Soil is the most important gardening basic there is, so it pays to get it right. Here are Don’s tips on working out what kind of soil you have, how to test the pH of your soil and how to keep your soil healthy.
Soil texture test
There’s a simple test you can do to determine the texture, or mineral fraction present in your soil. First, dig up some soil. Now wet it. Grab a handful and scrunch it up in your palm. If you end up with a firm sausage shape that you can bend into a circle, that’s clay soil. If the soil barely forms any shape and just crumbles, that’s sandy soil. And if the soil holds together but can’t be bent without cracking, you have a loam.
Improving your soil
Many people think that the worst soil you can have in the garden is heavy clay. However, Don explained that any soil can be a good garden soil. Indeed, most natural soils in Australia are suitable for growing a garden. In most instances it is better to improve your existing soil than to add bought soil to your garden.
Clay soil is made up of millions of tiny particles, giving it a very fine texture. This has advantages and disadvantages. On the plus side, clay soils are rich in nutrients (the particles provide multiple surfaces where nutrients can ‘stick’) and they hold water well. However, they are also prone to compaction, waterlogging, and can be sticky when wet and tough when dry. These problems are made worse if a clay soil is cultivated when wet.
Improving clay soil
- Apply gypsum to the soil. This is a natural substance, which binds with the very tiny clay soil particles and turns them into larger, better-draining particles. Gypsum is available as fine granules to sprinkle on and dig into the soil, or as an easy-to-apply, spray-on liquid treatment. Don’t apply gypsum when clay soils are wet; wait till they are ‘damp-dry’. Tough clay soils may need several gypsum treatments over the years, not just one.
- Dig in plenty – and we mean lots – of organic material (compost, mulch, manure), and keep on doing this for the next decade or two.
- Add a 75mm layer of mulch: it breaks down and improves soil structure.
- Choose plants that like clay soils. Roses love clay. Lots of native plants don’t.
- Create a raised garden bed (with retaining walls of treated pine sleepers or bricks) then add some new soil mix and plant into that.
Sandy soils don’t retain moisture and they are low in nutrients. On the plus side, they are well aerated and they’re loose and easy to cultivate. They are often classed as ‘poor’ or ‘coarse’ soils.
Improving sandy soil
- Grow plants that love free-draining, sandy soil. Australian natives are the prime candidates, so too are South African natives such as proteas. This is your best short-term option.
- Add lots of organic matter (compost, mulch, manure) to the soil at least twice a year, dig it in well, and keep doing this for the next 10 years or longer. Your sandy soil will slowly turn into better soil, but it takes time.
- Mulch well (to 75mm depth), to help it retain moisture by preventing evaporation. As the mulch breaks down, it improves soil structure. Keep topping up the mulch regularly.
- Add well-rotted manure to keep up its nutrient levels. Also apply slow-release fertiliser granules, as they will supply a steady stream of nutrients to plant roots.
The term ‘loam’ covers all the soils between sandy soils and clay soils. While a loam is said to be the perfect soil for growing plants, in many areas of Australia it is unlikely that you would find a ‘true loam’, halfway between a sandy soil and a clay soil. Some loams will have propertiessimilar to sandy soil, while others will be closer to clay soil. Whatever type of loam you have, you can improve it and keep it healthy by adding organic matter, such as compost, mulch and manures.
All about pH
Some plants like to grow in acid soil, others prefer alkaline soil, and many others aren’t so fussy. For instance, if you put lavenders into a highly acid soil they don’t do all that well; Japanese maples also like an alkaline soil. When soil pH is right for a plant, it will absorb nutrients from the soil because all the conditions for the various chemical reactions are OK. When soil pH is wrong for a plant, it simply won’t absorb nutrients, even if its roots are surrounded by nutrients.
How do I test soil pH?
If you don’t want to buy a soil test kit and test the soil yourself, your local nursery can test a soil sample for you (but talk to them first about it, to find out what they need). It’s much better (and very easy) to test your soil yourself. This is useful, as soil pH can vary from one part of your property to the next (say, front garden and rear garden). There are several different soil pH test kits available.
How do I adjust soil pH?
Your local nursery can give you expert advice on this – what to add and how much, etc. Adding lime and dolomite to soil raises its pH. Adding iron sulfate to soil can lower its pH. An alternative is simply to leave the soil pH as it is, and only use plants that can cope with your soil’s pH level. Here are some examples of plants that tolerate acid soil, and plants that tolerate alkaline soil:
Plants that tolerate acid soil
- Abies spp.
- Ajuga spp
- Arbutus unedo (strawberry tree)
- Camellia spp.
- Clethra arborea (lily-of-the-valley tree)
- Cryptomeria spp
- Daphne genkwa
- Enkianthus spp
- Erica spp
- Gardenia spp
- Gordonia axillaris
- Kalmia latifolia (mountain laurel)
- Michelia doltsopa
- Michelia figo
- Picea spp
- Pieris japonica (lily-of-the-valley shrub)
- Rhododendron spp (azaleas and camellias)
- Stewartia spp
Plants that tolerate alkaline soil
- Acer palmatum (Japanese maple)
- Acanthus mollis
- Agonis flexuosa
- Buddleia davidii (cvs)
- Correa pulchella
- Dianthus spp.
- Eschscholzia californica (Californian poppy)
- Lavendula spp (lavender)
- Magnolia spp
- Robinia pseudoacacia ‘Frisia’
- Rosemarinus officinalis (rosemary)
- Freesia refracta
- Nerium oleander
- Thymus spp (thyme)
- Multicrop GroundBreaker is available from nurseries and garden centres, cost $15.65 for 2 litres. Gypsum is available from nurseries and garden centres, cost $11.65 for 25 kilos.
‘Gardening Down-Under – A Guide to Healthier Soils and Plants’ by Kevin Handreck (Landlinks Press, $39.95, ISBN 0643066772). Available from major book stores or mail-order from CSIRO Publishing, PO Box 1139, Collingwood, Vic 3066, on the web at www.publish.csiro.au, via email at email@example.com or phone (03) 9662 7666 in Melbourne, or 1800 645 051.
An article titled ‘All About Soil’ is featured in the April edition of the Burke’s Backyard Magazine. The magazine is available at newsagents and supermarkets for $5.50.