We often refer to the soil pH in our garden advice, and although it sounds somewhat technical, the pH is actually vital to plant growth and needs to be tested before planting out a new bed. It’s also important to check the pH of any soil being imported into the garden.
pH generally ranges between 1 and 14 with 1 being very acidic and 14 being very alkaline. A pH of 7 is regarded as neutral. Most plants do well between 5.5 and 7.5 and plant roots will only grow in a range of pH levels between 4 and 9, but at either extreme only specially adapted plants survive.
It’s worth understanding that pH is a logarithmic scale, so that a pH of 6 is 10 times more acidic than one of 7, while a pH of 5 is 100 times more acidic than one of 7. Common vinegar has a pH of about 2.5. Knowing the pH of your soil is critical because this determines what nutrients your plants are able to take up via their roots from the soil.
The majority of nutrients are available between pH levels of 5.5 and 7.5. Acidic soils with a lower pH increase the availability of trace elements such as boron, copper, manganese and zinc while making it harder for plants to absorb calcium and molybdenum. If the pH gets as low as 5 then concentrations of these elements may be high enough to be toxic to the plant.
At the other end of the scale, as the pH gets higher and more alkaline then copper, iron, manganese and zinc become less available. The pH can also affect the micro-organisms and fungi in the soil.
How To Check pH
It’s easy to check the pH using a kit purchased from a nursery or hardware store. Instructions are on the packaging. Take samples from several different points in the garden bed or area you want to test. The pH can vary from the top of the soil, to the deep soil. I usually take samples from the top 10cm in six different positions, carefully mix them together and then test a teaspoon of this sample. If your reading is between 5.5 and 7.5 then no changes are needed to grow most plants. There are a few plants that do better in alkaline or acidic soils, so you may want to change the pH a little. See the table below left for examples.
Once you have determined your soil pH, you can either decide to grow only plants that do well at that pH or you can change the pH of your soil in specific areas to allow you to grow a larger range of crops. Regular additions of a variety of organic matter, including aged cow, horse and sheep manure, as well as worm castings, compost and mulch, will cause soils to gradually become neutral or slightly acidic.
Adjusting the pH
To raise the pH add lime (calcium carbonate) or dolomite. Dolomite also contains magnesium carbonate. Only mined calcium carbonate and dolomite are certified organic. In his book Gardening Down-Under, Kevin Handreck describes the following proportions needed: to increase the pH of the top 10cm, say from 5 to 6, apply 100g per square metre on sandy soils and 400g per square metre on clay soils. Wood ash, mushroom compost and biochar are all slightly alkaline so can be added to make small changes. To lower pH add sulphur from natural sources – the most common being gypsum – at 25g per square metre for sandy soils and 100g per square metre for clay soils.
PLANT pH LEVELS
Optimum pH range for some common fruit and vegetables
pH 4.0-6.0 : Blueberry
pH 5.0-6.5 : Apple, blackberry, chestnut, kiwifruit, mango, pineapple, pomegranate, olive, potato, strawberry, sweet potato
pH 5.0-7.5 : Banana, carrot, cauliflower, cucumber, pumpkin, raspberry, sweetcorn, tomato, turnip
pH 6.0-7.0 : Broccoli, cabbage, celery, lettuce, nectarine, peach, spinach
pH 6.0-7.5 : Apricot, artichoke, bean, beetroot, cherry, crabapple, garlic, grapevine, lemon, mulberry, onion, orange, pea, radish
pH 6.0-8.0 : Avocado, apricot, asparagus, pecan, plum, walnut
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