Australia is a very old continent, and after aeons of rainfall, much of the natural limestone has been leached from the soil. Native trees and plants have adjusted to the acid conditions and grow vigorously without needing any modification of the natural pH level. However, because many of our vegetables, fruits and ornamentals originated from Europe these plants prefer soil conditions similar to those of their homeland.
The term pH is the measure of soil acidity or alkalinity. The acidity-alkalinity scale ranges from 1 to 14. The low end of the scale indicates acid, the high side is alkaline. Neutral soils have a pH of 7. Soils are usually between pH 4.5 and pH 8.5.
Soils between 4 and 5 are very acid or sour, and relatively few plants will tolerate them. Similarly, soils of over 7.8 are too alkaline for most plants to thrive. In general, most common vegetables, field crops, fruits and flowers, do best on soils that have a pH of 6.5 to 7, in other words, soil that is slightly acid to neutral. Acid soil results in plants being unable to absorb nutrients from the soil.
You can buy kits for testing pH from most garden suppliers, and they come with full instructions on how to use them. To test your soil take several samples from different areas in the garden, mix them together, and then test.
To correct low pH (acid) soil, apply ground limestone or dolomite annually at a rate of 2 kg per 30 square metres of garden area. Increasing the organic matter in the soil will also help in correcting acid soil.
To correct alkaline (sweet) soil, gypsum can be used, but because gypsum is not strictly a natural product, it is avoided by some organic growers. A better way to acidify soil is to add peat moss.
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