i want to know if measuring these cation using electro-chemical methods give a reliable results, and if there is a protocol of how to use these techniques in Soil chemical analysis?
There isn’t a standard method for using ion selective electrodes for analyzing soil solution extraction for potassium and sodium as far as I know.
It is my opinion that if you want to use ion-selective electrode you have to do comparative analyses of soil solution extraction for sodium and potassium between measurements via flame spectrophotometer and ion selective electrodes. Via this procedure you will create reference curves.
You have to be careful with the following things
You have to see if there is any difference between different type of soils and
You have to check these reference curves for very large and very low concentrations.
There are several way to extract the soil solution and the reliability of the ion selective electrodes depends from the equipment you are using. Also ion selective electrodes are not 100% selective and may have some interferences depending on the concentration of other ions. Please see if this study can help:
Article Assessment of Ionic Interferences to Nitrate and Potassium A...
the paper that you had attached Dr Di Gioia fit well my questionning , and as you put it very well in your introduction, ISE are rapid alternatives to the classical measurement of K concentration using the Atomic absorption spectrophotometry and there are a lot of laboratory in the world that don't afford an Atomic Absorption spectrophotometer. So it's interesting to see how accurate is the ISE method for assessing K, Na or other ion in soil solution like in your study ( nitrate).
What I can say, based on the results of our study, the reliability of ion selective electrodes is higher at lower concentrations. So you can use dilutions. Nevertheless, these measurements should be considered as a rapid estimate and cannot substitute laboratory analyses. So if the purpose is to evaluate the amount of nutrients available in the soil and you need a quick estimate, then I will go with the ion selective electrodes, just keeping in mind that what you are getting is an estimate. Then we have to consider that in the soil there is also a lot of variability, so you need multiple samples, which becomes very expensive for laboratory analyses. So what I would do is sample the soil, do an extraction 1:2 (soil:water by volume) according to Sonneveld and Voogt 2009 (Plant Nutrition of Greenhouse Crops) and measure the concentration of the nutrient you are interested in with ion selective electrodes or other rapid methods. Doing this overtime, or in plots with different soil fertility, you should be able to see how reliable your estimate is, and if you can go for it or not.
http://link.springer.com/book/10.1007/978-90-481-2532-6
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