The common method of determining soluble sodium (Na) in soil is

The common method of determining soluble sodium in the soil is Flame photometric.
Flame photometry is also known as Flame emission spectrometry (FES) and it is an atomic optical emission spectroscopy technique that uses the flame as the source of atoms excitation.
Flame photometry is highly empirical, in comparison to an absolute, method of analysis such as gravimetry. Sodium gives yellow flame in flame photometry.