Soil acidification

Most plants and micro-organisms have defined ranges of pH for optimal growth. The optimal range for plants is between 5.5 and 8 (pH in CaCl2) whereas most soil organisms function best between pH 6 and 7.

In acid soils:

  • aluminium (Al) becomes toxic to many plants
  • microbial activity decreases
  • nodulation and nitrogen fixation are decreased in legumes. This can be overcome by selecting more acid-tolerant strains of rhizobia, by lime-coating the inoculated seed and/or by applying lime at or before sowing.
  • plant nutrients can become deficient, particularly phosphorus, nitrogen and sulphur, as well as micro-nutrients

Soil acidifies (soil pH gradually decreases) as alkaline compounds are removed either by the sale of agricultural products or by leaching of nitrogen (as nitrate N) from the soil.

All agricultural systems acidify the soil. 

Acidification rates are higher in production systems that remove more plant or animal product, for example hay cutting compared to grazing, or prime lambs compared to wool sheep.

Pastures with annual grasses and a high proportion of clover pose the greatest risk of soil acidification through the leaching of N from the soil. Acidification rates are relatively low in well-managed perennial pastures.

Superphosphate does not have a direct acidifying effect, but leads to acidification by encouraging legumes to fix nitrogen that can be leached from the root zone.

Little can be done to prevent the slow acidification caused by product sale, but much can be done to prevent the leaching of nitrogen from the root zone.

Applying lime is the only recognised method of correcting soil acidification. However, results with liming pastures are often inconclusive, with no clear link between the application of lime and pasture composition.

Pasture production responses are variable. Livestock usually prefer grazing limed areas and graze them harder, but there is little evidence of significant weight gain advantages to stock on limed paddocks, independent of any pasture production response.

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