Agriculture Reference
In-Depth Information
The high concentration of ammonium NH
4
+ in theAAsolution readily displaces
(exchanges for) the base cations Ca, Mg, K and Na from negative exchange sites.
This is due to
three factors governing ion exchange: The relative
concentration of the ion, the electrostatic charge of the ion, and the radius
(size) of the ion.
NH
4
+ has a molecular weight of only 18 (N=14 H=1), less than
Na at at.wt. 23, and a small radius which allows it to fit into small spaces such as
between clay layers in expandable clays. In solution, NH
4
+ has approximately the
same diameter and charge as K+. NH
4
+ can fit between clay layers, and when it is
in a higher concentration in the soil-water solution than the other cations, will
readily replace Ca, Mg, K, Na, and H+ on exchange sites.
In 1848, Harry Way poured an ammonium solution, ammonium sulfate, through
a column of soil. When he analyzed the solution that had leached through the
bottom of the soil column, he found it contained less ammonium but high
amounts of Ca, Mg, and K. This was the first scientific description of ion
exchange.
The AA8.2 Test VS the Mehlich 3 Test
If theAAsolution has a higher pH than the soil sample, the acetic acid will be
neutralized and will not react chemically with alkaline minerals. The base cations
in the resulting soil extract will largely be those that have been exchanged for
ammonium NH
4
+, and their amounts can be used to accurately estimate CEC.
Real World Examples
Following are two soil tests reports comparing the results of a Mehlich 3 and an
AA8.2 test on the same soil sample. This first example is from the garden of a
giant pumpkin grower in Walnut Grove, California.
Location: Walnut Grove, Sacramento County, California
Element
Mehlich 3
AA 8.2 pH
Ideal Soil Ratios
C
ation
E
xchange
C
apacity
19.05
10.26
10.26
pH of Soil Sample
7.80
7.80
Organic Matter %
13.13%
Sulfur
S- ppm
37
K x 0.50 =
100
Boron
B ppm
1.36
1.71
Base Cations
%CEC
ppm
%CEC
ppm
%CEC
ppm
Calcium
Ca++
59.06%
2256
64.38%
1321
83.50%
1713
26.12%
21.28%
10.00%
Magnesium
Mg++
597
262
123
Potassium
K+
13.47%
1001
13.40%
536
5.00%
200
Sodium
Na+
1.21%
53
0.93%
22
1.50%
35
The first thing to notice is that calculated CEC has dropped 46%, from 19.06meq
to 10.26meq, because theAA8.2 test has extracted many fewer base cations than
