Soil and Water-Holding Capacity

Max Strain
Heritage Elementary School
1700 E. Pawnee
Olathe, Kansas 66062

Overview: This lesson will help students gain an understanding of the relationship between organic matter in the soil and water holding capacity. Normal field soil water- holding capacity is 60-80% of its total capacity; that is 60-80% of the water filled pore spaces are filled. This corresponds to the optimal biological activity of 60-80% of the water-holding capacity. When the water-holding capacity falls below 55-60%, organisms could suffer from dryness; and when the capacity is over 80%, they begin to suffer from a depletion of soil oxygen.

Grade Level: 6-9.

Connection to the Curriculum: This lesson is to be used in conjunction with a soil ecology unit that explores all aspects of soil use.

Time Required: The total time required including class discussion and follow up will be approximately two days.

Materials and Equipment: Soil samples (oven dried), Humid Chamber, Hilgard soil cups, Drying oven, Spatulas, Mortar and pestle, Triple beam balance, Filter paper.

Objectives: The student will complete a hands-on activity that will allow them to determine the water-holding capacity of soil. Each student will use the process of observation, classifying, exploring, recording, predicting, inferring, investigating, and valuing.

Procedures:

  1. Place a moist Whatmen #2 filter paper on the screen inside the Hilgard soil cup. Weigh and record the cup dry with filter.
  2. Prepare the soil sample by crushing it to a fine state using the mortar and pestle. (This will require soil that has been dried in an oven at 110 C for 24 hours to remove all moisture.)
  3. Fill the cup gently with oven-dried soil so that it is even with the lip of the cup. If necessary use a flat blade to obtain an even surface.
  4. Place the cup into a shallow pan of water allowing only the bottom few cm of the cup to become wet.
  5. Allow the soil to become saturated.
  6. Remove the cup from the pan of water and place it in a humid enclosure until drainage is complete. Then weigh and record data.
  7. Complete the data sheet to obtain the water-holding capacity of the soil sample tested.

Body of Presentation: This will work well with any lesson on soil and water capacity or use. It works well with previous soil lessons that may have required the drying of soils for other experiments. This will allow the students to use other predried samples from previous experiments rather than dry samples for this one particular exercise.

Assessment: This will be accomplished by the students completing a correct data information sheet used during the course of this experiment.

Lesson Extension: This lesson can be extended by using other soil types. Soils used for comparison can range from those in cities, construction sites, grasslands, and fields in and out of production. Charts can be used to compare various soils for water holding capacity.


							Soil A			Soil B

Weight of dry cup with wet filter		_______________		_______________

Weight of dry cup and dry soil			_______________		_______________

	Gross weight (same as above)		_______________		_______________

	Dry soil weight				_______________		_______________	

Saturated soil and cup weight			_______________		_______________

	Minus weight of cup			_______________		_______________

	Weight of wet soil after drained	_______________		_______________

	Minus weight of dry soil		_______________		_______________

		Water (g)			_______________		_______________

		% Water holding capacity	_______________		_______________

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