Fractions of potassium in soil are (a) total potassium, (b) nonexchangeable (but plantavailable) potassium,
(c) exchangeable potassium, and (d) watersoluble potassium. The total potassium comprises the
mineral potassium and potassium in the soil solution and in organic matter. Soil solution potassium
plus organic matter potassium represent only a small portion of the total in mineral soils. The total
potassium depends much on the proportion of clay minerals and on the type of clay minerals.
Kaolinitic clay minerals, having virtually no specific binding sites for K
^{+}, have low potassium concentrations
in contrast to soils rich in 2:1 clay minerals. Mean total K
^{+} concentrations, exchangeable
K
^{+} concentrations, and watersoluble K
^{+} are shown Table 4.8
(93).
Soils with mainly kaolinitic
clay minerals have the lowest, and those with smectitic minerals, which include also the 2:1 clay
minerals with interlayer K
^{+}, have the highest potassium concentration. The K
^{+} concentration of the
group of mixed clay minerals, kaolinitic and 2:1 clay minerals, is intermediate. Watersoluble K
^{+}
depends on the clay concentration in soils and on the type of clay minerals. As can be seen from
Figure 4.9, the index of soluble K
^{+} decreases linearly with an increase in the clay concentration in
soils and the level of soluble K
^{+} in the kaolinitic soil group is much higher than that of the mixed
soil group and of the smectitic soil group
(94).

The determination of total soil potassium requires a dissolution of potassiumbearing soil minerals.
The digestion is carried out in platinum crucibles with a mixture of hydrofluoric acid, sulfuric
acid, perchloric acid, hydrochloric acid, and nitric acid (95). Of particular importance in the
available soil potassium is the exchangeable K ^{+}, which is obtained by extracting the soil sample
with a 1M NH _{4}Cl or a 1M NH _{4}^{+} acetate solution (96). With this extraction, the adsorbed hydrated
K ^{+} and some of the nonhydrated K ^{+} (K ^{+} at edge positions) is obtained. In arable soils, the
exchangeable K ^{+} ranges between 40 to 400 mg K/kg. Soil extraction with CaCl _{2} solutions
(125 mM) extracts somewhat lower quantities of K ^{+} as the Ca ^{2+} cannot exchange the nonhydrated
K ^{+}, in contrast to NH _{4}^{+}of the NH _{4}^{+}containing extraction solutions. 
For the determination of the
nonexchangeable K
^{+}, not obtained by the exchange with NH
_{4}^{+}and consisting of mainly interlayer
K
^{+} and structural K
^{+} of the potassium feldspars, diluted acids such as 10 mM HCl
(97) or 10mM
HNO
_{3} are used
(98). These extractions have the disadvantage in that they extract a K
^{+} quantity and
do not assess a release rate, the latter being of higher importance for the availability of K
^{+} to plants.
The release of K
^{+} from the interlayers is a firstorder reaction
(83) and is described by the following
equations
(99):
. Elovich function:
y =
a +
b ln
t
. Exponential function: ln
y = ln
a +
b ln
t
. Parabolic diffusion function:
y =
b t^{1/2}
where y is the quantity of extracted K
^{+}, a the intercept on the Yaxis, and b the slope of the curve.




FIGURE 4.9 Potassium solubility of various soils related to their type of clay minerals (Adapted from A.N.
Sharpley, Soil Sci. 149:4451, 1990.) 
In this investigation, soils were extracted repeatedly by Ca
^{2+}saturated ion exchangers for long
periods (maximum time 7000 h). Analogous results are obtained with electroultrafiltration (EUF), in
which K
^{+} is extracted from a soil suspension in an electrical field
(100). There are two successive
extractions; the first with 200 V and at 20°C (first fraction) and a following extraction (second fraction)
with 400 V at 80°C. The first fraction contains the nonhydrated adsorbed K
^{+} plus the K
^{+} in the soil
solution, whereas the second fraction contains the interlayer K
^{+}. The extraction curves are shown for
four different soils in Figure 4.10, from which it is clear that the K
^{+} release of the second fraction is a
firstorder reaction
(101). The curves fit the firstorder equation, the Elovich function, the parabolic
diffusion function, and the power function, with the Elovich function having the best fit with R
^{2}>0.99.