Activity XVII
Please read before attempting.
Soil Chemical Properties.
Soil clay particles have the ability to retain both cations and anions in an
exchangeable form. The charge
on clay minerals develops in two ways, by isomorphous substitution and by the broken edge
effect.
Notice for portion of a clay mineral crystal what happens if an
Al+++ ion substitutes (isomorphous substitution) for the silica ion.
A. [=O - Si++++ -
O=]o
B. [=O - Al+++ - O=]-1
The positive and negative charges are balanced in "A" but not in
"B". A cation would be attracted
to the site in a clay crystal where "B" occurs.
The broken edge effect is similar but occurs with a change in soil pH.
When excess H+ ions are present a positive charge develops on
the edge of the clay crystal as shown in "C". When the pH is above 7.0,
the clay crystal will develop
extra negative charges at the edges as shown in "D".
Cation Exchange Capacity (CEC)
The positive charge on clays is expressed in cmol (+)/kg of soil (= centimoles of cations per kg of soil). A cmol is equal to 1/100 of an element's molecular weight. A cmol(+) charge is equal to 6.02 X 10^21 charges, 1/100th of Avogadro's number. See the Table 3.1 below. When elements combine to form a molecule the positive and negative charges must be exactly equal in number.
The soil clay minerals have an excess of negative charge that is balanced with an equal amount of positive charges from cations swarming around the clay particle. Many of the cations are essential nutrients for life, i.e. Ca++, Mg++, K+, NH4+, and Zn++.
Plants trade H+ for the essential elements held by clay particles. To better understand the process consider the following table.
Table 3.1 Common Soil Ions and Equal cmol Weights
ion At.Wt. # of
atoms # of
ions
*cmol wt. charges/cmol
H+ 1.0g
6.02 X 10^23
6.02 X 10^23
0.01g
6.02 X 10^21
K+ 39g
6.02 X 10^23
6.02 X 10^23
0.39g
6.02 X 10^21
Na+ 23g
6.02 X 10^23
6.02 X 10^23
0.23g
6.02 X 10^21
Ca++ 40g 6.02
X 10^23
12.04 X 10^23
0.20g
6.02 X 10^21
Mg++ 24g 6.02
X 10^23 12.04 X
10^23
0.12g
6.02 X 10^21
Al+++ 27g 6.02 X
10^23 18.06
X 10^23
0.09g
6.02 X 10^21
NH4+ 18g 6.02 X
10^23 6.02
X 10^23
0.18g
6.02 X 10^21
Cl- 34.5
6.02 X 10^23
6.02 X 10^23
0.345g
6.02 X 10^21
The cmol weight here means the weight of an element or molecule needed
to supply 6.02 X 10^21 charges,
either negative or positive in a reaction.
Notice to form HCl, hydrochloric acid, requires the combination of 1.0g
of hydrogen (H+) and 34.5g of Cl-. Both amounts supply equal amounts of
positive and negative charges (Avogadro's number 6.02 X 10^23).
Only 1/2 to 1/3 of the atomic weight of elements having two or three
charges respectively are needed to supply the same number of ions in a reaction.
If a clay sample holds 5 cmol K+/kg, determine the weight of potassium held by the clay.
5 cmol K+/kg X 0.39g/cmol K+ = 1.95g K+/kg clay
A soil analysis reveals a sample holds 6 cmol (1/2Ca++)/kg. Determine the kilograms of plant available calcium held in a hectare-furrow-slice (2 X 106kg) of the soil. (Notice for calcium with two charges, a (1/2Ca++) is used as a reminder that only 1/2 of the atomic weight (40/2) of calcium is needed to supply 6.02 X 1023 positive charges).
The calculations are as follows:
6 cmol (1/2Ca++)/kg X 0.20 g/cmol = 1.20 g
Ca++/kg
1.20g Ca++/kg X 2 X 10^6 kg/hfs = 2.40 X 10^6 g Ca++/hfs
2.40 X 106 g Ca++/hfs X 1 kg/1000g = 2400kg Ca++/hfs
According to Table 13.1 in the text, a crop of corn removed from the soil 58 lbs/ac (26kg/hectare) of calcium. This soil retains a 92 year supply of plant available calcium (2400 kg/26 kg per year).
Soil testing is commonly done to determine if the soil retains enough nutrients to meet
the needs of a crop. It is a valuable tool for taking the "guess work" out
of applying fertilizer plant foods to soils.
Activity XVII
1. A soil was found to retain the following quantities of exchangeable ions. Use the information in Table 3.1 above to determine the quantity of each element held by the soil in kg/HFS. Use data from Table 2, page 262 in the Laboratory Manual to determine if the elements K and Mg are deficient for the production of potatoe tubers. Show a summary of results in a table.
cmol(+)/kg
dS/m
Ca++ Mg++
Na+ K+
Al+++ CEC
pH E.C.
4.2 3.6
0.2 0.1
3.3
11.4 5.6 ...4.5
2. What percent of the CEC is sodium? (ESP)
3. The first four ions are considered basic cations. Aluminum and hydrogen are
acid cations. What portion of the CEC is bases?
Find the kg of calcium held per hectare furrow slice of soil.
(2 million kg)
4.2 cmol (1/2 Ca++)/kg soil x 20 mg Ca/cmol (1/2 Ca++) =
80.4 mg Ca++/kg soil or 0.804 g Ca++/kg soil 0.804 kg Ca/kg soil =
1,608,000 g Ca/HFS soil or 1608 kg Ca/HFS
4.Find the pounds of Mg, Na, K, and H stored in an hectare-furrow-slice of the above soil. Show all your calculations.
Salts in soil are measured by determining the ability of an extract of a
soil paste to conduct electricity. The more salt in the soil/water paste extract the
more current will flow through the extract. Plants exhibit a range of
tolerance to salts in soils. Generally, as the soluble salt content in soil
increases yields decrease. Salts are measured in units of either decisiemens/m or
millimhos/cm.
Extract Electrical Conductivity Plant response to salt in soil
dS/m or mmhos/cm
0 - 2 few plants affected
2 - 4 sensitive plants
affected
4 - 8 many plants affected
8 - 16 most crops affected
16 + few plants grow
See Table 9.4 on PP 305 in the text for plant tolerance
to salt in soil.
Strawberries are the very sensitive to salt in soil while date palms, barley, and tall wheat grass are examples of plants more tolerant of salt in soil.
5. Explain in your words the two mechanisms by which soluble salts in soil
affect plant growth? (see text pp302)
6. What physical effect does sodium have on soil that is harmful to plants?
(See text pp304)
7. What methods are commonly used to dispose of salts removed from the root
zone of soils? (salt sinks)
8. According to the text pp267, what are five common sources of the hydrogen
ions that make soils acidic?
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