What instrument are you going to use to analyse K ?. it is better to have the same matrix in your standards and samples if you are concerned about any matrix effect. In that case you can use the extracting reagent to prepare your standards.

You must show your technique , for example when use spectrophotometery the solvent have a significant effect on determination of lambda max, where a shift may be occur with different solvents , also, with HPLC measurements . Standard addition or external addition method also have an effect on  results.

regards

I'll take reading in flame photometer @ Keerthy and Quasim

Soil Analysis is different from others. Look Following procedure

Extractable Potassium

This fraction of soil K is the sum of water-soluble and exchangeable K. The method uses a neutral NH4+ OAc solution (1 N) to replace the cations present on the soil exchange complex. This is considered as plant available K in the soils, and is commonly measured by the flame photometer.

However, the cation concentrations determined by this method are referred to as “exchangeable” for non-calcareous soils. For calcareous soils, the cations are referred to as “exchangeable plus soluble”.

Reagents

         Ammonium OAc Solution (NH4OAc), 1 N

Add  57  mL  concentrated  acetic  acidCH3COOH) solution to 800 mL DI water, and then add 68 mL conc. NH4OH solution, mix well, and let the mixture cool.

             Adjust the mixture to pH 7.0 by adding more CH3COOH or NH4OH, and bring to 1-L volume with DI water.

        Standard Stock Solution

·         Dry 3 g potassium chloride (KCl) in an oven at 120 °C for 1-2 hours. Cool in a desiccator, and store in a tightly stoppered bottle.

·         Dissolve 1.907 g dried KCl in DI water, and bring to 1-L volume. This solution contains 1000 ppm K (Stock Solution).

·         Prepare a series of Standard Solutions from the Stock Solution as follows:

Dilute 2, 4, 6, 8, 10, 15 and 20 mL Stock Solution to 100-mL numbered flasks by adding 1 N NH4+ OAc solution, and then bring to volume. These solutions contain 20, 40, 60, 80, 100, 150, and 200 ppm K, respectively.

Procedure

         Extraction

·         Weigh 10 g air-dry soil (< 2-mm) into a 250-mL flask.

·         Add 50 mL 1 N NH4OAc solution (ratio 1:5).

·         Shake for 30 minutes on a reciprocate shaker at 200-300 rpm.

·         Filter suspension using a Whatman No.1 filter paper to exclude any soil particles, and bring the extract to a 50-mL volume with 1 N NH4OAc solution.

Or as an alternative procedure

·      Weigh 5 g air-dry soil (< 2-mm) into a 50-mL centrifuge tube.

·      Add 33 mL 1 N NH4OAc solution, and shake for 5 minutes on a reciprocating shaker at 200-300 rpm.

·      Centrifuge at 2000 rpm until the supernatant liquid is clear and collect the extract in a 100-mL flask through a Whatman No. 1 filter paper to exclude any soil particles. Repeat this process two more times and collect the extract each time.

·      Dilute the combined 1 N NH4OAc extracts to 100-mL volume with 1 N NH4OAc solution.

Note

For both two methods above, the tubes or flasks should be stoppered with a clean rubber or polyethylene stopper, but not corks, which may introduce errors.

        Measurement

·         Operate Flame Photometer according to the instructions provided.

·         Run a series of suitable K standards, and draw a calibration curve.

·         Measure K in the samples (soil extracts) by taking the emission readings on the Flame Photometer at 767-nm wavelength.

·         Calculate K concentrations according to the calibration curve.

Calculations

 Extractable K (ppm)=〖ppm K〗_ from callibration curve ×V/Wt

Extractable K (ppm)=meq/L K from callibration curve ×V/Wt×39.1

Where:

 V= Total volume of the soil extract (mL)

Wt = Weight of air-dry soil (g)