To facilitate student investigations, it helps to have a basic understanding of what makes soils "tick" and to learn some of the lingo of soil science.
As rocks are weathered and broken down by forces of nature such as rain, freezing and thawing, glacial action, and flowing water, small mineral particles remain that form the basis for soil. These particles range in size from fine, silky clay particles to mid-sized silt to relatively large, coarse sand particles.
The proportion of these different-sized particles affects the amount of air, water, and nutrients available, and how the soil "behaves." The smaller the soil particles, the more they stick together when wet. Soils with a lot of clay hold water tightly but drain poorly and can be sticky and difficult to work. Because clay particles pack together tightly, there are few air spaces, and roots may suffer from a lack of oxygen. Although sandy soils are easy to work, they don't hold water or nutrients well.
Soil scientists feel and observe soil to help identify soil types, then describe them in terms of rough proportions of sand, silt, and clay. A loam soil, the ideal medium for growing most garden plants, has a balance of the three types of particles.
One of the main differences between soil and mere dirt is that soil is buzzing with life -- microorganisms like bacteria and fungi (billions in a single teaspoon!) and larger animals such as worms and sowbugs.
Many of these underground inhabitants feed on remains of plants and animals, breaking down their tissues. In so doing, they release nutrients that plants need for growth and development and the cycle begins again. Some soil microbes are also able to "fix" nitrogen from the air and make it available to plants. Worms constantly ingest soil and organic matter, digest it, then excrete crumbly, nutrient-rich "castings," which improve drainage and air circulation. Without these underground workers aerating and returning valuable nutrients, the plants we depend on for food couldn't grow, and the life-sustaining cycle would be broken.
The resulting organic portion of the soil, a dark, crumbly component called humus, holds water like a sponge, serves as a storehouse for plant nutrients, and makes soil particles clump together, helping to aerate the soil. Gardeners who want to increase humus add compost, leaves, grass clippings, and other forms of organic matter.