Soil conductivity: How do I get a near equivalent between meq/100 g to mS/cm? I have two soil reports, one that reports CEC as 45 meq/100 g and another that reports 6 mS/cm using 1:1 water soil.

I can see that the two are, if not apples and oranges, they are at best tangerines and grapefruit — the meq/100 g is the source for the ions in a water extract, but an equilibrium solution may not contain all the ions.

Can a meaningful conversion between meq/100 g and mS/cm be made?

E.g: few plants will grow in soil that has conductivity greater than 2.5 mS/cm and generally you start getting concerned at 1.  What is the equivalent measure in meq/100 g?

FWIW main ions of concern are $\ce{K+},$ $\ce{PO4^3-},$ $\ce{Mg^2+},$ $\ce{Ca^2+},$ $\ce{CO3^2-},$ $\ce{HCO2-}.$
The mineral ions (nutrients) in soil can be expressed as two ways: (i) Cation Exchange Capacity (CEC), which has units of $\pu{meq}$/$\pu{100 g}$; and (11)  Electrical Conductivity (EC), which has units of $\pu{mS/cm}$.

When the conductivity is measured in 1:1 mixtre of soil:water, it is called $\mathrm{EC_{1:1}}$ (This is a way of measuring electrical conductivity of soil water or $\mathrm{EC_{sw}}$). 

According to USDA Publication, $\mathrm{EC_{1:1}}$ readings less than $\pu{1 dS/m}$ ($\pu{1 mS/cm}$), soil are considered non-saline and do not impact most crops. This publication also gives you a easy procedure for how to measure $\mathrm{EC_{1:1}}$ of your soil.

My best bet is you don't have to worry about your second soil sample, which has CEC of $\pu{45 meq}$/$\pu{100 g}$, because none of the cations (exchangeble cations in the soil) in concern is $\ce{Na+}$ (the given CEC is the sum of all exchangeable cations in equivalents). According to above publication, soils with a high concentration of sodium salts (sodic conditions) have given problems.

To my understanding, there is no direct relationship between $\pu{meq}$/$\pu{100 g}$ and $\pu{mS/cm}$. For more info on Cation Exchange Capacity (CEC) and units of $\pu{meq}$/$\pu{100 g}$, read:


J. P. Reganold, J. B. Harsh, “Expressing cation exchange capacity in milliequivalents per 1 100 grams and in SI units,” J. Agronomic Educ. 1985, 14(2), 84–90 (https://www.agronomy.org/publications/nse/archive/vol-14).