Plant species are continuously exposed to their ambient soil environment during their life cycle. Sometimes the presence, application and/or formation of certain substances in soils increase to the extent that they become phytotoxic - the property of a substance at a specified concentration that restricts or constrains plant growth. Phytotoxic substances have long been accumulating in the terrestrial ecosystem including soils. Phytotoxicity in soils can be grouped into five broad categories: (1) pesticide toxicity, (2) metal and metalloid toxicity, (3) soil acidity and aluminum and manganese toxicity, (4) soil flooding and manganese and iron toxicity, and (5) soluble salt and sodium toxicity.