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Sunn hemp is most commonly used as a green manure, where it is grown for 2 to 3 months before cash crop planting and then incorporated into the soil at early blooming stage. Besides use as a green manure, sunn hemp can also be used as organic mulch where the cover crop is mowed and left on the soil surface. The advantage of using a leguminous cover crop as an organic mulch rather than green manure is that this practice will slow down the release of nutrients from the crop residues and allow the nutrients to be available for the subsequent cash crops over a longer period of time. It had been demonstrated that sunn hemp can be grown as a winter cover crop in Alabama, and leaving the residues on the soil surface over the winter resulted in the release of 67 to 71 lb N/acre (Reeves et al. 1996). Research done in south Florida suggested that when sunn hemp is used as green manure and organic mulch, it should be seeded at high rates (50 lb/acre) and the crop terminated at 10–12 weeks from the planting date (Abdul-Baki et al. 2001).
Work on maximizing biomass and seed production at low seeding ratings has been a focus due to issues with seed availability and cost. Research in south Florida concluded that cutting sunn hemp stems at 1 ft above soil level 100 days after planting (when plants were about 5 ft tall) and allowing the plants to grow for an additional 70 days resulted in the highest quality of green manure harvested as compared to uncut or cutting at a higher stem height. This is because cutting at 1 ft increased leaf yield (Abdul-Baki et al. 2001). This increased the N content of the biomass harvested because the leaf tissues contained higher concentrations of N (3.96%) than the stem tissues (0.88%) and whole-plant tissues (2.5%) (Marshall 2002).
Since most of the macro-nutrients in sunn hemp are found in leaves and flowers, use of sunn hemp as green manure or organic mulch would be most beneficial at the early to mid-blooming stage (Marshall 2002). Short-day sunn hemp cultivars will only flower in fall when day length becomes shorter in north Florida. However, sunn hemp is very susceptible to frost kill, and so opportunity for growth and biomass increase is limited by cool temperatures in the fall. Therefore, immediate use of most sunn hemp residues is limited to supplying nutrients only to benefit winter vegetable crops. Another option for use of a sunn hemp cover crop is to harvest the cover crop residues, air dry them, grind up the residues, and store them as organic fertilizer for later use (Marshall 2002). While this form of application is time consuming, it offers opportunities to manipulate fertilizer application rate and timing. The introduction of day-neutral sunn hemp cultivars may also expand the window for growing this crop, opening opportunities for green manure before crops not grown in winter.
Seaman et al. (2004) reported that frequent harvest of the top 18 inches of new growth by clipping sunn hemp at 16 to 32 inches height above soil line produced an organic fertilizer of 4% N. This means that if 3 tons/acre of dried sunn hemp clipped biomass is harvested as described above, it will contain 240 lb of N. Therefore, this high concentration of N in clipped sunn hemp materials has great potential as an organic N fertilizer.
The whole plant sunn hemp residues harvested at early blooming stage contained N-P2O5-K2O in amounts of 123–42–80 lb/acre, which gives a ratio of 3:1:2 (Marshall 2002). Using this ratio, one can formulate a fertilizer according to the specific crop nutrient requirement. Marshall (2002) demonstrated that sunn hemp residue supplied N levels comparable to those derived from inorganic N for bush bean (Phaseolus vulgaris), lima bean (P. lunatus), okra (Abelmoschus esculentus), cucumber (Cucumis sativus), cowpea (Vigna unguiculata), sweet corn (Zea mays), and squash (Cucurbita pepo)
