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Intro Soils – Lab 3 Soil Colloids – Cation Exchange Capacity
o Lecture and Text Materials: Soil Colloids (Chapter 8)
o Labs submitted without advised instructions will result in a 3 point deduction:
Proper document name (LastName_SoilsLab3)
Name included in document
Legible numbering and spacing including questions with answers
Use of spell and grammar check
o Labs submitted early will receive feedback to aid in exam preparation with the opportunity to resubmit the lab. Do not miss out on a great opportunity to be ensure understanding of the materials and increase your lab grade.
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 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.
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