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(This article is written by Jerry Coleby-Williams and first appeared in the ABC's 'The Organic Gardener' magazine, winter issue, 2006)

ImageAbout half of our gardening problems can be prevented by having healthy soil, the foundation of every successful garden. Organic gardeners know that all soil types can be improved by regularly feeding the soil, not the plants. But how do you know if this is eliminating any mineral deficiencies? Or - in the unlikely event that your garden has no deficiencies - if these minerals are actually available for plant use?

Fortunately creating healthy soil is really simple and saves money in the long term. Imagine youíre making a firework. You need 1. the right materials, 2. to prepare them so that the required chemistry works and lastly 3. a flame to set it off. In gardening terms you can achieve this with 1. organic matter and minerals, 2. soil conditioners and 3. seaweed. The firework that organic gardening aims to set off is, of course, healthy soil and itís this that feeds your plants.Healthy soil is brimming with life. Itís far more than just soil particles with decomposing organic matter, moisture and minerals. Itís a living matrix containing vast quantities of living organisms. A single teaspoonful can contain up to 20,000 different species of microorganism and up to 2 kilometres of microscopic fungal threads. The wonderful thing is that most of these are beneficial and they actively manage soilborne diseases.

Organic matter and minerals

Whatever you grow, adding organic mulches provides the key to successful gardening. Mulching suppresses weeds, reduces erosion and keeps soil cooler in summer and warmer in winter. Worms prefer mulched soil and their burrowing helps aerate soil and assists water to penetrate it. All these factors promote strong root growth.

ImageSoft, nitrogen rich mulches like sugarcane, straw, lawn clippings and lucerne encourage beneficial soil bacteria. Hard, woody, carbon-rich mulches like tea tree, bark or tree prunings encourage beneficial fungi. Both bacteria and fungi are essential, helping to release minerals that feed plants. By varying the type of mulch used soil microorganism diversity improves. And mulching with compost made from both woody and soft materials encourages really healthy soil which is easier to work and is harder to erode.Plastic sheeting and synthetic weed mats suppress weeds, but they donít encourage strong roots or keep soil healthy. Inorganic mulches, like stones, gravels or pebbles, may be fashionable, look decorative and last longer than organic mulches, but they do nothing to create healthy soil.The warmer and more humid your local climate is, the faster organic matter decomposes, so you must match the frequency itís applied with the rate that it decomposes. You can dig organic matter in but spreading a surface layer of organic mulch on garden beds is the easiest way to add organic matter where beds are filled with the roots of established plants.Drought aside, many regions experience rain as brief bursts, often causing erosion on conventionally managed soil. In eastern and northern coastal Australia sometimes a monthís average rain can fall in one hour. Healthy, organic rich soil helps soak up rain like a sponge, slowly releasing it for plant use. It helps bridge the gap between showers and reduces rain and soil shed as runoff and polluted storm water.

Adding artificial fertilisers excessively accelerates microbial activity, rapidly depleting the organic content of soil, and their regular use reduces the condition of soil and plant health.

As a rule of thumb most unmodified soils are low in minerals, such as nitrogen, potassium and phosphorous and trace elements (aka micronutrients) such as boron, magnesium, selenium and zinc. Once youíve added too much of a mineral itís almost impossible to remove. Trace elements are only needed in minute quantities and thereís a fine line between too little and too much, either of which can be equally detrimental to plant health.

If youíre faced with starting a new garden or have the opportunity to redesign one, invest in a full laboratory analysis. The upfront cost, between $300 and $500, seems steep. For this, expect to get information on your exact soil type, organic content, texture and condition, minerals present (deficiencies or excesses) and pH (a measure of the acidity or alkalinity of soil). Ask for testing to advise about contaminants, like heavy metals, too.

For example, unacceptably high levels of lead, arsenic or cadmium  may preclude growing certain crops, including turnip, silverbeet, swede, beetroot, radish and kohl rabi. These crops bioaccumulate (a natural process of absorbing and concentrating a compound) these heavy metals. By growing a test crop of one of these plants on suspect land and having them analysed by the Australian Governmentís National Measurement Institute you will know which, if any, heavy metals are present and if they are within nationally accepted levels.

In the long term laboratory testing eliminates guesswork, allowing you to make considered decisions about what needs addressing and will definitely save money and disappointment.

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