Method of expressing acidity or alkalinity : Soil Acidity

Method of expressing acidity or alkalinity

Equivalent quantities of all acids or alkalies contain the same number of total H or OH ions, respectively. But, when they are dissolved in water they do not ionize to the same extent. The amount of acid or alkali ionized depends upon the content of free H and OH ions. When the dissociation is high e.g., hydrochloric acid (a strong acid), it dissociates to a larger extent than the weak acetic acid. Acetic acid dissociates only to about 10 % as compared to hydrochloric acid. In a 1N solution of hydrochloric acid there will be 1 gram of H ions per liter, while in a normal solution of acetic acid there will be 1/10 gram of H ions per liter. But in titration 1 ml of 1 N hydrochloric acid and 1 ml of 1 N acetic acid will require 1 ml of 1 N sodium hydroxide for neutralization separately, because the total acidity is the same and titration determines both the ionized and unionized H or OH ions.

Soil acidity: There are three kinds of acidity.

(i) Active acidity is due to the H+ ion in the soil solution.
(ii) Salt replaceable acidity represented by the hydrogen and aluminum that are easily exchangeable by other cations in a simple unbuffered salt solution such as KCl and
(iii) Residual acidity is that acidity which can be neutralized by limestone or other alkaline materials but cannot be detected by the salt-replaceable technique. Obviously, these types of acidity all add up to the total acidity of a soil.

i. Active acidity: The active acidity is a measure of the H+ ion activity in the soil solution at any given time. However, the quantity of H+ ions owing to active acidity is very small compared to the quantity in the exchange and residual acidity forms. For example, only about 2 kg of calcium carbonate would be required to neutralize the active acidity in a hectare-furrow slice of an average mineral soil at pH 4 and 200/0 moisture. Even though the concentration of hydrogen ions owing to active acidity is extremely small, it is important because this is the environment to which plants and microbes are exposed.

ii. Salt replaceable (exchangeable) acidity: This type of acidity is primarily associated with the exchangeable aluminum and hydrogen ions that are present in largest quantities in very acid soils. These ions can be released into the soil solution by an unbuffered salt such as KCl.

Al3+ + 4KCI +AlCI3+HCI’ L~~~ H + L~~~~ ~ (Soil Solid) (Soil Solution) (Soil Solid) (Soil solution), in moderately acid soils, the quantity of easily exchangeable aluminum and hydrogen is quite limited. Even in these soils, however, the limestone needed to neutralize this type of acidity is commonly more than 100 times that needed for the soil solution (active acidity). At a given pH value, exchangeable acidity is generally highest for smectites, intermediate for vermiculites, and lowest for kaolinite. In any case, however, it accounts for only a small portion of the total soil acidity as the next section will verify.

iii. Residual acidity: Residual acidity is that which remains in the soil after active and exchange acidity has been neutralized. Residual acidity is generally associated with aluminum hydroxy ions and with hydrogen and aluminum atoms that are bound in non exchangeable forms by organic matter and silicate clays. If lime is added to a soil, the pH increases and the aluminum hydroxy ions are changed to uncharged gibbsite as follows. OH- OH- AI (OH) 2+ 7 AI (OH) 2+ 7 AI (OH) 3 In addition, as the pH increases bound hydrogen and aluminum can be released by calcium and magnesium In the lime materials [Ca (OH) 2 is used as an example of the reactive calcium liming material] The residual acidity is commonly far greater than either the active or salt replaceable acidity. Conservative estimates suggest that the residual acidity may be 1000 times greater than the soil solution or active acidity in a sandy soil and 50000 or even 100000 times greater in a clayey soil high in organic matter. The amount of ground limestone recommended to at least partly neutralize residual acidity is commonly 4-8 metric tons (Mg) per hectare furrow slice (1.8-3.6 tons/AFS).

It is obvious that the pH of the soil solution is only "the tip of the iceberg" in determining how much lime is needed. Buffering and Soil Reaction Buffer action: Buffering refers to resistance to a change in pH. If 1 ml HCI (of 0.1 N) is added to one liter of pure distilled water of pH 7.0, the resulting solution would have a pH of about 5.0. If on the other hand, the same amount of acid is added to a liter of soil suspension the resulting change in pH would be very small. There is, a distinct resistance to change in pH. This power to resist a change in pH is called buffer action. A buffer solution is one which contains reserve acidity and alkalinity and does not change pH with small additions of acids or alkalies. Buffer capacity: The colloidal complex acts as a powerful buffer in the soil and does not allow rapid and sudden changes in soil reaction. Buffering depends upon the amount of colloidal material present in soil. Clay soils rich in organic matter are more highly buffered than sandy soils. Buffer capacity of the soil varies with its cation exchange capacity (C.E.C.). The greater the C.E.C. the greater will be its buffer capacity. Thus heavier the texture and the greater the organic matter content of a soil, the greater is the amount of acid or alkaline material required to change its pH. Importance of Buffering to Agriculture: Changes In soil reaction (pH) have a direct influence on the plants and it also affects the availability of plant nutrients. Deficiency of certain plant nutrients and excess availability of others in toxic amounts would seriously upset the nutritional balance in the soil. Buffering prevents sudden changes and fluctuation in soil pH, so it regulates the availability of nutrients and also checks direct toxic effect to plants.

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