Green manures

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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:

USE green manures to:
- IMPROVE your soil
- MANAGE weeds and
- BENEFIT following crops
DESCRIBE the advantages of green manures and how to use them; and
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
Better gas and water exchange
Reduced soil pest and disease problems
Soil protection
Other soil benefits
Weed control

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:

CARBON from carbon dioxide in the air
OXYGEN from carbon dioxide in the air and from water in the soil
HYDROGEN from water in the soil
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

Different plants have different abilities to get each of the major and minor nutrients every plant needs. If your green manure and your crop have different abilities, the green manure can get nutrients your crop can't. As the green manure breaks down, these nutrients come out of it and are more available to your next crop than they were before

Soil amendments are materials that improve the soil but are not mainly a fertilizer, that is they are not mainly for supplying nutrients. They include lime, gypsum, dolomite, organic matter, compost and some rock dusts.

Making more available the nutrients in fertilizers and soil amendments (see at right). By applying fertilizers (particularly slow-release ones such as rock phosphate and other slow-moving rock dusts) when you plant the preceding crop or the green manure, you increase the benefits from the fertilizer. Many soil improvers and fertilizers only move slowly into soil water or onto soil particles where they can be taken up by plants. The increased biological activity after a green manure will speed the availability. Also some of the fertilizer will get used by the green manure and will then be released as the green manure breaks down

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

Water flowing over the soil surface is slowed by the green manure and the rougher surface
Better structure: The soil forms into crumbs rather than blocks or individual grains and greater pore space (holes through the soil) mean there are tunnels for the water to run in
Increased organic matter soaks up and stores water as it arrives, freeing soil pores to allow more moisture into the soil.

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:

Plants give off (exude) substances that help to hold soil together and so reduce erosion
The canopy of leaves shades soils from intense heat that can kill many soil organisms. The ground should always be covered in any hot climate (large parts of Australia, the Americas, Africa and southern Asia) but needs a rest from continuous cropping by growing a green manure, a pasture or a fodder crop or by being covered with a thick mat or stand of crop residue
Leaves and stems act like umbrellas to slow and break up the raindrops and so reduce the effects of raindrop impact (and this is where water erosion often starts, as one drop of soil at a time is dissolved and made ready to wash away)
The soil surface is protected from wind and other damage
Soil cover in summer when the green manure is left as a mulch reduces evaporation.

Other soil benefits of

Improved buffering capacity: Just as the buffers that stick out on the end of a railway carriage protect it from damage when it bumps into the next carriage as the train chugs along, organic matter buffers your soil. When you have a high level of organic matter, your soil is more stable and less knocked around by acidity, salinity and sodicity and even by normal wetting and drying. Clays become softer, sands more able to hold nutrients and moisture and to form crumbs. So each gains many of the advantages of the other without losing anything
Increased depth of available soil by:
- "UPPENING" the soil (increasing its apparent height) by adding height to the soil or helping it to avoid becoming more compacted. Green manures retain or increase soil height by adding organic matter plus air spaces and better crumb structure that improve the bulk density
- DEEPENING the soil by growing tough deep-rooting plants that push through hard soil to open up the soil at depth. By using a green manure with tough roots and a tendency to go down deep (some of the larger radishes such as (Raphanus sativa) or oil radish that can send roots down 2 metres/yards or more) or some of the Japanese daikons, you can help to break up hardpans and as a result deepening the soil available to your crops.

Weed management benefits of green manures

By growing a green manure you are able to shade out or out-compete weeds for nutrients and water

Allelopathic effects are any effects that could be seen as chemical warfare between plants, such as those that:

reduce germination
restrict growth or
reduce the ability of plants to compete with the one putting out the chemical. For more info on how allelopathy works and how you can use it, see Allelopathy

You can get the benefit of the plants' natural allelopathic effects (see at right) to reduce weed germination and competition. Barley is high in allelopathic chemicals when it is growing and as the straw breaks down. Other cereals also have allelopathic effects

You can time the planting of the green manure to not suit the weed and to suit your control of it. You can plant the green manure at a time when the weed would be getting ahead of a crop or would be just getting started. This gives the weed a more difficult time and generally reduces the population of weeds
If you time it right when you end the green manure, you can prevent the weed flowering or storing nutrients for the next year. This can reduce weed numbers and the vigor of the weed for following years and also eliminate the current lot of weeds
If the green manure is turned in to the soil, then the weed is turned in with it and is destroyed by the normal methods of tillage and destruction of weeds
The tillage at sowing time and when turning in the green manure can stimulate germination of weed seeds to give you or the green manure the opportunity to control them and so reduce the bank of weed seeds in the soil
If you slash the green manure and leave it on top of the soil as a thick mulch, it can limit the growth of weeds later and provide homes and nutrients for a range of soil organisms. Like living plants, mulch can have allelopathic effects and further suppress weeds. Plus, the slashing destroys and damages weeds.
By turning weeds into mulch or soil organic matter, you convert weeds into nutrients.

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:

DECIDE what you want from your green manure. See the list of benefits
FIND a point in your rotation where you can slot in a plant for a few weeks to many months
Or tuck one in without interfering with preceding or following crops using the techniques described in Squeezing a green manure in
WORK OUT how you will sow the green manure. Will you broadcast it, drill it or use some other method?
WORK OUT what seed bed preparation you will do for your next crop. How will the green manure affect that?
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?
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
MATCH this to your cropping system using all relevant info
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:

WITH the previous crop?
AFTER the previous crop gets established but before you harvest it?
AFTER the previous crop is harvested and before the next is sown?

For each of these you will need to match your sowing method for the green manure to the conditions.

If you Sow with the previous crop, you will probably undersow the green manure, perhaps using the small seeds box on the seeder for the green manure and the main seeds box for the crop or the other way around. You could mix the green manure seed with the crop seed if your seeder handles that

One farmer towed a second seeder with the green manure behind the main one and used this to add a soil amendment (gypsum) at the same time
If you Sow after the previous crop gets established but before you harvest it, you may need to broadcast it by hand, aeroplane or a high-mounted seed spreader. But germination may be less effective. So it can be direct sown with a narrow hand-seeder in a small bed or with a toolbar implement in broadacre growing. It could also be banded as part of a tillage operation, particularly if the crop is on beds or in wide-spaced rows
If you Sow after the previous crop is harvested and some time before the next is sown, you could combine seeding with slashing the stubble or turning it in.

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?

A common mix for autumn, winter or spring in temperate areas is oats and vetch. This gives plenty of organic matter plus plenty of nitrogen. The two plants grow tall enough to suppress many weeds and the oats produces plenty of long-chain molecules that build soil structure
A common mix for spring, summer or autumn in temperate areas is cow peas (or another similar large summer legume) and millet, such as french white millet. These provide a large bulk of organic matter, good nitrogen levels and the millet produces plenty of long-chain molecules that build soil structure
A common mix for tropical and subtropical areas is peanuts (or other suitable legumes) and local grasses.

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:

If your aim is to kill weeds, you may decide to end it early:
- so you can turn the weeds in with the green manure
- so you can encourage an extra germination of weeds to give you another go at weed control
- you may have two quick green manures to give you a second go at germinating and then swamping the weeds
- you may end it at a time when the weeds will be most disadvantaged.
If you want a fine seedbed, you may also decide to end the green manure early to allow time for extra tillage and for plant residues to break down before sowing.

The next decision is usually how to end the green manure:

cut it and leave it on the surface?
turn in the residues?
or do the first and let it sit for a while before doing the other?

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:

What the next crop will be
How the next crop will be sown. You may need to combine seedbed preparation with green manure finishing
What time you have available and when
What is the easiest way to do the job using existing tools. Often the job is done after another job, using whatever tool is on the tractor
And the situation or mood:
- You are on your way back from something else
- you've already thought about how you will end the green manure
- the implement you have mounted may not be ideal but will do the job
- you have enough time to do it right now and
- it is getting close to sowing time for the following crop.

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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:

very weed prone (such as carrots)?
started out small but grew to cover a large area (such as cabbages or lettuces)?
was small-seeded and the resulting seedling was small and relatively fragile (carrots, sesame, alfalfa/lucerne)?

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
http://www.OrganicExchange.com.au/all/fcmmgm00.htm

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

Litter

Hard pan

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