“Green manure” is the name given to cover crops that are planted for the purpose of adding nutrients and organic matter to the soil. These plants can be as effective as animal manure in producing humus, thereby increasing soil fertility and structure. Cover crops are inexpensive to plant, and serve multiple purposes. Best of all, they can be put to work during times of the year when growing food crops is not feasible.
A central concept of organic gardening is the notion of feeding the soil. We literally add organic matter to the soil to provide food for the organisms that live in the soil. They break down organic matter and minerals (and other elements) into forms that our crops are able to take up as nutrients. We want them to be able to do their work efficiently and to the best effect.
Aside from adding organic matter to the soil, cover crops have many other purposes:
They reduce soil loss from water erosion.
They maintain soil surface infiltration, so it does not compact.
Cover crops improve soil tilth (structure).
They scavenge nutrients that might otherwise leach from the field.
They feed and provide shelter for birds, wildlife, and beneficial insects.
They fix nitrogen in the soil.
This hardy annual grows so fast it can reach 1m (3’) tall in only three weeks. Then it blooms with white flowers that attract pollinating insects and beneficial hoverflies. Buckwheat grows so densely that it can be used to smother out competing weed species. Within ten days of blooming (or at any time before) it can be cut and tilled under to improve tilth and add organic matter. Buckwheat is a succulent, brittle plant that can break down completely into the soil in a matter of days. When breaking ground for a new garden, growing two consecutive crops of buckwheat and digging them in will provide ample organic matter to stimulate the soil biology.
Grasses & Grains
Oats, barley, wheat, and rye are all cereal grasses that produce a dense, fibrous root mass and a great deal of carbon-heavy biomass above ground. They are particularly well suited to protecting soil over winter, and some varieties are hardier than others. Even if they die back under frost, their roots will remain intact to prevent erosion. These plants also tie up quite a lot of soil nitrogen, which is then incorporated back into the soil as they break down.
Of all the nutrients needed to grow food crops, nitrogen is one of the hardest to maintain in soil. The Earth’s atmosphere is made up of nearly 80% nitrogen, but the element is not found in any parent rock, so all the nitrogen made available to growing plants comes, one way or another, from the air. Plants can’t make use of atmospheric nitrogen (N2), but they can absorb it from soil in the forms of nitrate (NO3) or ammonium (NH4). For the organic grower, the problem of getting nitrogen into the soil is solved through the use of legumes—members of the pea family, Fabaceae.
These plants have co-evolved with certain types of bacteria known as Rhizobia to form a symbiotic relationship that benefits both the bacteria and the host plant (and, happily, the organic grower). The roots of all legumes have tiny bumps called nodules that contain colonies of Rhizobia. The Rhizobia convert atmospheric nitrogen into ammonium, which benefits the host plant. In return, the plant supplies the Rhizobia with carbohydrates, proteins, and oxygen.
It is Rhizobia, by the way, that we introduce into the soil in large numbers when we use seed inoculant on legume crops. While some Rhizobia may already be present in the soil, using seed inoculant will kick-start that nitrogen-fixing process.
So instead of spreading chemical fertilizers, the gardener or farmer can simply plant nitrogen-fixing plants as part of their regular crop rotation. All that stored nitrogen in the soil will result in strong growth in any crop that follows. It makes particular sense in farming systems where fields might be vast—why not take advantage of a naturally occurring phenomenon, and let the plants and their bacteria do most of the work?
While there are nearly 20,000 species of plants in the family Fabaceae, there are a handful of these that work particularly well in organic farming systems. They have been selected ease of growth, hardiness, and the ease with which they can be turned under.
This hardy annual can be planted any time between late March to early October, and will survive winter freezes down to around -23°C (-10°F). For overwinter growing, sow in September through October. Crimson clover does best in well-drained soil with a fairly neutral pH, and wants to be sown fairly thickly. Whenever you choose to sow crimson clover, it will flower in April, and that’s the time to till it under or pull it out. As it sets seed, the stems become fibrous and tougher to break down. This plant is particularly succulent and is easily tilled or hoed under. You can remove the tops to the compost, or simply turn the whole plants under. They will be broken down in about ten days, and the soil will be ready for planting nitrogen-loving crops.
White Dutch clover
White-flowering Dutch clover is a perennial that does best from spring or fall sowing, and it will overwinter even in Zone 4. It spreads by underground runners that call for more thorough tillage in the spring. Allow two weeks for this clover to break down in the soil.
Small seeded fava produces a taller plant than the broad beans you might plant for eating, so that increased biomass produces abundant green manure. Plants stay brittle until they begin to set seed, and can easily be scythed down or ploughed under even when they are four to five feet tall. You can cut the tops down to ground level and add them to your compost, and then till in the root stubble. Fava beans can be started as early as the end of January, or as late as early November. Plants grow slowly, and are hardy down to Zone 7.
Using Cover Crops
Cover crops can be planted to improve just about any kind of soil and render it more fertile. The trick is to choose the right plant for the job. Remember that the different cover crops can be mixed, as well. Barley and white clover can be inter-planted in the fall and grown all winter to produce a massive amount of organic matter and fixed nitrogen for crops the following spring.
Like all other crops, it’s useful to think of cover crops in terms of space as well as time. You need to calculate how much seed you will need for a given area, and how deeply they need to be planted. And you also need to think about the right time to plant, the right time to dig the crop under, and the amount of time it will take to break down. The charts below will help with your calculations.