Green Manure & Cover Crops
Green manuring is the process of growing annual, leafy plants which are then dug into the soil once mature. This process provides organic matter and nutrients to the soil. The plants used for green manure are often ‘cover crops’ grown primarily for this purpose.
Cover crops are usually planted over winter or whenever the ground is bare however, cover crops can also be grown under fruit trees or amongst crops. Try alternating rows of edibles with cover crops that wont compete with your main crop for water, nutrients and sunlight e.g clover or cow peas which are low growing, deep rooted and fix nitrogen.
Remember anything you grow and turn back into the soil is technically a green manure, so if in doubt plant whatever bulky vegetables you have, peas, beans etc. In Australia, Green manure packs are available from Green Harvest, Diggers, The Lost Seed
A cover crop is a crop planted primarily to manage one or more of the following: soil erosion, weeds, soil health (fertility, quality, water retention, biodiversity), pests, diseases, and wildlife in an agricultural setting but the practice can also be useful in the home garden, if you have the space.
Soil erosion & weed control
Cover crops offer a method of holding nutrients in the soil that would often wash out over winter. This keeps the nutrients available for spring crops. Soil is protected from winds and heavy rains.
Cover crops shade the soil (which reduces weed germination) and compete with weeds for light, water and nutrients. Where weed control is important, a cover crop should germinate quickly and develop a rapidly growing and dense herbage that can effectively “shade out” weeds.
Green manures increase the soils organic matter which enhances the capacity of the soil to store nutrients and water. Almost all of the nitrogen and much of soil phosphorus and sulfur resides in the soil organic matter. The capacity of soil to hold macronutrients, such as potassium, calcium, and magnesium is enhanced as a result of green manuring. Micronutrients, such as iron, zinc, copper, manganese, and molybdenum, form complexes with organic matter, which increases their availability for plant uptake, as well.
Organic matter provides carbon as energy for microorganisms which helps make many soil nutrients more available to plants. Organic matter increases soil water holding capacity in sandy soils and improves water percolation in heavy, clay soils.
Choosing a cover crop
Determine your requirements from a cover crop.
Disease management: Some cover crops can be used to control pest and disease either directly (mustard is particularly good for this) or indirectly by breaking disease cycles i.e. cover crops known to harbour pests and disease of the crop to follow should not be used.
Nutrient requirements: Cover crops vary in the nutrients they accumulate. You may wish to choose one that contributes the nutrients most needed by the edible crop to follow. Deep rooted plants can retrieve nutrients which have leached into the sub-soil and are unreachable for most annual fruit and vegetable crops. Legumes such as peas, beans and clover contribute nitrogen.
Building soil: Bulkier plants produce a greater volume of organic matter (biomass) which in turn is returned to the soil.
Attracting beneficials: Flowers of some cover crops attract beneficial insects.
Usually a combination of cover crops is the best option e.g. Wooly pod vetch (nitrogen fixer) with oats (high biomass)
Warm season cover crops
Quick growing options for spring/summer include:
- Mustard: Turned in 6-7 weeks after sowing. Bio-fumigant. Protect from hot sun. Plant as a monoculture.
- Buckwheat: Turned in 5 weeks after sowing. Accumulates phosphorus.
- Cow Peas: Nitrogen Fixing. Nematode control. Living mulch which is particularly good under corn.
Cool season cover crops
Options for autumn/winter include:
- Clover: Nitrogen fixing. Dies back naturally early summer where it can be left place as mulch or solarise 3 weeks before plot is needed.
- Fava bean: Nitrogen fixing. Adds significant organic matter.
- Mustard: Bio-fumigant. Adds significant organic matter. Plant as a monoculture.
- Lupin: Nitrogen fixing. Deep rooting. Accumulates phosphorus.
- Wooly pod vetch: Excellent nitrogen fixer. Combine with oats for support. Can be slashed to provide “in situ” mulch for spring vegetables, especially beneficial for tomatoes. Attracts predatory/beneficial insects.
- Oats: Combines well with all cool season green manures. Nematode control. Adds significant organic matter.
Legumes for nitrogen
As well as holding and retrieving nutrients in the soil, some green manures (legumes) have the ability to fix nitrogen from the air and return it to the soil. Legumes usually used as cover crops include: clover, vetch, soybeans (peas and beans are also legumes).
Clover is a low growing, deep rooted legume that can be grown between rows of crops. Adjust row spacing accordingly.
Using legumes effectively
It is important to keep in mind that most of the nitrogen benefit from the green manuring of a legume crop is only obtained if the crop is turned under at the stage of 50% flowering or before it produces seed. Likewise, legume cover crop residue that is killed and incorporated into the soil decomposes rapidly and most of the nitrogen is released in the first 30 days. This does not correspond with the nutrient needs of most subsequent crops in many cases.
Grasses and grains usually produce more biomass than legumes. Since grass is usually lower in nitrogen content than legumes, it decomposes more slowly. Crops planted soon after large amounts of grass herbage have been incorporated into the soil may require additional nitrogen fertilizer to prevent nitrogen immobilization, which robs the subsequent crop of needed nitrogen.
Combining or cycling legumes with grain crops
Mixing a grain crop with a leguminous crop (e.g. 30% hairy vetch-70% winter rye mix) may prove beneficial provided the crops complement each other in terms of growth and nutrient capture (e.g. nutrient mining vs. nitrogen fixation). In the case of the above mix, the residue of the mix decomposes faster than pure rye, thus reducing the risk of nitrogen immobilization.
Also, legumes such as clover and hairy vetch contain waxy substances that can contribute to hydrophobic soils if used excessively over extended periods of time. Therefore, if not combining legumes with a grain crop, it’s best to alternate between legumes and other green manures to prevent this from happening.
Mustard for biofumigation
Mustard establishes quickly and easily, is fast growing and can be sown into empty beds at any time of the year. It needs 5-6 weeks growth for best results and produces significant organic matter and importantly has been proven to kill pests and diseases in the soil.
- To be successful as a biofumigant, mustard needs to be dug into the soil where it releases natural gases as it decomposes.
- Plant as a monoculture. It will retard growth of other crops.
- It should be tilled in for a minimum of three weeks before planting a subsequent crop. Decomposing mustard will reduce seed germination and affects plant growth. Rain or irrigation after tillage is required to release glucosinilates (disease killing gases).
- Cabbage white butterfly is a pest of Brassias including mustard. Regularly spraying mustard plants with Bacillus thuringiensis (Dipel), when pest is present will help keep populations down.