A pH of 7 is neutral, while acid solutions are less than 7 and alkaline ones above 7. As pH is a logarithmic scale every unit change, say from 5 down to 4, represents a tenfold increase in acidity.
Acid soils occur naturally in Victoria through processes of weathering and leaching.
Agricultural activities such as continued removal of alkaline plant material, excessive use of nitrogen fertilisers and nitrogen leaching have accentuated the rate of acidification. Anions such as sulfate, chloride, bicarbonate and in particular nitrate are leached beyond the plant root zone and into the subsoil. Associated cations of calcium and magnesium are also leached out leaving an over-abundance of hydrogen ions in the topsoil. As the number of hydrogen ions increases and the soil becomes more acid, concentrations of aluminium and manganese may enter the soil water solution. Many plants can only tolerate small quantities of these elements before the soil becomes toxic and restricts plant growth.