Cover Crops and Green Manures

"Only by the use of legumes can a significant proportion of the natural reserves be brought into use for the fertilizing of the ground. This, together with the treatment of manure and compost, constitutes one of the most important aspects of the bio-dynamic method of agriculture. Therefore, the legumes must always form part of a healthy crop rotation as a curative factor; but balance and moderation must be maintained even with legumes ( Pfeiffer, 1940)".

Nature, left to her own devices, does her utmost to maintain ground cover. We would do well to emulate her. Cover crops could be considered the backbone of any annual cropping system that seeks to be sustainable. The following article seeks to summarize the principal uses and benefits of cover crops and green manures. Brief descriptions and examples are provided for cover crops, green manures, living mulches, catch crops, and some forage crops. To impart a sense of the importance of these practices in sustainable farming, a summary is included of the effect of cover crops and green manures on: organic matter and soil structure, nitrogen production, soil microbial activity, nutrient enhancement, rooting action, weed suppression, and soil and water conservation.

Green “manuring" involves the soil incorporation of any field or forage crop while green, for the purpose of soil improvement. A cover crop is any crop grown to provide soil cover, regardless of whether it is later incorporated. Cover crops are grown primarily to prevent soil erosion by wind and water. In addition to providing ground cover and, in the case of a legume, fixing nitrogen, they also help suppress weeds and reduce insect pests and diseases. When cover crops are planted to reduce nutrient leaching following a main crop, they are often termed "catch crops."

Winter Cover Crop
A winter cover crop is planted in late summer or autumn to provide soil cover during the winter. Often a legume is chosen for the added benefit of nitrogen fixation. Cool-season legumes include clovers, lupins, medics, faber beans and peas. They are sometimes planted in a mix with winter cereal grains such as oats, barley, or wheat.

Green Manure Crop
A green manure occupies the land for a portion of the growing season. These cover crops can be used to fill a niche in crop rotations, to improve the conditions of poor soils, or to prepare land for a perennial crop. Legumes such as peas, beans, clovers, or lupins may be grown as green manure crops to add nitrogen along with organic matter. Non-legumes such as oats and barley are grown to provide biomass, smother weeds, and improve soil tilth.

Living Mulch
A living mulch is a cover crop that is interplanted with an annual or perennial cash crop. Living mulches suppress weeds, reduce soil erosion, enhance soil fertility, and improve water infiltration. Examples of living mulches in annual cropping systems include no-till planting of seedlings into subclover, clover drilled into small grains, and annual ryegrass broadcast into vegetables. Living mulches in perennial cropping systems are simply the grasses or legumes planted (or self seeded) as ground covers over the beds and in the alleyways between rows to control erosion and prevent leaching.

Catch Crop
A catch crop is a cover crop established after harvesting the main crop and is used primarily to reduce nutrient leaching from the soil profile. For example, planting cereal rye following corn harvest helps to scavenge residual nitrogen, thus reducing the possibility of groundwater contamination. In this instance, the rye catch crop also functions as a winter cover crop. Short-term cover crops that fill a niche within a crop rotation are also commonly known as catch crops.

Forage Crop
Short-rotation forage crops function both as cover crops when they occupy land for pasturage or haying, and as green manures when they are eventually incorporated or killed for a no-till mulch. Examples include legume sods of lucerne, sweet clover, trefoil, red clover, and white clover, as well as grass-legume sods like fescue-clover pastures. For maximum soil-improving benefits, the forage should not be grazed or cut for hay during its last growth period, to allow time for biomass to accumulate prior to incorporating.

BENEFITS

Organic Matter and Soil Structure

A major benefit obtained from green manures is the addition of organic matter to the soil. During the breakdown of organic matter by micro-organisms, compounds are formed that are resistant to decomposition—such as gums, waxes, and resins. These compounds—and the mycelia, mucus, and slime produced by the micro-organisms—help bind together soil particles as granules, or aggregates. A well-aggregated soil tills easily, is well aerated, and has a high water infiltration rate. Increased levels of organic matter also influence soil humus. Humus—the substance that results as the end product of the decay of plant and animal materials in the soil—provides a wide range of benefits to crop production.

Sod-forming grass or grass-legume mixtures are important in crop rotations because they help replenish organic matter lost during annual cultivation. However, several years of sod production are sometimes required before measurable changes in humus levels occur. In comparison, annual green manures have a negligible effect on humus levels, because tillage and cultivation are conducted each year. They do replenish the supply of active, rapidly decomposing organic matter.

Nitrogen Production

Nitrogen production from legumes is a key benefit of growing cover crops and green manures. The amount of nitrogen available from legumes depends on the species of legume grown, the total biomass produced, and the percentage of nitrogen in the plant tissue. Conditions that encourage good nitrogen production include adequate soil nutrient levels and appropriate soil pH plus good nodulation, and adequate soil moisture.

The portion of green-manure nitrogen available to a following crop is usually about 40% to 60% of the total amount contained in the legume. It is estimated that 60% of the tissue N is released when the cover crop is incorporated as a green manure rather than left on the surface as a mulch. Lesser amounts are available for the second or third crop following a legume, but increased yields are apparent for two to three growing seasons.

Forage legumes are valuable in rotations because they generate income from grazing or haying and still contribute nitrogen from regrowth and root residues. A high percentage of biologically fixed nitrogen is in the top growth.

Percent nitrogen in legume tops and roots.

Crop Tops (% N) Roots (% N)

Peas             84          16
Red clover   68          32
Lucerne       58          42

Soil Microbial Activity

A rapid increase in soil micro-organisms occurs after a young, relatively lush green manure crop is incorporated into the soil. The soil microbes multiply to attack the freshly incorporated plant material. During microbial breakdown, nutrients held within the plant tissues are released and made available to the following crop.

Factors that influence the ability of micro-organisms to break down organic matter include soil temperature, soil moisture, and carbon to nitrogen (C:N) ratio of the plant material. The C:N ratio of plant tissue reflects the kind and age of the plants from which it was derived. As plants mature, fibrous (carbon) plant material increases and protein (nitrogen) content decreases. The optimum C:N ratio for rapid decomposition of organic matter is between 15:1 and 25:1.

C:N ratios above 25:1 can result in nitrogen being "tied up" by soil microbes in the breakdown of carbon-rich crop residues, thus pulling nitrogen away from crop plants. Adding some blood and bone to aid the decomposition process may be advisable with these high carbon residues. The lower the C:N ratio, the more N will be released into the soil for immediate crop use.

The important point is that lush green manures are richer in nitrogen relative to carbon, especially in comparison to highly lignified crop residues like corn stalks. It will take a lot longer for soil microbes to break down corn stalks than fresh clover.

Nutrient Enhancement

In addition to nitrogen from legumes, cover crops help recycle other nutrients on the farm. Nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), and other nutrients are accumulated by cover crops during a growing season. When the green manure is incorporated, or laid down as no-till mulch, these plant-essential nutrients become slowly available during decomposition.

Certain broad-leaved plants are noted for their ability to accumulate minerals at high concentrations in their tissue. For example, buckwheat, lupin, and sweet clover are noted for their ability to extract P from soils. Likewise, lucerne and other deep-rooting green manures scavenge nutrients from the subsoil and translocate them upwards to the surface rooting zone, where they become available to the following crop.

The breakdown of green manures in soil influences mineral nutrient availability in another way. During decomposition of organic matter, carbonic and other organic acids are formed as a by-product of microbial activity. These organic acids react with insoluble mineral rocks and phosphate precipitates, releasing phosphates and exchangeable nutrients.

Rooting Action

The extensive root systems of some cover crops are highly effective in loosening and aerating the soil. In local wheat experiments, the taproots of a blue lupin cover crop performed like a "biological plough" in penetrating compacted soils. When cover crops are planted after a subsoiling treatment, they help extend the soil-loosening effects of the deep tillage treatment. The rooting depths of several green manures grown under average conditions are listed below.

Typical rooting depths of several green manure crops.

Depth (metres) Green Manure Crop
1.5 to 2.5           Red Clover, lupin, radish, turnips
1.0 to 1.5           Rape, vetch
0.3 to 1.0           White clover

Weed Suppression

Weeds flourish on bare soil. Cover crops take up space and light, thereby shading the soil and reducing the opportunity for weeds to establish themselves. The soil-loosening effect of deep-rooting green manures also reduces weed populations that thrive in compacted soils.

The primary purpose of a non-legume green manure—such as rye, barley or oats—is to provide weed control, add organic matter, and improve soil tilth. They do not produce nitrogen. Thus, whenever possible, annual grain or vegetable crops should follow a legume green manure to derive the benefit of farm-produced nitrogen.

Soil and Water Conservation

When cover crops are planted to prevent leaching, they should provide a high percentage of ground cover as quickly as possible. Most grassy and non-legume cover crops, like oats and barley, fulfil this need well. The soil conservation benefits provided by a cover crop extend beyond protection of bare soil during non-crop periods. The mulch that results from a cover crop in no-till plantings increases water infiltration and reduces water evaporation from the soil surface. Soil cover reduces soil crusting and subsequent surface water runoff during rainy periods.