A cover crop incorporated into the soil while still green and growing is considered a green manure. Many farmers include green manures in their rotational plan because they improve the soil’s organic matter content and conserve or provide nutrients for subsequent cash crops.
Legumes, such as clovers, vetches, peas and alfalfa, are known for their ability to capture atmospheric nitrogen and convert it to a form that plants can use. This process of nitrogen fixation is a joint effort, and credit must be given to the soil-borne bacteria that make it possible. I won’t bog you down with the details on transmembrane receptors, infection threads and mitosis, but I will say the legume’s roots, in reaction to the invading bacteria, form tumor-like nodules that are visible to the naked eye. The bacteria fill these nodules by the billions and become nitrogen-fixing factories, converting nitrogen gas (N2) absorbed from soil air pockets to ammonia (NH3), the form of nitrogen living organisms can utilize. The nodules are connected to the plant’s nutrient transport system so it absorbs the ammonia and dedicates it to protein development. Meanwhile, the bacteria thrive on sugars and nutrients fed by the plant. It’s a win-win situation, a truly symbiotic relationship.
There are roughly 35,000 tons of inert N2 gas over each and every acre of land on this planet, and an acre of leguminous plants can fix, on average, 100 pounds of it in a growing season. The amount fixed per acre varies according to a number of factors, including the species of legume grown, the bacteria’s effectiveness, soil conditions and climate. Crops with a high nitrogen-fixing capacity include alfalfa (100 to 200 pounds per acre), red clover (50 to 200 pounds per acre) and white clover (50 to 150 pounds per acre). To reach these capacities, each legume species develops a symbiotic relationship with a specific strain of bacteria in the Rhizobium or Bradyrhizobium genus. If that bacteria strain isn’t present in the soil or isn’t present in sufficient quantity, the nodules will not form and the crop’s nitrogen-fixing capabilities will not be realized. Such a scenario can be avoided by inoculating the seed, a process that involves either coating the seed with viable bacteria or planting the seed in a furrow lined with a peat-bacteria mixture. Bacteria inoculants are considered natural and for the most part are approved for use on certified organic farms. Exceptions include those produced through genetic engineering or that contain prohibited materials. If you have never planted a legume, or it has been a few years since you’ve planted one, inoculation is a form of insurance and not something to shy away from.
The extent to which a legume crop fixes nitrogen also depends on the amount of nitrogen present in the soil. It takes less energy to absorb nitrogen from the soil than it does to fix nitrogen from the air, so if your soil contains significant levels of it, the legume’s roots will tap it and slow or halt the process of removing it from the atmosphere. Also, if other essential plant nutrients are limited, that too could slow bacteria, nodule and plant development, which would, in turn, limit the nitrogen-fixing process.
The catch is that most of that fixed nitrogen remains in the plant. In order for the soil and subsequent crops to benefit, the leguminous cover crop must be incorporated into the soil. If harvested, no nitrogen is added to the soil. If left on the surface to decompose, up to 75 percent of the nitrogen will return to the atmosphere as volatile ammonia. Even when turned under, soil temperatures and moisture levels and the depth to which the plants are worked in influence the rate of nitrogen release. Cold and excessively wet soils are not associated with high levels of soil microbe activity, so the rate at which the microbes break down organic matter and release nutrients is slower under such conditions. Also, if the green manure crop is tilled too deeply, to a depth beyond roughly eight inches, the rate of decomposition will be significantly slowed.
The ideal leguminous green manure plant should grow rapidly and produce considerable top growth. In other words, it must be suited to your site conditions and microclimate. If it’s thriving, you’ll notice nodule development within a couple of weeks. Nodes pinkish or reddish in color are an indication that oxygen is flowing to the bacteria and the process of nitrogen-fixing has begun. If largely white or pale green, the process has either not begun or halted, and you may need to assess the growing environment to determine whether you can take steps to improve it.
You’ll want to turn the crop under at full bloom, when its growth has been nearly maximized. This is the point at which it will contribute the most nitrogen. Also, if it is allowed to grow past this point, the plant material will become tough and decomposition will be slowed. You’ll want to turn the plants under before cool weather has arrived so they can begin the process of decomposition. You want them to break down sufficiently and in a timely manner so the following crop may benefit from what they have to offer.
Nitrogen sourced from the atmosphere is not free. Purchasing seed, planting it and turning the crop under takes time and money, but it is a viable option, particularly if you grow nitrogen-hungry crops.
The author is a biologist who lives and farms in Vermont’s Northeast Kingdom. Comment or question? Visit www.farmingforumsite.com and join in the discussions.