Green manure as nitrogen source for sweet corn in a warm-temperate environment

Legumes utilized as GM may provide on-farm organic N. Nonetheless, data regarding GM use on sandy soils in warm-temperate environments remains scarce. We conducted a 2-yr field study to evaluate growth and decomposition of sunn hemp (Crotalaria juncea L.), blue lupin (Lupinus angustifolius L., winter 2001-2002), and Cahaba white vetch (Vicia sativa L., winter 2002-2003) used as GM to provide N for sweet corn (Zea mays L. var Rugosa) in a reduced tillage system in northern Florida. Intercepted solar radiation accounted for most variability in total GM dry weight and N content regardless of species or year. Sunn hemp exhibited substantial growth and N accumulation (<= 12.2 Mg ha(-1) and 172 kg N ha(-1) in 14 wk). However, rapid N loss (45-58% within 4 wk after death) occurred after sunn hemp death. Winter GM growth (2.04.0 Mg ha(-1) and 51-104 kg N ha(-1) in 18-20 wk) appeared limited by low LAI response to growing degree days (GDD) and was not affected by previous sunn hemp GM. Sunn hemp residues and living winter legume together contained 120 to 125 kg N ha(-1) at time of final sampling before sweet corn planting. Corn rotated with sunn hemp plus winter GM and supplemented with 133 kg synthetic N ha(-1) produced marketable ear yields similar to monoculture corn fertilized with 200 kg synthetic N ha(-1), but the practical value of this benefit is low. In northern Florida, winter and/or summer GM use may substitute only for a relatively small portion of synthetic N rates.