size distribution, bulk density, and field-saturated water content, as well as infiltration and hydraulic conductivity measured at water supply potentials of -40, -20, and +0 mm H2O. In 2003 and 2004 in Twin Falls, ID, radish were grown in a Portneuf silt loam (Durinodic Xeric Haplocalcid) for about 10 weeks in the fall, then incorporated later that fall by disking, followed by moldboard plowing. In early May of the following year, sugarbeet were planted, irrigated, then harvested for yield and quality. In the spring and fall of each sugarbeet growing season, soil samples were collected from two depths, 0 to 5 and 5 to 50 mm, on which we measured aggregate stability and size distribution by wet sieving. Soil cores were collected from 0 to 34 mm to measure bulk density. Also in spring and fall, we used ponded and tension infiltrometers placed in the row to measure steady-state, unconfined infiltration rates and, from those rates, to calculate near-surface hydraulic conductivities at each supply potential. Radish had either few or inconsistent effects on soil structure or hydraulic properties at potentials ≥-40 mm H2O. There were, however, two exceptions. Fall-incorporated radish increased the field-saturated water content by 10% to 0.446 m3 m-3 in spring 2003 but had no effect in spring 2004, compared to the control (no oilseed radish). Most importantly, compared to the control, radish as a fall-incorporated green manure consistently increased the proportion of flow-conducting soil pores ≤0.75 mm in diameter, likely increasing water retention.