3.11 Soil pH

Soil pH

pH is a measure of the hydrogen ion concentration acidity or alkalinity of the soil. Measured on a logarithmic scale, a soil at pH5 is 10 times more acidic than a soil at pH6 and 100 times more acidic than a soil at pH7. A neutral soil has a pH of 7. A soil is acidic if the pH is less than 7 and alkaline if the pH is greater than 7. pH can affect the availability of plant nutrients and toxins and the activity of many essential micro-organisms. The pH of a soil may influence the choice of crops grown and the type of soil organisms that are present and function in the soil.

The majority of agricultural species prefer approximately neutral pH levels. Soils that are excessively acid or excessively alkaline cause reduced productivity.

Acidity

Soils are considered acidic if the pH measured in water is less than 7 and strongly acid if the pH is less than 5 (both measured in water).

Some soils, particularly those with low clay content in high rainfall areas, are naturally acidic. Induced acidification, caused by accelerated accumulation of acids under certain land management practices, can be a significant problem. Areas most affected are the higher rainfall districts such as the Mount Lofty Ranges, South East, Kangaroo Island and Lower Eyre Peninsula.

Acidification is caused by:

• Accumulation of organic matter, which produces organic acids as it decomposes. Increasing the organic matter of a soil provides many benefits and liming can be used to maintain good soil pH.

• Addition of nitrogen to the soil via ammonium fertilisers or fixation of atmospheric nitrogen by leguminous plants, both result in the production of nitrates, which can hydrolyse to nitric and nitrous acid.

• Removal of alkaline elements (cations) in crops. Figure 24 illustrates the equivalent weight of lime required to replace the alkaline elements removed by a range of crops.

• Therefore, intensive cropping will result in a fall in soil pH over time. In alkaline soils this could have positive effects on production, but if soil is lower than pH6, crop performance can be reduced.

The susceptibility of a given soil to acidification is determined by its ‘buffering capacity,’ or ability to resist pH change. Soils with high clay content, high cation exchange capacity and organic matter levels generally have higher buffering capacities.

Alkalinity

Alkalinity is usually an inherent characteristic of soils, although irrigation can increase the alkalinity of saline soils. Soils made alkaline by calcium carbonate alone rarely have pH values above 8.5 and are termed ‘calcareous’.

Alkaline soils with pH values higher than 8.5 usually have significant exchangeable sodium (sodic soils).

Consequences of Soil Acidity and Alkalinity

Many of the adverse consequences of excessively high or low soil pH values are related to plant nutrient availability.

As Figure 25 indicates, the availability of individual elements varies withy pH. For example, availability of aluminium increases in acid conditions.  This can lead to toxicity.

Plants vary in their ability to extract nutrients from the soil and, for this reason, some plants are able to perform better in acid soils, while others prefer a higher pH (see Figure 4).

There are also some specific consequences of acidity and alkalinity.

In acid soils:

The rhizobia bacteria responsible for nitrogen fixation have specific pH requirements and most are less active at low pH, except for strains with lupins and sub-clovers. Reduced vigour of many leguminous plants is a major consequence of soil acidification.

Aluminium and manganese become so readily available at low pH (approx <pH5 (H2O)) that toxicity may occur.  Stunted growth and leaf necrosis, respectively, can result.  Extractable aluminium levels of less than 2ppm affect very sensitive species, such as lucerne, while tolerant species, such as oats, will withstand values up to 13ppm.

In alkaline soils:

Root growth is inhibited in strongly alkaline soils, possibly due to the plant's inability to absorb nutrient elements, such as zinc, which cannot move through the soil and must be present at the growing root tip.

Soil pH is an important factor when considering what species to grow where.

Measuring soil pH

Soil pH should be monitored regularly in the surface and subsurface, as small changes can result in changes in nutrient availability.

Lime should be applied to cropping soils where acidity is <pH5.8 (H2O).

In the field, pH can be tested using either a pocket pH meter or a field test kit. Using a meter requires that the soil be in a suspension of water pH (H2O) or calcium chloride pH (Cau2), whilst the test kit uses a powder sprinkled on the soil sample. Indicator strips are also available, but a recent study by CSIRO Land & Water comparing field tests for measuring pH found these to be less accurate, especially for high pH soils.

If your field results suggest pH is near a critical level, samples should be tested in the laboratory.

When a pocket pH meter is used, it is crucial to calibrate this correctly using a standard buffer solution. This can be ordered from the supplier of the meter.

3.11 Soil pH

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