Causes of soil acidity

Page last updated: Monday, 17 September 2018 - 11:35am

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Export of produce

In WA, the two main causes of soil acidification are inefficient use of nitrogen and export of food and fibre from the farm.

Plant roots take up nutrients as either cations, which are positively charged (such as ammonium, potassium, calcium or magnesium) or as anions, which are negatively charged (such as, nitrate, phosphate or sulfate). When a cation is absorbed by a plant, a positively charged hydrogen ion is be excreted into the soil to maintain electrical balance. When an anion is absorbed, a negatively charged hydroxide ion is excreted into the soil.

Plants absorb more cations than anions, which means that most plant material is slightly alkaline. In a natural system, when plants die they are decomposed and returned to the soil, balancing the acidity caused by the hydrogen ions.

In agriculture, if plant material is removed by grazing or harvest or relocated by the concentration of dung into stock camps, rather than being returned to the soil, there is a net export of alkalinity and residual hydrogen ions remain in the soil contributing to soil acidity (Figure 2). Over time, as this process is repeated, the soil becomes acidic. A translocation of alkalinity can occur in windrows with the soil off the windrow becoming more acidic.

Process showing how hydrogen ions added in the carbon cycle contribute to soil acidification.
Figure 2 Hydrogen ions added in the carbon cycle contribute to soil acidification (not the removal of the base cations)

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Gaus Azam
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Chris Gazey