Acid sulphate
in soil is rare but might be found in coastal regions.
The chemical composition of soil changes over time and depends on a range of factors, including climate and the geology of local rock. Although rare, acid sulphate is sometimes found in waterlogged soils in coastal, estuary and swamp regions. The chemical composition of soil is crucial for farming and important for gardeners, and acid sulphate soils can cause problems for both.
Formation
The acid sulphates in soil form when minerals and chemicals from the substrate rock leak into lower soil levels and accumulate. The sulphate soil is benign when buried underground below the level of the water table. Weathering, erosion and draining of land for construction can expose the sulphates. Once exposed, the sulphates react with the oxygen in air, and this causes acidification of the soil.
Acidification
Acid sulphate soils contain iron pyrites. The iron sulfides in iron pyrites react with oxygen in the air to produce sulphuric acid in the soil. The sulphuric acid then reacts with other minerals in the soil to release aluminum and iron.
Effects
Acid sulphate soils can acidify groundwater -- including lakes, rivers and garden ponds -- and kill fish and other aquatic life. Highly acidic soil can also kill vegetation, including shrubs, trees, flowers and food crops in agricultural areas. Acid sulphate soils can also degrade steel and concrete structures, damaging buildings and highways.
Identification
Gardeners can use an inexpensive soil chemical testing kit from a nursery or garden center to find the acidity of their soil. The test is simple and involves placing a small amount of soil in a test tube or container and then adding the chemical testing solution. Check the color of the solution against the supplied color chart to find the acidity of the soil. Sulphate soils have a pH level of less than 4 and usually look yellow and mottled. Gardeners can reduce the problem of acidic soil by adding alkaline substances such as crushed limestone.
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