What’s soil pH?

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 acid 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.
The only way to find the exact pH of garden soil is to test it. Test kits for this purpose are available at most large nursery and hardware stores for around twenty dollars, or so. The Manutec Test Kit is quite economical to use, and Bunnings are selling them, at the moment, for $15.50. 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 (–see image below).
***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.

pHtable.jpgpHkit.jpg
Black sections show level of availability to plants.

2 thoughts on “What’s soil pH?

  1. Hi,
    I am setting up an above ground organic garden. I have bought expensice organic soil but the PH is around 8. How do you lower the PH levels? JB
    Elemental sulphur (sometimes sold as flowers of sulphur) is the easiest way to lower the pH of slightly alkaline soil. The following amounts will lower the pH of topsoil approximately 1 pH unit.
    If you have sandy soil apply 25 grams sulphur to each square metre of bed.
    For good quality loams, apply at 50 gm p.s.m., and for heavy clays, apply at 100 gm p.s.m. Test soil again in a couple of weeks. If soil pH is higher than 7.5, dig some horse or cow manure through the top soil. As the manure breaks down it will release hydrogen ions, which will replace calcium ions on the exchange sites in the soil and lower the pH – but this won’t happen overnight. – Lyn

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