Green manure

Green manure


Home Gardening

We all know that organic matter — anything that was once living — is the perfect soil food. We also know that the quality of our plants relies on the quality of our soil. Synthetic fertilisers can feed the plant — but not the soil. Our garden soil should be high in organic matter to improve soil structure and allow water, air and roots to penetrate.

By Jane Varkulevicius

Organic matter

Organic matter helps the soil to form aggregates, or soil crumbs, that can hold both water and nutrients and encourage soil microorganisms to thrive.

Heavy clay makes it hard for roots to penetrate. It dries to a crust and is like mashed potato when wet. With the addition of organic matter it can be transformed into a dark, crumbly soil that holds water and allows air and roots to penetrate easily (picture at right). A healthy soil is really the ultimate slow-release fertiliser but it needs your help to develop and be the best it can be.

A “green manure” is a crop that is grown to be cut down when still soft, just before it flowers, and dug into the soil. Green manure crops can add organic matter, smother weeds and protect the soil from erosion and compaction from heavy rain, as well as add nitrogen when legumes are planted. They can loosen and enrich compacted soil, which is the perfect preparation for a new garden bed. What’s more, if you plant members of the mustard family they will act as bio-fumigants that help rid the soil of pathogens such as verticillium after a tomato crop. That’s a lot of benefit for merely scattering a few seeds — top value for effort, I say.

No-dig green manure

The down side to green manure is it does take up space that could be used for winter crops. A way around this is to plant green manures between the rows of existing plants. This, of course, rules out the digging-in part of the process. Instead, just cut down the sappy stems with hedging shears and use them like mulch — sort of a compost “in-situ”. Leave theroots to rot and create tunnels for worms and other soil life.

What to sow

The most economical, if not the most direct, method is to clean out your seed drawer and plant all the old seed that may have declined in germination. Some will come up, some won’t, but you should then have a nice, neat seed storage area with no halfopened old packets! If you want to use green manures for a particular purpose, choose from the following or make a mix that suits your needs.

To suppress weeds:
Barley is the ideal seed to plant to suppress weeds. It contains and exudes compounds that inhibit the growth of plants around them. This harmful effect of one plant on another is known as allelopathy. Just as walnut trees inhibit the growth of any plant in their root range, so does barley. Barley grows fast and supplies plenty of soft, sappy growth to dig in. Of course, barley is not suitable for growing between rows of existing crops as it will inhibit their growth as well.

For extra nitrogen: All legumes (members of the pea and bean family) have the ability to fix nitrogen from the air. They store it in nodules on their roots. The earth’s atmosphere is made up of more than 70 per cent nitrogen — far more abundant than life-giving oxygen or the ever-increasing carbon dioxide. It makes sense to sow plants that can tap into this resource and convert it to a plant-usable form. Peas and beans fix nitrogen from the air and store it in plant-usable form on the roots. Lupins, vetch, tick beans, clovers or any peas and beans have this capability. Never plant a crop of peas or beans after you have dug them in; instead, the bed will now be ideal for green, leafy vegetables such as lettuce, silverbeet, and spinach. Legumes are perfect for sowing between rows of other crops as they will supply a steady flow of nitrogen for the present as well as the future crop. The nodules even look like commercial slow-release fertiliser.

To clean the soil: The mustard or Brassica family — broccoli, cabbage, Brussels sprouts etc — exude compounds that control or suppress the growth of pathogens (the bad guys) in the soil, including nematodes and fungi. They also, of course, supply plenty of organic matter to be dug in or used for mulch. Choose from canola, mustard greens and brown mustard. Due to the volatile oils of these crops, wait six weeks after digging them in to plant in the soil.

To break up the soil: When preparing a new garden bed or to break up compacted soil and enrich it at the same time, sow a mix of legumes, brassicas and cereal crops. Oats and rye corn produce plenty of leafy growth without the plant-inhibiting exudates of barley. Any member of the pea family will provide a store of slow-release nitrogen, but add tap-rooted species to open up the soil. Turnips and radishes, both members of the Brassica family, are ideal. Green manure seeds are available from your local produce store or mail order companies such as Goodman Seeds, Green Harvest, New Gippsland Seeds and Bulbs or Diggers Club.

Dos and don’ts

• Do cut down or dig in green manure just before it flowers.
• Don’t plant until six weeks after digging it in — a good argument for cutting it down and using it as mulch.
• Follow single species plantings with crops that benefit from that species:
• After brassicas such as broccoli, mustards and cabbages, plant tomatoes, capsicums, chillies, eggplant and root crops.
• After legumes such as peas and beans, plant leafy vegetables, including brassicas.
• After cereals such as oats, barley and rye corn, plant peas and beans.