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Ah, the joys of Geography. While some might not find the concept of soil classification very interesting and some may in-fact wonder why we need it all, it is actually a very important factor that people such as farmers and town planners have to take into account. By classifying soils it is easier to make predictions about how they will behave over time, their capabilities and limitations as well as their management needs. As we all know - if things aren't managed properly they don't tend to last very long, which is a big concern when it comes to soils, as they are an indispensable part of our world.

As with all classification systems, the groups must be divided such that there is minimum diversity within each group and maximum diversity between groups. However, also as with many other systems, there does not seem to be one universal standard that people can agree on. There have been many changes over the course of history, and even today there is more than one classification in use. This is complicated by the use of so many different criteria to classify each type - the fact that different soil forming process take place on the global scale, the regional scale and the local scale make it very hard to find objective ways of measuring characteristics and placing them in a group. Also, indigenous people of certain lands have different classifications according to what they perceive to be the most important factors - they are after all, the ones who use the land. Many countries have system's that have been developed within the country for it's specific needs, for example Germany, South Africa, Australia, Brazil and Switzerland. There are also people outside of the earth science and humanities fields that have a need for accurate information, so there are also specific engineering classifications like the Unified Soil Classification used by civil engineers. This uniqueness of each soil has lead to many increasingly large and complex classifications, each with it's own terminology.

The earliest classification was published in Russia in the 1880's and was based upon the major soil types found in that region. Because this system takes climate as being the single most important factor in soil development it is still widely used in accounting for the distributions of soil types on a global scale. Some types of soil, even in other systems, have taken their names from the Russian terms used here - such as 'podsol' and 'chernozem'. It uses a soils distinctive zones and profiles to determine type and tends to ignore local factors such as parent rock type and changes in altitude.

The next major classification came out of Britain in the 1940's and was based on six main soil types found regionally in Britain by the Soil Survey Of Great Britain. Predictably this system's classification is very appropriate for the British Isles, but does not work well when applied on either a global or a small local scale. In the 1970's there were attempts to rectify this by taking into account factors other than climate. They looked at local properties of the surrounding areas such as parent rock, composition of the soil, relief and drainage patterns. In the end the declared 7 main groups with 35 subgroups.

In 1975 the United States Department of Agriculture (USDA) published what became known as 'The 7th Approximation Classification' of it's Soil Taxonomy due to the fact that it was the 7th revised edition that they had come up with (it is now in-fact in the 8th). The system was mainly based on descriptions of soil profiles and contains 10 main classes with 47 subtypes (see Soil Order). This classification is still in use today.

However, it was also in 1975 that The United Nations Food and Agriculture Organization (FAO) came out with their own classification. It was originally intended to be just a legend for The Soil Map of the World, but grew into an entire system. The study that produced the system was seeking the greatest amount of clarity between groups and accuracy that had not been thought necessary before, thus it is very detailed and consists of 26 major groups and over 240 subgroups. The last revision came out in 1988 and it is still the classification used most widely today for all forms of agricultural planning and management, when dealing with human settlement planning and management and by scientists/geographers/archaeologists when studying their surrounds.

A recent collaboration between the International Soil Reference and Information Centre (ISRIC), The International Union of Soil Science (IUSS) and the FAO via its Land & Water Development division has lead to what is called the World Reference Base. They stress that an international standard soil classification system is urgently needed. However, they also say that this system is not intended for use in semi-detailed or detailed mapping. While it borrows from all modern classification systems and even some older ones(the Russian concepts) it's focus is very broad and it ignores some local factors which may be very important. It is rather intended to provide a base of reference for people working in field in every part of the world.

"A world reference system for soil resources is a tool for the identification of pedological structures and their significance. It serves as a basic language in soil science and facilitates the scientific communication; the implementation of soil inventories and transfer of pedological data, elaboration of different systems of classification having a common base, interpretation of maps, etc.; the international use of pedological data, not only by soil scientists but also by other users of soil and land." - FAO Land & Water Division

For those of us who hold just a passing interest or only need to know the difference between quick-sand and concrete there is another classification that just takes the basic characteristics and splits the soils up into broad categories. This is the system most commonly learnt in schools and even some university courses(depending on the subject of course). It consists of three major categories with a few more minor subgroups. Due to the nice and easy nature of the system it's here presented to you so that on future occasions when the topic of conversation is soil, you can - using your vast background knowledge (see above) - come off sounding very well informed on this most expansive and important of topics.

Zonal Soil

These soils tend to form in areas were the landscape and climate have been very stable for long periods of time. Areas where there are no extremes of weathering, relief or drainage and the effects of the climate and vegetation have had a long time to exert their efforts on the parent rock. Thus, they are said to be mature soils and have had the time to develop very distinct profiles and clear horizons. These are divided into 8 types: tundra, podsols, brown earths, chernozems (prairie and chestnut), Mediterranean, red yellow desert, ferruginous and ferrallitic.

Azonal Soil

These types are in direct contrast to zonal soils. They have a much more recent origin and are said to be immature because they occur in areas where soil-forming process have not had much time to operate on the parent rock. Due to this, the soils show very distinct characteristics of their parent rock and agent of deposition. They do not have clearly defined horizons and are not associated with a specific climate. The number of groups in this category fluctuates mildly between regions due to the climatic conditions - however the core group consists of: scree, alluvium, till, sands and gravels, desert and marine sand dune, salt marsh and volcanic/tectonic.

Intrazonal Soil

These types of soil represent extremes and show the near total dominance of one single local factor, this is normally parent rock type or extremes of drainage. Because they are not related to general climatic controls they are not found in zones - that is, because they are due to say extreme rainfall, they can occur in any of the zonal/azonal types. There are 3 major types, 2 of which have 2 further sub-types each.

  • Calcimorphic or calcareous soils develop from a lime-stone parent rock. It has two sub-types:
    • Rendzina - This is where grasses form the surface vegetation. The leaf-litter from them is very rich in bases and encourages a lot of activity from organisms. The underlying limestone is thus very susceptible to chemical weathering and together with it's naturally permeable nature this leads to a very thin soil with limited moisture.
    • Terra Rossa - This usually occurs in depressions within the limestone or in Mediterranean areas where the vegetation is garrigue(eg. prickly shrubs: Rosemary. There must be a high rainfall volume, even if it is only seasonal, this causes leaching with in turn leaves behind a residual deposit that is rich in iron hydroxides - this is what gives it its red colour and its nickname of red rendzina.

  • Hydromorphic soils are those which have a constantly high water content. It also has two sub-types:
    • Gley soils - These occur when the pore spaces between the grains become saturated with water and contain no air. This lack of oxygen leads to anaerobic conditions which reduce the iron in the parent rock. This gives the soil a characteristic grey/blue colour with flecks of red.
    • Peat - This occurs where the soil is waterlogged but the climate is too cold or wet for organisms to break down vegetation completely. Peat is only regarded as a soil type when the layer of partially decomposed matter is greater than 38cm deep.

  • Halomorphic soils have very high levels of soluble salts and develop through the process of salinisation. They usually occur in hot, dry climates where the lack of leaching brings mineral salts to the surface. Any water quickly evaporates to leave a thick crust in which only halophytes(salt-resistant plants) can grow.
  • So there you have it, next time you're walking on the beach and get sand stuck in your swimming costume or a large puddle of mud decides to splash itself all over your newly washed car, you can closely examine it and after much deliberation tell it just where it should go stick itself - you're the expert.

    Sources:Geography - An Integrated Approach by Daid Waugh
    My old A-Level notes

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