Soil pH is the measure of the acidity or alkalinity of soil from an extremely acid pH of 0 to an extremely alkaline pH of 14. A soil pH of 7 is neutral, neither acid not alkaline. Knowing the pH of your garden soil is important because soil pH controls the availability of nutrients and the number of microorganisms that improve soil structure. Plants can only absorb nutrients as electrically charged “ions” that attach themselves to clay and organic matter ions with the opposite electrical charge. Depending on the level of acidity or alkalinity of soil, varying amounts of different nutrients can be taken up by plant roots. At some pH levels, nutrients can become bound to other elements, or to soil, and become “locked out” and unavailable to plants. All the major nutrients are only freely available to plants within a narrow soil pH range of 6.5 to 7.5, where essential trace elements are also available, and aluminium is locked out. (See pH Table below) Most vegetables and exotics will remain healthy if grown in a pH range of 6.0–7.0, but potatoes and strawberries do best when pH is around 5.5, and Brassicas and beetroot require a pH close to neutral. However, few plants will survive when the soil pH is below 4.5 where major nutrients are strictly limited and trace elements become available in toxic quantities, or above 9.0 where calcium becomes insoluble.
On the pH scale, the “p” stands for potential, and “H” is the chemical symbol for hydrogen. The more acidic your soil is, the more hydrogen ions in your soil. As hydrogen ions are replaced by calcium ions on the charged sites, soil pH rises. Just to make it confusing, the pH scale is shown as a negative logarithm so that the more hydrogen ions in topsoil, the lower the pH number. Because soil pH is expressed as a logarithm, a pH of 6.0 is ten times more acid than a pH of 7.0, and a pH of 5.0 is a hundred times more acid than 7.0. Adjusting pH without the buffering effect of decomposed organic matter is difficult.
Testing for pH level
The only way to find the exact pH of garden soil is to test it. The Manutec Test Kit is quite economical to use, is available from most large nurseries. I’ve found this test kit to be very reliable and have used it since it was first developed by the CSIRO.
Testing involves taking samples of topsoil from across the growing area, and mixing them thoroughly in a bucket. A small sample of the mixture is placed on a supplied sheet and moistened with a liquid dye. The damp mixture is then dusted with barium sulphate, and the resulting colour matched to a pH range on the kit’s colour chart.
***All garden soils should be tested at least annually, because exudates from plant roots and the decomposition of organic matter release hydrogen ions into the soil, replacing calcium ions and increasing acidity.