Moisture, pH Meters

Soil pH is an indication of the alkalinity or acidity of soil. It is based on the measurement of pH, which is based in turn on the concentration of hydrogen ions (H⁺) in a water or salt solution.
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When in balance (pH 7) the soil is said to be neutral. The pH scale covers a continuum ranging from 0 (very acidic) to 14 (very alkaline or basic). It is however uncommon to find soils at either extreme of this range. Under many conditions soils tend to become more acid or alkaline over time if steps are not taken to maintain a balance.

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 acid or alkaline 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.

During the acidification process the decrease in pH result in a release of positively charged ions (cations) from the cation exchange surfaces (organic matter & 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 wetted), 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.

It is thus important to realise that there exists no simple relation between soil solution concentration of Ca, Mg and K and reasonable pH-values. The reason for this is that Ca, Mg and K are base cations, i.e. cations of strong bases and strong bases are fully dissociated at the pH-ranges occurring in most natural waters. However, as the soil solution pH is dependent on mineral weathering and mineral weathering increase pH by releasing Ca, Mg and K a soil which is rich in easily weatherable minerals tends to have both a higher pH and higher soil solution concentration of Ca, Mg and K. On the other hand deposition of sulphate, nitrate and to some extent ammonia decrease pH of soil solution essentially without affecting Ca, Mg and K concentrations whereas deposition of seasalt increases Ca, Mg and K concentrations without having much of an effect on soil solution pH.

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