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What is acidic soil | Forms acidity in soil

What is acidic soil | Forms acidity in soil

Soil acidity

Soil acidity: Soil acidity is due to the acid content of the soil solution and to the presence of acidic salts in the soil in addition to the presence of hydrogen and aluminum ions. Soil acidity directly affects soil fertility through its effect on the concentration of available nutrients and also affects the activity of microorganisms and plantings


Forms of acidity in soil: 

1- Active acidity: 

It results from the presence of hydrogen ions in the soil solution and is measured by the amount of PH or the number of milliliters of alkali equivalents required to neutralize the hydrogen present in the soil solution. The PH of the soil solution is between 3.5 to more than 9

  • The most acidic soils are the Turbine swamp and podzolic soils, where the PH is between 4 to 6. 
  • Soil pH varies, but most agricultural plants need soils of light acidity and light alkalinity. The activity of microorganisms is related to the pH of the soil solution, where the fungus is activated in an acidic soils, but in an alkaline and neutral environment, the bacteria are activated 


Active acidity in most soils arises from the presence of carbon acid and its acidic salts, and the degree of its ionization is determined by the following equation:

 Co2       H+ =K--------------Ca(HCo3)2 

The presence of calcium bicarbonate reduces the acidity.


2- Potential acidity

The cause of potential acidity is the presence of adsorbed hydrogen. When the soil is exposed to solutions of neutral salts, a reciprocal  reaction takes place as follows:

 soil colloid]]H +KCl ⟶ soil colloid]]K+HCl 

The reason for the potential acidity is sometimes due to the adsorbed aluminum, where the  reciprocal reaction takes place as follows:

soil colloid]]Ca,Mg,H,Al + 4KCl ⟶ soil colloid] ]Ca,Mg,4K + HCl + AlCl3


Aluminum chlorine hydrolyses to form hydrochloric acid according to the following reaction: 

AlCl3 +3H2⟶ Al(OH)2 +3HCl 

According to the previous equations, it can be said that the potential acidity arises from the presence of adsorbed  hydrogen or aluminum or both of them,

hydrogen may be the main cause of it in some soils, and aluminum is the main cause in other soils, and this acidity does not appear except when the soil is treated with salty solutions and according to the type of salts with which it is treated Soil, we have two types of latent acidity: exchange acidity and hydrolytic acidity.

Exchange acidity:

It appears when treating the soil with neutral salty solutions, especially potassium chloride, as in the previous equations, as the chlorine acid resulting from the exchange process or from the hydrolysis of aluminum chlorine gives the exchange acidity and is usually measured by the pH of the saline extract of the soil. . We observe this phenomenon in podzolic and red soils, but in light soils, neutral or alkaline soils, this acidity does not appear.


Hydrolytic acidity: 

this acidity arises from the neutralization of the soil with solutions of alkaline salts such as sodium acetate that are able to displace hydrogen ions  that adsorbed on soil colloids to a degree greater than the ability of neutral salts to do so.

adsorption complex]]H + CH3COONa ⇄ adsorption complex]]Na + CH3COOH

The formed acetic acid expresses hydrolytic acidity and is estimated in millimeter equivalent in 100 g of soil 

Adsorption complex = soil colloid


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