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Soil acidity, lime requirement and aluminium toxicity

Increasing soil acidity (lower pH) reduces crop and pasture yields, and is recognised as a major problem facing agriculture. Yields are reduced by two factors: below-optimum nutrient availability (some nutrients become chemically locked up at low pH) and the presence of soluble aluminium and manganese, which are toxic to plant roots. Soils become acidic through the removal of nutrient cations and their replacement with hydrogen ions; through a build-up of organic matter, which is acidic (specifically, organic acids); and through the application of acidic fertilisers, such as ammonium sulphate.

Therefore there are two main reasons why we might want to raise the pH of a soil: to improve nutrient availability and to overcome Al toxicity. Al is the most common metal in Earth’s crust and is found in most soils, principally as a component of clay minerals. We can’t remove it, but we can control it.

Dealing with soil acidity

Historically there are three main approaches to dealing with soil acidity:

1. Grow acid-tolerant plants. For example, choose an acid-tolerant lucerne or move away from lucerne and grow an acid-tolerant clover and ryegrass pasture.
2. Add only just enough lime to reduce the soluble Al to an acceptable level for the crop.
3. Lime to obtain a certain pH. For example, we can measure fairly accurately how much lime is needed to bring pH up from say 4.5 to 6.5.

To support these decisions, a wide range of tests are available. Every lab seems to offer a different type of test, and often these vary by country, state and even region!

The SESL approach

SESL has researched this area extensively and has devised a package of standard tests that can answer all these questions cost-effectively:

1. pH in water and pH in CaCl₂. These two tests give us an estimate of the intensity of soluble acidity and “exchangeable acidity”. Soil pH (acidity level) is simply a measure of the intensity of acidity present. It does not indicate how much lime might be needed to overcome the acidity. To do this we need to perform a separate lime requirement test.

2. Exchangeable aluminium. If pH is less than 5.2, we measure exchangeable Al (Al is not soluble above pH 5.2, so we know there won’t be any). We can then calculate exactly how much lime is needed to neutralise this Al and thus eliminate Al toxicity. This is appropriate for extensive agriculture enterprises such as grain, oilseed, pasture, fodder and other low-input types of farming. We can do this test for no extra charge as it is done automatically.

3. Lime requirement test. Rather than determine a separate “buffer pH” (which would cost significantly more for a whole new extract), we can measure “total exchangeable acidity” in the existing Al extract for a small extra cost. By calculation we can then predict how much lime to add to obtain a given pH. Obviously this will be more than that required just to get rid of the Al.

Terminology notes

Buffer pH: The pH obtained when a soil is shaken in a chemical solution (the buffer, which tends resist pH change). The difference between this and the normal soil pH in water can be used to calculate a weight of lime to apply.

Total exchangeable acidity: Measured by a method called titration, whereby the amount of acidity present is measured by adding known amounts of a standard alkali. (This is sometimes called exchangeable hydrogen percentage, which is incorrect, since the acidity is not present in soils as hydrogen, but as Al and other ionic species.)

Further information

http://www.dpi.nsw.gov.au/agriculture/resources/soils/acidity/publications/acidity-liming

http://www.agric.wa.gov.au/content/LWE/LAND/ACID/SOILACIDITY_INDEX.HTM

http://www.regional.org.au/au/roc/1986/roc198601.htm

http://en.wikipedia.org/wiki/Soil_acidity

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