Agriculture Reference
In-Depth Information
carbonic acid are what replaces the cation nutrient on the exchange site.A
Calcium ion that is held to the exchange site has a double-positive charge, written
Ca++. When enough H+ ions surround it that some of them get closer to the
exchange site than the Ca++ ion is, two H+ ions replace the Ca++ ion and the
plant or microbe is free to take the Ca++ up as a nutrient.
How the CEC is measured, and what to do with that information once you
have it.
Exchange capacity is measured in milligram equivalents, abbreviated ME or meq.
Amilligram is of course 1/1000th of a gram, and the milligram being referred to is
a milligram of H+ exchangeable Hydrogen. The comparison that is used is 1
milligram of H+ Hydrogen to 100 grams of soil. If all of the exchange sites on that
100 grams of soil could be filled by that 1 milligram of H+, then the soil would have
a CEC of 1. One what? One ME, one milligram equivalent (meq), the ability to
adsorb and hold one milligram of H+ Hydrogen ions.
Let me repeat that: 100 grams of a soil with a CEC of 1 could have all of its
negative (-) exchange sites filled up or neutralized by 1/1000th of a gram of H+
exchangeable Hydrogen. If it had a CEC of 2, it would take 2 milligrams of
Hydrogen H+, if its CEC was 120 it would take
120 milligrams of H+ to fill up all of the negative
(-) exchange sites on 100 grams of soil.
meq/100g
vs
cmol
c
/kg
The current modern
notation for scientific
audiences is cmol
c
/kg
(centimoles of charge
per kg soil). The “c”
subscript before the
slash in cmol
c
/kg
denotes “charge”. The
magnitude of the
numbers remains the
same. 10meq/100g =
10 cmol
c
/kg. Many soil
testing laboratories still
use meq/100g, and we
will be using meq/100g
in this topic because
that is the notation used
byAlbrecht and what
will be found in the
older research that
much of our knowledge
of exchange capacity is
based on.
The "equivalent" part of ME or meq means that
other positively (+) charged ions could be
substituted for the Hydrogen. If all of the sites
were empty in that 100 grams of soil, and that
soil had a CEC of 1, 20 milligrams of Calcium
(Ca++), or 12 milligrams of Magnesium (Mg++),
or 39 milligrams of Potassium (K+) would fill the
same exchange sites as 1 milligram of
Hydrogen H+.
Why the difference? Why does it take 20 times
as much Calcium as Hydrogen, by weight? It's
because Calcium has an atomic weight of 40,
while Hydrogen, the lightest element, has an
atomic weight of 1. One atom of Calcium weighs
forty times as much as one atom of Hydrogen.
Calcium also has a double positive charge,
Ca++, Hydrogen a single charge, H+, so each
Ca++ ion can fill two exchange sites . It only
takes half as many Calcium ions to fill the (-)
sites, but Calcium is 40 times as heavy as
Hydrogen, so it takes 20 times as much Calcium
