The soil in your garden is a very complex structure and its cultivation depends on many different elements. There are several different soil types that all have advantages and disadvantages. For example, the soil may be acid or alkaline; it may be heavy or light; it may drain well or badly; it may be very rocky.
Soil Profile
What you see in your garden is simply the surface of the soil. Soil is made up of three layers: topsoil, subsoil, and the soil parent matter. Topsoil is formed over the years by the addition of organic matter that follows the decomposition of dead plants or animals. It is inhabited by a wide range of living organisms, and it is in this layer that the majority of the feeding roots of plants exist. Topsoils can be improved and deepened by the regular addition of organic matter.
The second layer is the subsoil, which is low in nutrients, generally contains few or no microorganisms, and is therefore inhospitable to roots. Thus, when digging deeply, it is advisable to bring to the surface only very small amounts of subsoil; these can be mixed with organic matter and will, eventually, turn into topsoil. Double digging breaks up subsoil and improves drainage without bringing the subsoil to the surface.
The nature of the subsoil has a profound effect on the water-holding capacity of the soil in general. If you have light sand or chalk subsoil, which drains very freely, you will need to increase the bulky organic matter content, and thus the water-holding capacity, of the topsoil. On the other hand, heavy clay subsoil, which drains poorly, may necessitate the installation of an artificial drainage system.
The third layer—the parent material—is the original mineral from which the soil was formed. This layer is normally deep enough not to concern the gardener, but may, on high ground, be comparatively near the surface. If this is the case, try to increase the depth of the topsoil by adding organic matter to the top layer.
Soil Types
There are five main soil types: clay, sand, silt, chalk, and peat. Generally, it is the nature of the original rock and the size of the mineral fragments that determine the soil type.
It is important to know what kind of soil you are dealing with in your garden because the way in which you manage it, the timing of cultivations, and the plants you grow will depend to a large extent on the nature of the soil. However, having said this, most soils contain a mixture of minerals.
If a soil is referred to as, for example, "clay," then this indicates its major constituent. Soil mixtures are known as loams; for example, a soil made up of 50 percent clay and silt and 50 percent sand is a "medium loam". Similarly, a soil which contains a high proportion of sand might be described as a "sandy loam," while one which contains a relatively large amount of clay might be described as a "heavy loam".
Practical Characteristics
Soils can also be heavy or light. A heavy soil contains a much higher proportion of clay. This type of soil has very small particles that tend to pack together, preventing free passage of water. Heavy soil is often very difficult to work initially because it tends to be either very wet and sticky or very dry and hard.
Eventually though, when it has been ameliorated by the natural drainage afforded by plant roots and the addition of organic matter, heavy soil becomes an excellent moisture- and nutrient-retaining medium.
Light soils, on the other hand, are easy to dig and warm up quickly in the spring but allow very free drainage, which has its own problems. Water and nutrients disappear through the topsoil, go into the subsoil, and eventually out the drainage system. Light soils require constant additions of organic matter to form a topsoil that retains moisture and generally need more applications of fertilizers than heavy soils.
Acidity And Alkalinity
Soil may also contain lime, which will cause it to be either "acid" or "alkaline", depending on the amount. The lime content will make a considerable difference to the fertility of the soil and will govern the range of plants you can grow because it has the ability to make some nutrients unavailable to plants.
Rocks
The proportion of rocks or gravel in your soil does not influence its texture classification, but may affect its fertility and drainage. Rocky soil has the advantages and disadvantages of a free-draining soil and it may need regular applications of bulky organic matter to improve water retention.
If you are lucky enough to have a heavy topsoil and a very rocky subsoil, you have the best of both worlds, with surface moisture and nutrient retention, plus good drainage of excess water.
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