They're called "green manure" because they provide nutrients to the soil much like manure does.

 

Sunn hemp (Crotalaria juncea L.)

• Do you want a cover crop that will increase organic matter, provide nitrogen, grow in low fertility sandy soils, and does not harbor nematodes?
• Sunn hemp, because of its rapid growth and relatively short growing season requirement, can be an excellent alternative. It grows best on well-drained soils with a pH from 5 to 7.5. S
• unn hemp has been used extensively as a soil improvement or green manure crop in the tropics because of its ability to produce large amounts of biomass in as little as 60 to 90 days. Because of this, it has the potential to build organic matter levels and sequester carbon. Also, as a legume it can fix large amounts of nitrogen.
• Used as a cover crop, sunn hemp can improve soil properties, reduce soil erosion, conserve soil water, and recycle plant nutrients.
• Suited to the warm summer rainfall areas (or irrigation).
• Plant as soon as soil temperatures are higher than 16 degrees Celsius (October – November).
• Sowing Rate: 40-50kg/ha.