Mr.Yacine,good question.Some details are available in research gate link provided by Dr.Antoun .As remarked by Dr.Schnug, one has to decide, is it convention or convenience.Determination of soil pH  in saturation paste gives soil pH values nearer to field moist condition. Preparation of saturation paste for routine analysis of a large number of samples is very difficult (But one has to use this procedure for characterizing saline and sodic soils).Next alternative is use of 1:1 soil: water suspension which is convenient for measuring soil pH.I understand that many laboratories use 1:1 ratio procedure in several states  of USA.In India we use 1:2 soil : water ratio.In European counties 1:5 ratio is used(from literature I looked into).As I understand a wider soil: water ratio (1:2.5 to 1:5)is used in organic soils as they absorb more water because of the presence of more organic matter.There are reports which indicate pH change of 0.1 unit for dilution from 1:1 to 1:2.5 and a change of 0.25 units from 1:2 to 1:5 ratio.These pH changes are variable in diverse soils.This increase in pH is due to surface ionization of the colloidal particles and hydrolysis of exchangeable bases(Puri and Asghar 1938 Soil Science 46,(3),249-258).There may not be much variation in pH when one uses 1:2 or 1:2.5 soil water ratio.But what is the reason for using the wide ratio between soil and water in Europe?Is it organic matter status in soils? any other reason?

not really, its simply a convention!

this beg another question , How is that that a convention become norm in science ? 

Dear Louadj Yacine

Actually pH of 1:2.5 was introduced for ease where large number of samples were to be tested and also to be extracted for other soil properties, e.g. farmer advisory services as well as exploratory surveys. Latter for such situations, even 1: 5, 1: 7.5 and even 1: 10 soil water ratios were followed. And scientists calibrated these soil : water ration with saturated paste and its extracts for different soil properties. However, I do not think any scientific reason for doing so. Still the best method is pH of saturation paste and its extract for other soil parameters.

We here in Punjab, Pakistan recorded higher conc of Ca, Mg, EC, TSS etc as the soil to water ratio increased. The pH1:1, 1:2.5, 1:5, 1:7.5 and 1:10 soil to water ratios was lower than pHs for calcareous saline and /or sodic soils but of reverse was true for acidic soils. The Na could increase with higher soil to water ratios where soils contain zeolite clay minerals.

In Langlier Precipitation Index, Gapon equation and its modifications, pHs is used and recommended, because the saturated solution of lime has pH of 8.2 – 8.4, that is why Bower in 1972 & 1975, Rhoades and Suarez in 1981, in their publications sometimes pH of 8.2 or 8.4.

 Mr.Yacine,good question.Some details are available in research gate link provided by Dr.Antoun .As remarked by Dr.Schnug, one has to decide, is it convention or convenience.Determination of soil pH  in saturation paste gives soil pH values nearer to field moist condition. Preparation of saturation paste for routine analysis of a large number of samples is very difficult (But one has to use this procedure for characterizing saline and sodic soils).Next alternative is use of 1:1 soil: water suspension which is convenient for measuring soil pH.I understand that many laboratories use 1:1 ratio procedure in several states  of USA.In India we use 1:2 soil : water ratio.In European counties 1:5 ratio is used(from literature I looked into).As I understand a wider soil: water ratio (1:2.5 to 1:5)is used in organic soils as they absorb more water because of the presence of more organic matter.There are reports which indicate pH change of 0.1 unit for dilution from 1:1 to 1:2.5 and a change of 0.25 units from 1:2 to 1:5 ratio.These pH changes are variable in diverse soils.This increase in pH is due to surface ionization of the colloidal particles and hydrolysis of exchangeable bases(Puri and Asghar 1938 Soil Science 46,(3),249-258).There may not be much variation in pH when one uses 1:2 or 1:2.5 soil water ratio.But what is the reason for using the wide ratio between soil and water in Europe?Is it organic matter status in soils? any other reason?

Louadj

As previous contributors have said it sometimes comes down to convenience and efficiency (here is Australia, we use a standard 1:5 ratio that doubles for rapid measurement of pH and EC). But as also pointed out - due to the presence of difference salts etc, these quick methods have limitations and different ratios will give you different results. The purpose of "standardising" at a given ratio is that it allows comparisons between samples (within the known limitations) even if the absolute value is an estimate. To avoid some of the suspension effects caused by differing soil to water ratios, some advocate pH measured in a dilute electrolyte (such as CaCl2 - you might might want to read Scholfield and Taylor 1955. The measurement of soil pH in the Soil Sci Soc Am Proc Vol. 19 No. 2, p. 164-167. for some background) and the difference between water and electrolyte suspensions gives you some Information about the surface chemistry/charge characteristics of your soil. So the methods you use should be fit for purpose. If you are looking for changes in time - then it matters not what ratio  you use as long as you keep it the same. If you are exchanging data between researchers - again standardisation is the key.

Regards, Andrew

Based on the initial work of Puri and Asghar (1938) and Schofield  and Taylor,1955),the 1M KCl and 0.01M CaCl2 methods  received the attention of researchers and now both the methods are recommended in several countries.They are being used for research purpose but I am not aware of the extent of their adoption in routine soil testing laboratories.While 0.01M CaCl2 method can be used in all soils,1M KCl method can be of some advantage in acid soils.I appreciate the comments of  Dr.Andrew.

l

Thank you dear Researchers for your answers, 

if i understand you , from the scientific perspective , the best way to characterize the soil solution ( pH, EC, Cation and Anions) or to replicate the condition in the field is Saturation paste extract, but For convenience purposes , we use dilution extract. 

i should add that for For pH , it always good to give two value for each sample, namely the pH of water and pH with salt ( KCl or CaCl2). this will tell us the potential for the soil solution to become acid and this will play a crucial role for example in carbonate chemistry , or even in absorption of some element by plants.

Kind Regard Yacine

Mr.Yacine,good conclusion points.