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POROSITY OF SOIL
In activity No. C142.07, we packed some soil in a glass jar, and carefully (but quickly) poured in some water. Remember the air bubbles which came from the soil as the water seeped through?
1. We can use this procedure to compare visually the amounts of air in different soils, by obtaining several types of soils to pack into different jars.
You will be able to see that the amounts and sizes of pore spaces space (porosity) influences the drainage and water retention of soil.
2. There are several methods of measuring airspace in soil.
a. Fill a large glass jar with dry pebbles from a beach or a creek bed. Estimate how much space there is between the pebbles. Be sure to notice whether the pebbles are sharp or rounded, and the manner in which they are placed in the jar. These two factors will influence the sizes and shapes of the pore spaces.
By filling the jar to the brim with measured amounts of water, you can see how accurate your estimates are.
The airspace (pore spaces--porosity) in other materials, such as sand or gravel, can also be measured.
b. Put 8 ounces of water in a quart jar, then add dry sand, 1 ounce at a time, until the sand's surface is even with the water's surface. Count the number of ounces of sand that were added, and compare the two measurements. (There are usually 8 ounces of pore space in 32 ounces of sand, which means that 25 percent of the sand is pore space.)
c. Put a two-hole stopper in a wide-mouthed jar filled to the brim with a soil sample. Tap the jar to settle the soil, then insert the stopper, leaving no airspace above the soil.
Place a thistle tube that extends to the bottom of the jar in one hole of the stopper. In the other hole, place a U-tube with some rubber tubing attached to it.
Invert a second jar of water in a pan of water, and place the end of the rubber tubing under it. Add water to the thistle tube until the jar of soil is filled to the stopper. Compare the amount of air collected in the second jar with the amount of water poured into the first. Keep a record of the amounts. Test other soils, both topsoils and subsoils, in the same way.
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