Timing Is Everything

Green manures are a selection of fast-growing plants commonly used in agricultural rotations to prevent soil erosion, enhance soil health and improve crop yields. Regular crops deplete soil nutrients, which green manures help replenish through nitrogen fixation and/or the addition of organic matter. They are grown specifically to be incorporated back into the soil at the end of their growth cycle rather than being harvested.

Most often cut before blooming and setting seed, green manure’s decomposing biomass feeds beneficial microorganisms. Over time, this significantly enhances soil structure, promotes nutrient retention and increases water infiltration.

Green manures tend to be part of the legume (Fabaceae) family. This includes clovers, vetches, cowpeas, soybeans, and alfalfa. These plants fix nitrogen in the soil through the rhizobium bacteria living in their root nodules.

Non-legume plants such as grasses (rye, oats), buckwheat and mustard are also commonly used. While they don’t fix nitrogen, they help improve soil structure, add organic matter, and can suppress weeds or pests. They are often used in combination with legumes.

Because of their rapid growth, green manures not only suppress weeds but their roots penetrate and open up heavy soils for better drainage. Also, plants such as phacelia and buckwheat are known to attract beneficial insects and provide shelter for predators such as frogs and beetles.

Are Green Manures the same as cover crops?

The main difference between green manures and cover crops lies in their emphasis and timing. Within agriculture, cover crops are grown between cash crops primarily to protect the soil from erosion during the winter months. 

Green manures, by contrast, are planted during the main growing season in place of a cash crop. The focus here is on adding organic matter and nutrients to the soil. Both green manures and cover crops contribute to soil health by preventing erosion, adding organic matter, attracting beneficial insects, and improving soil quality.  The distinction between the two practices is primarily based on when they are grown and the specific goals.

For the market or home gardener, green manures are used less due to space limitations. On a small allotment, home gardeners are rarely able to tie up a whole bed during a prime growing season. For a severely depleted bed, however, it may be worth giving up that space for a short period.

Common Green Manure Crop Profiles :

Legumes (e.g., vetch, clover, beans, peas): Leguminous roots develop nodules in symbiosis with the nitrogen-fixing rhizobium bacteria. This enhances soil fertility for subsequent crops.
Grasses (e.g., annual ryegrass, oats, winter wheat, winter rye): These quick-growing grasses smother weeds, prevent erosion and reduce compaction. Incorporated into the soil, they enhance its structure while decaying biomass restores nutrients.
Brassica Species (e.g., Rapeseed, Mustard, Radish and Turnip): Brassicas improve soil health in several ways. Their fibrous roots, such as those of rapeseed and mustard, bind soil particles into aggregates, boosting tilth, porosity, and water retention. Radishes and turnips excel at capturing leftover nutrients from previous crops, preventing nutrient loss and recycling them into the soil as they decompose. Many brassicas also develop deep taproots. These can reach up to six feet, break up compaction and result in enhanced water infiltration while drawing-up nutrients from deeper layers. When winter-killed, brassicas decompose quickly, creating a loose, easy-to-plant seedbed for the next crop.
Buckwheat: Buckwheat is known for its rapid growth and weed-suppression qualities, making it a beneficial green manure choice to enhance soil health and structure. It also attracts beneficial pollinators.
Phacelia: Phacelia does not fix atmospheric nitrogen like legumes, but it is an excellent nitrogen holder or “catcher crop”. Its dense root system helps prevent nitrogen from leaching out of the soil during fallow periods. A quick-growing plant, Phacelia can provide soil coverage in 6 to 8 weeks. Its extensive fibrous root system helps improve soil structure and drainage when the plants are incorporated into the soil. If left to bloom, the purple/blue flowers of phacelia are very attractive to bees, hoverflies, and other beneficial insects that aid in pollination. This can boost yields of subsequent crops.

Benefits For A Small Garden:

If you have limited space and can only choose three green manure crops, I’d recommend selecting ones that can be grown in different seasons. This allows you to make the most of your limited space throughout-the-year. The following example of three green manure crops demonstrates a range of benefits, including weed suppression, soil structure improvement, and nutrient enhancement.

Spring or fall: Crimson clover (Trifolium incarnatum) is a cool-season annual and nitrogen-fixing legume. It can be planted in the fall or early spring. In addition to its nitrogen-fixing abilities, crimson clover provides good ground cover, suppresses weeds, and attracts beneficial insects. It’s also relatively easy to cut down and incorporate into the soil before it sets seed.
Late spring or summer: Buckwheat (Fagopyrum esculentum) is a fast-growing, warm-season annual typically sown in spring or summer. It’s excellent for quickly suppressing weeds, improving soil structure, and attracting beneficial insects, both pollinators and predators. Buckwheat is also a good choice for gardeners with poor soil, as it can grow in low-fertility conditions and does not require much water. It decomposes quickly, making it easy to incorporate into the soil.
Fall: Winter rye (Secale cereale) is a hardy, cool-season grass that planted in the fall can grow over the winter months. It is a good choice for preventing soil erosion while suppressing weeds and improving overall soil structure. Winter rye produces an extensive root system that helps break up compacted soil and scavenge nutrients. It can be challenging to incorporate into the soil due to its tough fibres. Cutting it down while still young and tender can make the process more manageable.

Considerations

While green manures offer many benefits, there are also some potential drawbacks to consider:

Time and effort: Growing and incorporating green manures requires more time and effort than leaving garden beds fallow.
Temporary nutrient lock-up: During crop rotations, it is essential to be aware as green manures decompose, they can temporarily bind nutrients like nitrogen in the soil. This can make it unavailable to other plants nearby for a short period.
Seed production and weediness: If you don’t cut down and incorporate green manures before they set seed, they can spread throughout your garden. Species like clover can be difficult to remove once established.
Allelopathy: Certain green manure crops, such as mustard and rye, produce chemicals that can inhibit the growth of other plants. While beneficial for weed suppression, it may negatively impact nearby or subsequent crops if not properly managed.
Water competition: In dry climates or during droughts, green manures can compete for water with other plants and may require additional irrigation.
Potential pests and diseases: It’s essential to choose green manure crops that are resistant to common pests and diseases in your region.
Reduced space for food production: Using green manures means dedicating garden space to a non-food crop for a portion of the growing season. This may not be ideal for gardeners with limited space or those who want to maximize food production.

To mitigate these drawbacks, carefully select green manure crops based on your garden’s needs, climate, and available space. Properly managed, cutting them down before they set seed, and incorporating them into the soil at the right time, green manures can go a long way towards maintaining soil health and balance.