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CATION
EXCHANGE
Cation
Exchange Capacity (CEC)
Base
Saturation
Soil
Solution
Supply
of Nutrients to Plant Roots
Nutrient
Availability and Mobility
Cation
Exchange
Soil
colloids, clay minerals and soil organic matter account for cation
exchange properties of soils. See Chapter 5 of text for discussion
of soil colloids, structural features, and other characteristics
of soil colloids. Soil organic matter means the organic fraction
of the soil but does not include undecayed plant and animal residues.
Estimates of the average age of the carbon in soil organic matter,
based on radiocarbon dating, varies from a few hundred years to
more than 25,000 years. (See page 217 in text.) Soil organic matter
does not consist primarily of recent crop residues. It is a recalcitrant
mixture of organic "residuals" that resist decomposition.
Generally, it takes many years to change the organic matter content
of a soil from its current "equilibrium" value. Tillage
is the major agronomic practice that affects soil organic matter
content. Reducing tillage is the most effective way to maintain
or attempt to build back organic matter content that has been severely
depressed due to intensive or non conserving agricultural practices.
However, climate exerts a natural control or limit as to the amount
of soil organic matter that can be achieved and sustained.
Cation
exchange capacity (CEC)
The
CEC of a soil depends upon the amount and type of soil colloids
present. The clay content, the type of clay minerals present, and
the organic matter content determine a soil's CEC.
|
Colloid
|
CEC,
cmol(+) /kg*
|
|
kaolinite
|
3
- 15
|
|
illite
|
20 - 40
|
|
montmorillonite
|
60
- 100
|
|
soil
organic matter, humus, etc.
|
100
- 300
|
*Unit
is centimole of charge per kilogram of colloid; another
common unit for expressing CEC is me/100g (milliequivalents/100
grams). Note that 1me/100g = 1cmolc /kg.
Kaolinite is the dominant clay mineral in soils of this region,
but some soils contain significant amounts of montmorillonite, a
clay mineral of the smectite group.
CEC
for various soils: a typical range of CEC for soils in the state
and the region is 3-5 me/100 g. See page 147 of text for a range
of CEC for a variety of soils from various regions of the U. S.
You may encounter a CEC expressed as "sum of the bases"
(Ca+Mg+K+Na) or a CEC based on "sum of the bases and exchangeable
acidity" (H+Al). For soils such as those in the Piedmont there
is usually a large difference. Generally CEC is not used directly
to manage soils and their fertility. The effect of CEC and its significance
in managing soils is incorporated into management of pH and lime
requirement. The larger the CEC the more buffering capacity a soil
will have and the more lime that will be required to raise the soil
pH by a specific amount, for example, from 5.5 to 6.5. By the same
token soils with a large CEC will have more K supplying power for
a given degree of K saturation. Following proven soil test methods
and fertilizer recommendations is the best way to manage plant nutrient
supplies.
Conversion
of me/100 grams to pounds/acre and other facts relating to acreage:
|
Element
|
Factor
|
Example
|
|
Ca
|
400
|
1
me Ca/100 grams = 400 lbs /acre
|
|
K
|
780
|
1
me K/100 grams = 780 lbs/acre
|
|
Mg
|
240
|
1
me Mg/100 grams = 240 lbs/acre
|
|
Na
|
460
|
1 me Na/100 grams= 460 lbs/acre
|
These
conversions are based on an estimate that the surface 6 inch layer
of soil over the area of an acre weighs 2,000,000 pounds. The actual
weight depends upon the soil bulk density which commonly varies
from about 1.3 to 1.7 g/cc. For example, a cubic foot of water weighs
62.4 lbs. If the bulk density of a soil were 1.3 g/cc (1.3 times
heavier than water) then the soil would weight 62.4 x 1.3 or 81.1
lbs/cu. ft. on a dry weight basis. Soil properties such as clay
content and gravimetric water content are always expressed on a
soil dry weight basis.
Other
useful numbers to remember and examples of how to use them:
1
acre = 43,560 sq ft
the weight of an acre foot of soil with a bulk density of 1.47
= 43,560 sq ft x 1 ft x 62.4 lbs/cu ft x 1.47 = 4,000,000 lbs
1 part per million (ppm) nitrate N in the top 12 inches of this
soil = 4 lbs/acre
If you had a row spacing of 36 inches,
one row 14,520 feet long would be an acre (43,560 ft2/3 ft);
if the row spacing were 40 inches,
the row would be 13,068 feet long (43,560/(40/36) or 43,560/3.333).
- 454 grams = 1 pound; 1 ounce = 28.4 grams
- 1 acre inch of water = 3,630 cu ft = 27,154 gallons = 226,512
pounds
- 1 cu ft weighs 62.4 pounds
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Base saturation
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Note
differences in base saturation of kaolinite and bentonite over the
pH range 5.5 to 6.5. Humic acid and illite have pH-base saturation
ralationships similar to bentonite.
The figure "Base Saturation" is based on data published
in Soil Testing and Plant Analysis, Edited by Leo M. Walsh and James
D. Beaton and published by the Soil Science Society of America,
Madison, Wisconsin in 1973. The original data was developed by A.
Mehlich and published in 1943.
Base
saturation is the amount of the CEC that is occupied by the basic
cations such as Ca, K, Mg, and Na. The portion of the CEC that is
occupied by acidic cations, primarily H, Al, and Fe, is called the
"exchangeable acidity". Note differences in base saturation
of kaolinite and bentonite over the pH range 5.5 to 6.5. Humic acid
and illite have pH base saturation relationships similar to bentonite.
Unless
one knows some specific characteristics about a soil such as the
dominant clay minerals, the amount of clay in the soil, and the
amount of organic matter, soil pH will not tell how much is the
lime requirement. The pH is a measure of the amount of hydrogen
ion in solution. As is true of all cationic species, acidic as well
as bases, only a small amount of the total exchangeable ions are
present in the soil solution at one time.
Soil
test methods that have been developed to quickly measure exchangeable
acidity must be relied upon to estimate the amount of lime required
to raise the soil pH to a desired range.
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Soil
Solution
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.
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Supply
of Nutrients to Plant Roots
ForReferences:
text, page 282
There
are three main processes by which nutrients are supplied to
plant roots:
1.
Mass flow of soil solution to the plant root as a result
of water uptake.
2.
Diffusion of ions from solid phases or regions of high
concentration towards the plant root as concentrations become
depleted due to nutrient absorption.
3.
Root interception as a result of the root growing and
occupying more space.
The table below shows the relative amounts of several nutrients
supplied to corn roots by the three processes.
|
Nutrient
|
Total
Uptake
|
Root
Interception
|
Mass-Flow
|
Diffusion
|
|
Ca
|
23
|
66
|
175
|
.
|
|
Mg
|
28
|
16
|
105
|
.
|
|
K
|
135
|
4
|
35
|
96
|
|
P
|
39
|
1
|
2
|
36
|
|
Mn
|
0.23
|
0.10
|
0.05
|
0.08
|
|
Zn
|
0.23
|
0.10
|
0.53
|
.
|
|
Cu
|
0.16
|
0.01
|
0.35
|
.
|
|
B
|
0.07
|
0.02
|
0.70
|
.
|
|
Fe
|
0.80
|
0.10
|
0.53
|
0.17
|
From:
Soil Testing and Plant Analysis. 1973. Editors, Leo M. Walsh and
James D. Beaton. Soil Science Society of America. Madison, Wisconsin.
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Nutrient
Availability and Mobility
See
pages 261 274, 279 287, and 291 297 of the text for discussion
of reactions that affect plant availability of N, P, and K in
soil.
A.
General mechanisms that affect availability of various elements
and relevant terms.
Adsorption-desorption
or ion exchange; Note: adsorption refers to an attraction
to a surface while absorption refers to being incorporated into
something. See diagram on page 144 of the text.
Mineralization:
organic ---> inorganic
Precipitation-dissolution: when dissolved substances
form solids (solid phases) and drop out of solution, the process
is called "precipitation"; it is not the same as adsorption;
dissolution is the opposite process whereby solids go into the
solution phase and become "solutes".
Fixation:
elements or certain ions such as NH4 become physically
and chemically bound in a nonexchangeable form. An example is
the entrapment of potassium between silica layers of clay minerals.
Some clays also fix ammonium in the same way. Phosphorus can
also become "fixed" in unavailable forms but the mechanism
is different than ammonium or potassium fixation. Phosphorus
is fixed by being bound chemically to iron and aluminum compounds.
Denitrification:
biochemical reduction of nitrate or nitrite to gaseous nitrogen
Mineralization:
conversion of an organic form of an element into an inorganic
form.
B.
Nitrogen
Most
of the nitrogen in soil is present as soil organic matter and
is unavailable to plants. Organic matter is about 50% carbon
and 5% nitrogen, thus it has a carbon/nitrogen ratio of about
10. Nitrogen is released very slowly from soil organic matter
as a result of soil microbial activity. This process is affected
by moisture, temperature, tillage or any physical disturbance
such as soil wetting and drying. The amount of nitrogen released
by this process is also dependent on the soil organic matter
content. A soil with 1% organic matter will supply much less
plant available nitrogen than one that is 5% organic matter,
under similar conditions.
The
decomposition of crop residues is usually limited by the C/N
ratio of the residue as well as environmental conditions. Soil
microorganisms have first choice of available nitrogen. Higher
plants get what is left over. Alfalfa will decompose more quickly
in soil than corn stalks because of its higher nitrogen content
(lower C/N ratio).
Nitrate,
being an anion, is the most mobile form of N in soil; it moves
within the water as water percolates through soil and is easily
lost through this leaching process. Ammonium and ammonia forms
are cationic and held by cation exchange sites. However, ammonium
and ammonia forms are converted to nitrate by soil microorganisms.
The reaction is fast in warm moist soils. The reaction rate
becomes slower as the soil becomes more acidic. From spring
through fall most ammonium fertilizer is converted to nitrate
within a few days to a week or two. Urea nitrogen is hydrolyzed
to ammonium nitrogen by a soil enzyme and then quickly converted
to nitrate provided conditions are right.
C.
Phosphorus
Phosphorus
is strongly held by soil clays and iron and aluminum compounds
associated with soil clays; also, phosphorus forms very sparingly
soluble compounds, precipitates, in soil. These reactions proceed
rapidly after fertilizer is applied. Most phosphorus in soil
is precipitated, fixed, or adsorbed. These solid phases form
phosphate reserves that can replenish the soil solution when
phosphorus is taken up by plants, but the reaction is slow.
Only small amounts of phosphorus are present in the soil solution.
Because of these reactions with soil, P is very immobile, not
subject to significant leaching losses. However, P is readily
lost through erosion of surface soil and the associated P.
D.
Potassium
Potassium
exists as exchangeable K and in K-bearing minerals. In soils
that contain mica-type clay minerals and vermiculate some K
exists as "fixed" K within the clay mineral structure.
Fixed K is not exchangeable.
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Reference
Materials
Soils:
An Introduction to Soils and Plant Growth. 6th Edition. Donahue, Miller,
and Shickluna.
Soil Fertility
and Fertilizers, 3rd Edition. Tisdale and Nelson. 1975. Macmillan Publishing
Co., Inc. New York, NY.
Soil Acidity
and Liming, 2nd Edition. F. Adams. 1984. ASA, Madison, WI.
Soils and
Soil Fertility. Troeh, Frederick and L.M. Thompson. 1993. Oxford University
Press. New York, Ny.
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