What causes soil acidification?

  • Acidification of the soil is a slow natural process and part of normal weathering. 
  • Many farming activities cause an increase in the rate of acidification of the soil. 
  • Changes in soil pH under agricultural use are measured in tens of years rather than thousands of years as in the natural environment.

There are four ways that agriculture contributes to the accelerated acidification of the soil and these are:.

  1. Removal of produce
  2. Use of fertilisers containing ammonium or urea 
  3. Movement of nitrate nitrogen sourced from nitrogen fixation or from ammonium fertilisers
  4. Build-up of soil organic matter.

Removal of produce

Using nitrogen fertilisers

Build up of organic matter In some older texts the removal of “base” cations, that is calcium, magnesium, potassium and sodium, is given as a cause of soil acidity. This is misleading, and in a similar vein acidity cannot be corrected by applying calcium.

Rate of acidification

It is possible to estimate the rate of acidification for a given paddock enabling budgeting for future liming programs, but it is best to use lab test results of your soil to determine liming requirements.
Rainfall is the climatic feature that has the greatest effect on the rate of soil acidification as it influences:
*  plant productivity (which includes the amount of nitrogen fixed by legumes)
* movement of nitrate nitrogen
These factors interact in their influence on the rate of acidification.

In the past there has been a misconception that perennial pastures can be used to prevent soil acidification. This is not the case for reasons previously explained in the section What is an acidic soil? and Removal of produce above.