Green Manure

A green manure crop is grown primarily for the purpose of being plowed down to add nutrients and organic matter to the soil. Organic farmers consider green manures to be an essential part of the farm ecosystem.

Many field crops can be used as green manure. Legumes such as yellow sweetclover and alfalfa are commonly used, but white clover, red clover, peas, Indian Head lentils, black medick and certain vetches are also used to add nitrogen and improve the soil. Non-legumes that perform a multitude of functions include oats, barley, forage grasses, mustard, canola, buckwheat and fall rye.

Green manures play a role in soil improvement, nutrient management and pest management:

• They are effective in controlling erosion, adding organic matter, improving soil structure, stimulating biological activity in the soil and reducing compaction.
• Legumes such as alfalfa and sweetclover can fix over 200 pounds per acre of nitrogen.
• Effective green manures smother weeds, break insect and disease cycles, and provide a habitat for bees, parasitic wasps and other beneficial organisms. Fall rye and oats are particularly competitive. Some crops such as yellow sweetclover and mustard are allelopathic and produce natural chemical toxins that retard germination and inhibit the early growth of weed species.

The value of green manure can vary with the type of crop and the timing of the plowdown process. For example, most legumes turned under as green manure at the blossom stage will contribute in excess of 100 pounds of nitrogen per acre; a mixture of grass and legumes turned under at the blossom stage will contribute 50-100 pounds of N per acre; grass and legume residue after harvest will add less than 50 pounds of N per acre. The rate of decomposition also varies with soil and climate conditions.

Incorporating green manure with a discer-type implement into the top 3-4 inches of soil allows a favorable rate of decomposition. Deeper levels of incorporation will slow down the rate of decomposition; incorporation levels below 6 inches should be avoided.

As microbes break down the green manure residue, the micro- and macronutrients from these plants are made available over a number of years. An added bonus is that organic acids are released in the microbial process, resulting in lower soil pH and increased plant-available phosphorous.