Forage Radish and Rye Cover Crop Effects On Phosphorus Cycling and Soil Test Phosphorus.

The growth and decomposition of cover crops influence nutrient cycling in the agroecosystem.  Forage radish (Raphanus sativus L. var. longipinnatus) is a unique cover crop in terms of phosphorus (P) cycling because of its high tissue P concentration, rapid dry matter production in fall, and rapid decomposition in winter/spring.  We compared P uptake by forage radish and cereal rye (Secale cereale L.) cover crops and measured soil test P stratification following cover crops in a 2 year study at each of 3 locations near Beltsville, Maryland.  In each year the cover crops were planted in late August. Their P uptake ranged from 5.8 to 32 kg P ha^-1 for forage radish shoots, from 3.5 to 7.0 kg P ha^-1 for forage radish tap roots, and from 3.0 to 25 kg P ha^-1 for rye shoots.  Cover crops resulted in slight vertical stratification of soil test P at two sites. At one site, soil was sampled in 1 cm increments around radish root holes, revealing large increases in soil test P concentrations within 3 cm of forage radish root holes.  At this site, the average Mehlich 3 soil test P in the 0 – 2.5 cm depth was 73 mg P kg^-1 for the no cover crop treatment, 85 mg P kg^-1 in the radish treatment > 5 cm from radish holes and 132 mg P kg^-1 in the radish treatment < 3 cm from radish root holes.  These findings have important implications for soil sampling and management practices aimed at measuring and managing P fertility and environmental risk of P pollution.