pH & EC Values of Soil Samples
of the Parameters , EC & pH.
A saline soil contains sufficient soluble salts adversely affect crop growth & production. On visual inspection of fields , saline soils often show white patches , spotty crops stands , irregular crop growth stunted deep green plants and in some cases visible signs of salt injury , such as tip burn of leaves and chlorosis of leaves.
It is how ever necessary to know the amount of salt in the soil for further remedial measures. The amount of salt in a soil can be precisely determined only by complete chemical analysis. But a close estimate can be obtained relatively easily by measuring Electrical Conductivity of Soil : Water mixture. The more the salts , higher the electrical conductivity. The Electrical Conductivity of a soil solution is usually expressed in milli mohs/cm and that of irrigation water in milli mohs/cm at 25°C.
The pH value of soil is indicative of acidic/sodic nature of soil. The pH value of a solution is the negative logarithm of the Hydrogen ion activity. When the exchangeable sodium percentage becomes high (exceeds 10 - 20% of exchange capacity) , the pH value of such soil sample is above 8.5 and soil show physical conditions of low permeability , sticky when wet and hard when dry , nutritional disorders etc which visually display in reduction of crop growth and water logged conditions.
Procedures to determine EC Value of Soil Sample.
Measurements of Electrical Conductivity is determined on a saturation extract of soil or supernatent liquid of 1:2 soil water suspension. Electrical conductivity is measured with the help of Electrical Conductivity Meter. The Conductivity Meter is to be calibrated and cell constant be determined with a Standard Solution of 0.7456 gm of dry potassium chloride of 1 liter of distilled water (at 25°C , this solution gives Electrical Conductivity of 1.41 milli mohs/cm)
A 20gm of soil sample is shaken with 40ml of distilled water in a 250ml conical flask for 1hr. The conductivity of the suppernatent liquid is determined with the help of conductivity meter.
Procedure for Determination of Soil pH.
Definition : The pH value of solution is the negative logarithm of Hydrogen Ion activity.
Instrument : A glass electrode , pH meter with calomel reference electrode.
Procedure : Standard Buffer Solution
These may be of pH 4.0 and in other ranges of expected soil pH values. In case of buffer tablets (available commercially) , a single piece is to be dissolved in double distilled water and made up to 100ml. A 0.05 m solution of Potassium Hydrogen pthalet gives pH 4.0 & can also be used. It is necessary to prepare fresh solutions after a few days as the buffers do not keep for long.
20gm of soil is taken in 100ml beaker to which 40ml of distilled water are added. The suspension is steered at regular intervals for 30 minutes. Then pH is recorded. The suspension must be steered well just before the electrodes are immersed. pH meter is calibrated with 2 buffers , one in the acidic side and the other alkaline or neutral range. The glass & calomel electrodes are inserted in suspension and pH measurement is made. Mr.Vyas , Research Assistant.
|" Ranges in Soil pH values "|
|Soil pH Value||Soil Reaction|
|more than 7.0||Alkaline|
|less then 7.0||Acidic|
|6.5 to 7.5||Neutral for Practical purposes , maximum availability of all the essential plant nutrients|
|7.5 to 8.2||Normal of calcareous or saline soils.|
|8.50 and above||Alkali or Sodic soils.|
|3.5 to 6.5||Acid Soils of warm to humid and high rain fall areas. (Laterite soils)|
|less than 3.5||Acid Sulphate soils (Kerala costal belt)|