Protect your land - use cover crops

Vegetable farmers use their soil intensively. There are pressures to grow more and more crop - in volume or value - to maintain profits. Despite this, it is still important to grow non-vegetable cover crops. Cover or "break" crops include grasses such as oats and millet and legumes such as cowpeas and vetch. They may be ploughed in and are called "green manure" crops. These crops may also be mulched, slashed or sprayed, then later turned into the soil (incorporated).

Although cover crops do not normally produce income, they are important because they protect the soil and give other benefits.

Bare soil is easily damaged, it is best to protect it by maintaining plant cover.

The benefits and disadvantages of cover crops, and the opportunities to use them, are explained in this Agnote.

Bare soil is easily damaged

Bare soil is easily damaged and washed away by rain. When raindrops hit bare soil, the impact results in a surface crust. When soil is washed down a slope it can be caught in silt traps. However, the richest part of a soil is the fine particles and organic matter. These are usually not caught by silt traps - they are lost forever.

 The solution to all these problems is to keep soil covered by plants. Plants absorb the force of raindrops and slow the flow of water in a paddock.

Cover Crop Bulk

A patchy cover crop will protect your soil almost as well as a bulky crop. To benefit from a cover crop it is not necessary to use water and fertiliser to produce healthy and bulky plants.

 Even if you grow three crops a year and have little time between crops, there may still be opportunities to sow a quick-growing cover crop and turn it in before it is mature - there will still be advantages.

In many ways, a bulky crop is more difficult to manage than a less bulky one, and the benefits of a less bulky crop are almost the same.

Grass or legume?

Legumes are broadleaf plants that are special because they make their own nitrogen with the help of certain bacteria that live in their roots. If these bacteria are not already in the soil, which is likely if a chosen legume has not been grown in a paddock recently, then it is helpful to add the bacteria at sowing. This addition of bacteria is called "inoculation". The nitrogen that legumes produce adds to soil fertility. Grass straw is also good but not quite as nutritious. On the other hand, grasses grow faster, control weeds better and produce more bulk. If broadleaf weeds are a problem, then it would be safer to choose a grass.

Late-sown legumes are usually slow to establish. In many circumstances, a grass cover crop or a grass and legume mix will be the best choice.

To Plough-in or not?

Ploughing in - incorporating - a cover crop adds organic matter and speeds its breakdown. However, some of the existing organic matter in the soil "burns up" and is destroyed when it is exposed to the air by cultivation. Also, after cultivating the soil is more easily eroded. Often it is better to mulch or slash a cover crop. Alternatively, spray it with a knockdown hebicide then let it stay there, still protecting the soil, until it is time to cultivate for planting.

The choice depends on the amount of time available before planting again, the kind of crop and its bulk, and the likelihood that the cover crop will become a weed. Cover crops typically achieve maximum bulk and legumes maximum nitrogen build-up around the time of flowering.

Biological tillage

Biological tillage (cultivation) is the term used to describe plants loosening the soil as their roots grow and as they dry out the soil and it cracks. Many cover crops have strong root systems and will reduce the amount of cultivation needed to prepare for the following vegetable crop.

Advantages of cover crops

• Protect the soil: much less erosion - soil washing away - and less surface crusting.

• Maintain fertility: by maintaining organic matter levels in the soil and, if a legume, adding nitrogen.

• Weed control: a healthy cover crop keeps a paddock free of weeds.

• Disease control: can provide a "break crop" that helps reduce disease, nematode, and perhaps pest, levels. For vegetable production, grasses rather than legumes tend to be best for this benefit.

• Biological tillage: less cultivation is needed because cover crops loosen the soil.

• Improved paddock access: cover crops can dry out a soil underneath and help farm operations to be timely. This drying out also makes nitrogen in the soil more available.

Disadvantages of cover crops

• Loss of land for cash crops. If it is necessary to avoid this, do not grow the cover crops to maturity and grow only occasionally. A cover crop only once every two years still provides worthwhile benefits. On the other hand, once a year is better.

• Cost of seed and sowing: these costs are unavoidable but small. The costs associated with growing, such as watering, are usually avoidable.

• Can become a weed: usually not a problem in vegetable production.

• Bulky crops: can temporarily tie-up nitrogen and perhaps increase disease and have other effects. Their trash can also get in the way. If nitrogen deficiency in the following crop may be a problem, incorporate the cover crop earlier - for example 4-6 weeks before planting - or apply a nitrogen fertiliser to speed-up breakdown of the cover crop.

Summary

Vegetable growers often use cover crops and they should be used more! They are very effective at protecting the soil from erosion and they improve soil structure.

It is not necessary to grow a healthy cover crop to get a good result.

Cover crops are cheap and easy to manage. Depending on the price of seed, they can typically cost just $50 to $100/ha ($20-40/acre) to grow and only 10 hours/ha (4 hours/acre), or less, of work is involved.

Even in intensive vegetable production, there is opportunity to use cover crops and their use is important. Ask your produce agent or seed supplier, NSW Agriculture Horticulturist, or other growers for advice on how you can benefit from growing cover crops on your vegetable farm.

Acknowledgments

Thanks to representatives of pasture seed companies, resellers, and NSW Agriculture Agronomists and Horticulturists for their expert advice. Thanks to Tony Wells for advice on article layout and content.

For more information contact:

Ashley Senn, District Agronomist, NSW Agriculture, Windsor phone: (02) 4577 0600 or Tony Wells, Research Horticulturist, NSW Agriculture, Gosford phone: (02) 4348 1900

Case studies

Paul Grech, Theresa Park

We always grow oats in winter now to keep the paddock free of weeds and loosen the soil. When we go to plant in spring we can get onto the paddock because the ground is dry underneath. We can prepare a seedbed with only one extra pass and we save many cultivations when the oats are in the ground.

Paul Grech, Theresa Park

Joe Sultana, Freemans Reach

We don’t grow much in summer. Before, if we got a storm we’d lose a lot of soil and the ground would go hard. But by growing sorghum the dirt stays in the paddock. The water that runs off is clear - not muddy.

Joe Sultana, Freemans Reach

Tony Wells, NSW Agriculture, Somersby Research Station.

Growing a cover crop infrequently - in our case an oat crop every two years - still has a large effect on reducing soil erosion. Here on the Central Coast we have been comparing conventional vegetable production systems with alternative approaches. We have found that small affordable changes in management can have big effects on reducing soil and nutrient loss.

Tony Wells, NSW Agriculture, Somersby Research Station.

Cover crops for Sydney vegetable growers