In late summer and early fall — as harvest time celebrates the fruits of our labors — we begin looking ahead to next year, and planting green manures now can help improve your soil in preparation for next spring. In Part 1, the team at WSU Extension – Stewardship Gardening
gives us the rundown on green manures and how they work.
Successful gardening depends upon soil quality. Organic matter is a vital part of soil quality. Organic matter makes soil easier to work, improves water and nutrient retention for easy absorption by plants, improves soil aeration, and helps the soil warm up earlier in the spring. Since soil organisms constantly break down organic matter, gardeners need to build and regularly replenish their soil organic matter. Green manure crops are high in nutrients and are an important and inexpensive way to produce organic matter for the garden. Instead of buying and bringing home bags or truckloads of compost or manure, bring home some seeds to plant a green manure crop.
What Are Green Manures?
The name green manure is given to any crop which is grown only to be tilled back into the soil. As it rots, the nutrients in the crop leaves and roots will be taken up by the next crop planted in the same place. Instead of tilling the crop into the soil, cut the green manures, compost them, and then return them to the garden in the form of finished compost. Green manures also act as “cover crops”, plants grown at times when other crops do not occupy garden space. They protect the soil from compaction and erosion caused by wind and rain, and they reduce the extent that weeds take over on bare soil.
How Do Green Manures Work?
Green manures can be grown any time of year. Winter is an excellent time to grow them in western Washington when other crops take up less garden space and rain can do the most damage to uncovered soil. During the rainy winter, many nutrients may be leached out of the root zone to the water table and soil may be washed away by surface erosion. Green manures prevent this loss by absorbing those nutrients. They also soften the impact of the rain drops on the soil, reducing compaction and erosion.
In addition to preventing loss of nutrients, green manures add important elements to the soil. All plants undergo photosynthesis, a process that captures carbon dioxide from the air and transforms it into sugars in plant tissue. Green manures that are in the legume family, such as peas, beans, and clovers, have an added bonus. Nitrogen-fixing bacteria living around their roots can take nitrogen form the air and convert it to a form the plant can absorb.
Additional nitrogen-fixing bacteria may be added to the soil by treating the seed, or “inoculating” the seed to increase the amount of nitrogen fixed. Most seed dealers sell the inoculum which is actually bacterial spores. Apply by coating seeds, according to manufacturer’s directions, with fresh inoculum before sowing. This nitrogen can become available to subsequent crops. Legumes break down fairly quickly when turned into soil or composted. Legumes work well when mixed with a grass crop.
The nutrients held in the green manure plant tissue is available to future crops through decomposition. As the harvested green manure rots, the carbon captured from the air through photosynthesis becomes an important part of soil organic matter. The various nutrients absorbed into the plant tissues from the soil or from the nitrogen-fixing bacteria are broken down into forms that crops can readily absorb.
Next time: Planting and Growing Green Manures
Photos of winter wheat and crimson clover courtesy The Oregonian