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

About 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 you’re 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 it’s this that feeds your plants.Healthy soil is brimming with life. It’s far more than just soil particles with decomposing organic matter, moisture and minerals. It’s 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.

Soft, 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 don’t 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 it’s 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 month’s 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 you’ve added too much of a mineral it’s almost impossible to remove. Trace elements are only needed in minute quantities and there’s a fine line between too little and too much, either of which can be equally detrimental to plant health.

If you’re 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 Government’s 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.{mospagebreak}

Raised beds

Raising beds is a simple way to prevent diseases that can affect plants during prolonged wet weather. Low lying ground, clay soils and high water tables all risk waterlogging.Raised beds drain freely and assist air percolate into the soil. Conversely, poor drainage increases the risk of roots becoming weakened in saturated, airless soil and vulnerable to disease. Installing drainage is another solution. However well prepared soil is, it’s essential to limit the time it remains saturated. Even moisture-loving plants benefit from drainage.

Soil conditioners

If laboratory analysis seems too complex a commitment, it’s very important to at least understand you soil pH - whether it’s acidic or alkaline. An annual pH test allows you to track the progress of soil conditioning work.

Knowing how acidic or alkaline your soil is will determine which type and how often you should be adding soil conditioners to maintain good growing conditions for most plants. To find out whether your soil is acidic (sour) or alkaline (sweet) - and how extreme this is - use a basic soil pH test kit available from good garden centres. Theoretically the pH of soils varies from 1 (ultra acidic) to 14 (ultra basic). Both extremes are only encountered in a laboratory. Most Australian soils tend to be naturally acidic. A pH reading of between 6.5 (mildly acidic) and 7 (neutral), makes the widest range of minerals available and this suits the widest range of plants. In gardening terms this is our holy grail. Very acidic or alkaline soils tend to lock away certain minerals and mineral deficiencies become likely. Minerals may be present but become chemically unavailable in these conditions. Merely adding extra minerals doesn’t make them available and wastes money and effort.Altering the soil pH means adding soil conditioners. Iron chelates and sulphur powder will acidify alkaline soils. Garden lime or dolomite sweeten acidic soils.

Gypsum is another soil conditioner used on heavy clay soils to make them more workable - it improves their texture. It’s physical action encourages individual clay particles to flocculate - to gather together - and  form crumbs which help air and water to percolate through and make soil easier to dig. Instructions for the use of soil conditioners will be on the pack.

One group of clay soils, known as acid-sulphate soils, are associated with recently cleared or ancient mangroves. These have a special soil chemistry and adding gypsum to them makes them stickier and harder to work. The CSIRO, your state government or local council should be able to advise if such soils occur in your suburb. Acid sulphate soils exist countrywide, but are predominantly coastal. Unfortunately accurate mapping work is incomplete in some areas.

Thorough soil conditioning may take several years because in terms of soil chemistry changing the pH reading by one point is a jump by a factor of ten. A pH reading of 7 is neutral, and pH 6 is ten times more acidic than pH 7, while pH 5 is a hundred times more acidic than pH 7. The same applies going up the scale in terms of alkalinity. The effect of soil conditioners is temporary so conditioning should be routine. The end result - thoroughly conditioned soil - is a slow process and that’s good because plants and soil life respond best to gradual change.

Seaweed

A good quality seaweed fertiliser is a complete, gentle, balanced source of minerals. Once your soil is organic rich and the soil chemistry balanced, this provides the spark that gets your horticultural firework going. Amongst many other benefits, regular use stimulates soil microorganisms, helping to nourish plants.{mospagebreak}

To dig or not to dig?

Soil is easier to dig when moist and digging moist soil is far less likely to affect their structure. Sandy soils are easily cultivated but clay soils may only be workable for a short period after heavy rain. Digging clay when it’s too dry or too wet can ruin their workability for a season or more. But there’s varied advice on whether regular digging harms or helps soil. Most of our degraded croplands show that regular mechanical cultivation - just one aspect of contemporary poor soil management  - can easily result in compaction, a loss of soil structure and erosion.Permaculture and ‘no dig’ gardening are two sustainable approaches that take a long term view of soil improvement. Here conditioning and feeding soil tends to favour plant, animal and biological interactions to transform and transfer added organic matter and cycle minerals through soil rather than relying on regular digging to achieve the same end.

The most profoundly productive gardens ever were those of the 19th century kitchen gardens, usually found on large private estates. Those gardens were regularly dug by hand (or by farm animals) and considerable quantities of manures, composts and conditioners added. Far from losing their structure, those soils thrived. The depth and workability of their topsoil increased measurably. Digging is central to accelerating this result - the finest of healthy soils capable of intensive, all year round cropping.

Two special situations
   
Old neglected gardens or new subdivisions present two different gardening challenges.

New developments built on natural soils may lack the minerals necessary for growing most exotic plants. But they may be adequate for growing some native plants.

Such new gardens may have had new topsoil added after construction. This may be very different from the natural soil in your suburb. New subdivisions may also lack adequate topsoil because it’s sometimes removed to level a homesite, leaving just compacted subsoil for creating a new garden.

If you need to buy topsoil, especially in bulk, first have a sample tested by a soil laboratory to see that you’re getting something worthwhile. Some cheap topsoils are extracted from rivers and contain high amounts of silt. Apart from this being an unsustainable source, silt hardens into a water-repellant crust when dry and becomes an airless, gluey sludge when wet.
Topsoil sold by accredited suppliers is likely to have a lower environmental impact, be of superior quality and reduces the risk of introducing weeds and serious soil diseases into your garden. Before adding topsoil, ensure the existing soil is deeply cultivated. The old and new topsoil should then be deeply cultivated to blend them thoroughly.

Old gardens that have been regularly fed may be good for general, exotic flowers and vegetables, but could be too phosphorous-rich to grow plants like Grevillea, Banksia, Protea and Leucadendron. These members of the Proteaceae family are easily poisoned by added phosphorous because they have evolved roots that are super-efficient at gathering phosphorous from our naturally phosphorous-deficient soils. Sometimes nutrient-rich runoff from heavily fed gardens carries these minerals downhill. Occasionally this can make downhill soils so phosphorous rich that they poison native and exotic displays. So if you’re gardening for the long term, planning and checking can be critical.Resources

National Measurement Institute: Chemical and biological measurement
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NSW
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