Agriculture Reference
In-Depth Information
Back to Soil Tests and Estimating CEC
When we add a
strongly acidic extractant
to a
soil with a pH below 7
, shake it
in a test tube for 20 minutes, and then filter off the resulting liquid, most of the
base cations Ca, Mg, K, and Na found in the extractant will have come from
negative exchange sites. We can measure their amounts, and then, knowing their
positive charges, add those charges together and get a rough estimate of CEC.
Knowing the soil's measured pH, we can estimate what percentage of the
negative exchange sites are occupied by H+ ions and other bases, and come to a
close approximation of actual, functional exchange capacity.
However, when a
strongly acidic extractant
is added to a
soil at pH7 or above
,
the acid will attack and dissolve base cations from alkaline minerals in the soil, not
just from exchange sites.
The Fizz Test:
Asimple way to determine if a soil has an excess of base
cations that could be extracted by the soil testing solution and cause error
in estimating CEC is to pour a small amount of ordinary household vinegar
on a sample of dry soil. If the soil fizzes and bubbles, there are excess
cations and the soil will need another test in addition to the Mehlich 3 test
in order to accurately measure exchangeable bases. Most soils below pH
7 will not fizz or bubble, but some soils, especially calcareous sands which
are often used on golf courses, can have an overall pH below 7 and still
have undissolved limestone particles.
The commonly used multi-element extractants used for soil testing are either
acidic or neutral pH. The Mehlich 3 test is pH 2.5, the Mehlich 1 is pH 1.25. The
Morgan solution used in the Lamotte test is pH 4.8; the NeutralAmmonium
Acetate test is pH 7.0. Obviously, if any of these tests are used on a soil with a
higher pH than the test extractant, they can dissolve alkaline elements from soil
particles, not only from exchange sites.
Calculating CEC
As we learned earlier in this topic, the classic formula for calculating Cation
Exchange Capacity is
ppm Ca
+
ppm Mg
+
ppm K
+
ppm Na
= CEC in meq
200
120
390
230
The parts per million of Calcium extracted, divided by 200, is our estimate of the
amount of Ca being held in exchangeable ionic form.As we wish to know only the
amount of base cations held on negative exchange sites, any excess cations
extracted will give a false high estimate of CEC.
Our true goal is to
measure the total number of negatively - charged sites that
are potentially available
to hold and exchange base cations. There are soil tests
that will do just that, but they are complex, expensive to perform, and may contain
