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Understand Your Soil Test: pH-Excess Lime-Lime Needs

The relationships among pH, soil type, and lime requirements are explained.

Accurate soil tests can be an excellent management tool. Misuse of soil tests leads to increased production costs,  yield losses, or both. The elements required by plants for proper growth have been determined by experimentation. Experience has shown that soils differ greatly in their capacity to supply these elements. The amount of each element supplied by a soil depends on several factors. Two important ones are: (1) the type of material from which the soil was formed, and (2) the treatment the soil has received since being placed under cultivation. Not all of a particular element in a soil is available to a plant. Thus, the soil test must be able to  predict whether a soil contains sufficient amounts of available nutrient elements for a specific crop.

The acid and base levels of a soil solution are important because microorganisms and plants are responsive to  their chemical environment. Three possible chemical reaction conditions for the soil solution are acidity,  neutrality, and alkalinity. The reaction of the soil solution can be defined by an index using the concentration of  hydrogen ions in the soil solution. This index is called the pH. A pH of 7 is neutral, pHs less than 7 are acid, and pHs greater than 7 denote a basic (alkaline) condition. Soil pH can be an indicator of the kind of nutrient problems to expect in a soil. Obviously the pH is not a “cure-all” analysis, but may indicate a possible problem,
which may then be investigated with additional analysis. In mineral soils, pH is a general indicator of soil  nutrient availability, presence of free lime (calcium carbonate), presence of excess sodium, and excess hydrogen. Almost all soils are mineral soils; thus, soil pH is a good indicator for possible nutrient problems. For example, sulfur is available from pH 5.5 to 8.5; boron, copper, and zinc are most readily available from pH 5 to 7; and iron and manganese are abundant below pH 5, but moderately available from pH 5 to 7. Iron chlorosis frequently  occurs at pH above 7.7.

Factors Influencing pH

Initially, factors such as parent material, rainfall, and type of vegetation were dominant in determining the pH of soils. Under cultivation, however, organic acids from plant roots, repeated use of acid-forming fertilizers, plant  removal, and replacement of calcium and magnesium by hydrogen eventually lowers the pH of topsoil. Most of the nitrogen and phosphorus fertilizers used today are acid forming. For example, about 1.5 pounds of lime is required to neutralize the effect of applying 1 pound of anhydrous ammonia to the soil. Some irrigation water in contains substantial quantities of calcium and magnesium bicarbonates (lime) which help neutralize the acidifying effects. Thus, soils (without free lime) under production become increasingly acid unless lime is artificially applied or is present in the irrigation water. This means farmers need to frequently check soil pH to determine whether they are maintaining a proper soil acidity level.

(Source – http://www.milwaukeeinstruments.com/pdf/Master_AG900_Manual.pdf)

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