We use green manures primarily to increase organic matter in the soil. The living and/or decaying plant material feeds and increases earthworms and beneficial micro organisms which alternately feed and create new, healthy soil and therefore plants (+ animals and people). In the process, carbon and nitrogen is increased and as the soil structure is improved and can act like a sponge, improved moisture retention and air penetration is achieved along with better root penetration for the vegetables we grow.
Oats, then garlic then neighbour's paddock of cow pasture.
Our paddock above in the foreground was virtually bare excluding patches of weeds when we first moved onto this land a couple of winters ago, having been mainly grazed and compacted by cows with the odd hay crop grown on it for many years. Improved soil structure, including the increased and deeper plant roots, also stabilises the soil to prevent erosion. Erosion has become a major factor in the degradation of Australia's agricultural lands.
Currently we are working with our winter green manure crop of oats. Oats are great because they are very leafy and so produce lots of organic matter/bulk and carbon sequestration. This is particularly important for us with our red volcanic soil which is very fertile by nature but also lacks organic matter and so tends to dry out easily during summer. They grow well in the cold and wet. Our oats have gone wild this winter with all the rain we've been getting.
Our wild oats!
You can either dig in a cover crop whilst still green for a quick nitrogen boost to the soil just before planting vegetable crops, like we've been doing recently with the oats or you can slash it and leave it a while until the litter and stubble browns before digging it into the soil as more of a slow release, carbonaceous fertiliser, which we'll be doing prior to planting out our tomatoes in a few months. Green manure crops also regenerate the soils and nourish food crops subsequently planted via their ability to bring to the surface deep minerals. The oats' root depth matches their large leaf size and so they are good for mining the deep soil minerals. The increased soil coverage and types of plants used also provide habitat and food for beneficial insects which in turn reduce the populations of soil and plant pests.
Our preferred summer green manure is cow pea, which is good for fixing nitrogen into the soil being a legume. It also handles our hot, dry summers better than many other green manure varieties. It grows more thickly with regular rain but is drought tolerant which is handy in what have become dry summers for us.
The other huge benefit of green manure crops is that over time they out compete and effectively smother weeds, partly by their mere physical presence and partly due to the improved structure and nutrient status of the soil rendering it unsuitable for many persistent weeds to proliferate. This is the other major benefit on our farm...weeds are possibly the biggest thorn in an organic farmers' side and take a lot of time and labour to manage. Our oats are currently doing a great job of suppressing the wild radish and Cape weed which are our major pernicious and persistent weeds!