Soil Acidity

The level of soil acidity has a fundamental affect on plant growth and nutrient availability. Soil pH is a measure of the alkalinity of the soil. It measures how many active hydrogen ions there are in the sample. The pH scale goes from 0 - 14, 0 being the most acid to 14 being the most alkaline. The pH scale is logarithmic. This means that a soil with pH of 5 is 10 times more acidic than pH of 6, and a pH of 6 is 100 times more acid than a pH of 7. A pH of 7 is neutral. Most plants grow well in a pH range from 5 - 7. In this range most nutrients are available for plants to use. Large areas of our agricultural soils are acid and continuing to decline. Some soils (e.g. acid sulphate soils) are naturally acid others have become acid due to faming practices. Soils become more acid as a result of successful farming. The addition of fertilisers containing ammonium and nitrate, organic matter, the removal of produce containing alkalis, and the use of legumes to build up organic nitrogen, are increasing the development of acid soils. Acid soils reduce production by causing deficiencies of the major elements toxicities such as aluminium. A low pH inhibits the symbiotic relationship between rhizobia and legumes in which they form nodules that fix nitrogen from the atmosphere (in a form the plants can use). Lime in the form of calcium carbonate CaCo2) is used to reduce soil acidity. This allows use of problem soils but it does not cure the problem. Alkaline soils are less common in Western Australia. Soils with a pH of over 7 have problems where many of the elements such as copper, manganese, iron and zinc needed for plant growth are not available, or boron toxicity may be problem. Alkaline soils are often associated with salinity. Alkaline soils can become sodic where there is an excess of sodium ions that can adversely affect soil structure. The application of gypsum is recommended on these types of soils. How can you tell if your soil is acid?

Failure of legumes or yellowing or death of older leaves, in a good season.

Deficiency symptoms of sulphur, phosphorous, molybdenum and calcium and magnesium.

Root growth poor, stubby with few fine roots.

There are three pre requisites for a soil to have a risk of becoming acid or more acid;

The soil may be acid to start with;

The buffering capacity, which is the soils ability to resist changes in pH, may be poor: or

The agricultural system on that particular soil produces a lot of acid.

Rate of lime to raise the soil pH one unit for various soil textures.
A good soil test will give you exact rates

Soil texture Tonnes of lime per/ha

Sands/Loamy Sands 1-2.5

Loams, Sandy Clay Loams 2.5-4

Clay loams 3.5-5

References:
Farm Monitoring Handbook, UWA Press Natalie Hunt and Bob Gilkes
Farmnote: The Importance of soil pH, Dept. Agriculture No 8/2000

Back to the Soils Page

Back to the Home Page

Back to the top>