The Role of Green Manure in Soil Fertility

We can be only as healthy as our soils are. Once our soils lose their fertility, we lose our ability to cultivate crops and plants we need to sustain our good health. Maintenance of healthy soils has been a critical aspect of our life for thousands of years, but even with such extensive knowledge from our ancestors, it is not an easy task.

One method of improving soil fertility and productivity could be tracked throughout the Ancient Greek practices, the Roman Empire, the early North America colonization, up to this day’s sustainable agriculture. It is a green manure cropping, also known as “fertility building” cropping.

Despite its name, green manuring has nothing to do with animal waste. It is a practice of planting crops that will be turn into the soil with the purpose to increase organic matter and replenish nutrients. Green manure crops are in general grown between the main crops to shield soils from erosion – as winter cover crops, or to restore productivity of exhausted land.

With increasing challenges in agriculture, in terms of climate change, extreme weather events, soil degradation and land contamination by the overuse of agricultural chemicals, many farmers are implementing green manuring into their practice. Growing green manure proves to be a practical and economical method of securing long-term productivity of farmed lands.

How does green manure work?

The main purpose of a green manure crop is to prepare the soil for subsequent crops. Green manures work by drawing nutrients from soil and storing them in their bodies. These crops are not harvested and taken away from the land as this would remove the nutrients, but are tilled into the soil while they are still green. When turn back into the soil, plants slowly decompose and gradually release these nutrients to the next crop.

At the same time, green manure serves as a source of food for numerous soil microbes and organisms. The abundance of soil fauna is extremely important for soil health. Their movement and activity helps build good soil structure and by feeding on the organic matter, they allow for its distribution in soils.

Green manuring is an easy method, but to achieve the best outcomes, there are still a few rules to keep in mind:

• A green manure crop needs to be dug in before plants reach maturity. This allows for plants to decompose easier, as they will not be too woody. It also prevents the release of seeds, which wards off unwanted regrowth of the green manure when the main crop is planted.

• It is not recommended to till green manure too deep into the soil (like the traditional deep ploughing does). The best method is to turn plants maximum 15 centimeters or approximately 6 inches deep. Soil microbes are the most active in this upper soil layer beneath the surface and will speed up the process of decomposition [1].

• Do not sow a green manure crop from the same family as the main crop. The key is to plant species that are not related, because plants from the same family tend to use the same nutrients and are likely to host the same pests and diseases.

• Allow soil rest for 20 days after tilling to let the organic material decompose properly. This will ensure the best conditions for sowing of the following crop.

Green manuring is a method that can be easily utilized on large farming plots as well as small vegetable gardens. To learn more about the benefits of sowing green manure crops, read the following section.

Advantages of green manure

Amazing interconnectedness of processes in nature triggers a set of positive effects green manures have on soils and their long-term cultivation. Most advantages of this method stem from adding nutrients back into the soil and boosting soil fertility, but there are even other more unexpected advantages that can affect surrounding ecosystems as well.

#1 Provision of nutrients and organic matter to the soil

The use of green manure results in increased levels of key plant nutrients. Leguminous green manures (e.g. clover, alfalfa, vetch) have the ability to fix nitrogen from the air and add it into the soil, where this key nutrient promotes a healthy growth of following crops.

Some green manures, such as buckwheat, lupin and oilradish, enrich soils with phosphorus. Scientists measured the phosphorus uptake of lupin. They found out that the plant can draw in and utilize 10 times more phosphorus than a common grain crop, wheat, does. This means that once lupin is incorporated into the soil, phosphorus from its body will be released to the subsequent crop [2].

Many green manure crops have the ability to supply even potassium, calcium, iron and other trace minerals [1].

Turning in green manures also increases soil organic matter. High amount of organic material ensures soil fertility by improving its biological and physical properties. Examples of some include proper aeration, better water infiltration and improved soil structure.

#2 Prevention of nutrient leaching and erosion

Green manures are often called cover crops, because they are mainly planted to cover soils during the winter or the hottest summer months to avoid leaving soils exposed to elements. Roots hold soil particles in place and plant bodies provide a shield from rain or scorching sun, thus, preventing erosion.

At the same time, green manures minimize nutrient leaching into the environment. Green manures draw nutrients into their bodies and lock them in until the crop is dug into the soil. When plants start decomposing, nutrients get released slowly and gradually into the soil, just in time for the following crop to utilize them for its growth.

If soils were left uncovered, many of these nutrients would be washed off into the environment, leaving soils deprived of nutrients and causing damage to our water systems.

#3 Improvement of soil structure

By adding organic matter into the soil, green manures significantly help improve soil structure. Organic matter binds soil particles together and creates soil aggregates. These clusters of larger particles enable formation of pores, which allows for proper soil aeration, water retention and nutrient distribution. Plants grown on such soils have suitable conditions to develop a strong root system and utilize available resources with higher efficiency.

When turning green manure in, we are loosening compacted soil clumps, which are impenetrable for some gentler crop roots and often inhibit their growth. Some green manures, such as alfalfa, chicory, or red clover, have sturdier tap roots that break compacted soil already during their growth [2].

#4 Suppression of weeds

Green manure crops are favored by many farmers for their ability to suppress weeds. They do this by:

• Disrupting the growing pattern and cycle of weed plants;
• Outcompeting weeds for space, nutrients and water;
• Some species of clover and rye release chemicals from their roots that inhibit germination of seeds in the soil [2]. This effect is also known as the allelopathic effect.

#5 Interruption of pest lifecycles and diseases

Green manure crops are in some instances used to break the lifecycle of pests and diseases.

For example, rye sown in the fall is effective in decreasing populations of a major pest of potatoes and vegetables, root knot nematode [3]. Roots of a cereal rye act as a trap for nematodes. When these pests enter rye roots, as they would with any other crops, they find themselves trapped inside without a chance for escape.

Other example are some varieties of mustard that have high glucosinolate content. When the conditions are right, these compounds deter or even kill pests and diseases.

However, the effectivity of green manure as a measure against pests and diseases depends on the proper management of these crops, as they can easily become a habitat for some pests, such as slugs or bean seed fly [2].

This can be prevented by a careful selection of the crop species combined with the assessment of a particular field and its pest history. For example, long-term observations confirmed that ryegrass and alfalfa do not encourage occurrence of slugs as much as clover or vetch does.

#6 Habitat for natural predators and pollinators

One strategy of the biological control of pests is to boost the numbers of their natural predators. Green manure crops often serve as a winter home for beneficial predatory insects, including ground and rove beetles. Both beetle species are known for being skilled hunters of various pests and caterpillars [3].

But green manure does not only serve as a winter home to beetles, even summer manure crops attract natural predators. For example, blue flowers of Phacelia attract hoverflies which in turn feed on aphids – recognized by gardeners and farmers as one of the most widespread and resistant pest.

Flower nectar and pollen of many commonly planted green manures also supplies abundant source nutrition for many pollinators. White, red, pink flowers of clover, yellow flowers of mustard, white, yellow or blue flowers of lupin – all these flowering species attract pollinators and encourage their beneficial activity.

#7 Support of beneficial microbes and other soil organisms

Abundant populations of beneficial soil microbes and other organisms are important for the formation of good soil structure. The activity of these organisms helps the creation of soil aggregates, enhances soil porosity and mixes in the organic matter. One of the main advantages of green manuring is that it increases numbers and encourages high diversity of many of these beneficial organisms.

During the growth of green manure crops, the root exudate serves as a source of nutrition for soil microbes. Once the crop is tilled into the soil, the decomposition of green matter stimulates even further microbial activity in the soil.

Some green crops also support healthy populations of soil mycorrhiza. These symbiotic fungi often play an important role in the crop nutrient intake, overall resistance and growth. Their presence benefits soil structure as well, therefore, the maintenance of their presence in the soil should be desirable by all caring farmers and gardeners [3].

Examples of green manure crops

All green manures have two main characteristics in common. Chosen plants have to be easily established and grow fast. Other criteria for green manure selection depend on the expected purpose, climatic factors, budget, farming practices and the soil type. Every farmer or a gardener must evaluate the state of his plot and decide what green manure crop to choose accordingly.

In general, we recognize between two types of green manure crops:

1. Legumes: Used for their ability to fix nitrogen from the air and add it to the soil;
   Examples: Clovers, lupins, vetches, alalfa, peas, beans, soybeans;
2. Non-legumes: Mainly serve as cover crops and enrich soils of organic matter;
   Examples: Phacelia, buckwheat, chicory, mustard, turnips, ryegrass, oats, barley, rye [2].

Farmers may choose a suitable crop based on its main purpose, or even combine two different crops to utilize more of their benefits. For example, planting together barley and white clover in the fall supplies organic matter and enriches soils of nitrogen for the spring sowing of the main crop. Other commonly used mixes include oats, peas and vetch, or rye and vetch, as they effectively conserve nitrogen in the soil.

But let’s get back to discussing different functions of green manure varieties for farmers.

Green manure crops are classified according to their purpose as follows [1]:

• Cover crops: Crops sown to cover soils and prevent erosion.
  Examples: Oats, winter rye, Sirius peas, lentils, clovers, vetch

• Break crops: Crops that interrupt the lifecycle of pests or diseases.
  Examples: Mustard, rye, brassica, alfalfa

• Nitrogen-fixing crops: Leguminous crops planted to enrich soils of available nitrogen.
  Examples: Clovers, lupins, vetches, alalfa, peas, beans, soybeans

• Nutrient conserving crops: Crops that minimize nutrient leaching, and add more nutrients into the soil.
  Examples: Oilradish, red clover, buckwheat, rye grass

• Smother crops: Crops grown to smother weeds by outcompeting them in growth.
  Examples: Buckwheat, oilradish, winter rye, yellow sweet clover

In the end, the choice of a green manure requires a bit of study to be sure that the best outcome is achieved. However, once a well-working farming system is established, green manures can significantly boost the health of cultivated soils and maintain it that way for a long time, which is something everyone, who is blessed with taking care of soils should aim for.

Long term productivity with minimum degradation rate and chemical burden equals healthy and abundant future for all us.