Some researcher used, Electrical conductivity, some Chloride and some Sodium. Are all these parameters right? What if their contents are highly varied within the sampling unit?
If total salts are the only issue, electrical conductivity is the easiest and least expensive measure. We have found good correlations between soil electrical conductivity (and plant performance) and soil sodium, sulfur and chloride.
soil quality is management related. I believe you should select the main indicator of your interest. However, you may rely on multiple indictors and later assess their relative importance.
Electrical Conductivity is the best measuring technique for soil salinity and the least expensive. I have included some points concerning your current project:
Soil salinity (accumulation of salts in the surface zone) has a number of causes, which differ in different geological and climatic regions. The causes can be natural, can be due to clearing of native vegetation (‘dryland salinity’), stem from irrigation. Mitigation of soil salinity and its impact on plants must therefore be considered somewhat differently in the context of these three scenarios. Salinity is often accompanied by other soil properties, such as sodicity, alkalinity, or boron toxicity, which exert their own specific effects on plant growth. Waterlogging often accompanies salinity due to clearing or to irrigation.
Low salinity (ECe*=2 to 4 dSm-1) Natural salinity; often seasonally dry
Irrigation salinity; can be waterlogged after irrigation. Cropping: Low-moderate salt tolerance.
Moderate salinity (ECe= 4 to 8 dSm-1) Dryland salinity; often seasonally waterlogged
Crop-pasture rotations Moderate-high salt tolerance
High salinity (ECe > 8 dSm-1) Discharge areas; can be seeping or dry according to season Grazing or revegetation Halophytes.
Salts in soils are primarily chlorides and sulfates of sodium, calcium, magnesium, and potassium. Symptoms of soil salinity include slow and spotty seed germination, sudden wilting, stunted growth, marginal burn on leaves (especially lower, older leaves), leaf yellowing, leaf fall, restricted root development, and sudden or gradual death of plants.
It is important to consider that the EC measurement should be made in the saturation extract of the soil, salinity classes (0-2, 2-4... dS/m) are estimated according to this point. Soil extracts with other soil:water ratio should be corrected. An interesting paper related is Simon et al (1999) Geoderma, 90:99-109 (attached file).