Soil Reaction: Types, Factors and Influence | Soil Science

In this article we will discuss about:- 1. Types of Soil Reaction 2. Soil pH 3. Factors Controlling Soil Reaction 4. Influence.

Types of Soil Reaction:

One of the outstanding physiological characteristics of the soil solution is its reaction. Soil reaction influences many physical and chemical properties of soil. The growth and activity of plant and soil organisms depend on soil reaction and the factors associated with it.

There can be three types of soil reaction as follows:

(i) Acidity:

Soil acidity is common in regions where precipitation is high enough to leach appreciable amounts of exchangeable bases from the surface layers of the soil. The two adsorbed cations such as Hydrogen and Aluminium are largely responsible for soil acidity. The acid soil is generally found in humid region. The factors which will help in the release and removal of bases will help in the development of acidity of soil. If the hydrogen (H⁺) ion becomes more than hydroxyl (OH^– ) ion in the soil solution, the soil becomes acidic.

A highly acidic soil may have pH 4.5 and low calcium and magnesium, high solubility of iron, manganese, aluminium etc., but low availability of nitrogen and phosphorus. The activity of microorganism responsible for nitrification is adversely affected in acid soil. Generally limes are used for reclamation of acid soil.

(ii) Alkalinity:

The soil that contains absorbed sodium to interfere with the growth of most crop plant is known as alkali soil. The amount of exchangeable sodium in great quantities in the soil makes the soil alkalinity. The sodium ion easily displaced the calcium ion from clay colloid and makes the sodium mixed clay particles. This sodium is converted into sodium hydroxide by hydrolysis as per the following reaction-

The OH^– ion thus formed increases the soil pH.

(iii) Neutrality:

In those areas, where the soil contain hydrogen and hydroxyl ion almost in equal quantities, the soils are neutral in character.

Soil pH:

pH is the negative logarithm of hydrogen ion concentration of soil solution and it is usually written as –

pH scale is used to measure the acidity or alkalinity of a soil solution (or other solution). This scale runs from 0 pH to 14 pH. In this scale, the 7 units level is known as neutral i.e. neither acidic nor alkaline. Pure water has a pH-7.0. All values below pH 7.0 denotes acidity and the values above pH 7.0 represents alkalinity. The degree of acidity increases as pH decreases below pH 7.0. Soil showing pH 5 is ten times more acidic than showing pH 6.0. Likewise, the degree of alkalinity increase as we go higher from pH 7.0. The alkalinity at pH 9.0. unit is ten times more than that pH 8.0 units.

Factors Controlling Soil Reaction:

There are some factors that control soil reaction are as follows:

(i) Nature of Soil Colloid:

Soil colloid influences soil reaction to a very great extent. Soil colloids when dominated by adsorbed hydrogen (H⁺) ion, the reaction of soil becomes acidic. On the other hand, soil colloid when dominated by hydroxyl (OH^–) ion, the reaction of soil becomes alkalinity.

(ii) Nature of Ion:

The soil that contains more hydrogen ion than hydroxyl ions becomes acidic in reaction. When the aluminium ions are present in the soil, they react with water to liberate hydrogen ions, which increases the soil acidity.

(iii) Percentage Base Saturation:

A low percentage base saturation of soil means soil acidity. In humid areas, the basic elements have been leached down from the soil, the percentage base saturation decreases much below 80 and they become acidic in reaction. If the percentage of base saturation is above 80 and at 90, then they become neutral in reaction and alkaline reaction respectively.

(iv) Rainfall:

Rainfall plays important role in determining the soil reaction. The soils that are developed in high rainfall areas, becomes acidic in nature due to leaching of some nutrients such as calcium (Ca⁺⁺), magnesium (Mg⁺⁺) etc. from soil solution. So leaching encourages the development of soil acidity. On other hand, the soils that are developed in low rainfall areas, becomes alkaline in nature.

(v) Fertilizers:

The continual use of fertilizers is responsible for a marked change in soil pH. Acid forming fertilizers such as Ammonium sulphate, Urea, Ammonium nitrate etc. when applied in the soil in large quantities makes the soil acidic. On the other hand, basic fertilizers such Sodium nitrate, Basic slag etc. makes the soil alkaline.

Influence of Soil Reaction on the Availability of Nutrients:

Soil reaction affects the availability of nutrients to plants. Soil reaction affects indirectly in availability of nutrients to plants as soil organism do not function well in acid and alkali soil. Soil organism do their function at their best within a pH range 6.0-7.5.

The influence of soil reaction on the availability of nutrients to plant is as follows:

1. Nitrogen:

Nitrogen is most important nutrient for plants. Plant absorbs nitrogen in the form of Ammonium (NH₄⁺) and Nitrate (NO₃^–). Out of these two forms, plant absorbs most of their nitrogen in the form of nitrate (NO₃^–). The availability of nitrate nitrogen depends on the activity of nitrifying bacteria’s. The microorganism responsible for nitrification are most active when the soil pH is between 6.5 and 7.5.

The activity of nitrifying bacteria is adversely affected if pH falls below 5.5 and more than 9.0. The activity of nitrogen fixing bacteria (e.g. Azotobacter) falls down at below soil pH 6.0. In acidic condition, the decomposition of organic matter, the main source of nitrogen, is also slow down.

2. Phosphorus:

Phosphorus is an essential constituent of every living cells and for nutrition of plant and animal. The availability of phosphorus depends on the soil pH. In strongly acidic soil (pH 5.0 or less), iron, aluminium, magnesium and other bases remains in soluble form and phosphorus reacts with these bases are converted into insoluble form and become unavailable to plant.

3. Potassium:

Potassium is an essential element for the development of chlorophyll. The availability of potassium does not influence by soil reaction to any great extent. In acid soil, potassium is lost through leaching. Application of lime for reclamation of acid soil result in an increase in potassium fixation of soils and the potassium remains in the soil in the form of non-availability.

4. Sulphur:

Sulphur is an important element for oil seeds, cruciferae, sugar and pulse crop. The availability of sulphur is not affected by soil reaction. In acid soil, it is more soluble and is subjected to loss by leaching.

5. Calcium:

Calcium as calcium pectate is an important constituent of cell wall and requires in large amounts for cell division. Acid soils are poor in calcium. In alkali soil (pH not exceeding 8.5), the availability of calcium remains high. The availability of calcium decreases when soil pH is above 8.5.

6. Magnesium:

Magnesium is an essential constituent of chlorophyll. Acid soils are poor in magnesium. In alkali soil (pH not exceeding 8.5), the availability of magnesium remains high. The availability of magnesium decreases when soil pH is above 8.5.

7. Manganese:

Manganese is an essential constituent of chlorophyll and also formation of oils and fats. Soil pH has decided influence in the availability of manganese. At high pH values, all cations are unfavourably affected. Over liming or a naturally high pH is associated with deficiencies of manganese and such conditions occur in nature in many of the calcareous soils of West Bengal.

8. Iron:

Iron is necessary for the synthesis of chlorophyll. In very acid soil, there is relative abundance of ions of iron. Iron deficiency of plant due to high pH is not uncommon. At high pH i.e. in alkali soils, ferrous (Fe²⁺) ion is converted to ferric (Fe³⁺) and precipitated as Ferric oxide (Fe₂O₃). The availability of iron increases as the pH of soil decreases.

9. Zinc:

Soil pH affects the availability of Zinc. The zinc deficiency occurs on soils that are slightly acidic to neutral. High pH reduces the availability of zinc by precipitating zinc as zinc hydroxides.

10. Boron:

Boron occurs in most soil in extremely small quantities. The availability and utilization of boron is determined to a considerable extent by soil pH. Boron is most soluble under acid condition. It apparently occurs in acid soil in part as boric acid and this is readily available to plants. The high soil pH causes boron deficiency in plants forming complex compound. A specific Ca : B is required for every crop. When calcium level is high, boron content should be high and if no, plant will show boron deficiency.

11. Copper:

Copper is an essential constituent of enzyme. In very acid soil, there is relative abundance of copper. The solubility of copper decreases as pH increases particularly in sandy soils. The decrease in solubility with increasing the soil pH may be result of precipitation of copper in the form of cupric oxide (CuO). The oxidized state of copper i.e. hydroxides or hydrous oxides is insoluble. Copper deficiency is induced by heavy liming and excessive application of nitrogen and phosphorus.

12. Molybdenum:

Molybdenum is a constituent part of the enzyme, nitrate reductase. Molybdenum availability is significantly dependent on soil pH. It is quite unavailable in strongly acid soil and becomes available by liming of acid soil. As the pH is raised to 6.0 or above, its availability increases.