by Sammy Stein

Created on: April 15, 2011

The pH or potential hydrogen level of a soil is a measure of the concentration of hydrogen ions in the soil water. This, in simple terms, tells you how acid or alkaline the soil is. pH is measured on a logarithmic scale from 1 (totally acidic) to 14 (totally alklaine or base).

Because it is a logarithmic scale, each value is measured to the power of 10. Therefore a soil with a pH of 6 has ten times less hydrogen ions in its soil water than one with a pH of 5 so pH values matter.

How acid or alkaline a soil is affects several factors. First, it effects the way calcium is taken up by plants. Plants which need high levels of calium or tolerate high levels of it are suited to alkaline soils where thereis less hydrogen in the soil water, as this affects calcium uptake. These are calcicoles. Those plants who cannot tolerate calium are those preferring low pHs and are said to be acid loving plants or calcifuges. 

No plants (other than a few algae insulphur springs) live in pH less than 4 or above 9 so most plants live in soils where the pH is between 4.5 and 8.5 and soil with a pH of 6.5-7.5 will grow the largest range of plants. So ,understanding a plant's pH requiremtns is important for achieving its horticultural potential (best growth for the species).

pH also affects other factors. It affects the way plants become susceptible to diseases. Brassicas (cabbages and the family) are susceptible to a disease called club root. This not only thrives in soil with low pH but the plants growing in lower pHs are also more susceptible to the disease so, for a successful brassica crop, you need to keep the soil pH above 6.5 and often, lime is added to do this.

Certain soil additives will raise or lower the pH over time. Flowers of sulphur and animal manures will lower pH over several years whilst lime or gypsum will raise it but soils, because they originate from a parent rock which will ultimately determine their texture (proportions of minerals), and pH , tend to revert to their natural pH with additives being washed out over time. Therefore,it is usually best to choose plants which are suited to the soil pH you have.

pH is measured using a chemical kit or samples can be sent off to laboratories. Commercial growers analyse pH carefully to make sure they can grow crops successfully and add copious amounts of additives to control it but for most gardeners, either using additives over small areas or choosing the right plants for their soil type is far better

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