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Soils - Part 4: Soil pH

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Soil pH: The Nebraska Perspective

Active and Reserve Acidity

The relationship between active and reserve acidity is not constant across soils, and is influenced primarily by type and amount of clay and organic matter content of the soil. As the clay and organic matter content increase, the ratio of reserve to active acidity also increases. This relationship gives rise to the buffering capacity of the soil. The buffer capacity, or reserve, of a sandy soil is much less than that of a soil containing more clay such as a silt loam. Buffering, reserve, and active acidity are part of a complex system, which may be better explained by the following analogy.

Consider two full coffee urns, one with a 50-cup capacity and the other a 10-cup, both having the same size indicator tube and spigot. Coffee in the indicator tube represents the active acidity (measured by regular pH), and that in the urn represents the reserve acidity (measured by buffer pH). Let the large urn represent a clay soil high in organic matter, while the small urn represents a sandy soil. Both urns have equal amounts of coffee in the indicator tube; i.e., the same active hydrogen, so the same pH. Now, open the spigot and remove one cup of coffee from each urn. Removing one cup of coffee from each urn could be equated to adding small amounts of limestone to an acid soil. Opening the spigot will cause the level of coffee in the indicator tube to drop below the level in the urn, but will return to almost the original level (clay soil) when the spigot is closed. The momentary drop of coffee in the indicator tube represents the initial increase in pH when lime is added (affects the active hydrogen); but reserve hydrogen (similar to coffee in the urn) soon equalizes the effect from the lime, and the pH returns to essentially its original level, (clay soil, Figure 2C).

If the pH is 6.2 or lower, a buffer pH is run to measure the reserve acidity. The result of the buffer pH shows the amount of lime required to neutralize a major portion of the reserve acidity. The relative amounts of coffee in the two urns (Figure 2C) show why a sandy soil and a clay soil with the same pH result in different lime requirements. For example, removing one cup of coffee from each urn (like adding a small amount of limestone to an acid soil) reduced the total coffee (reserve acidity) by 10 percent in the small urn (sandy soil), but by only 2 percent of the large urn (clay soil). In a similar manner, one ton of agricultural limestone will make a greater change in the pH of a sandy soil than of a clay soil.

Figure 4.1

1. Herbicide activity. Soil pH markedly influences performance of s-triazine herbicides  (AAtrex, atrazine, Lexone, Princep, pramitol and Sencor). As soil pH increases,  particularly above 7.2, herbicides become more available to both weeds and crops, so it takes less herbicide to control weeds and also injure crops.

2. Activity of soil bacteria. Symbiotic Rhizobium bacteria associated with legume nodulation are reduced in acid soils. (Reduction starts at pH levels below 6.5 and worsens as soil pH becomes lower.) Acid topsoil may present problems for nodulation on new alfalfa seedings; but, once the plant roots reach the subsoil where the pH is usually higher, nodulation returns.

3. Availability of plant nutrients. High pH soils usually have excess calcium carbonate which reduces the availability of phosphorus by forming less soluble calcium phosphate compounds. Availability of micronutrients, like copper (Cu), manganese (Mn), zinc (Zn), iron (Fe) and boron (B), may be reduced, while calcium (Ca) and molybdenum (Mo) will be more available. Some plants may suffer lime-induced chlorosis (some soybean varieties and sorghum). At low pH values, iron, aluminum (Al) and manganese become very soluble and may become toxic. For example, some acid sandy soils in Kansas have toxic levels of aluminum, especially those on which wheat is grown. Fortunately, aluminum toxicity on acid sandy soils in Nebraska has not been a problem. Phosphorus availability may be reduced on acid soils, while zinc is increased.

4. Possible soil chemistry problems. Low pH (< 6.3) indicates the need to run buffer pH to determine if lime is needed; high pH indicates excess carbonates which may be accompanied by saline or sodic conditions.

5. Activity of soil microorganisms. Usually, at lower pH values, the microorganisms that decompose organic material are less active, resulting in reduced release of  plant nutrients, such as nitrogen (N), phosphorus (P), sulfur (S) and zinc (Zn). The activity of organisms causing plant diseases also may be influenced by soil pH.

6. Plant growth. Crops have a pH range where they grow best. These generalized ranges are shown in Table 4.1.

7. Reduced amounts of Ca and Mg.  At low pH values, less calcium and magnesium are held on the soil particles. Calcium deficiency has not been a problem in Nebraska soils, even at pH values below 5. However, on very acid, low cation exchange capacity (CEC) soils, excess application of potassium fertilizer could induce magnesium deficiency for some crops.

8. Changing the pH. Lime is used to raise the pH of acid soils. One milliequivalent (me) of calcium is required to remove 1 me. of hydrogen. The speed of the reaction depends on how finely the lime is ground. Beware of products with claims such as “200 pounds of product X, a unique and highly effective blend is equal to 2,000 pounds of 60 percent ECC Ag Lime.” Changing the pH is discussed in more detail later in this chapter.


 Table 4.1.  Preferred soil pH range of various crops.



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