Soil pH: Encyclopedia II - Soil pH - Nutrient availability in relation to soil pH

Soil pH - 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.

When interpreting soil solution pH values it is essential to take into account the method by which pH has been measured. Depending on whether or not the water has been equilibrated with ambient CO₂ pressure or not the pH reported from the same site may be either high or low. This is simply because the carbon dioxide pressure deep down in the soil might be 10-20 times higher than the ambient pressure due to decomposition of organic material. The higher carbon dioxide pressure result in more carbonic acid and hence a lower pH. Furthermore, soil solution can be extracted from the soil in many ways, e.g. by lysimeters, zero-tension lysimeters, centrifugation, extraction with CaCl₂, overhead shaking of soil sample with added water, etc. The CaCl₂ extraction method do not give the actual soil solution pH but rather a mix between soil solution pH and what is easily available e.g. through cation exchange. Also when mixing soil samples with water and using overhead shakers (or similar) the result is a mix between actual soil solution and cation exchange, although the hope is that the extracted water will be similar to the actual soil solution in most respects. If centrifugation or pressurised lysimeters are used, care must be taken that the extracted water do not include water that is not readily available (think wilting point and crystal water). Naturally, taking a sample introduces a disturbance of the system, which can e.g. result in a change in nutrient uptake and decomposition rates (e.g. due to cutting of fine roots when placing the lysimeter).

Many nutrient cations such as zinc (Zn²⁺), aluminium (Al³⁺), iron (Fe²⁺), copper (Cu²⁺), cobalt (Co²⁺), and manganese (Mn²⁺) are soluble and available for uptake by plants below pH 5.0, although their availability can be excessive and thus toxic in more acidic conditions. In more alkaline conditions they are less available, and symptoms of nutrient defficiency may result, including thin plant stems, yellowing (chlorosis) or mottling of leaves, and slow or stunted growth.

pH levels also affect the complex interactions among soil chemicals. Phosphorus (P) for example requires a pH between 6.0 and 7.0 and becomes chemically immobile outside this range, forming insoluble compounds with iron (Fe) and aluminium (Al) in acid soils and with calcium (Ca) in calcareous soils.

Adapted from the Wikipedia article "Nutrient availability in relation to soil pH", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki