Agriculture Reference
In-Depth Information
1.1.1
THE SOLID PHASE COMPONENTS
Apart from the organic horizons of peat soils, inorganic materials form the bulk of most
soils both in terms of mass and, more importantly, surface area. This inorganic fraction
of the soil may include mixtures of rock (or other parent materials) in all stages of
weathering and physical disintegration, materials transported from elsewhere by wind,
water, volcanism or gravity together with a variety of secondary components that
have been formed by transformation
in situ
, or moved from other parts of the profile
and elsewhere. These secondary components may be distributed throughout the profile
or occur as local concentrations as nodules or concretions formed by reaction or
precipitation from solution. Alternatively, they may form discrete layers or impermeable
or slowly-permeable pans that modify soil drainage and other properties.
The average concentrations of the major elements in the surface of the earth's crust
are presented in Table I.1 (Taylor and McClennan, 1985). It should be noted firstly that
this is the average composition of the parent materials that weather to form soils
(Chesworth, 1992) and, secondly, that the remaining, non-included elements of the
periodic table comprise only a very small part of the total mass. The data presented in
this Table are given both as oxides and as elemental percentages; these data are conven-
tionally presented as oxides since oxygen is the major balancing ion (Birkeland, 1984).
Two of the major elements critical to all biological organisms (nitrogen and sulphur)
do not appear in Table I.1 and are ultimately derived from the air and minor minerals,
respectively. As considered later in this topic, shortages of these and other less-abundant
elements commonly limit the growth of all organisms.
