• Students will discover that soil has solid, liquid and gas constituents.
• Students will discover that particle and pore size affect water movement and holding capacity of soil.
• Students will discover that soil particles and gravity affect water movement.
• Texas Essential Knowledge and Skills, K-8  Adobe *.pdf file

Hypothesis: Soil has solids, liquids, and gasses present at all times

Methods:

I

A

Place golf balls into container.  Ask students if the container is full.

Discussion points: Some soil particles (sand) are quite large and leave large spaces among them. Other particles are smaller (silt) and will fit in the spaces among the larger particles. Clay particles are the smallest and can fit into very small spaces. However, there is still space among them. This space can be occupied by air or water.

Follow-up activities for older students:

I	D	Measure the amount of water added.
	E	Pour the water out, and measure how much was recovered.
	F	Add only golf balls to the container.  Measure the amount of water added and recovered.
	G	Repeat with marbles, etc.

Discussion points: Water exists in pores (voids) in the soils, and in films around the soil particles.  Less water is recovered because water is held on the surfaces of the objects, and in some of the small pores created by the groups of bb's or beads.  More water is recovered when larger objects are used because the large objects have less total surface area, and they create much larger pores.
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Hypothesis: Water exists in films around particles and in pores among particles.

Methods:

I	A	Place objects into the container, one layer thick.
	B	Place the dish onto an overhead projector, and project onto the screen.

Discussion points: Notice the water first fills the smaller pores and forms films around particles.  As more water is added the larger pores are filled.
 

I	D	Continue adding water slowly until most pores are full.
	E	Use the pipette or towel to gradually extract the water.

Discussion points: When a soil holds all the water it can against the force of gravity, some of the larger pores will be empty.  (This will become more obvious with the sponge demonstration that follows.)
 

I

F

Continue removing water slowly until no more water can be extracted.

Discussion points: Notice the first pores to empty are the larger ones.  Even pores on the other side from the extraction point will begin to empty.  The pipette or paper towel acts like a plant root.  As the root removes the water next to it, water from other places moves to the root.  Water movement occurs through the pores and in the films around the particles. Water movement is rapid when most of the pores are filled and slows as the pores are emptied. Eventually, only the smallest pores still contain water.  Water is also in thin films around the particles.  When the soil reaches this level of dryness, the root can no longer extract water. Water movement still occurs, but very slowly, in the thin films around the particles.  The soil is not completely dry, it still contains water.  But the water is not available to plant roots.

These images show the full and dewatered conditions. Follow the link at right to see what happens in between.	Soil and Water - This link has photos to show the design and process of these steps.

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Allow a sponge to soak in a pan of water. Lift the saturated sponge out of the water, holding it horizontally until no water drains out. Then change the axis to the next longest side until no water drains. Finally turn it so the longest direction is vertical until all water drains. Ask, "Is there water left in the sponge?" The water that has drained out was held in the largest pores. You may be able to see water in a saturated sponge. Next squeeze the sponge until all possible water is squeezed out. Again ask, "Is there water left in the sponge?" Note that the sponge is damp. The remaining water is not held in the pores (spaces) but on the particles (the fibers of the sponge). See The Sponge Model.
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Place a dry sponge with the long axis vertical into a pan of water. Observe as the water moves up into the sponge.
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