What is soil made of?
Garden soil consists of many different ingredients. These include varying proportions of clay, silt, and sand. A soil containing a high proportion of clay is considered to be heavy and it is often difficult to cultivate, especially when wet. Sandy soils, on the other hand, are light and easy to cultivate, even after rain. Decomposing plant remains, air, and water are other important soil constituents. Microbes by the million are also present; some can be seen only with the aid of a microscope. Other organisms, such as the earthworm, can be seen easily.
How does soil form?
During the course of millions of years, weathering reduces different types of rock to small particles. Freezing, for instance, causes rock to shatter and crack, allowing plantto penetrate; their expansion causes further crumbling. Wind blasts the rock fragments into smaller pieces, and rain swirls them together. Rainwater contains carbonic acid, which causes chemical reactions to take place in rocks. Carbonic acid is also produced by small plants that attach themselves to rocks. The plants die and, in turn, provide food for other plants. Running water also plays a part by grinding rocks and transporting fragments from one place to another.
Why do different soils vary in colour?
The colour of soils depends considerably on the minerals present. Various oxides of iron, for instance, are responsible for red, yellow, blue, and grey tints in different soils; and most people have seen the effect of chalk on down-land soil. Humus makes the soil darker, which helps soil to warm up faster in spring.
What is a soil pan?
When certain minerals are washed down through the soil by rain, they usually lodge some way below the surface. This often happens in sandy soil containing a high proportion of iron. Over a period of time the minerals weld together to form a hard layer impervious to water. This layer restricts the downward spread of plant, so that poor growth results.
A similar situation can occur if a rotary cultivator is used regularly and its tines are set at the same depth on each occasion: the action of the tiller blades causes soil compaction at that depth. This ‘mechanical pan’ is avoided by varying the depth of rotovation.