Acid Sulfate Soils


icon representing growing crops with hills in the backgroundWhat is an acid sulfate soil?

Acid Sulfate Soils (ASS) generally occur in low lying and flat locations which are often flood prone or swampy.

ASS are the common name given to soils containing iron sulfides. In Australia, the ASS of most concern are those which formed within the past 10,000 years, after the last major sea level rise.

When the sea level rose and inundated land, sulfate in the sea water mixed with land sediments containing iron oxides and organic matter. The resulting chemical reaction produced large quantities of iron sulfides in the waterlogged sediments. When exposed to air, these sulfides oxidise to produce sulfuric acid, hence the name acid sulfate showing scalding of creek bed by acid sulfate soils

The acid produced by oxidation of iron sulfides affects both soil and water, and can damage the environment severely. As sulfuric acid moves through the soil, it strips iron, aluminium and sometimes manganese from the soil. In some cases it also dissolves heavy metals such as cadmium. In the soil this mixture can make the soil so acid and toxic that few plants can survive. In some cases, where peat overlying the iron sulfide layer has burnt away, the iron sulfide layer is completely exposed to air. It produces so much sulfuric acid that nothing will grow, giving the soil surface a bare, scalded appearance as shown.

Affects of acid sulfate soils

ASS reduce farm productivity. The sulfuric acid lowers pH, which makes several soil nutrients less available to plants. The acid dissolves iron and aluminium from the soil so that they become available to plants in toxic quantities in soil water. These conditions reduce plant growth, and only acid-tolerant plants such as smartweed, shown below, can survive. This effectively means loss of drought-refuge swamp pastures used in the past by farmers.image of acid tolerant plants known as smart weed

Animal productivity is affected by acid sulfate soils. The acid discourages good quality pasture. Grazing animals may take in too much aluminium and iron by feeding on acid-tolerant plant species and drinking acid water.

Drainage of coastal wetlands for agricultural and urban development constantly releases enough sulfuric acid and aluminium to affect the aquatic food chain, fish populations, and the health of fish. Sulfuric acid affects waterways and the aquatic life in them. Most aquatic life needs a minimum pH of 6 to survive. The pH of acid water can be as low as 2, and is often around 4. The lower the pH the more acid the water. Fish and crustaceans try to avoid acid water, but if they cannot escape, they may die. Plants, unable to escape the acidified water, are often killed.

Massive fish kills can occur when sulfuric acid is washed into waterways. This is a particular problem after droughts, when the watertable has dropped and the iron sulfide layer has oxidised.

If you are considering developing on a site with ASS refer to Kiama Local Environmental Plan 2011 Section 6.1 Acid Sulfate Soils and Chapter 6 – Kiama Development Control Plan 2012 – Rural Development.


05 May 2016