Soil Characteristics

It is often necessary to test the characteristics of soil.  In the section on septic systems, we discussed testing the permeability, or perk, of the soil.  Here, we will be concerned with the density, percent moisture, and percent organic matter of soil.  These factors influence the ability of soil to compact, which is important whenever a structure (such as a water tower, a treatment plant, or even a distribution line) is being built in or on soil.

Density of Soil

Let's find the density of a sample of soil.  If you have a scales which will weigh small samples, you can try this experiment at home.

First, you will need to take a sample of soil.  Simply dig a hole in the ground, removing the soil as your sample.

Second, we weigh the sample to determine its weight.

To determine the soil's volume, we fill the hole with sand, measuring how much sand we put in as we go.

Finally, we can find the soil's current density by dividing the weight by the volume.  If the volume of the soil was 25 mL and the weight of the soil was 119 g, then the density would be:

Soil's Percent Moisture

The density affects how much a soil will compact, but so does the amount of moisture present in the soil.  To find the soil's percent moisture, we first weigh the soil, as we did above.  Then we dry the soil so that all of the water has evaporated out.  Finally, we weigh the soil again and use the following formula:

So, if the soil's original weight was 119 grams, and its weight after drying was 97 grams, then the percent moisture would be:

Moisture has a big influence on soil's ability to compact. Some soils won't compact well until moisture is 7-8%.  Soils with less moisture than this are like dry dust which floats up into the air rather than compacting.

Wet soil also doesn't compact well.  Consider what happens when you pick up a handful of mud and try to close your fist around it.  Rather than compacting into a ball of soil, the mud oozes out between your fingers.

Soil's Percent Organics

Soil is made up of a variety of substances.  Some of the soil is bits of sand or rock and some of the soil is organics such as decomposed leaves and other materials.

We can determine the percent organics in the soil by heating the dried soil in a muffle furnace at 550
°C.  This is like leaving your food in the oven until it burns up into ashes.   All of the organic matter in the soil will burn up and disappear.

Now we weigh the cooked soil.  The percent organics is determined by the following formula:

If the soil's weight after cooking was 70 grams, then the percent organics would be:

The organics in soil serve a variety of purposes.  They help plants since they are the fertilizer which the plants use to grow. They also make soil more permeable.  But organics in soil hurt compaction and cause slippage and stability problems. Soil high in organics may be good for growing crops or installing a septic system but may be bad for installing a large building.