I look at a lot of soil reports these days, and the sodium adsorption ratio (SAR) is one number that is very informative to me.
Simply put, SAR of a soil extract takes into account how much the adverse effect of sodium (Na+) is moderated by the other cations calcium (Ca2+) and magnesium (Mg2+). As you all know, calcium and magnesium can replace sodium on soil particles, subsequently permitting this toxin to be washed away from the plant roots. So the more free calcium and magnesium we have around in the soil, the better odds we have of mitigating sodium.
SAR levels below 6 are OK, levels which run above 10 mean trouble.
Although most soil reports will give you a calculated value for SAR, you can calculate SAR yourself. It's simple: SAR = Na+/((1/2(Ca2+ + Mg2+))1/2 ), where Na+, Ca2+ and Mg2+ are all measured in meq/L (milliequivalents per liter).