Excessive soil acidity

Clearly, nature plays a major part in increasing the acidity of soils In many climates, particularly the wetter ones, rainwater enters the soil, moves downwards through it, dissolving and taking into solution various of the soil chemicals, before finally passing on down to the groundwater. This removal of chemicals from the soil, including precious nutrients, eventually leads to the soils becoming increasingly acid. The place of the nutrients that are lost from the soil is taken by increasing numbers of H⁺ ions and elements such as aluminium, manganese and iron which can be toxic in strongly acid conditions. However, in most soils up to the present time it has taken hundreds of years of leaching with rainfall for the buffering capacity of many soils to diminish and for increasing acidity to begin to set in train a number of undesirable processes.

Since humans have had increasing influence on their environment, however, things have begun to change more rapidly. In the last 200 years, following the Industrial Revolution, human influence on the environment has led to increasing soil acidity. Acid rain (see Section on Acid Rain) is one of the environmental problems that is accelerating the process of acidification. It is caused by a range of manmade pollutants such as those caused by coal burning, smelting, car exhausts, which release into the atmosphere nitrogen oxides and sulphur dioxide. These gases are converted into nitric and sulphuric acids, both strong acids, which acidify the rainfall reaching the soil. In some parts of the world, in the lee of major industrial areas, the pH of the rainfall can be as low as 4, i.e. extremely acid. This has the potential to acidify the soils more rapidly and beyond their buffering capacity much more quickly than by nature alone. Both chemical and organic nitrogen fertilisers used in agriculture can also lead to an increase in acidity. Often these fertilisers are ammonium based and as the ammonium is transformed into nitrates, H+ is released, causing an increase in acidity. Crop removal also leads to a loss of nutrients, particularly calcium, magnesium and potassium, so that there are fewer nutrients relative to the acidifying H⁺ ions.

A rather different example of human influence increasing soil acidity is the acid sulphate soils. In the last few decades in the search for additional land on which to grow crops, there has been reclamation of low lying wetlands and marshes. Some of this land, often unbeknown to the reclaimers, turns extremely acid when drained. The land contains the mineral pyrite (iron sulphide) which in wet, poorly aerated conditions remains reasonably stable. However, when the land is drained and oxygen is more available in the soil the iron sulphide oxidises to sulphuric acid (H₂SO₄), a strong acid. The pH falls to below 3 and the land is unable to grow the crops for which it was intended. The occurrence of these acid sulphate soils is now well known in the Tropics, for instance in Thailand, and also in Western Europe, the United States of America and Australia.