Chapter 7
SOIL COLLOIDS

The soil colloids are the most active portion of the soil and largely determine the physical and chemical properties of a soil. Inorganic colloids (clay minerals, hydrous oxides) usually make up the bulk of soil colloids. Colloids are particles less than 0.001 mm in size, and the clay fraction includes particles less than 0.002 mm in size. Therefore, all clay minerals are not strictly colloidal. The organic colloids include highly decomposed organic matter generally called humus. Organic colloids are more reactive chemically and generally have a greater influence on soil properties per unit weight than the inorganic colloids. Humus is amorphous and its chemical and physical characteristics are not well defined. Clay minerals are usually crystalline (although some are amorphous) and usually have a characteristic chemical and physical configuration. Both inorganic and organic colloids are intimately mixed with other soil solids. Thus, the bulk of the soil solids are essentially inert and the majority of the soil's physical and chemical character is a result of the colloids present.

Cation exchange

One of the most important properties of colloids is their ability to adsorb, hold, and release ions. Colloids generally have a net negative charge as a result of their physical and chemical composition. This negative charge is balanced by thousands of cations. Thus, colloids can be viewed as huge anions surrounded by a swarm of rather loosely held cations. Water molecules are also adsorbed to colloid surfaces; they are present as part of the hydrated structure of the cations. The amount of water associated with a particular cation is important, because the effective radius of the cation changes with the amount of hydration, or associated water.

In humid regions, the cations associated with the colloids are dominated by Ca+2, H+, and often A1+3, resulting in acidic soils. As the soil becomes more acid, H+ and Al+3 become more predominant. The cations Mg+2, K+, and Na+ are usually found in lesser amounts, while NH4+ may be present in considerable quantities if the soil has been recently fertilized with ammonium fertilizers. In semiarid and arid regions, Ca2+ usually dominated the cations, but Mg2+ and Na+ are often found in large quantities. H+ and A13+ are usually present only in small concentrations.

Many of the other plant nutrient cations are found only in very small amounts as cations on colloidal surfaces. More often, they are found as chelates or in chemical combination. Such cations include Mn+2, Zn+2, Cu+2, Fe+2, and Fe+3 and generally make up only a small percent of the exchangeable cations. Anionic nutrients, such as NO3-, C1-, SO4-2, and PO4-3 are not held on the surfaces of colloids to any great extent. Instead, they exist as free anions in the soil solution or fixed within chemical compounds.

Cation exchange is the exchange of a cation in the soil solution for another on the surface of a colloid (Figure 7.1). Cation exchange is a phenomena which is constantly going in soils and is of great importance. Without some mechanism to temporarily hold cations in the soil, plants would be unable to obtain sufficient quantities of the essential nutrients to grow. Without cation exchange, the nutrients would simply be leached downward in the soil and lost. Cation exchange plays a role in other soil processes as well. Acidification is the process of exchanging basic cations, such as Ca+2, Mg+2, K+, and Na+, for acidic cations, such as H+ and A1+3. Liming acid soils results in a reversal of this process, H+ ions are exchanged for Ca+2 ions. If cationic fertilizer nutrients are not held by the soil colloids, the nutrients would be lost to percolation water.







Figure 7.1. Cation exchange between soil colloids and the soil solution.

Cation exchange capacity (CEC) is a quantitative measure of the ability of a soil to exchange cations with the soil solution and is expressed in terms of cmols(+) kg-1 of soil. Historically, soil scientists have expressed CEC in terms of meq/100 g of soil and these units were often encountered in textbooks and journal articles. The unit cmol kg-1 is equal to meg/100 g.

The cmol weight of the ions commonly found in soils is easily calculated by knowing:

1) the relative atomic mass of the ion divided by 100 and
2) the charge on the ion.

For example, the calcium ion has a relative atomic mass of 40 g mol-1 or 0.40 g cmol-1 and a charge of two. Because if it's charge, it will replace 2 of hydrogen (hydrogen has a charge of 1) atoms. Dividing the relative atomic mass of the ion by its charge gives you the cmol weight of the ion. For Ca2+, that is 40 g mol-1 or 0.40 g cmol-1/2 or 0.20 g cmol-1.

Table 7.1. Relative atomic mass, charge, and cmol weight for some common soil ions.

 

 

Relative

 

 

 

 

Ion

Atomic mass

Charge

cmol weight

 

 

g

 

 

g/cmol-1

Al+3 27 +3 0.09
Ca+2 40 +2 0.20
Cl-1 35 -1 0.35
CO-2 60 -2 0.30
H+3 1 +1 0.01
K+ 39 +1 0.39
Mg+2 24 +2 0.12
Na+ 23 +1 0.23
NH4- + 18 +1 0.18
NO3-2 62 -1 0.62
SO4 96 -2 0.48
ZN+2 65 +2 0.32.5

Cation exchange capacity (CEC) is an expression of the 'amount' of cations held in the soil. This is expressed as cmol of cations per kg of soil. When the CEC is combined with the cmol weight of a particular cation, then the 'amount' of that cation can be expressed on a weight basis. Three examples of this are given below for H+, Ca2+, and Al3+, respectively.

Assume a soil has a CEC of 20 cmol kg-1 g of soil (this means that 1 kg of soil will hold 20 cmol (the 'amount') of cations. To convert this 'amount' of cations to a weight basis, the cmol weight (g cmol-1) is multiplied by the CEC, as follows:

Assume all of the exchange sites are occupied by:

H+:

CEC x cmol wgt = 'amount' of cation on weight basis

20 cmol_ x .01 g H+___ = .20 g H+ kg-1 soil

1 kg soil 1 cmol H+

Ca2+:

CEC x cmol wgt = 'amount' of cation on weight basis

__20 cmol_ x _.20 g Ca+2 = 4.00 g Ca+2 kg-1 soil

1 kg soil 1 cmol Ca+2

A13+:

CEC x cmol wgt = 'amount' of cation on weight basis

_20 cmol___ x .09 g Al+3_ = 4.60 g Al+3 kg-1 soil

1 kg soil 1 cmol Al+3

Another calculation often required is the conversion of 'amount' in soil on a weight basis from (g kg-1 soil) to (pounds/acre furrow slice) or (kilograms/hectare 15 cm). Remembering that an acre furrow slice weighs 2 million pounds, we can say that pounds of nutrient per acre furrow slice is the same as parts per 2 million (pp2m). The next step is the conversion of g kg soil to ppm. Assume that a soil will hold .400 g of Ca+2 kg of soil. To calculate how much Ca+2 that could exist in the soil (expressed as pounds per acre furrow slice) (1b/AFS), first convert CEC to ppm, as follows:

0.400 g Ca+2 x 1,000,000 kg = 4000 kg = 4000 ppm

1 kg 1,000,000 kg 1,000,000 kg

Then change ppm to pp2m or pounds per acre furrow slice as follows:

4000 ppm x 2 = 8000 pp2m or 8000 lbs/AFS

The conversion of g of nutrient kg-1 soil to kg ha-1 is similar. A hectar furrow slice (15 cm deep) weighs 2,000,000 kg.


Flocculation and Dispersion

Soils are generally in an aggregated state. Aggregation, however, is dependent on the soil colloids and the cations associated with them. Soil colloids can be in either a flocculated or dispersed state. The normal situation is for colloids to be in a flocculated state. Individual particles stick together to form aggregates of particles or floccules. Such aggregates do not move in the soil solution and form the basis for soil structure. When soil particles are dispersed, aggregates do not form, and each particle behaves as an individual. Without aggregation, water, air, and root movement in the soil is inhibited. Thus, dispersion is not a desirable characteristic of productive soils.

The type of cations present in the soil solution determines whether a soil is dispersed or flocculated. Sodium cations cause dispersion while calcium, magnesium, aluminum, and hydrogen ions promote flocculation. Because colloids are simply large anions, they attract cations in order to neutralize their negative charge. Flocculating cations sufficiently neutralize the negative charge, allowing colloids to adhere and flocculate. The attraction of particular cations to the negatively charged colloids is a function of two things, the hydrated size of the cation and the charge of the cation. These two factors combine to determine the charge density on the cation, in other words, the distribution of charge over the surface of the cation. For example, with the highly hydrated Na+ cation, the hydrated size of the cation is relatively large, while its charge is only +1. So, that +1 charge has to be distributed over a relatively large area. With such a large cation having such a low charge, the negative charge on the colloids is not sufficiently satisfied and the colloids actually repel one another, resulting in dispersion.

Shrinking and Swelling

Soils shrink and swell as they dry and rewet. Shrinking and swelling is an important factor in the construction of bridges, roads, and buildings, because of the pressures exerted by swelling or expanding soils on the foundations of such structures. Shrinking and swelling is largely a function of the type of colloid present, particularly clay colloids. As water moves in and out of clay crystal lattices, they respond by expanding or contracting. Extreme expansion and contraction is exhibited by clays such as montmorillonite, which have expanding lattices. Clays with nonexpanding lattices, such as kaolinite and chlorite, have very little capacity to shrink and swell.

The exercises that follow contain a hypothesis relating to the concepts presented in the above discussion. Each hypothesis is followed by a simple experiment designed to test that hypothesis. The results of each experiment should be viewed qualitatively and evaluated as to whether they validate the hypothesis.

Laboratory Exercise

A. Cation Adsorption

HYPOTHESIS: Soils are able to adsorb cations from the soil solution. The amount adsorbed will be proportional to the amount of clay in the soil.

1. Place 5.0 g of coarse-textured (sandy loam) soil, 5.0 g of fine-textured (clay loam) soil and 5.0 g of field sample into separate test tubes.

2. Add 10 ml of solution containing Ba2+ cation to each test tube and hake for 1 minute.

3. Filter and save the extract.

4. Pipet 2 ml of each extract into 2 clean test tubes.

5. To a third test tub, pipet 2 ml of the Ba2+ cation containing solution. This will serve as a control.

6. To each test tube add one drop of Reagent A and mix. REAGENT A contains a chemical which precipitates the excess Ba2+ not adsorbed onto the soil colloids.

7. After 2 minutes observe the amount of precipitate in each tube and record on the data sheet. The greater the amount of precipitate, the less Ba2+ was held on the soils colloids. Since the coarser-textured soil contains fewer colloids, the extract from this soil should have a greater amount of precipitate.

8. Evaluate the hypothesis and relate the results to soil texture.

B. Clay Flocculation

HYPOTHESIS: Soil cations with a high charge density will flocculate clays and cations with low charge density will not.

1. Label eight clean test tubes: NaC1, KC1, HC1, CaC12, MgC12, A1C13, deionized water, and control.

2. Shake the bottle of clay suspension well.

3. Fill each of the test tubes to the same level (about 3/4 full) with the well mixed clay suspension.

4. Add 10 drops each of 0.5N NaC1, KC1, HC1, CaC12, MgC12, A1C13, and deionized water to the appropriately labeled test tube. Add noting to the test tube labeled control.

5. Mix each tube for 1 minute.

6. Observe and record on the data sheet the speed with which floccules form, their relative size, and settling rates.

7. Evaluate the hypothesis and relate results to cation charge density.

C. Clay Shrinking and Swelling

HYPOTHESIS: The ability of a clay to shrink and swell is a function of the clay type (2:1, 1:1) and its water holding capacity.

1. Fit a piece of filter paper into each of two water retention rings.

2. Fill one ring with a 2:1 type clay and the other with a 1:1 type clay. Be sure to level the tops off with the flat side of a spatula.

3. Set the filled rings in a dish of water so the level of the water just reaches the bottom of the clay. This allows the clay to take up the water slowly.

4. Over the laboratory period observe and record the response of the clays to water uptake.

5. Evaluate the hypothesis and relate results to clay lattice type.


DATA SHEET
Chapter 7

A. Cation Adsorption

 

 

Relative amount

Amount colloids

Soil texture

of ppt

Predicted by ppt

Coarse ____________________________________

Fine ____________________________________

Control ____________________________________



B. Clay Flocculation

Indicate relative flocculation time, size, and settling time by numbering the cations 1 through 8, fastest to slowest, largest to smallest.

 

 

Relative

Distinctness

Relative

 

 

time to

and floccule

settling

Cation

flocculate

size

time

 

 

"1st", "last", etc.

 

 

 

 

Na _______________________________________________

K _______________________________________________

H _______________________________________________

Ca _______________________________________________

Mg _______________________________________________

A1 _______________________________________________

Water _______________________________________________

Control _______________________________________________

C. Clay Shrinking and Swelling

Field sample vs. known samples.

Clay type Swelling

____________________________ ___________________________

____________________________ ___________________________

____________________________ ___________________________

____________________________ ___________________________