Hi João,

Measurement of pH for soil is method dependent; as you have eluded to above. Since your likely to place your findings in the context of the larger scientific community you will want to use the method, or methods, that allow you to compare and contrast with others results/findings. 

Furthermore, some inferences can be made between the results from different methods.

Personally, I believe the use of 1:1 H2O and 1:2 0.01 molar (M) CaCl2 are needed for all my studies that I collect pH  for/on. See references below for how the data for the largest soil data base (USDA NRCS) in the world was created.

Much exists freely as references:

From the USA (USDA-NRCS)

http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_054247

http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_053575

http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_053575

http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_054247

It is really hard to find appropriate standards or standards from different reputable instituitions and/or articles that agree with each other. As I suspected the 1 N KCl method is less apropriate for soil pH assessment, although, as I mentioned, there are a vast number of references that refer to this extraction as adequate for that very same purpose.

Thank you so much for your answer.

Your reccomendation is to do both 1:1 H2O and 1:2 0.01 CaCl2 for each soil sample?

How about the need for filtration?

Hi João

A classical use of  the measurements of pH in soil / solution with 1N KCL is to estimate the dominance of charges in the soil (therefore if the soil can hold cations or anions). It is an important property in many tropical soils (acric / geric soils) This evaluation is done from the pH (KCl) - pH (H20) . A classical reference about it is

MEKARU, T. & UEHARA, G. 1972. Anion adsoption in ferruginous tropical soils. Soils Science American Proceeding, Madison, 36(2):296-300. 

Thank you again for your asnwers.

Hi João,

Have a look in the pages 67 - 69 of ICP Forests (http://icp-forests.net), (attached):

There you can see the The pH of the soil is potentiometrically measured in the supernatant suspension of 1:5 (volume fraction). This liquid is made up of a 0.01 mol/l solution of calcium chloride in water for pH(CaCl2) or deionised water for pH(H2O) (ISO 10390. 2005. Soil Quality – Determination of pH).

Hi João,

Some people offered you information about methods to measure soil pH. I'm sure that they are suitable and useful for you.

On the other hand, I wish you had clear that the pH is not an isolated feature. As an example, soil pH is related to the availability of some components to the plants, from nutrients to toxic heavy metals. You can find a lot of information about this subject using search internet engines, i.e. Web of Science, Google Scholar and others.

hi João,

for measurment ph of the soil, using pH meter with application of suspension of 1:5 

Hi Joao,

I want to put my piece,

You have to know that variation of soil to water ration in the range of 1:1 to 1:10 doesn't alter the pH values (Piper, 2010). it was also reported that 1:2.5 and 1:5 soil to water ratio are equally reliable for determination of pH (Vogel, 1994).

On the other hand, there is about 1 unit difference between pH (KCl) and pH(H2O) of a given soil. 

As a matter of fact, there are no perfect method. All of them reflect different approaches and interpretations of soil acidity. As said, pH in KCl must be evaluated together with water in order to evaluate if you have CEC or AEC. pH in CaCl2 is interesting to avoid the effect of ionc strength variations due to recent fertilization. So, the best method depends upon your objective.

João,

To follow up on my "Much exists freely as references" statement above, go to:

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1253871.pdf

and the "Hydrogen Ion Activity" (4C) measurement method and information begins on page 265 and includes the "seminal" foundational references after the introductions with additional foundational references after each method in turn.

I paste the passage below so that if you have a sluggish connection you can see the relevance to downloading the entire pdf manual. (apologies for the line feed and carriage returns as it came from a pdf and I did not spend much time cleaning it up)

"Soil pH is one of the most frequently performed determinations and one of

the most indicative measurements of soil chemical properties (McLean, 1982).

Soil pH tells more about a soil than merely indicating whether it is acidic or

basic. The availability of essential nutrients and toxicity of other elements can

be estimated because of their known relationship with pH (Thomas, 1996). Soil

pH is affected by many factors, e.g., nature and type of inorganic and organic

matter; amount and type of exchangeable cations and anions; soil to solution

ratio; salt or electrolyte content; and CO2 content (McLean, 1982). The acidity,

neutrality, or basicity of a soil influences the solubility of various compounds; the

relative ion bonding to exchange sites; and microbial activities. Depending on

the predominant clay type, the pH may be used as a relative indicator of base

saturation (Mehlich, 1943). Soil pH is also a critical factor affecting the availability

of most essential elements for plants.

The KSSL performs several pH determinations. These methods include,

but are not limited to, NaF (1 N pH 7.5 to 7.8) (4C1a1a1); saturated paste pH

(4C1a1a2); oxidized pH (4C1a1a3); 1:1 water and 1:2 CaCl2

(final solution: 0.01 M CaCl2) (4C1a2a1-2, respectively); 1 N KCl (4C1a2a3); and organic materials,

CaCl2 (final solution ≈0.01 M CaCl2) (4C1a1a4).

An increase in the soil to water ratio or the presence of salts generally results

in a decrease in the soil pH. The soluble salt content of the soil can be overcome

by using dilute salt solutions, e.g., CaCl2

or KCl, instead of distilled water. The use of dilute salt solutions is a popular method for masking seasonal variation in soil pH. The pH readings are usually less with dilute salt solutions than with distilled water but may be equal to or greater in highly weathered tropical soils, i.e., soils with a high anion exchange capacity. When the pH values of various soils are compared, determination by the same method is important (Foth and Ellis, 1988).

The pH as determined by the 1 N KCl method is an index of soil acidity. This

method is more popular in those regions that have extremely acid soils and in

which KCl is used as an extractant of exchangeable Al. The KCl pH indicates the

pH at which Al is extracted. Similarly to the 1:2 CaCl2 pH, the 1 N KCl pH readings tend to be uniform regardless of time of year.

The saturated paste pH method is popular in regions that have soils with

soluble salts. The water content varies with the water storage characteristics

of the soil. The saturated paste pH may be more indicative of the saturated,

irrigated soil pH than is the soil pH measurement at a constant soil to water ratio.

The saturated paste pH method is also the method from which the saturation

extract is removed for salt analysis. The saturated paste pH is therefore the pH

and the dilution at which the sodium adsorption ratio (SAR) is computed (method

4E4b).

The 1 N NaF pH may be used as an indicator that amorphous material

dominates the soil exchange complex. The oxidized pH may be used to assess

the activities of soil microorganisms. In soil taxonomy, the CaCl2

pH is used to distinguish two family reaction classes in Histosols (Soil Survey Staff, 2014).

References

Foth, H.D., and B.G. Ellis. 1988. Soil fertility. John Wiley and Sons. New York,

NY.

McLean, E.O. 1982. Soil pH and lime requirement. p. 199–224. In A.L Page,

R.H. Miller, and D.R. Keeney (eds.) Methods of soil analysis. Part 2.

Chemical and microbiological properties. 2nd ed. Agron. Monogr. 9. ASA

and SSSA, Madison, WI.

Mehlich, A. 1943. The significance of percentage of base saturation and pH in

relation to soil differences. Soil Sci. Soc. Am. Proc. 7:167–174.

Soil Survey Staff. 2014. Keys to soil taxonomy. 12th ed. USDA–NRCS.

Thomas, G.W. 1996. Soil pH and soil acidity. p. 475–490. In D.L. Sparks (ed.)

Methods of soil analysis. Part 3. Chemical methods. No. 5. ASA and SSSA,

Madison, WI.Kellogg Soil Survey Laboratory Methods Manual: Current Methods

Hi Joao,  ,

A common method of measuring soil pH is performed by placing a glass electrode in a mixture of soil and deionized water. The most common ratio used for soil-water pH is  1:2.5 ratio of soil to deionized. Electrolyte solutions, such as 0.01 M CaCl2 or 1 M KCl, can be added to soil rather than deionized water with the resultant pH referred to as salt pH. 

To add to the use of 0.01 M CaCl2 or 1 M KCl: the idea is that one wants to have the pH that is relevant for your type of research. In the normal world, soil pore water is not demi water, but a solution of salts and organics. So if you are interested in the pH that plants or microbes will experience, it is more usefull to use a small salt solution (0.01 M CaCl2 is often used). 1M KCl is a very high salinity, but can be usefull for saline environments.

Most researchers use the 1:2 or 1:5 volume ratio with demi water. This is the most standardized one. The idea there is that pH is an equilibrium, so the exact volume ratio is not that important. I personally use 1:2 volumes, as that is most practical. Do not forget to shake the suspension at reasoable temperatures (and not in direct sun for hours and hours, you don't want the microbes working too hard).

The primary use of 1:1 1 N KCl solution in  is to test for the presence of exchangeable aluminum. So, it should be use in acidic soils with pH

The French standard method (AFNOR) is the one described by Said Nawar and used in routine.

Fulvio, very interesting and detailed answer. Is this the reference you talk about?

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2389.1950.tb00713.x/abstract

Hi Joao, I am of openinon same to Prf. Tegbaru Bellete. Hence 

So we might get down to two opinions here on the dilution effect for demi water: it exists, or it does not exist. In my -not to wide but existing- practical experience, the probe used to measure the pH is an important factor. They do age considerably, and so I am really wondering if the 1960-iues references are still relevant, given that current probes are much more precise. Theory, as stated out by Bellete, should be that the dilution doesn't matter on the lower ranges, so it all is more a practical discussion.

Maybe someone here has a nice student project to try out the effects, and some different probe combinations?

Dear All; If the soil is highly alkaline than 1:2 ratio is also best ratio. Otherwise we can take the pH of Saturation extract. Wihch is being used widely. Thanks

Fulvio,

I don't understand the relation between pH and the diffuse layer. Can you explain it more for me?

Does any one have the methods (e.g. weblink) mentioned above  ? Thanks a lot in advance for any information .. 

@ Dilojan, Which other methods than all the provided downloads in the answers would you like?

just would like to know what is "1 N KCl"? It is a very strange expression for me. How to make a "1 N KCl" solution?

Dear Yiyu Meng, a 1 N KCl is just another way of saying 1 mol L KCl solution

Thanks João. It is not a usual expression that I am familiar with. Good to know it.  

An increase in the soil to water ratio or the presence of salts generally results in a decrease in the soil pH. The soluble salt content of the soil can be overcome by using dilute salt solutions, e.g., CaCl2 or KCl, instead of distilled water. The use of dilute salt solutions is a popular method for masking seasonal variation in soil pH. The pH readings are usually less with dilute salt solutions than with distilled water but may be equal to or greater in highly weathered tropical soils, i.e., soils with a high anion exchange capacity. When the pH values of various soils are compared, determination by the same method is important (Foth and Ellis, 1988). The pH as determined by the 1 N KCl method is an index of soil acidity. This method is more popular in those regions that have extremely acid soils and in which KCl is used as an extractant of exchangeable Al. The KCl pH indicates the pH at which Al is extracted. Similarly to the 1:2 CaCl2 pH, the 1 N KCl pH readings tend to be uniform regardless of time of year. The saturated paste pH method is popular in regions that have soils with soluble salts. The water content varies with the water storage characteristics of the soil. The saturated paste pH may be more indicative of the saturated, irrigated soil pH than is the soil pH measurement at a constant soil to water ratio. The saturated paste pH method is also the method from which the saturation extract is removed for salt analysis. The saturated paste pH is therefore the pH and the dilution at which the sodium adsorption ratio (SAR) is computed (method 4E4b).

refers the following link and see the page frpm 265

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1253871.pdf

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1253871.pdf

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1253871.pdf

Hello everybody,

I followed this discussion panel a while. I think I need to do some product placement now, sorry for that. I am not working directly in science, but in a small company that is specialized in the fabrication of environmental monitoring systems. We developed a system to continuously monitor pH (and redox, by the way) in soils. It might be interesting for your current or future projects.

Right now, there is not much published on it, because it is rather new (utility patent). But this article makes refers to it:

Thiele‐Bruhn, Sören, Stefan Wessel‐Bothe, and Marc‐Oliver Aust. "Time‐resolved in‐situ pH measurement in differently treated, saturated and unsaturated soils." Journal of Plant Nutrition and Soil Science 178.3 (2015): 425-432.

If you are interested in it, just send me a private message. Thank you!

0.01M cacl2 suspension is the best method

This discussion is indeed very useful. But I have a query. Can we use 0.5M KCl instead of 1M KCl?

Mukul of course you can change or modify the state of art methods according to your needs. But you will lose the possibility to directly compare your values with the data already existing in literature. Inside your results dataset I assume the relationships between pH values should be linearly comparable using either proposed KCl solution.

right

Electric way - - - - -

It is the most common and accurate method - a pH meter is used. The dough is processed and then filtered to obtain an extract of saturated soil paste

Electric way - - - - -

It is the most common and accurate method - a pH meter is used. The dough is processed and then filtered to obtain an extract of saturated soil paste

fluctuation problems is common in pH meters

I followed the discussion and it was interesting.

I recommend Martin's answer

Best Regards

Thanks Luma Abd very useful answered to my field work. doing pH measuring during period of research.

I recommend to use electronic way

1:1 or 1:2 for soil to water are all good because they can both form paste. However, i recommend 1:1 for soil that is more of Sand and 1:2 for sand that is more of Clay.