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Green Manure

Boost fertility with green manure

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Green manures are the cheapest and easiest way to build fertility in the food producing garden, writes Justin Russell.

When I started growing my own food 13 years I go, I believed a common myth. Seems silly now, but I thought that once a vegie garden was made – the soil dug over and some horse poo thrown around (on the recommendation of my Mum) – all I needed to do to grow healthy plants from then on was provide some extra fertiliser from time to time.

What I failed to realise is that food producing gardens work hard. Nutrients get taken up from the soil by the plants and get stored in fruit, roots and leaves, so from season to season, these elements need to be replaced. Fertiliser alone isn’t sufficient to maintain a healthy balance of nutrients and garden beds can become exhausted sooner than you might think.

To build fertility, you’ve got a few options. You can go down the “blitz” track and import a big load of compost or manure, but unless you’re related to a farmer, this will put a serious dent in your gardening budget and you may end up bringing in a load of weed seeds. A second option is to make your own compost, or collect manure from your own animals. Every little bit helps, but if you’ve only got space for a compost bin and a couple of chooks, you’re unlikely to generate enough material to maintain a high level of fertility. The third option, to my mind, is the pick of the three. It’s the use of green manure.

A way of composting in situ rather than building an external heap, green manure is simply a crop that gets broadcast sown on a spare garden bed, is allowed to grow until lush and leafy, then before it sets seed, gets slashed to the ground and turned back into the soil. Within days the “manure” begins to decompose, providing food fungi, micro-organisms and earthworms, which in turn, increase the soil’s fertility with their castings.

I’ve been using green manures since I built my current vegie garden in 2007, and with excellent results. I find that once decomposition of organic matter is complete the soil becomes as light and fluffy as one of my wife’s pancakes. This fluffiness indicates to me that the soil is full of life-giving oxygen and healthy aerobic bacteria, and will retain moisture and nutrients like a sponge.

I’m getting ready to sow some cool season green manure this week. A good seed combination for autumn and winter, particularly in frosty zones, is a triumvirate of mustard, legume and grass. When mustard decomposes in the presence of moisture, it releases a natural form of mustard gas that helps cleanse the soil. It’s particularly effective on harmful nematodes. A legume such as clover or vetch will capture nitrogen from the air and “fix” it in special modules in the roots, boosting fertility. And a grass such as barley or oats will provide bulk organic matter to feed the worms.

The great thing about green manure compared to composts and animal dung is that it can be purchased very cheaply by the kilogram. If you’re unsure of quantities, you can buy ready-mixed green manure kits from some seed suppliers, but it doesn’t really matter whether one plant is more dominant than the others. The main thing is to give green manure a go. It’s the best way I know to keep your food producing garden humming along for years, and even decades to come.