measure salt in soil

[Wayne S. Ambler]

My greenhouse soluble salt meter measures salts in ppm, and millimhos.  
Some of the research I've found indicates salt levels in EC units.   
and/or dSm(exponent)-1.  Can someone explain this in high school math 
level terms?  I have not had chemestry and need some help understanding 
this.  

One study class the low salt level 3dSm-1, and the high salt level 
10dSm-1.  If the -1 is an exponent, it seems it should be visa 
versa.

Thanks. Emily