9.2 Soils
Lesson Objectives
- Discuss why soil is an important resource.
- Describe how soil forms from existing rocks.
- Describe the different textures and components of soil.
- Draw and describe a soil profile.
- Define the three climate related soils: a pedalfer, pedocal and laterite soil.
Characteristics and Importance of Soil
Thank goodness for mechanical and chemical weathering, because without these forces work-
ing to breakdown rock we would not have any soil on Earth. It is unlikely that humans would
have been able to live on Earth without soil! Your life and the lives of many organisms de-
pend on soil. We get wood, paper, cotton, medicines and even pure water from soil. So soil is
a very important resource. Even though it is actually only a very thin layer on Earth’s sur-
face over the solid rocks below, it is the place where our atmosphere, hydrosphere, biosphere
and the rocks of the Earth meet. Within our soil layer, reactions between solid rock, liquid
water and air take place. It is a mistake on our part to disregard this important resource, yet
we say things like “soiled” or “dirty” when we talk about ruining something. Our precious
soil resource needs to be carefully managed and cared for. If we neglect or abuse the soil we
have, it will not remain the renewable resource that we have relied on throughout human
existence.
We can think about soil as a living resource or an ecosystem all by itself. Within soil,
there are many elements. It is a complex mixture of different materials. Some of them
areinorganic,like the products of weathered rock, including pebbles, sand, silt and clay
particles. There are also bits of organic materials,formed from the partial breakdown
and decomposition of plants and animals. In general, the pieces of rock and minerals make
up about half of the soil, with the other half made of organic materials. In between, in the
spaces of soil, there are thousands or even millions of living organisms. Those organisms
could be earthworms, ants, bacteria, or fungi, as well as many other types of organisms. In
between the solid pieces, there are tiny spaces filled with air and water. In some soils, the
organic portion could be entirely missing, such as desert sand. At the other extreme, a soil
could be completely organic, such as the materials that make up peat in a bog or swamp.
The organic materials are necessary for a soil to be fertile. The organic portion provides the
nutrients, like nitrogen, needed for strong plant growth. We will learn about that organic
portion in just a bit.