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Moisture & pH Meters

Soil pH is the pH of soil water. It is based on the measurement of pH, which depends on the activity of hydrogen ions (H+) in a solution. There are many different methods to collect soil water, all which influence the measured soil pH in one way or another (see below). more...

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The soil pH is closely linked to the concepts alkalinity and acidity (see acid neutralizing capacity). A neutral solution has pH 7 while an acid solution has pH less than 7 (more H+ than OH-) and a basic solution pH larger than 7 (more OH- than H+) but there is, contrary to popular belief, no exact limit to the pH range. In natural soils and surface waters buffer systems make pH levels below 3 uncommon, but not impossible. The exposure of the soil to sunlight does not usually affect the pH of the soil.

(NOTE: While a basic solution always has a pH larger than 7, an alkaline solution (i.e. a solution with positive acid neutralizing capacity) does not necessarily have a pH larger than 7. For details on the relation between pH and ANC, see acid neutralizing capacity)

Soil pH is an important consideration for farmers and gardeners for several reasons, including the fact that many plants and soil life forms prefer either alkaline or acidic conditions, that some diseases tend to thrive when the soil is alkaline or acidic, and that the pH can affect the availability of nutrients in the soil.

Nutrient availability in relation to soil pH

The majority of food crops prefer a neutral or slightly acidic soil. Some plants however prefer more acidic (e.g., potatoes, strawberries) or alkaline (brassicas) conditions.

The above table gives a guide to the availability of several nutrients at various pH values

During the acidification process the decrease in pH results in a release of positively charged ions (cations) from the cation exchange surfaces (organic matter and clay minerals). In the short term acidification thus increases the concentration of potassium (K), magnesium (Mg and calcium (Ca) in soil solution. Once the cation exchange surface has become depleted of these ions, however, the concentration in soil solution can be quite low and is largely determined by the weathering rate. The weathering rate in turn is dependent on such things as mineralogy (e.g. presence of easily weathered minerals), surface area (i.e. the soil texture), soil moisture (i.e. how large a fraction of the mineral surface area that is wet), pH, concentration of base cations such as Ca, Mg and K as well as concentration of aluminium. The amount of plant available nutrients is a much more difficult issue than soil solution concentrations. The term plant available nutrients usually include pools other than soil solution but which are supposed to replenish soil solution pretty fast e.g. through cation exchange. One reason for including such pools is the plants capability of releasing organic acids which increase the total soil solution concentration of some cation nutrients that are important for the plant.


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