For most
elements only a very small fraction of that present in soil is available
to plants or other biological organisms. The soil solution, that is
the water surrounding the soil particles which contains dissolved
minerals and salts, typically contains only a few parts per million
of the various elements. The natural abundance of elements in a surface
soil is presented in Table 2.
Table
2. Elemental concentrations of surface soil from a location in the
Piedmont of South Carolina; means of 33 samples.
|
Element
|
Concentration
|
St.
Dev.
|
|
|
%
|
ppm
|
|
|
Br
|
0.0004
|
4
|
54%
|
|
Ga
|
0.0015
|
15
|
29%
|
|
Mo
|
0.0017
|
17
|
52%
|
|
Cu
|
0.0018
|
18
|
50%
|
|
Ni
|
0.0021
|
21
|
43%
|
|
Rb
|
0.0050
|
50
|
25%
|
|
Cr
|
0.0059
|
59
|
48%
|
|
Pb
|
0.0069
|
69
|
60%
|
|
Sr
|
0.0070
|
70
|
106%
|
|
Zn
|
0.0093
|
93
|
81 %
|
|
V
|
0.0099
|
99
|
37%
|
|
Cl
|
0.012
|
114
|
58%
|
|
Zr
|
0.0282
|
282
|
41%
|
|
Mn
|
0.0390
|
390
|
48%
|
|
N
*
|
0.0625
|
625
|
|
|
S
|
0.065
|
645
|
48%
|
|
P
|
0.114
|
1,043
|
55%
|
|
Ca
|
0.25
|
2,493
|
69%
|
|
Mg
|
0.35
|
3,507
|
43%
|
|
Ti
|
0.59
|
5,852
|
30%
|
|
K
|
1.48
|
14,799
|
39%
|
|
Fe
|
3.32
|
33,220
|
34%
|
|
Al
|
11.1
|
111,498
|
19%
|
|
Si
|
23.0
|
229,773
|
14%
|
*
Estimate based on 1% organic matter content.
Note
the large standard deviations. This is typical for soils. Also, soils
in this region are naturally low in soil organic matter. A typical
organic matter content for soils in South Carolina is 1%. Soils with
a higher organic matter content would have a correspondingly higher
total nitrogen content and higher nitrogen supplying power than most
soils of the Southeast.
Nutrients
are distributed between solid and liquid or water phases. The major
portion of the various elements are part of the structure of amorphous
and crystalline minerals, clay minerals, and organic matter. They
are not available to plants or microorganisms except through dissolution
and weathering processes. Exchangeable ions are held close to the
colloidal surfaces. They are not free to move about as are ions or
solutes in the soil solution but they can be replaced as a result
of an ion exchange reaction.
The concentration
of nutrients in the soil solution is constantly changing as a result
of many reactions proceeding simultaneously, including growth cycles
of soil microorganisms, decomposition of crop residues, dissolution
and precipitation of solid phases, uptake of ions by plant roots,
respiration of plant roots and release of metabolic products such
as carbon dioxide and organic acids, and the cycling of ions between
the various phases as a result of these reactions.
Measurement
of the amount of nutrients in soil which are available to plants has
been the subject of extensive research over the past 100 years. Most
estimates are based on extraction of the soil with various solutions
including acids, salts, and chelating agents. The amounts extracted
are then compared with the amount which can be taken up by plants.
The plant is the authority on what is available. Some plants are able
to extract more nutrients from soil than other plants.