Build Soil With Green Manure

Have you ever stopped to consider the difference between dirt, and soil? Dirt is, for all intents and purposes, dead. It’s devoid of life, depleted of nutrients and unable to adequately support plant life. In contrast, soil is teeming with worms and micro-organisms, it’s full of organic matter, aerated, and has the right mix of nutrients to support the growth of healthy plants.

When you look at soil this way, it’s suddenly apparent how vital the stuff is to human existence. If we want to eat, we need healthy soil. But what also becomes apparent is how willing we humans are to abuse our second most important natural asset (beside water). We let soil dry out, expose it sunlight, work it hard without replenishment, and generally treat it like…dirt. Crazy stuff really, akin to licking a toilet bowl and wondering why we get crook.

This is where organic and biodynamic practitioners come to the fore. They don’t treat soil like dirt, they treat it like gold. They’re obsessed with it. They recognise that it’s the lifeblood of a productive garden or farm, and do everything humanly possible to ensure that the soil is maintained in optimum condition. They’ve also learnt that the “building block” of good soil is humus.

The term humus refers to any organic matter that has decomposed to a point of stability, and once made, humus can remain stable in the soil for thousands of years. It can hold 80-90% of its own weight in water, naturally corrects problems of alkalinity or acidity, and feeds a vast array of microbes. Plants grow best in soils with a high humus content, yet most Australian soils contain around 1%. The mixes you buy from a landscape suppliers may contain three to five percent organic matter, but in the most productive, organically managed soils, the humus content can approach 20%.

Adding compost and well-rotted manure are two ways of increasing the humus content of your soil. But there’s another technique that’s easier to perfect than compost making and just as useful as the addition of animal poo. It’s the use of green manures and cover crops.

Put simply, a green manure isn’t related to animal dung at all, but is just a quick-growing beneficial crop that is allowed to reach a certain point before being dug back into the soil, where it will decompose. Cover crops are very similar, though they are sometimes cut and laid on the surface of the soil as mulch. Either way, the key concept of either technique is the building of humus.

But wait, there’s more. Recent research by the CSIRO has found that particular green manure crops in the brassica family can actually act as bio-fumigants, suppressing pathogens within the soil and boosting the yields of some vegetables by as much as 40%. Mustards and radishes have proven the most successful biofumigants, primarily because they contain chemicals that most people would recognise as the “hot” flavour of related plants like rocket or horseradish.

The best part about all this is that green manures, including biofumigant types, are an absolute piece of cake to grow at home. They are perfect as a rotational break in the vegetable garden. Simply prepare the ground by raking to a relatively fine and level tilth, then broadcast green manure seed across the surface. Lightly rake again to just cover the seeds then water well. Keep the water coming at least daily until germination, then as much as needed to maintain rapid growth.

Once the plants reach about knee high, dig them back in. A good chop with a spade helps, and you may need to dig a couple of times to really get the plants into the soil. This is important for mustard, as research has shown that the beneficial chemicals are released as the leaves break down. Once decomposition is well advanced, after about a couple of weeks, you’re set to plant.

It really is that easy. The only thing you need to think about is the varieties of seed you want to sow and when you will sow them. Seed suppliers are usually pretty helpful in this regard. Most have both warm and cool season green manures, and often these can be purchased as a mix of two or three different varieties. As an example, I’ve just dug in a green manure containing ‘Nemcon’ Mustard, and plain old barley. The mustard will do its bio-fumigating thing, and the barley will contribute bulk, or bio-mass.

Give green manure a go at your place. Your soil will thank you for it, and the vegies that get produced should be better than ever. In fact the results may prove so successful, that you could soon be sowing barley and mustard in the flower beds. And why not!

First published in the Toowoomba Chronicle 9th May 2009. Photo by Justin Russell.

Tagged as: green manure, humus, organic, soil