Acidic soils

Certain soils are naturally acidic, and others tend, through use, to become acidic. Soil acidity is common where rainfall exceeds evapo-transpiration. Soils in humid regions gradually become more acidic, under natural or cultivated conditions. This occurs as the hydrogen ions contributed by slightly acidic rain and from the decomposition of soil organic matter gradually replace soil calcium and other cations, which are then leached away. Virgin soils in humid temperate regions may have a pH as low as 4.8 (pH 7 is neutral); such soils have a very low natural fertility.

Crop plants vary widely in their tolerances of soil acidity. For example oyoya and many clovers require essentially neutral soils, having a minimum pH of 6.5. Soil acidity adversely affects the availability of soil nutrients for plants, it also increases the level of toxic ions in the soil. In particular, acidity leads to the presence of elevated levels of soluble aluminium, which is a common cause of stunted root systems. Very small amounts of aluminium, less than one part in a million in water, can sharply reduce root growth and uptake of water and nutrients. The fish are dying from a toxic aluminium build-up in the lake and river waters in these areas.

The Netherlands is the third largest agricultural exporter in the world, amounting to 10 percent of world agricultural exports in 1992. 40 percent of this export consists of livestock products. At any given moment there are 14 million pigs, 100 million chickens and 5 million cows in the Netherlands. Dutch livestock production causes an annual manure surplus of 40 million tonnes. This surplus is responsible for 61 percent of the acidification of the Netherlands, acidification being one of the three main causes of Dutch biodiversity depletion. Forest ecosystems especially suffer from acidification -- in 1992 only 43 percent of Dutch forests were considered to be healthy.

Tropical coastal lowlands are well suited for reclamation and development with wetland rise, but often such recovered areas abruptly develop acid sulphate soils. Without special provision for drainage and flooding in these areas, recurrent acidification is apt to persist for years, and acidified drainage water may adversely affect surrounding areas. While technologies have been developed to identify potential acid sulphate soils, and to keep acidification within managable bounds, the necessary information rarely reaches areas concerned.

According to a 1999 report, the soil of Europe's forests was in poor state, because of its level of acidity. This was particularly the case in central Europe - the Czech Republic, Slovakia, southern Poland and southern Byelorussia. Nitrogen deposition in the atmosphere leads to concentrations of nitrates in the subsoil. In nearly one out of five observation plots these concentrations reached levels beyond which ground water is no longer drinkable.

In Australia some 30 million ha of soils within the higher rainfall, improved pasture and cropping areas have been acidified, and have a pHwater of less than 5.5 (SCARM 1998). Acidification can lead to toxic soils, poorer water and nutrient uptake by plants, and thus reduced yields (SCARM 1998).