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Soil Acidity and Liming (Overview)

Measuring Acidity and Alkalinity

The term pH stands for the potential (p) of the hydrogen ion (H+) in water. It is actually a way of reporting the concentration of H+ in solution using an electrical "potential" to measure H+. The pH of any solution is one of the easiest laboratory measurements to make using a pH meter and an electrode specifically designed to measure hydrogen (pH electrode). Color indicators and litmus paper are a quick alternative for less precise measurements. By mixing a quantity of soil with demineralized or distilled water (usually a 1:1 mixture), we can measure the pH of the water solution in equilibrium with the soil.

The pH measurement is based on a scale from 1 to 14 (pH is reported as the negative logarithm of the hydrogen ion activity). At a pH of 7.0, there is an equal balance of hydrogen (H+) ions and hydoxyl (OH-) ions, and the soil (actually the soil-water suspension) is said to be neutral. Because the pH measurement is logarithmic, each unit change in pH represents a ten-fold increase in the amount of acidity or basicity. That is, a soil solution with a pH of 6.0 has 10 times as much active H+ as one with a pH of 7.0.

pH of solutionHydrogen ion activity (g/liter)
9.0 (very alkaline)10-9 (0.000000001)
8.0 (alkaline or basic)10-8 (0.00000001)
7.0 (neutral, pure water)10-7 (0.0000001)
6.0 (slightly acid)10-6 (0.000001)
5.0 (very acid)10-5 (0.00001)
4.0 (extremely acid)10-4 (0.0001)

Most Southeastern soils have a pH ranging from 4 to 8. With the exception of some native vegetation (e.g. pine trees) and a few acid-loving plants such as azaleas, blueberries, gardenias, and centipede grass, most plants do best in a slightly acid soil with a pH between 6.0 and 7.0.

Acid Soil Infertility

When the pH falls below 6.0, the availability of nutrients such as phosphorus, potassium, calcium, and magnesium decreases. The availability of the metallic micronutrients, however, like zinc, manganese, copper, and iron increases as the pH decreases.

Plants don't need aluminum to grow. It's not an essential plant nutrient. Aluminum, however, is one of the prominent mineral components of silt and clay. Therefore, the earth's crust is naturally high in aluminum. Like zinc, manganese , copper and iron, the more acid the soil, the more aluminum will be dissolved into the soil solution. If the pH is allowed to drop much below 5.5, the availability of manganese and aluminum is increased to the point that they could become toxic to plants. Aluminum toxicity to plants is the main concern we have with acid soils in our region.

Problems in very acid soils Problems in alkaline soils
*Aluminum toxicity to plant roots*Iron deficiency
*Manganese toxicity to plants*Manganese deficiency
*Calcium & magnesium deficiency *Zinc deficiencies
*Molybdenum deficiency in legumes*excess salts (in some soils)
*P tied up by Fe and Al*P tied up by Ca and Mg
*poor bacterial growth*bacterial diseases in potatoes
*reduced nitrogen transformations

Some plants such as alfalfa, spinach, and lettuce require high levels of calcium and potassium and can tolerate high salt levels that may occur in near neutral to alkaline soils. White potatoes will do well in near neutral to slightly acid soils but are usually grown in more acid soils (pH less than 6.0) because of "scab", a bacterial disease. As already noted, bacteria don't thrive in very acid soils.

Factors Affecting Soil pH

Soils are not homogenous and the pH can vary considerably from one spot in the field to another. It also varies with depth. Soils in different geographic regions, as already mentioned, may have different pH's because of the five soil forming factors: (1) parent material, (2) climate, (3) living organisms, (4) topography, and (5) time.

Parent material. Soils of the Piedmont and Sandstone Plateau regions of Alabama are very acid because of the acid nature of the rocks (granites and sandstones, respectively) which formed these soils. Limestone valley soils were formed from basic rocks (limestones) but may be acid on the surface because of time and weathering. Some Black Belt Prairie soils may be alkaline because the Selma chalk (soft limestone) which formed the soils is alkaline.

Rainfall/leaching. Rainfall also affects soil pH. Water passing through the soil leaches basic cations such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+) into drainage water. These basic cations are replaced by acidic cations such as aluminum (Al3+) and hydrogen (H+). For this reason, soils formed under high rainfall conditions are more acid than those formed under arid conditions.

Fertilizers. Both chemical and organic fertilizers may eventually make the soil more acid. Hydrogen is added in the form of ammonia-based fertilizers (NH4+) , urea-based fertilizers [CO(NH2)2], and as proteins (amino acids) in organic fertilizers. Transformations of these sources of N into nitrate (NO3-) releases H+ to create soil acidity. Therefore, fertilization with fertilizers containing ammonium or even adding large quantities of organic matter to a soil will ultimately increase the soil acidity and lower the pH.

NH4+ + 2O2 bacteria NO3-+ 2H+ + H2O

Plant uptake. Plants take up basic cations such as K+, Ca++, and Mg++. When these are removed from the soil, they are replaced with H+ in order to maintain electrical neutrality.

Raising Soil pH (Liming Acid Soils)

Soils are limed to reduce the harmful effects of low pH (aluminum or manganese toxicity) and to add calcium and magnesium to the soil. The amount of lime needed to achieve a certain pH depends on (1) the pH of the soil and (2) the buffering capacity of the soil. The buffering capacity is related to the cation exchange capacity (CEC). The higher the CEC, the more exchangeable acidity (hydrogen and aluminum) is held by the soil colloids. As with CEC, buffering capacity increases with the amounts of clay and organic matter in the soil. Soils with a high buffering capacity require larger amounts of lime to increase the pH than soils with a lower buffering capacity. Most soil testing laboratories use a special buffered solution to measure the exchangeable acidity. This is the form of soil acidity that must be neutralized for a change in soil pH. By calibrating pH changes in the buffered solution with known amounts of acid, the amount of lime required to bring the soil to a particular pH can be determined.

Lime reduces soil acidity (increases pH) by changing some of the hydrogen ions into water and carbon dioxide (CO2). A Ca++ ion from the lime replaces two H+ ions on the cation exchange complex. The carbonate (CO3-) reacts with water to form bicarbonate (HCO3-). These react with H+ to form H2O and CO2. The pH increases because the H+ concentration has been reduced.

Soil Colloid + CaCO3 Soil Colloid-Ca++ + H2O + CO2

Remember, the reverse of the above process can also occur. An acid soil can become more acid as basic cations such as Ca2+, Mg2+, and K+ are removed, usually by crop uptake or leaching, and replaced by H+.

Liming Materials

The most common liming materials are calcitic or dolomitic agricultural limestone. These are natural products made by finely grinding natural limestone. Since natural limestone is relatively insoluble in water, agricultural limestone must be very finely ground so it can be thoroughly mixed with the soil and allowed to react with the soil's acidity. Calcitic limestone is mostly calcium carbonate (CaCO3). Dolomitic limestone, according to most state laws, must have at least 6 percent magnesium, and is made from rocks containing a mixture of calcium and magnesium carbonates. Either will neutralize soil acidity. Dolomitic limestone also provides magnesium. Other liming materials which are less frequently used are listed in Table 2.

Because high quality, finely ground limestone is very dusty and difficult to spread, some companies market a "prilled" or "granular" limestone for home use. A small amount of clay or a polymer is added to the ground limestone so small prills are formed instead of dust. This makes it easier to spread by hand or with a garden fertilizer spreader. Once applied, soil moisture will cause the "prills" or "granules" to dissociate and disperse the limestone particles. The limestone can then react with exchangeable acidity in the following manner.

Application and Placement of Lime

Time of year. Lime may be applied at any time during the year. For the farmer or gardener, winter or early spring just prior to soil preparation is usually most convenient. Don't apply caustic liming materials such as burned lime, hydrated lime, or wood ashes to actively growing plants. Ground limestone will not harm plants.

Lime placement. The most important consideration is lime placement. Ground agricultural limestone is relatively insoluble in water so maximum contact with the soil is necessary to neutralize the soil acidity. Lime will not move into the soil like water-soluble fertilizers. Thoroughly mix the recommended amount of lime with the top 6 to 8 inches of soil. As soon as moisture is present, the lime will begin to react. Coarse lime particles react more slowly than very fine particles. Therefore, using very finely ground limestone and thoroughly mixing it are necessary to achieve the desired soil pH change within a few months. If the soil will be turned with a bottom plow, turn it first and then apply the lime and mix.

Some common soil liming materials.
Material Relative
Neutralizing value

----------- % ------------
pure CaCO3100 not generally available
Calcitic agricultural lime,
  (calcium carbonate,
  CaCO3 +impurities)
90 - 100easily available
Dolomitic agricultural lime,
  CaCO3 + MgCO3
95 - 108easily available; provides Mg
Ground oyster shells85 - 95 
Selma chalk/marl,
  CaCO3 + clay
50 - 85contains clay; keep dry
Burned lime, CaO150 - 175very caustic; don't use
Hydrated lime or
  builders' lime, Ca(OH)2
120 - 135caustic; use with caution; no Mg
Basic slag50 - 70 contains some P & micronutrients; byproduct
Wood stove or fireplace
40 - 70provides some plant nutrients
Boiler wood ash30 - 60 provides some plant nutrients
By-productsVariable use as specified by manufacturer
Gypsum and/or
  ground drywall, CaSO4

Prepared by: Charles C. Mitchell
Extension Agronomist-Soils & Professor
Auburn University

Return to Soil Acidity & Liming - Part II

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