Most of us think of soil as the black or brown substance in which we grow our vegetables in the back garden; the substance which gets wet and sticky when it rains and makes our shoes dirty. But what is it that gives the soil the special property which allows us to grow many of the plants that we and other animals need? Let’s know some facts about rocks and soils.
We have already seen that rocks can break down in various ways. When the products of this breakdown are mixed with a material called humus the result is known as soil. If you were to look at a quarry you would probably find that on top of the solid rock there is a band of broken and partly rotted rock, and then as you move upwards there are different layers of soil.
What you would actually see depends upon the type of rock, the climate and the way in which the water in the soil moves, and the type of vegetation growing there. Of course, plants also reflect the climate and the type of soil, so that you can see each depends upon the other in a very complicated relationship.
What is humus? Humus is a jelly-like acid material which results from the decay of the remains of plants and animals. You may ask where the first hummus came from if hummus comes from plants and animals. Animals need plants which grow in the soil.
Soil needs humus. As you read in the last question bacteria can break down a rock, and the substances thus released provide nourishment for the first specialized plants such as lichens and mosses. When these plants decayed their remains mixed with the rock particles to form the first soils.
Scientists who study the soil are called penologists. This worked is derived from the Greek, pedion, meaning the ground, and logos meaning discourse or speech. In an effort to understand the complicated world of the soil, penologists use soil profiles which are simply vertical sections through old, well-established soils showing the various layers.
These layers are formed as the soil ages by the action of rainwater moving material downwards (leaching), the concentration of humus by plant growth, and so on.
As climate influences soils, penologists recognize that there are a number of main types of soil that can be seen in the various climatic regions around the world. Podzols and brown earth, for example, are found in Britain and Europe where the climate is temperate.
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