Very nice explanation by Dr Tarafdar and Dr Thapa with good input from Dr Ghafoor..It will be interesting to know, what type of soil is to be subjected to saturation paste pH and when soil-water suspension is to be used.

Soil pH is defined as the negative logarithm of the active hydrogen ion concentration. In which the activity of H ion in the soil water suspension is expressed as g ions /L. A solution with H ion activity of 0.001 M will have a pH of 3.0, one with an H ion activity 0.0001 M, 4 and so on. The H ion  concentration has a ten fold change between each whole pH number. Thus a soil of pH 5.0 has 10 times more H ion in soil solution than a soil solution with a pH 6.0.

The soil pH is measured by electrometric method, in which involves a glass, H ion sensing (indicator) electrode paired with a reference electrode attached to a suitable meter for measuring emf, which is proportional to pH ( Jackson 1973; Page et al. 1982).

Take 100 ml of dry soil and cover will 200 ml of water stir the soil will water for one minute and then let it sit to settle out. Some researchers like to use 0.1 Cacl to make the pH measurement others just use distilled water.  After another one minute or so take a pH meter and read the value. Make sure your meter is well calibrated and accurate. Alternatively, the solution can be tested by litmus paper with a color reference chart.  J. C. Tarafdar gave a very good explanation. In dry areas, most pH problems are related to alkalinity and salts and these are problematic in soil pH of over 7.3 especially over 7.8. Under the alkaline condition the micro nutrients Fe, Zn and Mn can become limiting as well as P can be immobilized.  In areas where rainfall exceeds evaporation, the issues are usually associated with acid soil infertility which is related Mn, Al and Fe toxicity and low nutrient availability. If the soil is above 5.3 and 5.7 pH, respectively,  Al and Mn toxicity is usually not a problem. Optima for PNK is at about 6.5 to 7.0. Most crops and varieties have diverse optima for pH and nutrient needs. 

 Note to the master of poly reed is 100 ml dry soil + 200 ml of distilled water with a vibration at a certain temperature, then measure the pH of the mixture and then apply the extension of the law. That's what I understood from the text Is it true or not?

People in laboratories use temperature as room temperature which 25 C in the temperate areas. Many are using hand held meters and making readings right in the field where temperature will vary. In United States they use distilled water which is neutral in pH if this is not available the type of water used should be noted but still should give good data. Water is at room temperature unless the readings are being made in the field with portable instruments. On the web there is a review of 10 best pH meters for soil that is worth a look. In my conditions most of the pH readings will be acid I imagine in your much drier conditions the readings will mostly be alkaline. Hope some of this information was useful. 

There were very healthy discussion that took place in research gate earlier in this topic i am attaching the link below hope you will fond it helpful. 

Fulvio M. Rivano (https://www.researchgate.net/profile/Fulvio_Rivano) have presented an very good information. 

Schofield (ca. 1950) trying to keep a constant pH value, not modified by the diffuse layer, proposed to measure the soil pH in CaCl2 0.01 M solution. The following "tables" show the variation of pH using different soil/water and soil/CaCl2 solution ratios in a Chilean Haplohumult (Cudico Series):

Soil/water     measured soil pH

1/10                 5.32

1/5                   5.30

1/2.5                5.22

1/1                   5.09

Soil/Solution (CaCl2 0.01 M)      measured soil pH

1/10                                                         4.43

1/5                                                           4.42

1/2.5                                                        4.41

1/1                                                           4.42

Which are the best methods for the measurement of soil pH?. Available from: https://www.researchgate.net/post/Which_are_the_best_methods_for_the_measurement_of_soil_pH [accessed Jul 15, 2017].

https://www.researchgate.net/post/Which_are_the_best_methods_for_the_measurement_of_soil_pH

https://www.researchgate.net/post/Whats_the_most_accurate_way_to_measure_pH_of_soil

Good discussion. The pH of saturated paste is measured electro-metrically that mostly satisfactory for arid lands. Such pH has been well correlated with nutrient ion availability, ion exchange, salt precipitation/salt dissolution reactions and with soil physical/chemical/physico-chemical soil properties.

Raju presented typical data for non-saline soils, for which the 10 mM CaCl2 pH values usually agree well with those in saturated pastes, i.e. the pH values in water are artificially elevated due to artificially low ionic strength compared with field moist soils.  With particles of ~ 2 mm  diameter the shaking time for stabilisation is typically 30 to 60 min.  pH indicators can be used in addition to instrumental methods  

useful Link

vlab.amrita.edu/?sub=2&brch=193&sim=1549&cnt=1

Very nice explanation by Dr Tarafdar and Dr Thapa with good input from Dr Ghafoor..It will be interesting to know, what type of soil is to be subjected to saturation paste pH and when soil-water suspension is to be used.

Dr. Tarafdar nicely explained principle and the method used to measure the pH of soil.  But selection of the sensor as well as its regular calibration and maintenance are very important for correct estimation. 

Beside saturated soil paste, pH can also be measured in salt solution ( 1N KCL or 0.01M CaCl2). The main advantage of the measurement in salt solution is the tendency to eliminate interference from suspension effects and from variable salt contents, such as fertilizer residues.

soil solution or salt solution with a pH meter is enough

In a national study comparing four in-field soil pH determination methods by University of Arkansas, the hand-held pH meter produced results closest to the average from 82 laboratories while the pH probe provided the same soil pH for three sample groups tested . Results from the color kit were intermediate between the pH probe and hand-held meter.

The accuracy of the hand-held meters is offset by cost and complicated operation. The hand-held meter is a portable version of a laboratory meter and must have the same care and calibration with buffer solutions before each use. Fresh and accurate buffer solutions are needed for each time the hand-held pH meter is calibrated. The accuracy of the hand-held meter is dependent on the operator’s care and condition of the electrode. A hand-held meter kept in a truck with a dried electrode cannot be expected to perform adequately. Improper storage and treatment of an electrode will not only produce poor results but require replacement before the meter will again perform to standard.

Soil probe was unable to distinguish between samples with more than a 2.0 pH unit difference, making it unacceptable for use for estimating soil pH in the field. However, the color kit used in this study came with a whole unit (1.0) graduated scale. A smaller graduation pH scale of 0.5 units is necessary in many cropping situations. Sometimes 0.1 – 0.2 unit differences are critical when determining pH given the high cost of lime in Oregon. Solutions used in color kits usually degrade with time and heat. Care is needed to insure the solutions have not degraded. This method can be effective when used to determine which samples will need to be sent to a laboratory for further testing.

Another used method not utilized in this study is pH strip papers impregnated with pH sensitive dyes. Accuracy of pH strips is similar to the color kit and would prove to have nearly similar challenges other than the absence of solutions in pH strips. As with all in-field pH test methods, periodic lab testing is recommended to ensure accuracy with this method.

It is also important to know how the soil lab you use determines soil pH. This is because comparison of results requires same methodology for measuring pH primarily the amount of water mixed with soil. Three common methods for soil pH determination are used by laboratories in the western United States—saturated paste, 1:1 and 1:2 ratio of soil to water. It has been shown that soil pH increases slightly for acidic soils as soil to water ratio increases. Soil pH by saturated paste will be slightly lower than 1:1, which will be slightly lower than 1:2 soil-water ratio. The differences between water measurements will be smaller though at about 0.1 – 0.2 pH units. Excerpts from Measuring Soil PH,Sam Angima, Oregon State University Small Farms Program,Publish Date:  Fall 2010 VolNo:   4

Dried tobacco leaves that have been milled to 60 mesh size powder weighed as much as 10 g and placed in 250 ml beaker. After that add as much as 5% NaOH in 10 ml beaker and then stirred using a stirrer for 15 minutes. Then filtered

using Buncher glass wool.

Hi Dr @Paul Reed Hepperly

Can you give us more explication about your contribution : Under the alkaline condition the micro nutrients Fe, Zn and Mn can become limiting as well as P can be immobilized .

Thanks you