Green manures are crops cultivated exclusively to add nutritional value to the soil. They are dug deep into the ground before they flower and get solid and woody, and left to rot down and add organic matter. Some green manures are also nitrogen enhancers, capable making an effort adding nitrogen to the soil and make it more abundant. If you’ve got a spot of soil that will be empty for the next months, then sowing a nitrogen upgrading green manure will give your next harvest a big development. You can also grow a small spot of green manure to slice and use as mulch in other places, or to supply green content for the compost bin.
Green manure crops can have downsides if you don’t pick the crop wisely and take care of it properly. If you let a grass green manure expand until it becomes tough, it can take a quite a while for soil organisms to break it down. You can avoid this challenge by adding another source of nitrogen, or by thriving the grass with a legume. If you place seeds that can lie dormant in the soil for a couple of years before growing, or let a green manure go to seed, you can develop new weed issues.