Healthy soils are a complex web of life, teeming with earthworms, fungi and bacteria. They smell good and are moist and crumbly.
Roots are able to penetrate deep into the soil. Plants growing in healthy soils have fewer pest and disease problems.
If your soil doesn't match this picture of health but instead dries out to a cement-like texture, is devoid of life, plants look sickly and are plagued with pest problems, then your soil is in desperate need of organic matter.
Organic matter is literally the life of the soil. As organic matter decomposes, through the activity of soil organisms, nutrients are made available to growing plants.
Organic matter improves soil structure, allowing the free passage of air and water, both equally necessary to the growth of plants. It acts like a sponge, holding onto water and nutrients. Soils high in organic matter, like we would find in an undisturbed moist, mountain forest, can actually feel spongy to walk on.
There are many different ways of adding organic matter to our soils, from the recycling of food scraps and weeds, commonly known as composting, to the growing of crops specifically for organic matter production, referred to as green manuring.
To protect our soils from erosion and summer heat we need to mulch them, which also provides organic matter as it decomposes.
All organic materials will eventually rot, regardless of what you do. Composting is the art of producing a rich, sweet smelling decomposed product rather than a wet, smelly, fly attracting, rotting mess.
There are two main ways to go, using a bin or tumbler or building a compost heap. The bin is very useful for people with small backyards, and has the advantage of being rat-proof.
Management is necessary to get the right mix of carbon to nitrogen. Kitchen scraps alone are generally too wet and too high in nitrogen. As each bucket is added a drier material such as soil, shredded newspaper or sawdust should be mixed in. Worm composting bins work particularly well and can even be used in high-rise buildings.
If the space is available to have a compost heap, it should be covered to keep out rain, and placed to take advantage of any nutrient rich run-off, such as close to a citrus tree. A minimum size of about a cubic metre is needed for a heap to reach a sufficient temperature to kill weed seeds.
Never try to kill pests such as fruitfly in your compost heap as they are able to move away from the heat and complete their lifecycle.
Avoid adding lime to compost heaps, as this will increase the loss of nitrogen from the heap. If you can smell ammonia then nitrogen is escaping from the heap and more carbon should be added, such as straw, small amounts of sawdust or wet, shredded newspaper.
Incorrectly stored animal manures can lose 50% of their nitrogen during storage. If the heap is too dry, white threadlike strands will appear and fungi will become the main decomposers rather than bacteria.
The compost should be as moist as a squeezed out sponge. Turning the heap speeds up the process and gives a more even result. Simply digging the food scraps into different areas of the garden, such as under fruit trees, is useful and will improve the soil over time.
Chemical fertilisers supply nutrients but no organic matter, so these nutrients are easily loss to the soil to become pollution in our waterways.
The major advantage green manuring has over the use of inorganic fertilisers is the increased organic matter levels. Green manures also increase the soil life by providing a readily available food source to decomposing organisms.
If we use legumes in our green manures we provide a renewable source of nitrogen, the major element needed for plant growth. Inorganic nitrogen fertilisers are produced from fossil fuels by polluting processes, which contribute to the greenhouse effect.
Growing a green manure crop is as easy as throwing out a handful of seed onto freshly raked ground.
Digging in is not necessary, as by slashing and leaving the green manure crop on the surface we also create the mulch for the following crop. Plants should be slashed while still green and lush, usually just as flowers form.
A combination of a legume and a grass works well, the legume providing nitrogen and the grass, such as oats or ryecorn, the bulk of the organic matter.
Green manures can also be used to smother persistent weeds, such as couch grass. Good choices for weed suppression include lablab, cowpea, lucerne, sorghum and buckwheat.
Green manures can be used to interrupt pest and disease cycles in much the same way as crop rotation.
Particular green manures can be used to control root knot nematodes and root rot fungal pathogens, reducing the need to use toxic chemicals for soil fumigation.
When plants such as BQ Mulch are dug into the soil, exudates released from the decomposing plants suppress these diseases.
Green manures that can be sown in winter in SE Queensland include fenugreek, lupins, oats, BQ Mulch, rapeseed, field peas and woolly pod vetch.
Different mulches should be selected for different areas of the garden. Bark mulches are useful under native shrubs, but may retard the growth of sensitive, exotic plants. Straw or grass hay mulches are used in the vegetable garden.
Growing our own mulch reduces costs and the risk of weeds being imported into the garden. Shrubby, nitrogen-fixing plants such as pigeon pea and lucerne can be cut regularly and laid on garden beds and under fruit trees.
Lush, fast growing plants such as arrowroot Canna edulis, provide abundant supplies of mulch in the subtropics.
THE IMPORTANCE OF pH
On very poor soils it may be difficult to grow anything, including a successful green manure crop. In this case the pH should be tested for excessive acidity or alkalinity, as this will interfere with uptake of nutrients by plants. If the soil is too acid, then agricultural lime should be applied.
The amount needed will vary depending on the pH and the soil type. As a rough guide apply 120 g/m2 to a clay soil and 30 g/m2 to a sandy soil. Test again in a few months and apply more if necessary.
The addition of organic matter will help to make an alkaline soil more acid. Kits to test pH are available, easy to use and all gardeners should have one.
For clay soils that stay wet and sticky, the addition of gypsum will improve the structure. It should be applied at a rate of 500 g / m2.
Many Australian soils are deficient in a range of macro and micronutrients, all equally necessary for plant growth. The addition of composted animal manures or products such as Dynamic Lifter, will increase the supply of macronutrients such as nitrogen, potassium and phosphate.
Trace or micronutrients are only needed in small amounts but are just as important for healthy plants. Trace elements include calcium, magnesium, sulphur, iron, manganese, copper, zinc, boron and molybdenum. Initially in deficient soils it may be necessary to apply a trace element mix, but ongoing nutrients can be supplied by fish emulsion or a seaweed fertiliser such as Natrakelp.
The aim in applying fertilisers is to create sufficient fertility for initial growth. During periods of continued heavy rain, the leaching of some nutrients is inevitable. To keep plants healthy and growing vigorously at these times, the application of seaweed fertiliser as a foliar feed is recommended.
To maintain a healthy soil we need to constantly cycle nutrients within the garden, by the organic practices of composting, green manuring and mulching.
Article by Frances Michaels
Green Harvest Organic Gardening Supplies