Green manures

Why buy nutrients as fertilizer when you can grow most for free? A green manure gets nutrients from air and water and stores them in its tissues and the soil. While doing this it improves your soil in other ways and controls weeds. All you do is find a spot and a time to fit a green manure in to get a huge range of benefits. And you can use green manures in annual crops (wheat, rice, vegetables etc) or in perennials such as asparagus, herbs, orchards and vineyards.

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Green manures are plants grown for some benefit other than to harvest or graze. They are grown before a crop to improve the soil for it or to control weeds.

Green manures are grown until the best point for you, the soil, the rotation and your weed control. Then you slash the green manure and leave it on the surface or turn it in to the soil. You can incorporate fertilizers that have been applied at the same time or earlier and you can smother or turn in and kill weeds.

Green manures are common in organic vegetable production. They are being used more in broadacre cropping, particularly where there are few animals grazing the cropping area. Green manures cost little to put into your farm or vegetable rotation.

When you have successfully completed this and associated pages, you will be able to:
  1. USE green manures to:
  2. DESCRIBE the advantages of green manures and how to use them; and
  3. MANAGE pasture so it gives you the sort of benefits a green manure would.

To get full value from this web page and relevant associated web pages, you need a basic understanding of Farming and of How plants grow.

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What this means

Green manures are grown for a benefit and not for harvest and they are not grazed. They are commonly used to improve the soil or to control weeds.

Instead of harvesting or grazing, the green manure is slashed and left on the soil surface or is ploughed into the soil. Green manures take some time and planning, but cost little to put into your rotation.

Green manures have been used for centuries but went out of fashion over the last 50 years in many of the "more developed" countries because they were seen as a slow way to do what bought fertilizer could do.

But the best that fertilizers can do is improve the chemical fertility of soil in the short term. And fertilizers feed the plant instead of feeding the soil.

If instead you feed the soil with a green manure, you feed all the soil organisms and improve the biological fertility of the soil. Having a biologically fertile soil is like having a strong, fit and healthy body that is able to resist disease naturally rather than one pumped up on steroids. A healthy soil is more able to resist changes and cope with long dry spells or other extremes that could otherwise damage the soil flora and soil fauna (plants and animals that live in the soil).

By growing green manures you provide soil organisms with a home, moisture, nutrients and other conditions that benefit them. These organisms convert the nutrients in the green manure into available nutrients for crops. Because these organisms are more active with a green manure than without one, they also convert the unavailable nutrients in the soil into available nutrients. In the process, they improve the general soil conditions.

Benefits of a green manure

The purpose of a green manure varies with the farmer and the situation, but some benefits they offer are:

Increased soil nutrient levels through:

Nitrogen-fixing organisms include Rhizobium. These harmless and very beneficial nitrogen-harvesting bacteria live in the roots of legumes such as alfalfa (lucerne), clovers, beans, peas and certain trees such as acacia (called wattles in Australia) and tree lucernes (tagasaste and leucaena).
  • Nutrients added: Green manures collect from the air and water the four main nutrients that plants need:
    1. CARBON from carbon dioxide in the air
    2. OXYGEN from carbon dioxide in the air and from water in the soil
    3. HYDROGEN from water in the soil
    4. NITROGEN from the air (if the green manure includes a legume) through nitrogen-fixing organisms (see at right).
      All these nutrients are free and are useful to the green manure, to other plants that follow them and to the soil organisms.
  • Greater organic matter improves the soil's water-holding capacity, structure, buffering ability, nutrient-holding capacity, biological activity and just about everything else worth improving in most soils
  • Nutrients made more available
  • Deep-rooted green manures can bring nutrients up from deep in the soil
  • Higher soil biological activity leads to better biological fertility.

  • Better gas and water exchange through:

  • Loosening and opening the soil as the green manure's roots push through it
  • Better soil structure: As organic matter breaks down it releases compounds that help bind soil particles into crumbs that make better structure for plants to grow in. Soil with good structure is easier for roots to penetrate and allows air, water and waste gases to pass into and out of the soil
  • Biological fertility: As green manures rot down, their roots disappear leaving channels through which earthworms and other soil organisms plus air and water can pass. So green manures aerate the soil and increase the activity of organisms in the soil. Then the roots rot and become food for soil organisms
  • Deep-rooting plants get moisture from the subsoil that is not available to shallow-rooted plants. An example is alfalfa (lucerne) that can send its roots 12 or more metres down into the soil
  • Greater water-holding capacity: The increase in organic matter levels in the soil and the better structure of the soil can both lead to an increase in the water-holding capacity. Crops grown in soils that have had green manures can go without rain longer. If these plants have the opportunity to send their roots deeper (through the green manure opening up the deeper soil) they can be even more drought resistant
  • Greater infiltration of rainfall: The rain can soak into the soil because
  • Moisture capture: Green manures contain moisture and if they are handled carefully, much of this moisture will be kept in the soil and in soil organisms. So a green manure grown during your wetter season can increase the soil moisture for your next dry season.

  • Reduced soil pest and disease problems through:

    The breakdown of canola (rapeseed) and mustard crop residues releases compounds into the soil and these control or suppress a range of insects, nematodes and fungi. This table shows the amount of Take-all fungus in wheat after different crops:
    Preceding crop Amount of Take-all
    Canola (rapeseed) 0%
    Lupins 12%
    Another cereal 25%
    Pasture 25%
  • using plants that produce substances that slow down or kill pests or diseases. Canola (rapeseed), mustard or other brassicas grown before a wheat crop can kill off Take-all (see at right) in the soil. Other plants can kill or limit other pests or disease
  • using plants that do not get the disease and therefore do not host it (provide a home for it). This is called a "break crop". Diseases and other organisms that have no host will eventually die off
  • Greater biological activity can reduce or kill off fusarium, phytophthora and other soil diseases.

  • Soil protection through:

    Other soil benefits of

    Weed management benefits of green manures

    However, remember that every time you till the soil, you "burn up" organic matter that escapes into the air. In this way, you lose nutrients and the substances that give your soil its structure.

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    Using this

    To get the benefits of a green manure you could:
    1. DECIDE what you want from your green manure. See the list of benefits
    2. FIND a point in your rotation where you can slot in a plant for a few weeks to many months
    3. Or tuck one in without interfering with preceding or following crops using the techniques described in Squeezing a green manure in
    4. WORK OUT how you will sow the green manure. Will you broadcast it, drill it or use some other method?
    5. WORK OUT what seed bed preparation you will do for your next crop. How will the green manure affect that?
    6. DECIDE how and when you will end the green manure. Will you incorporate it or will you leave it on the soil surface?

      How will you make these two operations work in together, ending the green manure and preparing the seed bed?
    7. CHOOSE a variety that will give you the benefits you want. Decide whether your need is for a particular result more than any other - such as you want to break up a Hard pan. For more info on choosing varieties, see Choosing a green manure mix
    8. MATCH this to your cropping system using all relevant info
    9. CHECK your decision (if you need to) with a neighbor who has used green manures successfully or with your local advisor.

    Starting out: sowing a green manure

    Note, in some situations (particularly where there is only a small area and you want to get the green manure rolling quickly) it can be worth soaking seeds before sowing. This allows germination to get started.

    Soaked seed needs follow-up moisture in the form of rain, soil moisture or irrigation. Otherwise germination may run out of moisture and stop. Many plants have a low survival rate if they have to restart germination. Many others can't restart it and they die.

    The first decision is usually will you sow the green manure:
    For each of these you will need to match your sowing method for the green manure to the conditions.
    Green manures sown under a crop or not into their own seedbed are usually sown at lighter rates so they don't compete too strongly with the existing crop.

    Green manures sown on their own are often sown at a heavier rate to maximize the competition against weeds and to get more bulk in less time.

    Generally it is best only to undersow a green manure under a crop that is tall compared to what you are under sowing. In small-scale commercial vegetable production: Corn (maize) can be undersown with soybeans, clover or vetches. Tomatoes can also be undersown with clover or vetches and they can also have oats grown between the rows of tomatoes. For more information, see Green manures under sweet corn or tomatoes These methods can also be used on these and other crops in broadacre growing.

    In small-scale organic vegetable operations that are common (and often very profitable) in areas close to large towns or cities, a lot of this work is done manually and on a bed-by-bed basis. There it is easy to pick up a bucket of standard green manure mix and scatter it before turning the crop remains in or to scatter it between the rows of the crops still growing on the bed.

    What season and climatic conditions will this green manure grow in?
    Some other green manure plants include sunflowers, various beans, snail clover, subterranean clover, other clovers, buckwheat (later phosphorus uptake and different phosphorus recycling patterns to most of the cereals), fenugreek, canola (rapeseed) and more.

    Some farmers have found that it pays to think more broadly. Sometimes it is a good idea to try something completely different - potatoes in a broadacre system to get good weed killing and soil improvement. You will probably get a suitable mix by asking your local advisor or a neighbor who has used green manures successfully.

    Finishing a green manure off (the coup de grass)

    The best time to finish a green manure off depends on when it is best for you.

    If you have plenty of time before your next crop goes in, you can end the green manure when it has done the job for you and when you want to. Many farmers let the green manure die off and fall into place as a mulch. This is particularly common with green manures that are mostly annual plants. However, many green manures are ended early for various reasons:
    The next decision is usually how to end the green manure:
    Most plants are at their greatest bulk and best protein (nitrogen) level around flowering. Just when depends on the variety and on what you want. If they have started to head up (produce a seedhead and fill it with seeds), they are putting the energy into the seeds. Seeds are a concentrated energy store, not a bulky one. Most green manure growers want bulk, not concentrates. You could check with your advisor once you know what you want from the green manure and have some idea of your choices of plants.

    Preparing for the next crop

    The factors most farmers consider include:

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    Taking this further

    WORK OUT how you would get from your pasture the benefits a green manure would normally give if your pasture is not doing it quickly enough or well enough. Once you have done that (no peeking) have a look at Return a third to the soil

    EXPLAIN to someone how you or they could use green manures. It will be more valuable to you if you use your own words than if you just repeat material you have read here or elsewhere

    WORK OUT what you would want in a green manure when the following crop had very specific needs. For example, what would you want in a green manure if the following crop was:

    A suitable next step might be:
    FITTING a green manure in on each vegetable bed every two years

    TAKING that a step further by developing a rotation plan for the future that allows for a green manure crop every year or two in vegetable or fruit production or every three years in continuous broadacre cropping

    APPLYING this to your pasture so you can get similar benefits to those above. You could do this by grazing for a shorter time then slashing it so about a third of the pasture is returned to the soil rather than used for production. For more information on this, see Return a third to the soil

    GROWING a green manure before the next planting or fruiting cycle for one block of trees, one field or one paddock.

    If you are looking at a printed version of this page and you would like to visit it on the internet and get a stack of other info that may assist you, the full web address is

    Related info:

    Squeezing a green manure in

    Green manures in orchards and vineyards

    Choosing a green manure mix

    Green manures under sweet corn or tomatoes

    Return a third to the soil


    Hard pan

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    A good green manure can make it easy to:

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    This page was updated on December 27, 2007