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The tropical legume 'Tropic Sun' sunn hemp (Crotalaria juncea L.) may have potential as an alternative legume cover crop or as forage for cattle in southern temperate regions. This study determined dry-matter production, chemical composition, and N release from sunn hemp residue under conventional and no-tillage systems as might be used in corn (Zea mays L.) production. Sunn hemp was sown in mid-August and mowed in early December on a Norfolk sandy loam (fine-loamy, siliceous, thermic Typic Kandiudult) and a Lucedale fine sandy loam (fine-loamy, siliceous, thermic Rhodic Paleudult) in Alabama (1991 1992). Mesh bags were used to determine residue decomposition and N release. Average dry-matter production was 5.9 Mg ha" 1 9 to 12 wk after planting. At mowing, residue N content averaged 126 kg ha~'. Residue overwintered on the soil surface until early April. During the first 4 wk following mowing, N release from residue was 50%. In April, N remaining in overwintered residue was only 38% of that after mowing in December (45 kg N ha' 1 ). Nitrogen release from residue during the subsequent corn growing season was 13% in no-tillage and 43% in conventional tillage. Sunn hemp produced sufficient dry matter to cover and protect the soil from erosion and provided sufficient N to benefit a succeeding summer crop. In addition, forage quality of leaves was suitable to provide late summer and fall grazing. Sunn hemp has potential to be managed as an alternative to winter legume cover crops in warm temperate regions.

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An experiment was conducted on Norfolk sandy loam soil (Fineloamy, siliceous, thermic Typic Kandiudults) during two years to determine the feasibility of using field chlorophyll measurements for evaluation of corn (Zea mays L.) N status. Nitrogen was applied at rates of 56, 112, 168, 224, 280 and 336 kg ha -1 to establish a range of corn chlorophyll levels, tissue N concentrations, and grain yields. At the V10 and midsilk stages of growth, field chlorophyll measurements were taken with a hand-held chlorophyll meter (SPAD-502 Chlorophyll Meter, Minolta Camera Co., Ltd., Japan) 3 and tissue N was determined. A typical curvilinear grain yield response to N fertilizer was observed both years; maximum agronomic yields were obtained with 227 and 242 kg N ha 1 , respectively, in 1990 and 1991. Tissue N concentrations at V10 and midsiik were a good predictor of grain yield. Field chlorophyll measurements were highly correlated with tissue N concentrations at both growth stages during both years of the study. Field chlorophyll measurements had excellent grain yield prediction capabilities, even at V10, which shows promise for utilization of this tool for in-season N recommendations. However, further calibration of field chlorophyll measurements will be required prior to routine use for corn N recommendation purposes.

A reliable laboratory index of N availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a fi rst-order exponential

the winter, chemically terminated in the spring, and then corn is planted into the surface residue. Corn is typically The benefits of winter legumes as cover crops for corn (Zea mays planted early in the growing season, which forces an L.) are diminished by the earliness of corn planting in relation to early termination date. A termination date at least 14 d biomass and N production by the legumes. Tropical legumes may before corn planting enables soil surface water recharge offer an alternative to winter legumes because they produce adequate biomass before corn planting. We determined the suitability of 'Tropic by planting time (Hargrove and Frye, 1987); however, Sunn' sunn-hemp (Crotalaria juncea L.) as a cover crop for corn on as a result, biomass production for erosion control is a Compass loamy sand (coarse-loamy, siliceous, subactive, thermic limited and N release may not always synchronize with Plinthic Paleudults) in central Alabama using a split-plot treatment rapid N uptake of corn at the six-leaf stage (Magdoff, structure in a randomized complete block design with four replications 1991). Decker et al. (1987) reported higher N percentfrom 1991 to 1993. Main plots were winter fallow and sunn-hemp ages with increased legume top growth the longer the planted in mid-August, and subplots were N (0, 56, 112, and 168 kg legume was allowed to grow in the spring.