Soil pH

The PH is the negative logarithm of the hydrogen ion activity or the logarithm of the reciprocal of the H ion concentration.

PH = long 10 (H+) = log 10 ( H)-1-

The PH of pure water at 25 0C is 7 while that of acid is below 7 and that of base is above 7.

Significance of Soil PH Measurement:

Plant can grow on different types of soils with wide varieties of properties and wide ranges of PH. However plants do have a preference for a specific range of soil reactions or soil PH. In above figure we have seen different ranges of soil PH like from slightly acidic to very strong acid from slightly alkaline to very strongly alkaline and intermediate of both, neutral.

This soil reaction does have its effects on availability of nutrients and thus on growth of plants. Soil PH has influence on various soil properties like chemical, physical and biological.

Rating Chart for Soil PH:

Less than 6.0: Acidic
7.0 – 8.5: Normal
8.6 – 9.0: Tendency to become alkaline.
Able 9.0: Alkali

Methods PH Measurement:

I. Glass Electrode PH Meter:

The glass electrode generally used for the PH measurement employes a glass membrane of special, chemically pure, soft glass paired with a calomel reference electrode are used for measuring the soil PH.

II. Hydrogen Electrode:

In order to measure hydrogen ion concentrations of pH of a solution use of hydrogen gas as a metal electrode and dip in a solution under test and measure the electrode potential.

III. Quinhydrone Electrode:

It consists of equimolecular mixture of quinone and hydroquinone added directly to the solution under test. This electrode does not give correct reading in alkaline solution for PH grater than 9.0.

Factors Affecting PH Measurement

1. Soil Solution Ratio:

Condition being the same, the more water added to the soil sample the higher the soil PH.

2. Salt and Electrolyte Content:

A salt also increases the hydrolysis of Al and Fe ions, increasing the H+ in solution. The salts lower the PH. The use of 0.01 M cacl2 has been recommend as a solution which masks small differences in salt contents without displacing a large fractions of the hydrogen ions.

3. Degree of Dissociations of Hydrogen Ions:

Soil colloids differ in the amounts of discrete hydroxyl groups capable of ionizing the hydrogen ions.

4. Type of the Acidity:

Hydrogen ions adsorbed to electrostatic bands that have great tendency to dissociate than those from the OH groups.

5. CO2 Content:

The CO2 dissolved in water forming H2CO3 lowers the PH markedly.

6. Liquid Junction Potential:

Appreciable liquid junction potential ( Ej) may occur at the liquid junction particularly into very concentrated colloidal electro suspension. Liquid junction potential kept low by the use of a saturated KCL solution.

7. Equipment and Procedure:

Major difference in the PH appears due to inadequate standardization of the equipment rather than a limitation of the equipment.

Method of Soil PH Measurement - Glass Electrode PH Meter

The glass electrode generally used for the PH measurement employes a glass membrane of special, chemically pure, soft glass paired with a calomel reference electrode are used for measuring the soil PH.

Composition of corning glass electrode: 72 % SiO2, 6.0 % CaO and 22.0% NazO.
1. Vacuum tube volt meter
2. Hg.Hg2Cl2 Half cell
3. Saturated KCL.
4. Glass membrane
5. KCL crystals
6. HCL
7. Test solution

Hg. Hg2-Cl2, KCL saturated H+ (a1) KCL. Hg2cl2 H.
Glass electrode system for measurement of PH.

Across the glass membrane develops an electrical potential that is proportional to the difference in PH, between the two slide in accord with concentration cell in which the glass membrane and liquid junction. A systematical potential E develops across the glass membrane even, when solution of the same H+ ion activity is on the two sides. For this reason the meter must be calibrated with the standards buffer solution.

Advantages of the Glass Electrode PH Meter:

1. It does not expel dissolved gases such as CO2 from the system under measurement.

2. It is adaptable to thick fluid, paste and coloured solution.

3. It is not affected by oxidizing and reducing solution in contrast to quinhydrone or hydrogen electrode.

4. It does not require H2 gas or catalytic surface required by H electrode.

5. It has relatively low salt error.

6. After standardization give accuracy within 0.1 PH unit with care within 0.02 PH unit.

7. Rapid convenient in expansive and adaptable for long term recording.

Precaution in the Use of Glass Electrode:

1. Electrode is not allowed to remain in the test solution or suspension longer than necessary especially if more alkaline than PH 9.0.
      
2. Immediately after testing, the electrode is washed off with a strong stream of distilled water from a wash bottle. If the system was alkaline, the electrode should be dipped for a few second in the acid PH buffer or dilute HCL to remove the film of CaCO3 that some time forms.

3. For storage after cleaning the electrode is suspended in distilled water and the system is protected from to evaporation. Drying out of the electrode is avoided.

4. Failure of the glass electrode PH meter is indicated, when , after standardization it gives slow response to large PH changes.

Reference: https://www.agriinfo.in