Different Types of Garden Soil
How inappropriate and demeaning that, in certain parts of the English-speaking world, soil is known as “dirt”. First-rate gardens cannot exist without excellent soil. If yours is poor and infertile then it has to be improved. Drastic steps may be necessary, but first, since soils vary hugely in texture, structure and quality, it is vital that you begin by assessing its character. The soil in any area is the product of local geology. During the ice ages glaciation transported huge quantities of rock across the globe. Each type of rock responded differently to weathering so that limestone bedrock, for example, broke down to a very different material than volcanic rock. Some areas, such as the Rhine Valley in Germany, benefit from loess, a fine, fertile soil formed by wind erosion. However, the most fertile soils are found in flood plains, being the sediment deposited by rivers.
Soil is a living material. If healthy it contains billions of micro-organisms which live off the organic content which mainly consists of decaying vegetation. Good soil must also contain moisture and oxygen, and usually carries a high proportion of mineral particles. When very fine, the soil resembles clay; when coarse, a sandy loam. On fen- and peatland the topsoil may be composed almost entirely of organic material, the result of millennia of sedges or mosses living and dying, gradually forming a thick layer of fibrous material.
Most gardeners need only know whether their soil is clay-like or sandy. Clay retains moisture, is difficult to work and sticky when wet, and sets very hard with surface cracks in a dry summer. It needs regular breaking up over winter with a soil conditionier (for example, mushroom compost), although it is often very fertile in its own right. Sandy soil is easy to work and dries out quickly, but needs plenty of well-rotted manure or compost to improve moisture retention. Alluvial silt in a flood plain is an exception to the sandy rule; it is easy to work, fertile, and though free-draining, excellent at retaining moisture.
Acidity and Alkalinity
Plants manufacture their own food by converting carbon dioxide in the atmosphere into carbohydrates. Other essential ingredients come from mineral salts dissolved in the water that coats the soil particles. Nitrogen, phosphorus and potassium are needed in fairly large quantities, with minor but vital additions of magnesium, calcium, sulphur, oxygen, iron, manganese, boron, molybdenum, copper and zinc.
Plants differ in their ability to take up these mineral nutrients. Some are only efficient at absorbing iron, for example, in acid soil. Others can obtain everything they need even in the most alkaline conditions. That is why it is essential to know the character of your soil. If it is alkaline you will not be able to grow limehaters such as rhododendrons or camellias. In very acid soils limestone plants such as philadelphus, clematis and dianthus will flounder. You can easily buy cheap pH testing kits which you should apply to different parts of the garden since conditions will vary. (The pH refers to the negative decimal logarithm of hydrogen ion concentration expressed in moles per litre). A pH of 7 is neutral; anything higher, and the scale goes up to 14, is alkaline, and anything lower acid. Generally, most plants thrive at 6.4-7, vegetables preferring 7-7.5. To confirm your readings look around the neighbourhood to see what plants are growing well in other people’s gardens. If you want to increase soil alkalinity add lime. But note such a step tends to be irrevocable so think carefully before you act. It is not so easy to increase the acidity. The best way is to create raised beds, or special enclosures, filled with acid soil for ericaceous plants.
