Back to the Basics: Begin Your 2001 Fertilizer Program with a Good Liming Practice

February 2001
Dr. Martin B. Adjei - Range Cattle Research & Education Center

The Institute of Food and Agricultural Sciences of the University of Florida recommends lime application to an optimum pH of 5 and about 60 lb/A of nitrogen(N) fertilizer to bahiagrass pasture in south Florida. It is a known fact that repeated application of inorganic sources of N (ammonium nitrate, ammonium sulfate or urea) will eventually lead to acid soil conditions (pH <5). Soil acidity directly affects the growth and well-being of crop's roots due to aluminum and/or manganese toxicity. Acidity also indirectly affects the availabilities of soil nutrients to plants. The objective of this article is to discuss soil acidity, its importance, and steps to take to avoid related problems.

Theoretically, acidity refers to the concentration of active hydrogen ions (H+) in a system. Practically, acidity is measured by a term called pH which is defined as the reciprocal of the H+ activity i.e. pH= log 1/A H+. The lower the pH the more the active hydrogen ions present. A pH of 7 is neutral and is normally too high for most forages in Florida. A pH of 5 to 6 is slightly acidic and satisfactory for most Florida forages. A pH of 4 is too low, very acidic and will result in poor growth of most forages. Remember an acidic substance tends to increase active hydrogen ions of a system to which it is applied and conversely a basic substance tends to mop up (neutralize) hydrogen ions in a system. Another definition, that of buffers or buffer systems are compounds which can maintain the pH of a solution within a narrow range when small amounts of acids or bases are added. Buffers can either mop up or issue a few active H+ ions when the need arises.

The soil represents a very complex system with sources of acidity and buffering substances at the same time. Sources of soil acidity are humus or organic matter, clay minerals (aluminosilicates), hydrous oxides of iron and aluminum, exchangeable aluminum, soluble salts carbon dioxide and inorganic N fertilizer. Sources of soil buffering capacity are the amount and nature of the inorganic clay and organic matter.

Soil pH is the best single indicator of the general soil acidity conditions. However, as a measure of active soil acidity in a medium that has some buffering capacity, it gives no indication of the amount of lime to be applied to reach a target pH. The lime requirement of a soil is related not only to soil pH but also to its buffering capacity. Some soils are more highly buffered than others and a lime requirement determined for one soil will in all probability not be the same as that determined for another. Hence, soils classified as clays, peats and mucks have higher buffer capacity and, if acid, will have a higher lime requirements. Coarse textured soils with little or no organic matter will have a low lime requirement.

The implication of the discussion so far are:

  • Soil acidity will increase with repeated use of inorganic sources of N.
  • Increased soil acidity (below pH 5) could reduce your pasture production by more than a third, regardless of N fertilization and predispose the sward to damage by soil borne insects and exhibit grass yellowing.
  • However, indiscriminate use of lime on coarse-textured soils could lead to excessive alkaline conditions and a resulting deficiencies of iron, manganese and other micronutrients.
  • Adequate liming recommendations are based on a knowledge of the soil pH and buffer capacity which only the expert can provide.
  • The indiscriminate use of pH test kits by persons unfamiliar with the rudiments of soil chemistry is undesirable.

Liming as the term applies to agriculture is the addition to the soil of any calcium or calcium and magnesium containing compound that is capable of reducing soil acidity. Lime correctly refers to calcium oxide (CaO) but the term almost universally includes materials such as calcium-magnesium carbonate (dolomite) and calcium silicate. Most native soils in south-central Florida often have a pH of less than 4.5 and require lime application before pasture establishment. Bahia will grow satisfactorily at pH of 5 and all other warm season grasses require pH of 5.5.

If you have not applied any lime to your pasture within the past 3 years then as you plan your 2001 forage program, take representative soil samples from the fields and allow a reputable soils laboratory to analyze and make appropriate liming recommendation. You may consult with your county agents for the proper sampling and soil processing procedures.

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