Soil is a complex body composed of five major components (Fig. 1.3 namely:
mineral matter obtained by the distintergration and decomposition of rocks;
organic matter, obtained by the decay of plant residues, animal remains and microbial tissues;
water, obtained from the atmosphere and the reactions in soil (chemical, physical and microbial);
air or gases, from atmosphere, reactions of roots, microbes and chemicals in the soil
organisms, both big (worms, insects) and small (microbes)
According to its size, soil can be separated into various fractions. Two common systems of classification are given in Table I.
Table I.: Classification of soil particles according to two systems (U.S.D.A and International)
|Soil seperates||U.S. Dept. of Agric. |
|Number of |
particles per g
|Very coarse sand||2.00 - 1.00||)|
) 2.00 - 0.20
|Coarse sand||1.00 - 0.50||720|
|Medium sand||0.50 - 0.25||5,700|
|Fine sand||0.25 - 0.10||)|
) 0.20 - 0.02
|Very fine sand||0.10 - 0.05||722,000|
|Silt||0.05 - 0.002||0.02 - 0.002||5,776,000|
|Clay||below 0.002||below 0.002||90,260,853,000|
Fig. 1.1. Schematic comparaison of agriculture and aquaculture
Fig. 1.2. Soil profile
The clay fraction, because of its high surface area, is the most active part of the soil controlling many of the chemical and physical properties of the soil. It is the seat of soil fertility. The sand and silt fractions i influence mainly the physical properties of the soil. The elemental composition of the inorganic component (mineral matter) of two soils formed from two types of rocks is presented in Table II. It could be seen from the table that Si, Al and Fe are the three major cations present in the soil.
Table II.: Elemental composition (%) of two common rocks and soils developed from these rocks (Townsend, 1982)
The inorganic component (mineral matter) of the soil is composed of many types of minerals which influence the properties of the soil. The differences among soils are due mainly to the differences in the type and relative abundance of such minerals. Minerals are naturally occuring inorganic compounds having definite crystalline structures. They are classified into primary and secondary minerals. Primary minerals are those formed at elevated temperature and inherited unchanged from igneous and metamorphic rocks whereas secondary minerals are formed at low temperature reactions and either inherited from sedimentary rocks or formed directly by weathering in soils.
Some of the common types of minerals found in the various size fractions in the soil are shown in Fig. 1.4. It could be seen from the figure that the sand and silt fraction consists mainly of quartz and other primary minerals (feldspars, micas, pyroxene, olivine). In addition some small amounts of secondary minerals such as oxides of aluminium (gibbsite) and iron (hematite) are also found. The clay fraction is mainly composed of secondary silicate minerals such as kaolinite, illite and montmorillonite.
Soil organic matter could be considered to consist of two general groups: (i) fresh or partially decomposed plant and animal residues having some recognisable physical structures traceable to its origin; and (ii) the humus, which is a more resistant product of decomposition and colloidal in nature. The black or brown colour usually observed in the surface layers of soil profiles is due to the presence of humus. Humus is the most reactive part of the organic matter. Its capacity to hold water and nutrients greatly exceeds that of clay, its inorganic counterpart. The fresh and partially decomposed plant and animal residues generally occur in the sand and silt fraction of the soil and the humus occurs in the clay fraction.