Agriculture Reference
In-Depth Information
by weight to neutralize those (-) charges, or 12 times as much Magnesium, atomic
weight 24 (Mg++, also a double charge), or 39 times as much Potassium+.
(Potassium's atomic weight is 39, and it has a single positive charge, K+, so it
takes 39 times as much K+ as H+ to fill all the exchange sites, once again by
weight.) The amount of + charges, the quantitiy of atoms, of K+ or H+, is the
same.)
What We Have Learned
We have now learned the basics of CEC, cation exchange, in the soil. 1) Clay and
organic matter have negative charges that can hold and release positively
charged nutrients. (The cations are adsorbed onto the surface of the clay or
humus) That static charge keeps the nutrients from being washed away, and
holds them so they are available to plant roots and soil microorganisms
2)The roots and microorganisms get these nutrients byexchanging free hydrogen
ions. The free hydrogen H+ fills the (-) site and allows the cation nutrient to be
absorbed by the root or microorganism.
3) The unit of measure for this exchange capacity is the milligram equivalent, ME
or meq, which stands for 1 milligram (1/1000 of a gram) of exchangeable H+. In a
soil with an exchange capacity (CEC) of 1, each 100 grams of soil contain an
amount of negative (-) sites equal to the amount of positive (+) ions in 1/1000th of
a gram of H+.
Base saturation equivalents for H+, Ca++, Mg++, K+ and Na+:
Per 100 grams of soil,1 meq or ME=
1 milligram H+
20 mg of Calcium Ca++ (atomic weight 40)
12 mg of Magnesium Mg++ (atomic weight 24)
39 mg of Potassium K+ (atomic weight 39)
23 mg of Sodium Na+ (atomic weight 23)
Per Acre
, to a depth of 6” to 7”, 1 meq or ME=
20 lb Hydrogen H+
400 lb Calcium Ca++
240 lb Magnesium Mg++
780 lb Potassium K+
460 lb Sodium Na+
Per 1000 square feet
, 6” to 7” depth, 1 meq or ME=
O.46 lb of Hydrogen H+
9.2 lb of Calcium Ca++
5.5 lb or Magnesium Mg++
17.9 lb of Potassium K+
