Sandy soil analysis

[Shadihamdan]

I am running experiments on Electrokinetic soil remediation technology and my target contaminants are soluble salts. I am using artificial sandy soil " Silca sand" please see attachment for the properties of soil. To prepare salt affected samples, I spike the sandy soil with different salt solutions (0.01M NaCl , 0.1M NaCl, 0.5M NaCl) to saturation level. My question here: I am expecting to have salt concentration in the soil very close to the concentration of salt solution was used but it doesn't seem the case. The procedures I follow are:
1- Artificially contaminate the sandy soil with a desired concentration of salt solution "as mentioned above";
2- Take samples of contaminated soil and leave it for air day;
3- From air dried samples, I prepare extract of 1:5 dilution ratio.
4- I use ICS for testing for Chloride concentration and ICP-MS for Sodium concentration.
The results show much less concentration than the concentration of the salt solution used. For instance, concentration for sodium in the solution is 216 ppm and Na concentration in the soil following the previous steps is 15 ppm.
I believe this is not a reliable result. However, I explain that because salts "NaCl" precipitate when drying the soil and this resulting in low concentration is not because of the dilation process but because the fact that salts precipitated.
What would you advise me to follow to get reliable results?