What is Soil?
Thehas four main components — mineral or inorganic particles, vegetable or (humus), air and water, the latter usually being called the ‘soil solution’ because it contains traces of various salts which have been derived from the weathering of rocks. As well as being in part mineral and in part vegetable, the soil is also animal, this part being the teeming hordes of insects, worms, minute animals such as protozoa, as well as fungi, bacteria and other primitive life. All these constitute the population which gives the soil its animal or living nature.
The proportions of the four major components vary greatly from garden to garden and even with depth in the same soil. Inorganic soil particles occupy only about one half of the total bulk of most soils, and do not fit together like the pieces of a jigsaw puzzle, but have spaces between them. These inorganic or mineral particles are the residue of the rocks from which the soil is derived, and which have resisted the action of weathering. As they are the most unchanging part of the soil, they really represent its skeleton or framework. Many of these particles can be seen quite easily, but others are so small that they can be examined only with an extremely powerful microscope.
Soil scientists find it convenient to divide the mineral particles into fractions or separates, according to their size. These fractions or separates they call coarse or fine sand, silt and clay. Soil layers rarely consist of one fraction only, but usually contain a mixture of all three. The proportion of sand, silt and clay in each soil layer is called the soil ‘texture’, and as many recommendations for the management and improvement of garden soils are keyed to it, the texture should be judged by the gardener. The texture in most soils changes from layer to layer, the subsoil generally containing more clay than the surface soil above it or the layer below it.
Samples of soil can, of course, be sent to a laboratory for analysis, but with a little practice the relative amounts of sand, silt and clay can be estimated by rubbing a pinch of moist soil between thumb and forefinger.
Sands feel harsh and gritty, and the particles scarcely hold together at all, even when moist. Soils containing a high proportion of sand are called coarse-textured soils. They are well aerated and ‘light’ or easy to dig, but they hold little moisture and nutrients and therefore, for good plant growth, need constant replenishment with water and very frequent feeding.
Silt feels smooth, soft and floury and is intermediate in size between sand and clay. Being smaller than sand grains, the particles pack together with smaller air spaces, which slows up the movement of air and water in the soil. Thus very silty soils drain slowly and remain moist for long periods. They are difficult soils to improve since they are not granulated by and made less sticky by liming, by frost or by cultivation. Large dressings of bulky organic manures make working easier.
As opposed to light sandy soils, clay soils are termed ‘heavy’ because they are hard work to cultivate. Clay minerals are quite different from the sand particles. They are not merely fragments of the original rock, but are a secondary product of rock weathering, and are exceedingly fine, at least 1,000 times smaller than sand particles. Their peculiar structure gives unique properties to clay and is responsible for the plasticity and stickiness of the soil. Most gardeners think of clay as a nuisance, for excessive amounts of clay lead to a very bad structure, but it does play a vital role in the soil; in proper amounts it can act as a conditioner by binding soil particles into granules — a property which is normally associated with the condition known as ‘good tilth’.
Clay particles are able to retain chemical elements dissolved in the soil solution surrounding them. Many of these elements are plant nutrients, and when so held are readily available to plant roots. Thus clay and humus (which is very similar to clay in many ways) serve as a nutrient storehouse, the clay because its particles are so small, and the humus because of its spongy nature.
Soils with a high clay content are notoriously difficult to handle. Compression occurs if they are cultivated when wet; compact lumps are formed and these dry out into steely clods. When dry clays are moistened, the colloidal matter (very fine sticky particles) swells and tends to block their pores and make them impassable for water. Heavy clays, therefore, become easily waterlogged or ‘puddled’. When the clay dries, it shrinks again forming the great deep cracks so often seen in heavy soils in summer. These cracks admit air to the soil and help to improve the structure.