Our study is based on Salinity stress and its impact on the plant's biosynthetic pathway. We are treating pots containing plants with different concentration of NaCl water and analyzing the results by maintaining a control plant without NaCl. We want to check the sodium concentration which is already present inside the soil before NaCl treatment. Can anybody please suggest us a simple biochemical or any other method??
I am not sure if soil conductivity could fits you, because it indicates the total concentration of soluble salts present in a soil solution, not only NaCl, but it is what we use. For obtaining soil conductivity only weigh around 20 grams of soil sample, add 100 ml of water at 20 ºC and shake for 30 minutes. Then filter it and measure with a conductivity meter under constant agitation.
There are some modeling publication regarding soil salinity.
I think the majority of soil salinity is related to Na ions, and NaCl constitutes generally more than 60-95% salinity in soils.
As sodium salts are highly soluble in water, so water extraction can show the soil sodium. Just take 1:5 ratio of soil and water and after appropriate shaking then measure its conductivity for all samples then measure the Na concentration with flame photometry. Then you can make a curve and relationship between soil EC and Na concentration. Then you can measure the rough soil Na with a portable EC meter!
I recommend you read the article:
Modeling of soil exchangeable sodium percentage using easily obtained indices and artificial intelligence-based models
Thank you, Stay Safe!
Flame photo-metric method is an atomic emission method for the routine detection of metal salts eg. Na, K, Li and Ca+.