Growing with Acid Soils
All soils can be measured for acidity or alkalinity using a simple pH test kit. This test indicates on a scale of 0-9 the pH balance of the soil. A pH between 0-6.5 is acidic, 7 is neutral and 7.5-10 is alkaline or sweet. In our garden soils, nutrients necessary for healthy plant growth are most readily available around the pH 6.5 to 7 range. However, most Australian soils have been eroding and becoming acidic over millennia. Our native plants have adapted well to the acid conditions and will grow beautifully in a slightly acid pH.
If you want to make the most of what you’ve got and relish the thought of gardening with an acid soil, I have some great growing tips for you.
Changing the pH of your garden soil to meet the needs of your plants is not hard. Let’s look at how you can make those changes.
Testing and changing your soil pH
First, buy a soil pH testing kit, and follow the instructions in the pack to test your soil. If your soil is very acid, say pH 4.0, few plants will thrive in this acidity. A camellia for example grows best in pH5.5-6.5. So, you’ll need to add a liming (soil sweetening) agent to adjust the pH up to this level. Lime or dolomite are your choices and both are rock products. Lime is calcium carbonate, whereas Dolomite will give you added magnesium which is my application of choice most times. In a loamy soil, adding 1 cup of dolomite or lime per square metre will change the pH by 1 point from pH 4.0 to pH 5.0. You can see you’ll need 1,5 to 2 cups of lime or dolomite to bring the pH into the preferred range for camellias.
If your soil tests alkaline and you want a change it to acid, add agricultural sulphur. The amount depends on the soil type. Note that a heavy clay soil always needs more sulphur or lime to change the pH than a sandy soil.
Changing the soil in either direction does require patience. Several applications a few months apart may be required.
Making the most of your acid soil
In preparing your beds for acid loving plants, add lots of organic matter. It works like a sponge for water, holds nutrients in the soil and is a food source for beneficial microbes and worms.
Sources of organic matter include:
Used coffee grounds and tea leaves are slightly acid. Spread them under your plants
Animal manures, especially aged cow and pig manures
Seaweed, river weed and lake weed
Green manure crops like barley, millet and cow peas that you dig into the soil
Choose plants for your garden that love growing in acid soils.
Ornamental plants that love acid conditions (aim for pH 5.5-6.5)
Azaleas and Camellias
Pieris, Rhododenrdron and Daphne in cool climates
Acid loving fruit can be a little fussy, so get the pH just right
Blueberries pH 4-5
Citrus prefer pH 6-7
Strawberries pH 5-6
Many natives thrive in acid soils pH 5-6.5. Here are some of my favourites
Callistemons (Bottle Brushes)
Eucalypts and Corymbia
Leptospermum (Tea tree)
Pimelea and Ozothamnus (Rice flowers)
Keeping your soil healthy and slightly acidic
Maintaining an acid soil is not an issue unless you have imported alkaline soil from the landscape yard or if you live in an alkaline area. In this case, you’ll need to add sulphur regularly. Tips for maintaining acidity:
· pH test your soil twice a year to check the pH
· Add compost, manures and rotted leaf material to the soil or potting mix
· Mulch blueberries and strawberries with pine needles or Casuarina needles. (They’re naturally acid)
· Feed your acid loving plants with a fertiliser specially suited to acid lovers. Fertilisers like Searles Robust for Camellia and Azaleas; Searles Flourish Native Plant food (with low Phosphorous for sensitive plants), Yates Acitone, a tonic to maintain acidity or Yates Azalea and Camellia food. They are pH adjusted with a little sulphur added.
My article first appeared in an Express Gardening Publication in 2012.