This activity can be easily utilized in an introductory soils course but can also be modified to fit in an undergraduate hydrology course (water chemistry of the vadose zone) or geochemistry (soil chemistry/ near surface chemistry.

For use in its entirety, students must have been introduced to soil taxonomic classification terminology and the basic concepts behind cation exchange capacity. The exercise can be modified for use, however, without using the soil taxonomic descriptions.

This activity was used as a homework assignment. We did, however, spend 20 minutes in class with the students exploring the Web Soil Survey on lab computers with me providing some hints and instruction.

The goal of this activity is for students to learn how various physical and chemical properties of the soil affect the cation exchange capacity of the soil itself. The greater the cation exchange, the easier it is for primary producers to gain the nutrients they need to grow.
The activity will also give the students practice in utilizing soil taxonomic classification terminology to determine important aspects of the soil and the soil forming processes.

In order to complete the assignment, the students will need to critically evaluate several aspects of the data they extracted as to it relationship to the cation exchange capacity they extracted. They will need to consider this as well as other soil/environmental properties to determine they the data for cation exchange varies as it does.

Students will need to be able to use an online GIS based database, organize pertinent information into a table and write 1 to 2 pages of text to coherently describe trends of data and influences of one aspect of data on another.

To prepare for this activity, the students complete both background reading and attend lectures on both cation exchange capacity and soil taxonomy. The students are then introduced to the Web Soil Survey in class and have a chance to explore its functions with other students and the instructor in the room. For the assignment, students must locate an adequate area in the US that has enough soil data/information to completely answer the questions. This activity provides students a way to use real data to explore the influence of various soil properties on the soils ability to retain nutrients near the surface.

Cation Exchange Capacity Assignment (Microsoft Word 34kB Apr25 13)
Teaching Notes for Cation Exchange Exercise (Microsoft Word 33kB Apr25 13)
Cation Exchange Example Assignment (Microsoft Word 883kB Apr25 13)

Students will have met the goals of this assignment if they have successfully come to the conclusion that as both clay and organic matter content of the soil increase, the cation exchange capacity increases as well. The students will hopefully also see that as pH of the soil decreases, the cation exchange capacity in the soil will also decrease. The students should also have noted how some of the other soil properties or soil forming factors described in the taxonomic classification may have either directly influenced (clay type for example) or indirectly influenced (such as degree of saturation)the cation exchange capacity.