Agriculture Reference
In-Depth Information
toxic metals, e.g. the Barium chloride extraction, or reagents that are corrosive
to lab equipment, e.g. the SodiumAcetate/Ethanol CEC test.
WilliamAlbrecht determined the exchange capacity of the colloidal clay he used
in his BCSR experiments by running a DC current through the clay suspended in
water and filling all of the negative exchange sites with protons, i.e. H+, and then
displaced the H+ by adding various base cations to the suspension. Measuring
the
amount
of base cations
adsorbed when the H+ ions had all been
displaced
gave him the total CEC, the total permanent negative electrical
charge of the clay fraction he was experimenting with
.
For the first century of soil chemical analysis, each element was measured
individually through a separate series of steps with different reagents. Often a
completely different procedure was used to extract various elements; one
extractant and test for Fe, another for Cu, another for B, a time consuming and
complex process.
For a routine soil analysis we do not want to involve many separate steps and
different tests. Ideally we want a test that will extract all of the elements we wish to
measure, and will at the same time allow us to closely estimate CEC.
The two present-day "universal" soil tests that do extract most or all of the
exchangeable cations are theAmmoniumAcetate and the Mehlich 3 tests. The
Mehlich 3 and the neutral ammonium acetateAA7.0 will both extract about the
same amount of Ca, Mg, K, and Na. Both are strong extractants that will not only
strip exchangeable cations, but can etch and dissolve carbonates, or any other
rock that is more alkaline than they are.
Given the ability of both the M3 andAAextractants to extract more base cations
than are actually exchangeable,
which soil test can be used
to extract
exchangeable cations and only exchangeable cations from a high-pH soil? The
answer turns out to be
the ammonium acetate test,
but only after it has been
modified to have a pH higher than the soil sample being tested.
Ammonium acetate is made by mixing aqueous ammonia NH
3
with acetic acid
CH
3
COOH, the acid found in common vinegar.The pH of the resulting ammonium
acetate solution (NH
4
C
2
H
3
O
2
) will depend on the ratio of acetic acid to ammonia in
the mixture. If more ammonia is added the mix becomes more alkaline, more
acetic acid makes it more acidic. For soil extractant use the mixture has usually
been made at pH7 or pH7.2.Adding a pH7 solution to a soil of pH >7 will result in
alkaline mineral compounds being dissolved along with exchangeable bases. By
adding more ammonia to the solution, the pH can be raised to 8.0 to 8.5, above
the pH of most agricultural soils. For soils in the 7.0 to 8.0 pH range the
ammonium acetate extractant is commonly raised to pH8.2. This is known as
the
AA8.2 soil test
.
