The soil that we walk on is an underground city. In this
city, every organism does its part to make sure the city works together.
The actions of the animals, insects, and microorganisms that live in the
soil not only influence what happens in the soil but also what happens
above the soil. Soil is made of inorganic matter (mainly rocks and
minerals) mixed with organic matter. Soil is formed from the weathering of minerals derived from
bedrock and contains living organisms and the products of their decay.
Soil can be considered a mixture of mineral materials, organic matter,
water, and air in varying proportions. Topsoil, the A horizon, is
usually the upper ten inches of a soil in a well-developed soil profile.
Plant roots, bacteria, fungi, and small animals are abundant in this area
along with plants who thrive in this type of environment. Topsoil
has less organic matter than in the O horizon (the surface) which is the
reason that topsoil is lighter than surface soil. Topsoil is one
of our most valued commodities since it provides the nutrients and environment
for the growth of plants.
Subsoil or the B horizon, is the middle soil layer.
It has fewer organisms and less organic materials than both the A and O
horizons. Consequently, the B horizon cannot support the growth of
plants very well. If subsoils are clayey, they usually are harder
when dry and sticky when wet than the surrounding soil layers.
The C horizon is the lowest layer and is partially weathered
parent material from which the other horizons are formed. It is less
altered and weathered than the layers above and has less living matter.
The parent material is sometimes named the D horizon.
- The information given on the worksheet
should help students to write a paragraph describing the different horizons
- Save the worksheet for the lab portion of this
unit. Students will look at soil samples and describe the