Different soil-to-water dilutions would not yield a linear relationship (you're still working with 2 different phases). That is because different ions will be selectively retained by the soil surface differently depending on their concentrations in the solution phase. For a negatively charged surface, such as most soils, divalent cations are more strongly retained than mono-valent cations, upon dilution. You can find detailed info. about cation exchange reactions and selectivity in many soil chemistry textbooks.

By the way, if you are only interested in EC measurements, you can make a saturated soil paste and use a portable EC meter, which costs about $50, to read EC easily and quickly.

Hope this helps, NVH.

Dear Joao:

The two methods would yield different results. That is because of exchange reactions (and selectivity effects) involving a charged solid phase (soil) and a liquid phase (soil solution). The 2 dilution methods may be equivalent only if you work with a single phase.

Best, NVH

Thank you that is exactly my point, e.g, the saturated soil paste is regarded as a more appropriate extraction method but it can be unpratical because to yield an adequate volume of aliquot after filtration you would need more soil sample. What I'm suggesting is to filtrate the saturated soil paste, no matter how little volume it can be extracted, and the dilute it for analysis after filtration (and therefore after contact with the soil for exchange reactions) and therefore, although a similar volume, would be more realistic than 1 to 5 dilution (im guessing)

Yes. Your logic is correct. However, in order to get, say 5 ml solution from the saturated soil paste, you still must have at least 30-50 g of soil to start with. Once you've got the solution, you could dilute many times as you wish.

Thank you for your answer.

I agree with your other respondents. The two methods will yield different results. However, we assessed a range of methods a while ago (2001) and determined that extraction was considerably more reliable and repeatable.

See: FAULKNER H, WILSON B R, SOLMAN K AND ALEXANDER R (2001) ‘A comparison of three cation extraction methods and their use in the determination of the sodium concentration ratios of sodic soils’. Communications in Soil Science and Plant Analysis, 32 (11/12), 1765-1777.

I would like to draw attention to another aspect. From an ecological point of view, saturated extract seems to be a better way. The water content (saturation percentage -SP) mainly depends on the soil texture and organic matter content. Moisture conditions are individual for each sample and refer to the state, which can meet by the plants in the environment. Therefore ECe values ​​are often used to evaluate the salinity level of the saline habitat as well as for soil classification .

In the case of 1:5 extract all samples are treated in the same way without reference to their characteristics (texture and organic matter content).

I suggest the following publications:

RICHARDS L.A. (ED.), 1954; Diagnosis and improvement of saline and alkali soils, Agriculture Handbook No. 60, USDA, Washington.

RHOADES J.D., MANTEGHI N.A., SHOUSE P.J., ALVES W.J., 1989, Estimating soil salinity from electrical conductivity, Soil Sci. Soc. Am. J., 53: 428-433.

BRADY N.C., WEIL R.R.; 1996, The nature and properties of soils, eleventh edition, Alkaline and salt-affected soils and their management [w:], Prentice Hall International Editions, New York, London.

Rhoades J.D., Chanduvi F., Lesch. S.; 1999. Soil salinity assessment. Methods and interpretation of electrical conductivity measurement. FAO Irrigation and Drainage Paper 57, Rome.

Best regards

Piotr

Does anyone have experience with those new equipments that estimated the EC(a) in a non invase way ? These methods are getting popular in the precision agriculture mapping.

Dear Prof. Nugyen. Im trying to learn a bit more about EC measurements. I dont understood you post (above) where you mention that the differences of results will be equivalent only if I work with a single phase ? If I dilute soil with water in different relations (i.e 1:2,5, 1:5, 1:10) the results will not be linearly related ?

Different soil-to-water dilutions would not yield a linear relationship (you're still working with 2 different phases). That is because different ions will be selectively retained by the soil surface differently depending on their concentrations in the solution phase. For a negatively charged surface, such as most soils, divalent cations are more strongly retained than mono-valent cations, upon dilution. You can find detailed info. about cation exchange reactions and selectivity in many soil chemistry textbooks.

By the way, if you are only interested in EC measurements, you can make a saturated soil paste and use a portable EC meter, which costs about $50, to read EC easily and quickly.

Hope this helps, NVH.

Dear Nguyen. Im interested to use the the soil:solution that we used to measure pH (10 g : 25 ml de water) to measure EC. I was supposing that those results would be linearly related with saturated soil paste but I have learned that it will not be the case. Thanks for your comments.

Here is one more question in this line.

Will EC measured of saturation extract (soil solution) ECe exactly match with EC measured in saturated paste ECsp (Measurement in soil paste by dipping electrode)?

Plz quote!

The soil paste extrct and 1:5 extract is normally adopted under different situations. If you have comparatively lesser number of samples and you want more accuracy that is very nearer to the situation roots are facing as well time is not a constraint, you should go preferably for saturation past method. However, if you have large number of samplese and time is a constraint then you can select 1:1 or 1:5 dilution. Care must be taken to express the results based upon saturation paste method by adopting a correlation value calculated specifically for your case using different soils, rather it is better to calculate separate values for different textured soils.

Dilution of saturation paste extract can be done (1:1 or 1:2 ---1:5) if you have large number of determinations but you must be careful during determinations because your error will be multiplied by the dilution factor you selected.

Please remember that saturation past extract diluted by 1:5 and extract of 1:5 moisture are not the same.

Dear Nazir Hussaim

Interesting comments. Im looking for some procedures to perform analyses of EC in the same extract that we use to read pH (soil - water diluted 1;2.5). Normally we read the pH after 1 hour of the mixture. But I realized that EC measurements can be higher if I read with more time (2, 4 and 12 hours). Do you know some information or publication about this. I dont want to make new experiment if this is already established.

If you are using your results in calculations of soils the only accepted method is a saturated paste extract.  you need more sample to test the solution (to cover the probe completely) use more soil.  I doubled my soil to 200 grams and get 50ml of extract.  Adding extra water to your saturated paste will dilute your sample, diluting your sample decreases the concentration of the salts that you are measuring for and you will receive a lower conductivity reading. 

In regards to the timing and increased readings of electrical conductivity, the structure of soils you are testing can impact the speed in which the soil will become saturated.  The smaller the pores of the soil the longer this will take, especially wetland soils heavy in organic matter and clays. 

Saturated Paste Characteristics:

            No pooling of water

            Soil recovers from spatula impression by tapping

            Soil glistens

            Soil will flow slowly to the side when tilted 45 degrees

            Clay soil does not flow freely from spatula

After 1-2 hr then stirred to test consistency of the soil and left an additional 18-24 hours to ensure saturation in clay soils.

Once you compare the soils EC paste extract with the same soils diluted to a 1:5 ratio in a correlation graph, you will have a conversion factor specific to your site.  I have also heard of using a EC 1:10 to help the cations and anions from neutralizing the salts and lowering the reading but I have not investigated further.