Soil Acidification and Heavy Metal Contamination in Soils

Soils are acid/base systems which have the ability to absorb and release metals which have been introduced into the soil by anthropogenic or natural means. Soils are naturally acidic or alkaline based on their bedrock and climatic conditions. This measure of acidity or alkalinity in soils is important as it directly affects the growth of plants and the leach of heavy metals into soils which will cause crop contaminations.

In alkaline soils, i.e. sodic soils, plant growth is poor due to competition of sodium ions with other important ions such as magnesium and potassium for uptake in plant roots. However, alkaline soils have excesses of base cations which can bond with oxidised heavy metals (metal oxides) to fix heavy metals in soil. Alkaline soils thus are less likely to cause health problems when contaminated with heavy metals, but this is dependent on the type of metal oxides present in soil solution. However, these soils are too salty for crop growth. 

Picture showing Sodic Soils. Taken on 2 October 2014 in Inner Mongolia, China. Sodium salt levels are very high, with salts crystallizing to form a white layer on top. Nearby lake is salty as well. 

In acidic soils, the problem of heavy metal contamination is more serious than in alkaline soils due to the release of metals bonded on soil surfaces into the soil solution. In places where acid rain from nearby polluting factories acidifies soils, aluminium is leached into soils when pH<4. Aluminium ions are toxic to plants and retards plant root growth (Krstic et al. 2012). Besides, as aluminium has 3 free electrons for bonding, after being leached out, negative soil surface adsorbs other metal ions available and causes nutrient deficiency in plants. However, addition of hydrogen ions also replaces metal ions in soils and releases metals, but the overall effect of adsorption is higher as aluminium has 3 free electrons for bonding as compared to a hydrogen ion which only has one. 

Thus, places in the south - the southern part of China where acidic soils and acid rain are both present, aluminium toxification levels are high. For some crops, they will fail to grow, but crops with higher tolerance levels can bioaccumulate aluminium and pass on aluminium in the food chain. 

Krstic, D. et al. (2012) Aluminium in Acid Soils: Chemistry, Toxicity and Impact on Maize Plants. Collected in Food Production - Approaches, Challenges and Tasks. Aladjadjiyan, A. eds. InTech,  Croatia. 

Acid Rain - Soil Interactions (n.d.) Available at: http://www.elmhurst.edu/~chm/vchembook/196soil.html