Inescapably soil plays a key role in gardening and virtually all of horticulture. The questions which may be posed in the examination are likely to be fundamental ones to do with the maintenance or improvement of soil fertility. Soil fertility itself is the result of a great matrix of factors including:
(I) soil texture
(ii) the arrangement of these soil particles into a soil structure
(iii) the depth of the top soil and the depth and availability to plants of the sub-soil
(iv) the nutritional state of the soil
(v) its pH (acidity, neutrality or alkalinity)
(vii) soil temperature
(viii) the nature of the soil fauna and flora
(ix) its soluble salt concentration
(x) its water-holding capacity and content
(xi) its base exchange capacity (including the organic matter and clay concentrations).
This lesson is concerned with many of these factors. Subjects such as the soil population and lime are covered in other lessons.
Although the proportions of the components vary greatly, all soils consist of organic matter, mineral matter, water and air. It is the first of these four, the organic matter, which is distinctive. Pure sand, with some water and air in its interstices, could not be regarded as soil even if plants were growing on it – nor could artificially crushed granite. Usually, a soil contains enormous numbers of living bacteria and other minute organisms; also smaller numbers of easily visible organisms such as earthworms, centipedes, and ground beetles.
A Study of Soil: the soil profile
Soils differ widely according to the basic materials from which they originate and to the manner in which they have been formed. These differing qualities and characteristics have a profound effect on the plants which a soil will sustain so that careful study of the plant life will yield information about the nature of a soil. It can almost be said that soils have individuality, for they differ in their make up texturally, structurally, and physically.
The best way of finding out the nature of a soil is to examine its profile, I.e. from the surface downwards. This can be done most effectively from a soil pit, but a large amount of information can be obtained by using a soil auger. This is simply a boring implement which enables a core of soil to be brought up from any depth from 1 to 4 or 5 feet, (0.3 – 1.5 m), according to the length of the auger. Each successive core can be carefully examined and if necessary, subjected to analysis.
Such exami layer of so a gardener crowded wi micro-organ abundant in fertile lay from a few
nations usually reveal the fact that the upper il, the top soil (or top spit to use the words of ) consists of the finest particles: it is most th plant, most heavily populated by ic and macro-organic soil life and is most organic matter. It is, in fact, the most er of soil. The depth of this top layer may vary inches/centimetres, such as is found on many
chalk soils, to perhaps two or three feet (0.6 – 0.9 m) in the case of a well cultivated organic or peaty soil. Usually, the line of demarcation is clearly visible. In a cultivated soil, the sub-soil or second layer, begins at the deepest point of cultivation. In nearly all cases, the sub-soil is lighter in colour and contains much less organic matter.
Generally speaking the shade of a soil gives some indication of its organic content. The lighter the soil the less organic material it contains; organic material turns dark brown or black on decay and thus darkens the soil. A dark soil absorbs the heat of the sun better than a light one and loses its warmth more slowly.
Soils are composed of particles. The sizes of the particles which predominate in any soil primarily determine its texture. Texture is established by the geological processes responsible for the formation of soil and can only be changed over a long period of time. A sandy soil remains a sandy soil and a clay soil remains a clay soil in spite of any cultivational actions taken to alter it. Soil particles are usually graded as follows:-
Stones and Gravel 2 mm or more in diameter
Coarse Sand Between 2 and 0.2 mm in diameter
Fine Sand Between 0.2 and 0.02 mm in diameter
Silt Between 0.02 and 0.002 mm in diameter
Clay Less than 0.002 mm in diameter.
The proportions in which these ingredients occur determines the class to which a particular soil rightly belongs.