Apart from regional variations in subsoil type, top soils vary considerably. They can even be totally different in different parts of the garden.
Before attempting to do any planting, a prospective gardener should know what type of soil his plot contains.
Loam: This is the ideal soil to have and is every gardeners dream. It consists of a well-balanced mixture of large particles (sand), small particles (clay), medium small particles (silt), chalk and humus. A medium loam, which is the best for gardening purposes, has a proportion of about 50 or 60 per cent sand to 30 per cent clay and has twice as much humus as lime (chalk). Light loams have more sand in them than this, and heavy loams more clay. If a loam has a higher proportion of lime to humus than the above, it may well be alkaline (with a pH reading above 7). If there is a proportional imbalance the other way with the humus (i.e. more than twice as much humus as lime), the soil tends to be acid. The type of plants you grow depends on whether the soil is neutral, acid or alkaline. You can determine the pH with a soil-testing kit, available from garden shops and centres. In addition to having a pH indicator, the more sophisticated of them also contain chemicals to show whether a soil is deficient in any of the major elements — nitrogen, phosphorus and potash, generally referred to as NPK. As well as soil-testing kits which work by adding an appropriate liquid chemical indicator to a small amount of soil, shaking, and then comparing the resultant colour with a chart, meters are also available but are not so accurate. Soil pH and nutrients can be adjusted by adding certain substances which will be described later.
A light loam dries out rather more quickly than a heavy or medium one but you can improve the water-retentive properties by adding more humus-forming materials. You can also help a soggy, heavy loam by adding sand or other soil-improving substances (discussed below).
Clay: This type of soil is the gardener’s horror; it sticks to the boots when it is wet, and dries out to a rock-hard crust with deep cracks during dry spells in summer. It is composed of very small, even particles which fit closely together, leaving few and small air spaces in between. These spaces soon become filled with water which cannot drain off, and roots find it very difficult to grow healthily because of the airlessness of the soil. Cracks appear during hot, dry weather, so this soil dries like concrete, which is extremely bad for anything growing in them. The only advantage of clay is that it can be very fertile as it holds the soluble nutrients well and does not allow them to leach away.
You can improve a clay soil in several ways. You can add lime or gypsum to cause the particles to stick together into bigger ones which can hold more air between. Lime has the additional benefit of ‘sweetening’ them by increasing the alkalinity. Digging-in humus-forming vegetable matter will also aerate the soil by getting between the fine particles. Sand will do the same job. Heavy clods of clay will also break up and form a better texture if allowed to ‘weather’, especially during frost.
Silt: Silt soils are a cross between clay/sand. They also have many even particles, but these are larger than clay, whilst smaller than sand. They are often found near rivers and where ground has been drained. They too are very fertile because of their water-retaining properties but tend to ‘pan’ when they dry out after a period of wet weather if the surface has been broken down too finely. They can be improved by adding humus-forming materials while digging.
Sandy soils. These have a high proportion of large particles (sand) in them, so they are open and free draining. They are easy to dig at almost any time, being light and draining quickly after rain. Roots penetrate readily, and because these soils do not remain soggy in winter, they warm up quickly in spring. The disadvantage is that because water drains away so freely, a sandy soil soon dries out.