Soil pH

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Soil pH buffering

Sandy soils acidify quicker because of the lower buffering capacity but the pH can be recovered faster with the application of less lime compared to clay soils.

The buffering capacity of a soil indicates the capacity of the soil to resist pH change. Hydrogen ions in soil are present both in the soil solution and adsorbed onto the soil surfaces. pH measures the concentration of hydrogen ions in the soil solution. Soils differ in the number of surface sites able to accommodate hydrogen ions. Soils with large numbers of sites able to hold hydrogen ions are able to resist change in the concentration of hydrogen ions in the soil solution and therefore have a high buffering capacity.

Soils with a high proportion of clay or organic matter have a larger number of surface sites able to hold hydrogen ions and are able to resist a decrease in pH. However, once acidic, highly buffered soils are able to resist an increase in pH. When hydrogen ions in the soil solution are neutralised by lime, hydrogen ions from the soil surfaces are release into the soil solution to maintain equilibrium and resist increase in pH. Better buffered soils are slower to acidify but require more lime to lift pH when they do acidify.

Clays are generally better buffered than loams, which in turn are better buffered than sands (Table 1). Poorly buffered sandy soil types comprise more than 40% of the agricultural land in the south-west of WA Department of Primary Industries and Regional Development (DPIRD) soil-landscape map unit database, 2013.

Table 1 Rule of thumb indication of the pH change expected from the application of 1t/ha of pure calcium carbonate on different soil types with a starting pH (Ca) of 4.5
Soil type pH change
Sand 0.5 - 0.7
Loam 0.3 - 0.5
Clay 0.2 - 0.3

The naturally acidic peaty sands of the south coast have a high buffering capacity and would require more lime to increase pH than other wheatbelt soils.

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