Radish Cover Crop Growth and Compaction Alleviation Potential in Southeastern Coastal Plain Soils

Abstract

Soil compaction in the form of hardpans often restricts cash crop root growth in the southeastern U.S., reducing plant vigor and yield potential for crops with deep taproots such as cotton (Gossypium hirsutum). Planting forage radish (Raphanus sativus) as a cover crop has been suggested as a method to alleviate hardpans and preserve soil structure by reducing the need for deep tillage. Research is needed to determine if forage radish can alleviate compaction in Coastal Plain soils and provide basic information on radish management to determine appropriate planting dates and cultivars for the Southeast. To address this objective, a field study using five radish cultivars (i.e., ‘Lunch’, ‘Sodbuster’, ‘Nitro’, ‘Tillage’, and ‘CCS779’) planted on three planting dates (i.e., mid-September, mid-October, and mid-November) at two locations in the Coastal Plain region of Alabama was created to evaluate radish growth and soil compaction alleviation in cotton. Plant canopy width and foliage, root, and total dry matter were measured at five sampling times during the growing season. Root diameter and root length aboveground, belowground, and in total were also measured. Plots were evaluated for soil compaction using a tractor-mounted penetrometer after cover crop termination, which revealed that radish cover crops did not reduce penetration resistance compared to fallow plots. No differences were observed between cultivars for most growth parameters. However, planting date had a significant effect on radish growth—earlier planted radishes consistently produced larger canopy widths, more dry matter, and larger roots. In this study, Sep-planted, Oct-planted, and Nov-planted radishes produced a maximum of 19,373, 3246, and 307 kg ha-1 of dry matter, respectively. Radish growth was markedly different between the 2017-18 and 2018-19 growing seasons, suggesting that planting date and accumulated growing degree days are more important than cultivar selection for dry matter production and root growth. To test the ability of radish roots to alleviate compaction, a greenhouse study was conducted to determine the ability of radish taproots to penetrate compacted topsoil in PVC cylinders. Two radish cultivars (i.e., ‘Tillage’ and ‘Smart’) were planted into 40 cm PVC cylinders with and without a constructed hardpan (>1.7 g cm-3) located approximately 30 cm from the soil surface. Canopy width and aboveground root length data were collected weekly. Cylinders were opened after three months to observe root length (aboveground and belowground) and biomass for radishes in each cylinder. While no radish was able to penetrate into or through the hardpan, ‘Tillage’ radishes produced wider canopies and longer aboveground and total root lengths than ‘Smart’ radishes, while radishes grown in compacted cylinders produced more foliage and total dry matter than those grown in uncompacted cylinders. These results indicate that while radish cultivars may have marked growth patterns and morphological differences, there is little evidence that those differences may lead to greater penetration into compacted soil layers. Further research is needed to assess the bulk density at which radish taproot growth is restricted.