The soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm (base 10) of the activity of hydrogen ions (H+) in solution. It ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is basic. Soil pH is considered a master variable in soils as it controls many chemical processes that take place. It specifically affects plant nutrient availability by controlling the chemical forms of the nutrient. The optimum pH range for most plants is between 6 and 7.5, however many plants have adapted to thrive at pH values outside this range.
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The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows: [1]
| Denomination | pH range |
|---|---|
| Ultra acid | <3.5 |
| Extreme acid | 3.5 - 4.4 |
| Very strong acid | 4.5 - 5.0 |
| Strong acid | 5.1 - 5.5 |
| Moderate acid | 5.6 -6.0 |
| Slight acid | 6.1 -6.5 |
| Neutral | 6.6 - 7.3 |
| Slightly alkaline | 7.4 - 7.8 |
| Moderately alkaline | 7.9 - 8.4 |
| Strongly alkaline | 8.5 -9.0 |
| Very strongly alkaline | >9.0 |
[2] Acidity in soils comes from H+ and Al3+ ions in the soil solution and sorbed to soil surfaces. While pH is the measure of H+ in solution, Al3+ is important in acid soils because between pH 4 and 6, Al3+ reacts with water (H2O) forming AlOH2+, and Al(OH)2+, releasing extra H+ ions. Every Al3+ ion can create 3 H+ ions. Many other processes contribute to the formation of acid soils including rainfall, fertilizer use, plant root activity and the weathering of primary and secondary soil minerals. Acid soils can also be caused by pollutants such as acid rain and mine spoilings.
Basic soils have a high saturation of base cations (K+, Ca2+, Mg2+ and Na+). This is due to an accumulation of soluble salts are classified as either saline soil, sodic soil, saline-sodic soil or alkaline soil. All saline and sodic soils have high salt concentrations, with saline soils being dominated by Ca and Mg salts and sodic soils being dominated by Na. Alkaline soils are characterized by the presence of carbonates.
[3] Plants grown in acid soils can experience a variety of symptoms including Al, H, and/or Mn toxicity, as well as potential nutrient deficiencies of Ca and Mg.
Al toxicity is the most widespread problem in acid soils. Al is present in all soils, but dissolved Al3+ is toxic to plants; Al3+ is most soluble at low pH, above pH 5.2 little aluminum is in soluble form in most soils. Al is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through osmosis. Al damages roots in several ways: In root tips and Al interferes with the uptake of Ca, an essential nutrient, as well as bind with phosphate and interfere with production of ATP and DNA, both of which contain phosphate. Al can also restrict cell wall expansion causing roots to become stunted.
Below pH 4, H+ ions themselves damage root cell membranes.
In soils with high content of Manganese (Mn) containing minerals, Mn toxicity can become a problem at pH 5.6 and below. Mn, like aluminum becomes increasingly more soluble as pH drops, and Mn toxicity symptoms can be seen at pH's below 5.6. Mn is an essential plant nutrient, so plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves '
[4] Nutrients needed in the largest amount by plants are referred to as macro-nutrients and include nitrogen (N), phosphorus (P), and potassium (K), calcium (Ca), Magnesium (Mg) and Sulphur (S). In addition to macronutrients, plants also need trace nutrients. Trace nutrients are not major components of plant tissue, but are required for growth. These include Iron, (Fe), manganese (Mn), zinc (Zn), copper (Cu), Cobalt (Co), Molybdenum (Mo), and Boron (Bo). Both macro and trace nutrient availability is controlled by soil pH. In slightly to moderately alkaline soils, molybdenum and macro-nutrient (except P) availability is increased, but P, Fe, Mn, Zn Cu, and Co levels are reduced so low they may affect plant growth. In acid soils, micro-nutrient availability (except Mo and Bo) is increased. Nitrogen is supplied as ammonium (NH4) or nitrate (NO3) in fertilizer amendments, and dissolved N will have the highest concentrations in soil with pH 6-8. Concentrations of available N are less sensitive to pH than concentration of available P. In order for P to be available for plants, soil pH needs to be in the range 6.0 and 7.5. If pH is lower than 6, P starts forming insoluble compounds with iron (Fe) and aluminium (Al) and if pH is higher than 7.5 P starts forming insoluble compounds with calcium (Ca).
Most nutrient deficiencies can be avoided between a pH range of 5.5 to 6.5, provided that soil minerals and organic matter contain the essential nutrients to begin with.
Methods of determining pH include:
The most common amendment to increase soil pH is lime (CaCO3 or MgCO3), usually in the form of finely ground agricultural lime. The amount of lime needed to change pH is determined by the mesh size of the lime (how finely it is ground)and the buffering capacity of the soil. A high mesh size (60 - 100) indicates a finely ground lime, that will react quickly with soil acidity. Buffering capacity of soils is a function of a soils cation exchange capacity, which is in turn determined by the clay content of the soil, the type of clay and the amount of organic matter present. Soils with high clay content, particularly shrink-swell clay, will have a higher buffering capacity than soils with little clay. Soils with high organic matter will also have a higher buffering capacity than those with low organic matter. Soils with high buffering capacity require a greater amount of lime to be added than a soil with a lower buffering capacity for the same incremental change in pH.
Other amendments that can be used to increase the pH of soil include wood ash, industrial CaO (burnt lime), and oyster shells. White firewood ash includes metal salts which are important for processes requiring ions such as Na+ (Sodium), K+ (Potassium), Ca2+ (Calcium), which may or may not be good for the select flora, but decreases the acidic quality of soil.
These products increase the pH of soils through the reaction of CO32- with H+ to produce CO2 and H2O. Calcium silicate neutralizes active acidity in the soil by removing free hydrogen ions, thereby increasing pH. As its silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral solute.