In this article, our focus will be the traditional soil classification systems. Do you know that soil classification is a way of organizing soils in order to understand their important attributes and the relationships between the various attributes of such soils?
In fact, soil classification is an attempt to introduce order into a chaotic situation of complex and infinite variety of differences of soil types both in space and in time.
Generally, soil classification has been done by arranging them into assemblages according to selected characteristics properties.
There are several ways of classifying soils which relate to each discipline such as agronomy, pedology, engineering, geography, geology etc.
However, no classificatory system is mutually exclusive i.e. none stands alone.
Traditional Classification of Soils
As earlier indicate, soil classification could simple be defined as the arrangement of soils into assemblages according to certain inherent characteristic properties of such soils. Through these processes, their important attributes will not only be known, the relationships among the soil attributes will also be understood.
There are three main traditional classificatory approaches of soil worldwide. These are the empirical or generic system, the morphological system and the genetic system. In subsequently sub-sections, each of them will be considered in some details.
1. Empirical classification of Soils
Do you recollect that empirical classification of soils is also known as generic classification of soils? It is the oldest system of soil classification in which the properties that appear to be significant in the use of virgin soils for crop growths are used.
Two major soil properties have been used in the USA soil surveys. These are texture of the topsoil and the type of parent material.
The texture of the topsoil has been used primarily because the general ease of cultivating a soil has been closely related to it.
Soil texture is also perhaps the most important as far as soil fertility and productivity, including response to soil treatment, are concerned.
As for the parent material, it is considered to be the determinant of soil texture and, even soil structure, and so invariably of the fertility of virgin soils.
In the empirical soil classificatory system, some other observable properties of soil like colour, depth, pH, nutrient content, Base Exchange capacity, clay fractions etc. could also be used for this system.
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For example, Australia has successfully used soil colour in this regard. It has been observed that empirical soil classification based on chemical properties of soils such as pH and base content are very common in the agronomic literature.
Note that this early system of soil classification is still being used particularly at the local level such as series and soil type levels. Soil serves are usually distinguished on account of profile characteristics.
On the other hand, soil types are distinguished by the surface textual class such as silt, silt-loam, sandy loam and loam.
2. Morphological Classification of Soils
Morphological classification of soils is the system in which the structure and development processes of soil profile are considered. These may be internal or external.
The main aim of this system is to infer genesis from profile characteristics. Morphological classification of soils is mainly at the series level in which a number of similar soil types are grouped together.
This system also utilizes properties such as colour and texture. It is for this reason that this system of soil classification is frequently mixed up with the empirical system of soil classification.
3. Genetic Classification of Soils
Genetic classification of soils is based on the relationship between soils and the climatic characteristics.
This classificatory system is therefore based on the idea that each soil has a definite form and a structure that is relatable to a combination of soil forming factors which are found in certain climatic zones.
Important among soil forming factors are the parent material, weathering processes, leaching and the organic matter supply. Indeed, apart from the parent material, at the other factors are more or less climate controlled.
Local parent materials is important to the extent of imparting certain soil characteristics like the texture and the inherent chemical minerals.
However, climate determines the rate of weathering, leaching and the production of organic matter supply and its decay.
For instance, it is known that leaching and eluviation processes, rock weathering and decay of organic matter are more pronounced under moist, warm humid climates than under dry climates.
Therefore, given the same parent material under different climatic zones, different types of mature soils will likely evolve; and this is the basis and the sum total of zonal soil concept.
It is necessary to point out that the genetic soil classification was suggested by Dokuchaer, a Russian soil scientist as far back as 1880. Dockuchaev noticed that by moving from the polar area to the equatorial area, the soils in the different climatic zones vary, notwithstanding the nature of the parent material. However, this classificatory system has since been developed by the European and American soil scientists.
In conclusion, three traditional systems of soil classification are identified. They include the empirical,the morphologicaland the genetic systems. The oldest of these systems is the empirical or generic classificatory system of soil.
This system makes use of the properties of soil that appear to be significant especially soil texture and parent material. In the case of morphological system, the structure and the development processes of soil profile are usually considered. Genetic classification of soils is mainly related to climate.
Do you remember that empirical classification of soils is the oldest system of soil classification? Here, topsoil texture and parent material have been used mainly to achieve this soil classification.
Some attempts have also been made to use some other observable properties of soil such as colour, depth, clay fractions and pH.
Genetic classificatory system of soils is based on soil-climate relationships. It is clear from this classification that soil related so closely to the climate and that even given similar prevent materials the evolving soil type will be shaped by climate.
This brings about the idea of zonal soil concept. Note that it was Dockucheav, a Russian soil scientist that first suggested genetic soil classification.