Soil LaB

Intro Soils – Lab 3 Soil Colloids – Cation Exchange Capacity

o Lecture and Text Materials: Soil Colloids (Chapter 8)

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Lab 3 –Soil Colloids and Cation Exchange Capacity Soil colloids are the smallest size fraction of the soil particles and are the most chemically active portions of the soil; soil colloids include clays and humus. These particles are generally <0.1µm in size and are collectively called the soil colloid fraction. The soil colloids have very large per unit volume surface areas and thus are critical in attracting and holding water and nutrients in the soil profile. There are four types of soil colloids including crystalline silicate clays, non-crystalline silicate clays, iron and aluminum oxides, and humus (organic matter). The clay minerals are a result of the weathering or decomposition and recrystallization of primary minerals into secondary minerals. The composition of these clay minerals is contingent on the weathering conditions, parent materials, and climate under which they are formed. The surfaces of soil colloids carry electrostatic charges, most of which are net negative. Colloid charge can either originate from two main sources. Charge can be constant from isomorphic substitution of a higher charged ion for a lower charged one in the tetrahedral or octahedral sheets in the layer silicates. Charge can also be pH dependent originating from humus or protonation on broken edges of the clay crystals in layer silicates and the iron and aluminum hydroxides. As the pH in soil increases, to do these pH dependent charges. Net negative charge serves as the seat of soil chemistry and fertility. The negative charges are neutralized by positive cations in the soil solution and include calcium, magnesium, potassium, sodium, ammonium, and hydrogen. These cations are retained in the soil solution and used for plant and microbial nutrition. The mass of exchangeable cations sorbed per unit mass of soil is the cation exchange capacity (CEC). The CEC of soils is a good indicator of soil fertility, and the capacity of a soil to sorb and make available existing and applied plant nutrients. The exchange of cations is determined by several principles:

(1.) Exchange reactions are reversible and rapid. The cations in soil are exchangeable and will move in the direction of the most available product or reactant.

(2.) The reactions are charge equivalent. Ultimately, the negative charges created on colloid surfaces will be neutralized by cations in soil solution, but they are neutralized on a

 

 

stoichiometric basis not on an ion to ion basis. The soil ions have varying levels of charge per mole, discussed below and will be satisfied on a charge to charge basis.

(3.) The law of mass action will be obeyed. If the system is flooded with a particular cation, it will move onto the exchange sites. The law of mass action is utilized in determining exchangeable cations to calculate CEC.

(4.) Size and charge dictate which ions if available will move onto the exchange site. The higher the charge and smaller the radii of the ion the stronger it will be held. The lyotrophic series lists the order in which cations will be exchanged on the soil colloid surface based on complementary ions in the soil solution. Waters of hydration around ions give rise to the formation of outer-sphere complexes where the ions are more loosely held and are easily exchangeable. Ions that form inner-sphere complexes bond directly with the colloid surface forming a stronger bond with less exchangeability.

Cation exchange capacity is quantified by measuring the amount of exchangeable ions that can be replaced on the soil colloid surface. Simply, the soil sample is flooded with a high concentration of a cation which through mass flow displaces all of the soluble cations (most common in soils are sodium, potassium, magnesium, calcium, and in acidic soils hydrogen and aluminum) off the soil colloids and into solution. A benchtop method first uses ammonium to replace cations on the soil exchange sites, followed by a second exchange which moves another ion like sodium or potassium onto the exchange sites. The amount of ammonium can be quantified to calculate the chemical equivalent CEC (cmolc/kg) (Text Figure 8.22).

Soil testing laboratories do not generally directly measure CEC, instead CEC is estimated using the quantity of soil cations tested in a standard soil test. Soil testing facilities use standardized extractants (Meilich I or III, Bray-I) to displace the all of the exchangeable ions in soil to determine how much of those particular elements will be available for plant uptake during a crop season. Those determinations are then used to recommend a range of nutrient additions, fertilizer and lime, required to meet crop needs for an expected crop yield. Soil testing facilities routinely utilize inductively coupled plasma spectrometry (ICP) coupled with atomic adsorption (AA) spectroscopy to determine a wide range of elemental concentrations. Inductively couple plasma technologies heat the samples to a very high degree to create ionization; individual ions emit specific wavelengths of light which are quantified downstream by various detection methods including atomic adsorption, mass spectrometry and others. These tools can analyze for multiple elements simultaneously and can also be used for several matrices including plants, soils, manures, and water. CEC can then be estimated using the by summing up the contributions from the major soil cations in the extracted solution. More traditional benchtop methods analyze the elements individually using colorimetric assays for end point quantification. Again, the mass of these soluble, exchangeable cations per unit of soil and represent the capacity of that soil to exchange cations, CEC. At pH 7, neutral conditions, some soils do not have exchangeable hydrogen ions and aluminum ions, and some soils to not exhibit exchangeable sodium ions, so caution is taken to know what exchangeable ions are in the soils which are tested, reported, and utilized for CEC calculations. CEC estimation using soil test data is easy to generate using already measured soil test nutrients, but just as the name implies, it is an estimate, and should be interpreted as such. There are various means of determining CEC beyond the scope of this exercise, but again, it is important to note the method used to determine CEC and potential pitfalls and the agronomic ramifications of over or underestimation of plant nutrients, and thus CEC. The importance and value of CEC cannot be understated. CEC is the ability of a soil to sorb

 

 

ions and molecules, making them available or not to the plant and microbial community or ultimately to leaching or runoff, and is key to managing soil fertility. Estimating CEC using Soil Test Values (ppm) To calculate the CEC using soil test values, chemistry concepts, the charge for charge neutralization rule, and the units for CEC should be reviewed. The end goal is to convert a parts per million (ppm or mg/kg) quantity from soil test into CEC which is conveyed in cmolc/kg of soil. Recall from chemistry, each ion (element, metal) has a specific atomic weight found on the periodic table in units of grams per mole (reference pg. 923 in text). We can utilize that information as well as the equivalent charge per ion to make this conversion. It is important to be aware of the units used and understand the end point unit. Ultimately, the cmolc from each cation are summed together to determine the estimated CEC. Soil labs also utilize the units of meq/100 grams of soil but cmolc/kg is the standard international unit.

Table 1: Cations, Atomic Wt, Charge Equivalence

Cation Atomic Wt (g/mol) Equivalent

Charge/Valence

Calcium (Ca2+) 40 2

Magnesium (Mg2+) 24 2

Potassium (K+) 39 1

Sodium (Na+) 23 1

Hydrogen (H+) 1 1

Example calculations: Equation 1: Determining cmolc from the calcium ion contribution from the soil test calcium values. Sum the values from the ‘top’ (above the dividing lines) then divide by the sum of the ‘bottom’ (below the dividing line) to produce cmolc for each particular ion/kg soil.

Equation 2: Review of unit cancellations. Each member of the equation is utilized to convert one unit to another to ultimately end with cmolc/kg soil. Mark-thru lines are unit cancellations; in order for a unit to ‘cancel’ it must occur in the top and bottom of the overall equation.

Equation 3: Procedure for calculating the CEC contribution from the additional ions (calcium is shown in Equation 1). You will simply use the exact same equation (Equation 1) replacing each time the ppm (mg/kg) from the soil test for each ion, the molecular weight of the particular ion, and the equivalent charge of the particular ion (provided in the table above).

 

 

Equation 4: CEC is the sum of the contribution from each individual major soil cation. Again, calcium, magnesium, and phosphorus are always used and include sodium, hydrogen, or aluminum in soils with those exchangeable ions.

Calculating CEC using soil test values (lbs/acre) Many soil labs also report the various elemental analysis in terms of lbs/acre since fertilizer recommendations are still calculated in that manner. Here, the calculations for estimated CEC is still the summation of the contribution from each individual ion but using the equivalent weight in pounds per acre equal to 1 meq/100 g (older unit estimation, same as cmolc/kg soil) in one acre soil to a depth of 6 inches (Table 2). To obtain this value, divide the molecular weight by the valence (equivalent weight) and multiply by 20. To calculate the estimated CEC contribution from each ion, simply divide the lbs/acre of each ion by its meq weight in lbs/acre (far right value) from Table 2. For instance, if a soil test result is 1500 lbs/acre of calcium, its contribution to CEC would be calculated as (1500 lbs/acre / 400 meq) or 3.75 meq/100g of soil. Each of the ions would be calculated individually and summed to compute the estimated CEC using lbs/acre.

Table 2: CEC Calculations using lbs/acre

Cation Atomic Wt

(g/mol) Equivalent

Charge/Valence

Equivalent Weight

Amount in 1 acre soil 6-inch deep @ 1 meq cation/100g

Lbs/acre

Calcium (Ca2+) 40 2 20 400

Magnesium (Mg2+) 24 2 12 240

Potassium (K+) 39 1 39 780

Sodium (Na+) 23 1 23 460

Hydrogen (H+) 1 1 1 20

Estimating CEC using Soil Texture Cation exchange is based in the soil colloids, clays and humus, so CEC can actually be estimated using soil texture. Ranges of common estimates of cation exchange capacity of some of the major soil textural classes are included below. It should be apparent that increasing clays also increase CEC and thus the ability of a soil to maintain and provide soil nutrients for plants and the microbial community.

 

 

1.) Sands 1-5 cmolc/kg 2.) Sandy Loams 5-10 cmolc/kg 3.) Loams/Silt Loams 5-15 cmolc/kg 4.) Clay loams 15-30 cmolc/kg 5.) Clays > 30 cmolc/kg

Using knowledge of the clay percentage, organic matter percentage, as well as information of the parent material of the local soil type one can estimate CEC. For instance, if you have a Tennessee Alfisol known to contain 15% clay and 3% organic matter. You also happen to know the dominant clay in this area are kaolinites. At neutral pH, the CEC of kaolinite is approximately 8 cmolc/kg and OM approximately 200 cmolc/kg. Kaolinite: 15% or 0.15 kg x 8 cmolc/kg = 1.2 cmolc OM: 3% of 0.03 kg x 200 cmolc/kg = 6 cmolc Total Estimated CEC: 1.2 + 6 = 7.2 cmolc/kg

Intro Soils – Lab 3 Assignment Questions Soil Colloids – Cation Exchange Capacity

o Utilize Lecture and Text Materials: Soil Colloids (Chapter 8)

o Note: Again, if I cannot recreate how/where you came up with any calculated number in this exercise you will not get credit for that answer. If you utilize reference values for any of your calculations, please include the reference, i.e., table/figure number from the text.

1.) Farmer Brown has purchased a new area of land to add to his row crop operation. He has

collected soil samples to get a baseline assessment of the land to obtain soil test values and to determine how much lime and fertilizer will be needed for his corn crop. His soil test arrived back from Lab XX and included the amount of several soil cations in the soil, but did not estimate CEC of his new property. Below are the values reported of the soil major cations:

Calcium: 1800 ppm Magnesium: 450 ppm Potassium: 380 ppm Sodium: 25 ppm

Calculate the estimated CEC using the soil test ppm values using information from Table 1 and Equations 1 thru 4. Reminder to show your work!

2.) Farmer Brown decided his pasture was not performing very well either, so he sent this sample to another soil lab for similar assessment. This time, his pasture soil test values arrived and this lab too failed to estimate CEC, but this time, his cations were reported in lbs/acre. Below is a list of the cations and their test values:

 

 

Calcium: 2700 Magnesium: 344 Potassium: 218 Sodium: 14

Calculate the estimated CEC on this pasture soil using the above soil test values. Utilize the information from Table 2 for these calculations.

Review Questions

3.) Define what constitutes a soil colloid and list 4 main characteristics.

4.) Discuss isomorphic substitution: Include a definition, where it occurs, discuss what ions might be included in isomorphic substitution, and name three clays in which their charge is dependent on isomorphic substitution.

5.) List at least one major colloid from each of the four types of colloids, include their colloid type, and CEC; rank them in order of decreasing CEC, and include their major source of charge (constant or pH dependent).

6.) Rank the following soil orders highest to lowest based on expected CEC: Mollisols, Alfisols, Ultisols, Histosols, and Vertisols.

7.) Discuss the four main principles that govern cation exchange?

8.) Why are cations not exchanged ‘ion for ion’ but rather on charge equivalence?

9.) Clay type and amount in soils are the result of weathering of parent materials. In general, discuss how the weathering process shapes clay formation (Utilize Figures 8.16 and 8.28).

10.) When using a new herbicide, why might a famer or crop consultant want to understand the combination of the Kd or Koc and major soil characteristics (texture and CEC) prior to using this product? What information do the Kd or Koc provide?

11.) BONUS! Estimate the CEC of a Soil in Texas known for its shrinking and swelling smectititic clay. The soil contains 25% clay and 2% organic matter.