Green Manure Crops

Help you to:

1. Increase organic matter, earthworms and beneficial micro-organisms in your soil
2. Increasing the soil's available nitrogen and moisture retention capabilities
3. Stabilise the soil to prevent erosion
4. Bring deep minerals to the surface and break up hardpans
5. Providing habitat, nectar and pollen for beneficial insects and reduce populations of pests
6. Improving water, root and air penetration in the soil
7. Smothering weeds
8. Save Money

An ideal way to do this is by using a green manure crop.

Green manure crops are crops grown, not to be harvested by the grower, but to be incorporated into the soil before they reach maturity to contribute to the volume of organic matter in the soil. It is an old technique of soil management and improvement that used to be used widely but now is only used in a limited manner. We have used this method in a number of our gardens to help improve the soil.

Green manure crops contribute directly to the fertility of the garden through the supply of important plant nutrients. Many green manure crops contain Legumes. Legumes in particular supply a valuable amount of nitrogen since their roots form an association with soil-borne bacteria that can transform nitrogen from the atmosphere into nitrogen compounds that can be used by plants. The soil bacteria live in special nodules on the plants roots and can be easily seen if you pull up a mature legume. This is quite a complicated feat - and one which can save you the cost of fertilisers! Different nutrients such as phosphorous are supplied by other green manure crops.

Green manure crops also contribute indirectly to nutrient supply. Once the green manure crops is incorporated into the soil it begind to decompose and forms compost. Organic matter can also be supplied through incorporating compost inot the soil but typically the cost of compost is much higher than the cost of the seeds for the green manure crop. Many composts are also sterilised for health and weed reasons before they reach your gardens.

A good healthy soil should contain approximately 5% organic matter. While this may seem to be a small component of the soil, it is a vital one.

Why is organic matter so important? Because decayed organic matter, or humus as it is called, is the key to good soil structure, nutrient supply, water holding capability and the biological balance of the soil.

Drought resistance can also be improved in another way by the use of green manure crops. Many of the legumes used as green manures, such as alfalfa, lupins and sweet clover, are very deep rooted crops. Their roots can penetrate the subsoil and open it up which is an important improvement in compacted soils. Subsequent vegetable crops can use the channels in the subsoil to allow their roots to reach deep into the subsoil and obtain water from the lower levels. It is worth remembering that many common vegetable crops are capable of putting down a large root system if the soil is loose enough. For example, in a deep, well structured soil, tomatoes can put roots down 150cm!

Another benefit of a green manure crop is that while the green manure crop is growing it prevents weeds colonising the bare ground left after the previous crop has been removed. In general it helps protect the soil surface from erosion and leaching of nutrients. This means that green manure crops can be used as a nursery crop for a slow growing crop - such as native grasses.

Green manures can be grown in a number of ways:

1. As a crop during the main growing season, which, however, has the disadvantage of taking up valuable space at the most productive time of year
2. As an undercover crop or nusery crop grown with the main crop. If this approach is taken care should be taken to ensure that the green manure crop is a sterile and annual crop.
3. As a crop grown during the off season, or in garden beds during the hottest periods of the year where you want to prevent soil erosion and leaching but are unable to establish trees due to lack of rainfall..

It is important that green manure crops be dug in before seed set and before they turn woody. You will then need to wait a number of weeks before planting to enable the green manure crop to compost down in the soil. You can also leave the green manure crop on the top of the garden beds but incorporating it in will speed up decomposition.

GREEN MANURE CROPS

If you live in Tasmania please follow this link to this Green Manure Crop Calander .

If you live in Victoria click here to open the Victorian DPI Green Manure Brochure.

Legumes: (fix nitrogen)*

Broad Beans: Produce a large amount of organic matter. Can be sown late in Autumn. Will stand some water logging. Sow 35grns/sq m

Lupins : Effective phosphorous gatherers. Contribute lots of organic matter. Not usually susceptible to fungal diseases which may affect peas and beans. Sow 16grn/sq m. Usually sown in March in Melbourne.

Sub Clover : Very effective nitrogen fixer. Not large amount of foliage. Sow l gm/ sq m.

Tic Peas : Cheaper alternative to Broad Beans

Vetch: Large bulk. Competes well with weeds.

* Some lucernes may also be suitable

Non-Legumes:

Barley : Vigorous grower. Increases uptake of Phosphorous in following crop.

Oats : Grows in wide range of soils. Doesn't mind acidity. Tolerates very cold weather. Broadcast 10 gm/sq m

Ryecorn: Large amount of organic matter. Drought resistant. Sow similar to oats.

Mustard can be effectively sown as a fast growing, shallow rooting green manure. It is also particularly useful in controlling the soil pests: eelworms or nematodes. Mustard is a very sappy plant and will decompose quickly so it is an ideal crop for digging in.

Important Note on Planting

In an ideal world planting of green manure crops should be timed to be followed by natural rain. It is very important that the germinating seeds do not dry out before they have time to grow roots. If they do run out of moisture they will stop germinating. Many then die.

It is possible to soak the seed to improve germination but the seed should only be soaked for 20 minutes. Once the seed is sown it will again need to be kept mosit until roots have been established.

If you would like to develope a garden which is more sustainable please contact us.