A few chemical principles are important for you to fully understand soil fertility and plant growth/nutrition. You should know the name, chemical symbol, valence, and ion name of these elements. 

Atoms of chemical elements are the fundamental units of all matter.  Ions are atoms that carry an electrical charge.  Positively charged ions are call cations, and negatively charged ions are called anions.  The amount of this charge is determined by the number of electrons that an element has gained or lost, which indicates combining ability and is called valence.  With two valence states, “ic” denotes the higher state (ferric, Fe³⁺) and “ous” the lower state (ferrous, Fe²⁺).

Atomic weight is the relative weight of atoms of various elements (C is 12.0000 g). 

Mole is the amount of any substance containing 6.022 x 10²³ elementary units (Avogadro’s number).  Units may be atoms, molecules, ions, electrons, photons, etc.  For example, one mole of nitrate weighs 62 g and contains 6.022 x 10²³ NO₃^- molecules.  One mole of potassium weighs 39 g and contains 6.022 x 10²³ K atoms.  One mole of potassium ions weighs 39 g and contains 6.022 x 10²³ positive charges (the weight of the lost electrons is quite small).  One mole of calcium ions weighs 40 g and contains 12.044 x 10²³ positive charges (each ion has two positive charges).

Soil cation exchange capacity is normally expressed as milliequivalents (meq) per 100 g soil (or cmol per kg soil).  This expresses the amount of charge present.  One equivalent of charge (mole of charge) is equal to 6.022 x 10²³ charges, and a milliequivalent is equal to 1/1000 of an equivalent or 6.022 x 10²⁰ charges (when dividing 6.022 x 10²³ by 1000, one substrates the exponents--10²³-10³).  Therefore, a soil that has 30 meq/100 g of cation exchange capacity has 180 x 10²⁰ or 1.8 x 10²² negative exchange sites to which positively charged ions could be attracted (30 times 6.022 x 10²⁰ = 180 x 10²⁰).

Examples:

One mole of potassium ions weighs 39 g and has 6.022 x 10²³ ions, each with a single charge, or represents 6.022 x 10²³ charges.  The equivalent weight of K is 39 g.  One milliequivalent (1/1000 of an equivalent) of potassium ions weighs 39 mg (39 g/1000), and represents 6.022 x 10²⁰ charges.  Therefore, the milliequivalent weight of K is 39 mg.

One mole of calcium weighs 40 g and has 6.022 x 10²³ calcium ions, each with two positive charges.  Therefore, the equivalent weight of calcium ions is 40/2 or 20 g, which represents 6.022 x 10²³ charges.  Thus, the milliequivalent weight of calcium ions is 20 mg, which represents 6.022 x 10²⁰ charges.

One milliequivalent of potassium ions (39 mg) will occupy exactly the same number of negative exchange sites in soil as one milliequivalent of calcium ions (20 mg); i.e, 6.022 x 10²⁰ sites.

In general:

Milliequivalent weight = atomic weight (in milligram)

                                        valance

Verify:

(1) If a soil holds 400 mg per 100 g of Ca ions on cation exchange sites, the soil has a CEC of 20 meq/100 g.  

(2) If a soil has a CEC of 15 meq/100 g, it could hold 585 mg of K⁺, or 180 mg of Mg²⁺, or 0.345 g of Na⁺.