The pH of the soil is a measure of acidity or alkalinity. It basically indicates how ‘sour or sweet’ the soil is. The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate an acid soil, and above 7, alkaline. Because the pH scale is logarithmic, a pH change by 1 unit means it is 10 times more acidic or alkaline.
Typically, Western Australian soils have a pH range between 4 and 8.5. In the metropolitan area, soils are more alkaline near the limestone-based coastal sands. Soils further inland, and most agricultural areas, are naturally acidic. In agricultural regions soils which were not very acid when cleared have become increasingly so through the continuous application of fertiliser.
In home gardens, where mineral fertilisers are frequently used, the soil pH may also be acidic.
Significance of soil pH nutrition
Soil pH determines the nutrient availability to plants. Some nutrients become ‘tied up’ in the soil at certain pH levels. For example, acid soils can lead to deficiencies of phosphorus, calcium, magnesium and molybdenum, as well as toxic levels of manganese and aluminium.
Alkaline soils may lead to deficiencies in iron, manganese, boron, copper and zinc.
While most plants prefer neutral soil, some are suited to other pH levels. Examples of different preferences are:
| Soil pH | Ornamentals | Fruit | Vegetables |
|---|---|---|---|
| Acidic 4.5 to 6.0 | Azalea Gardenia Hydrangea | Blueberries Cranberries | Potato |
| Alkaline 7.0 to 8.0 | Sweet Pea Ivy | Melon | Beetroot Choisya Spinach |
A common pH-related condition in the metropolitan area is ‘lime-induced chlorosis’ which is an iron deficiency caused by high pH levels. It manifests as yellow-white leaves on plants growing in limestone-based coastal sands.
This condition is rectified by applying iron sulphate, which will also drop the soil pH or an iron compost. Check if yellow leaves are due to iron deficiency by spraying the plant with an iron chelate. If the leaves green within a week apply an iron compost.
Iron compost
Moisten compost, coco peat or animal manure until just wet. To each 10 litres mix in one cup of iron sulphate. Dig holes 20cm deep around the root zone of the plant and compact the iron compost into the holes. Use three holes for a rose bush and up to eight holes for a big shrub. The roots will grow into the organic matter and the treatment lasts for several years.
Pests, diseases and soil pH
Strong and healthy plants which get sufficient water, nutrition and sunlight, will build up a natural resistance against pests and diseases. It therefore pays to keep the soil pH around neutral to avoid certain nutrient deficiencies, which will weaken the plants and make them more vulnerable to pest and disease attacks.
There are a few diseases which are affected by soil pH. Clubroot, a disease of crucifers, can be controlled by increasing the soil pH, while common scab, a disease of potatoes, is more likely to cause problems in alkaline soils.
When and how to test soil pH
Test your pH in autumn when the soil is relatively dry and is free of excessive organic material. This will give you some time to do adjustments, if needed.
When sampling, follow these guidelines:
- Do not mix different soil types, and sample into clean containers.
- Do not sample after heavy rain or after watering.
- If you have applied fertiliser, wait three to four weeks for the soil to adjust.
- Take the soil from the top 20cm with a trowel, spade or auger and avoid weeds and plant roots in the sample.
- Select at least five random spots in the area to be tested, and mix the soil to get an average reading. Then take about a cup-full as a sub sample.
Inexpensive soil pH test kits are available from many garden centres and give home gardeners an approximate reading.
If you want more accurate results, take a sample to be tested to a soil laboratory like the Chemistry Centre WA or check for other services in the Yellow Pages directory.
Laboratories may report a pH reading in water (H2O) or in 1:5 calcium chloride (CaCl2 ). Readings in calcium chloride are considered a better indicator of acidity.
If flowering, hydrangeas (Hydrangea macrophylla) are good indicators of soil pH. A soil pH of 6 or below will produce blue flowers while a soil pH of 6.8 or higher will produce pink flowers.
Changing soil pH
Soil pH is generally corrected using iron sulphate (decrease) or lime (increase). There are some organic soil improvers on the market which are claimed to stabilise the pH on neutral. Make sure the soil additive is well worked and watered into the soil.
Acid soils
To increase the pH by one point, apply dolomite lime at 100g/m2 in sandy soils, and up to 250g/m2 in clay. The effect is noticeable after two to three months.
Alkaline soils
To decrease the pH, iron sulphate is available from nurseries and hardware stores. Follow label instructions for the correct rate. Compared to lime, the effect is immediate.