Understand Acidification

What is soil acidification?

Acidity is a major and often overlooked problem in Australia. There is approximately 8-10 times more land affected by acidity than salinity. To correct this problem, approximately 66,000,000 tonnes of lime would need to be spread nationwide.

Soil acidification is the build up of hydrogen cations, reducing the soil pH.
Soil acidification has been described as a sleeping giant. In many cases, symptoms of reduced growth in acid soils are just explained away as a poor season or lack of fertiliser.
In the case of grazing systems, acidification processes work unnoticed – until pasture and/or animal productivity declines.

The causes of soil acidification

Problems occur when pH levels fall below 5.5, as measured in a soil/water suspension (below 4.7 in a soil/calcium chloride suspension)
This occurs in old pasture lands with lighter textured (sandy) soils in high rainfall districts.
Decreased soil pH is indirectly related to the prolonged use of superphosphate (SSP) and other fertilisers, as they promote the growth of legume pastures.
Legume pastures increase soil nitrogen, organic matter and microbial activity.
This high level of organic matter is also responsible for soil acidification. (eg. Peat soils, which are composed almost entirely of organic matter, are higher acidic.

Visual symptoms of soil acidification

• Uneven pasture and crop growth
• Poor nodulation of legumes (e.g. medics, lucerne)
• Stunted root growth and a higher incidence of root disease
• Invasion of pastures by acid-tolerant weeds. (e.g. Capeweed, Patterson’s Curse, Sorrel, Geranium, etc.)
• Legumes such as medics and lucerne being more difficult to establish where they previously grew well.
• Formation of ‘organic mats’ on the soil surface; a direct result of reduced soil microbial activity.