Soils are formed by the weathering of rocks or materials deposited by rivers or wind. There are five groups of factors responsible for the kind, rate and extent of soil development. They are: Climate, organisms, parent material, topography and time. Soil from one place is different from another because of the differences in the influence of these factors.
The influence of climate is due to basically two factors: temperature and rainfall. Climate indirectly affects soil formation through its influence on organisms as well. High temperatures and rainfall increase the degree of weathering and therefore the extent of soil development.
Table VI. Ratings of soil nutrient values in agricultural soils (Loganathan, 1987)
|Nutrients||very low||low||moderate||high||very high|
|(available P, Bray and Kurtz No.1, ppm)||<3||3 – 10||10 – 20||20 – 30||> 30|
|(exchangeable K, meg/100g)||<0.2||0.2–0.3||0.3–0.6||0.6–1.0||> 1.0|
|(exchangeable Ca meg/100g)||<2||2 – 5||5 – 10||10 – 20||> 20|
|(exchangeable Mg, meg/100g)||<0.3||0.3–1||1 – 3||3 – 8||> 8|
Increase of rainfall increase organic matter content, decrease pH, increase leaching of basic ions, movement of clay etc. Increase temperature increase organic matter decomposition and decrease its accumulation. The influence of organisms is due to two factors: animal activities (humans, earthworm activity etc.) and influence of vegetation (grasslands produce soils very different from forests). Parent materials influence the type of minerals in the soil, nutrients, pH, texture etc. For example, soils formed on granite materials are more acidic and have less basic cations than those formed on basalt. Topography of the land influence the type of soil formation through its influence on drainage and slope. Young soils do not have much horizon differentiation and would not have reached the advanced state of soil development compared to older soils.
Among the 5 factors, climate has the greatest influence on soil formation. In areas of high rainfall and temperature, the soils formed are often similar even though the parent materials are different.
As the weathering of the parent materials progresses, with the various factors of soil formation acting on it, a more or less definite layering of the soil developes. A section from the surface through the various layers of the soil to the parent material is called the soil profile. The texture, depth, colour, chemical and physical properties, structure, and the sequence of the various horizons characterise a soil and determine its agricultural, aquacultural and other values.
The various layers are grouped into three headings A, B and C. The subdivisions are called horizons.
|A group -||(eluvial region) - zone of maximum leaching. Subdivided into Aoo, Ao, A1 A2, A3|
|B group -||(illuvial region) - zone of deposition of clay, Fe and Al oxides, Ca CO3, Ca SO4 (the last two materials generally occur in arid regions). B group is subdivided into B1, B2,|
|C group -||unconsolidated mineral mass from which the A and B are developed.|
In order to have a systematic study of soils and to transfer knowledge from one area to the other the soils are classified according to their morphological, chemical and physical properties. As in the case of the classification of plants which are grouped into Class, Order, Family, Genes, Species, the soils are also classified into categories such as Order, Suborder, Great Soil Group, Soil Series, Soil Types and Soil Phases. Order is divided into Zonal (determined primarily by climatic differences), Intrazonal (determined by difference in local conditions such as drainage, salinity which dominate the influence of climate) and Azonal (soils without horizon differentiation - alluvial deposite etc.). Great Soil Group differs in the expressions of some specific condition in the soil profile. Soil Series have similar profile characteristics except for differences in the textures of the surface horizon. They are named normally by the name of the place where the series is first observed. Soil Types differ in the texture of the surface horizon (e.g.: Miami silt loam and Miami sandy loam). There are many types of classification systems, namely Soil Taxonomy (U.S.A.), French system (ORSTOM), Belgian system (INEAC), FAO - UNESCO Soil Legend system and many others (Sanchez, 1976). These classifications are developed primarily for agricultural applications. At the lower levels of classification (Order, Suborder, Great Soil Group) the various classification systems do not have much use in providing information on suitability of the soils for aquaculture. Classification at the Series or higher level is required in order to determine its suitability for aquaculture.
Classification systems used for engineering purposes are generally based on particle size characteristics and same other soil physical properties like plasticity. One such classification system is the Unified Soil Classification (USC). There are three major soil units in the USC. They are:
Table VII.: Suitability of soils for construction of dikes and dams (Coche and Laughlin, 1985)
|USC group symbol||Descriptions||Suitability for dikes and dams|
|Coarse grained soils|
|GW||Well-graded gravels, gravel-sand mixtures, little or no fines||Very stable; permeable shells of dikes/ dam|
|GP||Poorly graded gravels, gravel and sand mixtures, little or no fines||Reasonably stable; permeable shells of dikes/dam|
|GM||Silty gravels, gravel and sand and silt mixture||Reasonably stable; not particularly suited to shells; but may be used for impermeable Cores or blankets|
|GC||Clayey gravel, gravel and sand and silt mixtures||Fairly stable; may be used for impermeable cores|
|SW||Well-graded sand, gravelly sands, little or no fines||Very stable; permeable sections; slope protection needed|
|SP||Poorly graded sands, gravelly sands, little or no fines||Reasonably stable; may be used|
|SM||Silty sands, sand and silt mixtures||Fairly stable; not particularly suited to shells; may be used for impermeable cores/dikes|
|SC||Clayed sands, sand and clay mixtures||Fairly stable; used for impermeable cores for flood control structures|
|Fine grained soils|
|ML||Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity||Poor stability; may be used for embankments with proper control|
|CL||Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays and Lean clays||Stable; impermeable cores and blankets|
|OL||Organic silts and organic silty clays of low plasticity||Suitable for low embankments with very low hazard only|
|MH||Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts.||Poor stability; core of hydroulic fill dams; not desirable in rolled fill construction|
|CH||Inorganic clays of high plasticity, fat clays||Fairly stable with flat slopes; thin cores, blankets and dikes sections|
|OH||Organic clays of medium to high plasticity, organic silts||Suitable for low embankments with very low hazard only|
|Highly organic soils|
|Pt||Peat and other organic soils|
The coarse and fine grained soils are subdivided according to liquid limit and plasticity index. Typical names and group symbols of USC system, their suitability for construction of dikes and dams are presented in Table VII. (Coche and Laughlin, 1985).