Somehow, already, we’re at the stage of sowing overwinter green manures at the farm. This is a bedding-down-for-the-cold-months sort of job and as such it feels eerie, to think only a few weeks ago we were still sweating and getting sunburnt out in the field!
So here’s a bit of info about our use of green manures and why we use them.
What is a ‘green manure’? Well the vaguely oxymoronic name does give a clue. It’s like spreading manure across the field… except it’s green (a plant)… and you grow it (from seed), rather than spread it. As well as increasing soil organic matter – which is good for a whole array of reasons – and improving soil structure, it is a fertility-management component within the farming system. There are two main ways green manures can play this role.
Lifting. Whilst taking a harvest from your crops off your field is an obvious way in which the fertility can be reduced, in fact a larger loss of fertility is generally associated with nutrient leaching. This is where excess nutrients in the soil are washed through by the rain. To prevent this we try to leave as small an area of bare soil as possible, and this is particularly where the overwinter green manures come in (such as the mixture of cereal rye and vetch we’re currently sowing). In effect these plants ‘mop up’ nitrogen left over in the soil after a summer or autumn crop, holding it in the aerial parts of the plant and releasing it gradually into the next crop when it is incorporated the following spring. Deep-rooted green manures can also lift nutrients from the sub-soil, again making them available to shallower-rooted crops as they break down.
Fixing. An obvious example of a green manure that you’ll know of, and which farmers and growers make much use of, is clover. Clovers, like other legumes (peas, beans etc.), carry out a rather magical process under the surface of the soil called nitrogen-fixation. This takes the nitrogen in the air all around us and ‘fixes’ it in the soil in a mineral form that plants can then use later. So rather than adding nitrogen to the soil yourself (either through animal manures or artificial fertilisers) the plant is doing it for you – using nothing but thin air and sunlight!
However, we should let credit go where credit’s due: the unsung hero of all this is the nitrogen fixing bacteria, who do the real work. They form a symbiosis with the leguminous plants by inhabiting their roots, providing mineralized nitrogen to the plants in exchange for compounds the plant can make through photosynthesis. A true co-operation!
Indeed this process – nitrogen from the air being fixed by bacteria – is responsible for the vast proportion of soil fertility around the world. Whilst industrially-produced nitrogen fertilizers have obviously had a huge transformative impact on agricultural systems (not to mention the environment), we shouldn’t kid ourselves: still today only one-third of fixed nitrogen is from manufactured sources. The rest is down to these little guys, the bacteria. And when animals go grazing, it’s this fertility that they munch up and concentrate, and the very same which comes out the other end.
Thus, pausing for a thought, the term ‘green manure’ is, in a sense, rather misleading. It seems to suggest, backwardly, that the use of clovers is some eco-hippy way of doing what one would normally do with animal manure. But, as we can see, if anything, spreading animal manure is the ‘new idea’ that needs proving – in effect, the ‘brown manure’ that we’re perhaps more familiar with just is the green manure that we’ve been sowing at the farm. More concentrated perhaps, but in many ways less efficient overall.
All this is not to say that at the Cultivate farm we’ve eschewed animal by-products. Maintaining soil fertility is a pressing and immediate requirement – and we have a source of horse manure right on our doorstep at the Earth Trust. However, it’s usage is something that we are trying to work towards reducing, to zero eventually, and it’s the green manures as well as their friendly bacteria chums that will help us get there.
VETCH GREEN MANURE