Soil Acidity
Liming Acid Soils
In a liming program, sufficient lime must be applied to neutralize both active and potential acidity. Liming acid soils results in better crop yields by: (1) raising soil pH, (2) improving overall nutrient availability, (3) reducing soluble aluminum, and (4) improving microbial activity. Overliming may, however, reduce the availability of phosphorus and cause deficiencies of micronutrients such as manganese, zinc, and copper. The purpose of liming fields is to maintain an adequate soil pH, not to produce an immediate increase in crop yield as with fertilizer applications.
Liming Materials
Liming materials are generally carbonates and bicarbonates of calcium and magnesium, and sometimes oxides and hydroxides. The anion in liming materials (chemically speaking, a “base”) reacts with soil acidity (H⁺) to neutralize it.
Carbonates
The carbonates include calcium carbonate (CaCO3), often referred to as lime, limestone or aglime, and magnesium carbonate (MgCO3), also called dolomite or dolomitic lime. Carbonates are the most widely used liming materials. They are generally less expensive than other lime materials. Dolomite is derived from deposits of calcium carbonate combined with magnesium carbonate and contains much higher levels of magnesium.
Lime Forms
Ground Limestone/Dolomite. Limestone and dolomite are pulverized into a dry, very finely powdered material (dust).
Pelletized Lime. Pelletized lime consists of very finely ground particles of limestone and/or dolomite formed into pellets with a soluble binding agent. Nearly all of the particles pass a 100-mesh sieve and 25 to 40 percent can pass a 200-mesh sieve. It is chemically the same as traditional agricultural lime and neutralizes soil acidity the same way.
Liquid Lime. Lime is sometimes sold as a suspension, often called “liquid lime.” It consists of fine lime particles mixed with water and a suspending clay. All the lime particles must be 100-mesh or finer. The main advantages are ease of handling and precise application. Although it is a fluid, this material does not react any faster than dry lime of the same particle size.
Oxides
Oxide liming materials include burned lime and slaked lime. Oxides reacts rapidly in the soil to raise the pH. Oxides are the most efficient of all liming materials on a pound-for-pound basis. Burnt lime is made by burning calcium carbonate which leaves calcium oxide. Water is then added, resulting in hydrated lime. These materials are fast acting in the soil, but do not provide a long-lasting effect.
Hydroxides
Hydroxides are simply oxide materials with water added. They are also known as hydrated lime, slacked lime, or builders’ lime. These materials are similar to oxides because they are powdery, quick acting, unpleasant to handle, and can easily burn plants that are already established. Hydroxides are also more expensive than carbonate materials. Many by-products of mining, refining, processing, and manufacturing processes are used as liming materials.
Lime Quality
Not all limestone is the same. The quality of aglime varies significantly and should be an important consideration in aglime management. Quality standards used to differentiate liming materials include calcium carbonate equivalence (CCE), total neutralizing value (TNV), effective neutralizing value (ENV), fineness, and moisture.
Calcium Carbonate Equivalence
Acid neutralizing capacity of a liming material is determined by its calcium carbonate equivalence (CCE). It is the acid neutralizing capacity of the material compared to pure calcium carbonate. In CCE comparisons, pure calcium carbonate has been assigned a value of 100 percent. The CCE is expressed as the percentage of pure calcium carbonate by weight. All other liming materials are compared with this standard.
Total Neutralizing Value
Total neutralizing value (TNV) refers to the ability of a lime material to neutralize soil acidity relative to pure calcium carbonate equivalence (CCE) and is expressed as a percentage.
Effective Neutralizing Value
The effective neutralizing value (ENV) is the fraction of the liming material’s CCE that will react with soil acidity in the first year of application. The ENV is calculated by multiplying the CCE with the fineness of the material. For example, a liming material with CCE of 90 percent and a fineness of 0.86 has an ENV of 90 × 0.86 = 77.4 percent.
Fineness
The neutralizing capacity of a liming material is determined by its fineness, i.e., the particle size which governs how quickly acidity will be corrected. The finer the material, the more surface area it has and the more quickly it will react when thoroughly mixed with the soil. Fineness of limestone or dolomite is determined by passing the material over a set of sieves (screens) of different sizes. Lime particles that pass-through a 100-mesh sieve are very fine and react rapidly—within a few weeks.
Moisture
Water is sometimes added to dry limestone or dolomite to improve the handling characteristics of the ground material. The moisture content of limestone or dolomite does not directly affect its effectiveness. However, moisture adds weight and will decrease the amount of effective liming material on a ton basis.
Buffer pH
Soil pH measurement reveals the amount of active acidity (H⁺) in the soil solution and whether or not lime should be applied. If the soil pH level is below the optimum for the desired crop, liming may be necessary. In order to accurately determine the amount of lime necessary to neutralize soil acidity to the desired level, the buffering capacity of the soil must be measured. Buffer pH measurement reveals how much reserve (exchangeable) acidity needs to be neutralized by the lime. Reserve acidity refers to the hydrogen and aluminum cations that are held by soil exchange sites. As buffer pH decreases, the amount of lime required to neutralize both active and reserve acidity increases. For example, soil with a buffer pH of 6 requires two to three times the lime of a soil with a buffer pH of 6.5.
Stratification of Soil pH
Soil pH stratification in the surface should be considered when liming. Stratification of pH occurs especially in no-till soils where anhydrous ammonia has been injected at a 4- to 8-inch depth for many years. At the depth of injection, an acidified layer is created due to hydrogen ions generated during the nitrification process.
Methods of Application
Dry Bulk Limestone
Dry bulk limestone is typically applied using fertilizer spreader trucks. Similarly, to a broadcast application of nutrients, make sure limestone is spread evenly throughout the soil surface by avoiding overlaps. Most limestone spreaders generate clouds of fine dust. This is the finest portion of limestone and, therefore, the quickest to react with the soil. Traditionally it has been claimed that up to 8 percent of the lime can be blown away. Some spreaders have a shroud to control the dust.
Liquid, Fluid or Suspension Lime
Liquid, fluid or suspension lime is a combination of very fine limestone (100 mesh or smaller) in water with 1 to 2 percent clay to form a suspension that is about 50 to 60 percent solids. The material is typically spread using a tank truck equipped with a boom and high-volume nozzles. With proper calibration, this enables very uniform product application with no dust. Like all liming materials, to be most effective fluid/suspension lime should be incorporated into the soil. Fluid lime is usually more expensive per ton than limestone applied dry due to increased costs for finely ground materials, freight and product application.
Precision Agriculture
There is often a large variation in soil pH across a field. Often a lime application rate is based on a single soil test representing the whole field. Ideally it is best to take a number of soil samples in a grid system across the field for a better indication of the soil pH however, this is time consuming and expensive (Section 11.3). Alternatively, some growers are using electrical conductivity sensors to map soil pH. These sensors take pH readings from numerous points in a grid system across the field. An electrical conductivity sensor coupled with a GPS receiver typically can generate 50 electrical conductivity samples per acre when pulled at a ground speed of 10 mph and 60 feet spacing per pass. This is a much denser dataset than grid soil sampling (usually one sample every 2.5 acres). It is possible to produce a greater resolution soil map than with a typical nutrient map. Mapping at this density will identify soil inclusions that are a quarter-acre in size or larger.
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