Soil is produced over many millions of years by rock breaking up. Each type of rock produces a different type of soil. There are five main soil types: clay, sand, silt, peat and chalk. All have different characteristics with some advantages and some disadvantages so it is important to understand your soil in order to build on its good points and improve its poor points. All types of soil can be improved by adding organic matter.
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Soil type based on struture
This is a cold and heavy soil made up of tiny particles which pack tightly together holding the water in, rather than allowing it to drain away. Clay feels sticky when wet, making it hard to work, and goes hard when dry. The drainage can be improved by adding lime and digging in organic matter. Both help to aerate the soil, allowing water and roots to pass through it more easily. Dig it over roughly in the autumn and leave for winter frosts to break it up. Clay usually contains reserves of plant nutrients and when improved will grow good crops of most fruit and vegetables. Root vegetables are the likely exception but digging deep beds should improve the chances of success.
Sandy soil is light and gritty, made up of larger uneven particles with space between them. This makes sandy soil easy to work, and quick to heat up in the spring. However, because it is free draining, there is little reserve of moisture for plants in dry weather and nutrients can be quickly leached out of the soil. Replace lost nutrients in the spring, shortly before planting, by spreading organic matter on top of the soil or by digging it into only a shallow top level — it will soon sink deeper. You will need to provide extra plant food with fertilizers. Mulch well with well-rotted compost to minimize evaporation of water and to further build up a fertile layer of top soil. A sandy soil improved in this way should grow peas, beans, cauliflowers, potatoes and fruit well.
This very fine soil packs down when wet in the same way as clay and so does not drain well either. Use the same methods as for clay to improve the drainage. Dig over roughly in the autumn and add lots of organic matter plus coarse grit to open up the soil. A well- cultivated silt soil will grow most vegetables and fruit well. Dig special deep beds if you intend to grow root vegetables, which will not otherwise do well on silt.
This pale coloured soil, which is often stony, is free draining like sand but contains high levels of calcium, which combines with iron, boron, manganese and zinc to make these nutrients unavailable to plants. It is often too alkaline for growing vegetables, which prefer a slightly acid soil. To counteract chalky soil’s high level of lime dig in plenty of organic matter. In an improved chalk soil brassicas, peas and beans and fruit will grow particularly well.
This dark brown or grey soil is derived from decayed plants rather than rock. It is usually acid and badly drained so you will need to improve drainage by digging deeply and incorporating plenty of bulky organic matter. Fork over regularly to improve aeration. Adding lime will lower the acidity. Many vegetables will flourish in a well-cultivated peaty soil, especially potatoes, celery and the onion family. Fruit is less likely to be successful.
Soil type based on acidity
How well plants grow also depends on how acid or alkaline the soil is.
In a slightly acid soil, ideal for vegetables, most nutrients dissolve slowly and can be taken up by the roots. If the soil is too acid, nutrients can be washed away altogether or lie in toxic quantities in the soil water while vital phosphorus becomes unavailable to plants. Earthworms, so important for improving the structure of the soil, will move out of a very acid soil.
If the soil is alkaline, trace elements become insoluble and cannot be taken up by plants.
Use one of the widely available testing kits to check the pH of your soil. A neutral soil has a pH of 7; anything below that is acid, anything above is alkaline. Most vegetables grow best on a soil with a pH value of 6-6.5. If your soil is too acid, adding lime will help. The quantity needed will depend on your soil type: clay will take more than a sandy soil. Be cautious when using lime, as over-liming can be very harmful, and only apply lime every three to four years. Adding calcified seaweed will also make the soil less acid. To increase the acidity of an alkaline soil, add large quantities of well-rotted organic matter.