Can Sunn Hemp Decrease Nitrogen Fertilizer Requirements of Vegetable Crops in the Southeastern United States?

Urban Farm DB summary

Sunn hemp (Crotalaria juncea) was tested as a summer cover crop in a Community Supported Agriculture vegetable operation in Southeast Florida, United States, from 2003 to 2005. The farm system compared sunn hemp (SH) with a conventional summer fallow, followed by winter/spring tomato and pepper and then sweet corn. Tomato and pepper received 0, 67, 133 or 200 kg N/ha in 2003, and 0, 100 or 200 kg N/ha in 2004/05; sweet corn received 133 or 200 kg N/ha in 2003 and 100 kg N/ha in 2004/05. Average SH biomass was 3.7 Mg/ha. In 2003, tomato yields following SH without supplemental nitrogen were similar to the fallow treatment fertilized with 200 kg N/ha. By the third year, tomato and pepper yields in SH plots were 25% and 26% higher, respectively, than conventional plots, while conventional pepper fertilized with 200 kg N/ha showed only 8% higher yield than plots given 100 kg N/ha plus cover crop. Sweet corn yields were low overall but increased when the preceding tomato/pepper crop received higher nitrogen rates: in 2003, sweet corn fertilized with 200 kg N/ha following an SH-fall vegetable rotation produced 17% higher marketable yield than the fallow treatment, and in 2004–2005, sweet corn within the SH–unfertilized tomato system produced 29% higher yields than a comparable conventional system. The results indicate that in this rotation, both fall vegetable crops and sweet corn benefit from residual nitrogen, with SH mineralization affecting not only the immediately following crop but also yields later in the year.