Soils
By Marjorie Henry, Contributor

A group of sixth formers at Frome Technical High School study together in this November 2005 file photo. - Claudine Housen/Staff Photographer

In the previous lesson, I indicated that the geography syllabus requires that you be aware of the characteristics of the soil found in equatorial, tropical marine and tropical continental regions. I also stated that the main soil types in these regions are latosols and rendzinas. The soil orders within which each soil type occurs must also be known. Do you know the soil orders? It is quite likely that some of you may not know what I am referring to, so let me take a little time to share with you on this.

Soil order is the classification of world soils. It links world soil types with the global climate and vegetation belts. There are three soil orders, namely zonal, intrazonal, and azonal. Although only two are relevant to our study just now, I will share the information on all three.

ZONAL - This is the predominant of the three soil orders. The well-developed characteristics of these soils are more largely determined by climatic conditions than by its parent rock. They are developed in areas where there are no extremes of weathering, relief or drainage and where the landscape and climate have been stable for a long time. Consequently, these soils have had time to develop distinctive profiles with, usually, clear horizons. Latosols found in hot humid climates is an example of zonal soils.

INTRAZONAL - These soils usually develop either in poorly-drained areas, or in limestone or chalk regions. In contrast to zonal soils, the development of intrazonal soils is influenced more by local conditions, such as drainage and parent material, than by climate and vegetation. Rendzina is an example of intrazonal soils.

AZONAL -This type of soil has no marked horizons, as it has not been sufficiently subjected to soil-forming processes. This occurs because either the horizons have not had time to develop or the soils are located on steep slopes, which prevent the development of soil profiles. Some examples of azonal soils are newly-deposited alluvial and volcanic soils, dune sands, and wind-blown soils such as loess and scree soils.

Let us now turn our attention to the two major soil types to be known, namely, latosols and rendzinas. We will first look at latosols. Most Caribbean soils can be classified as latosols.

• Latosols develop in humid tropical and equatorial latitudes as a result of excessive leaching under heavy rainfall and continuous high temperatures.
  
  
• The strong leaching washes out the silica content of the soil, leaving the iron and aluminium oxides behind, sometimes creating a hardpan. It is the concentration of these minerals also which give latosols their characteristic reddish colour.
  
  
• The natural vegetation of latosols is an evergreen rainforest.
  
  
• Intense bacterial activity makes the humus layer thin. The annual fall of leaves is high, but because of high temperatures, the leaves rot very quickly, leaving only a thin layer of litter covering the surface.
  
  
• The 'A' Horizon is about 300 mm deep. The 'B' Horizon goes down to 2100 mm. It is dark red in colour. The 'B' Horizon gradually fades in colour to the 'C' Horizon.
  
  
• Latosols quickly lose their fertility under crop cultivation because excessive leaching has removed the plant nutrients in all but a thin surface layer. However, the soil is favourable for the luxuriant growth of broadleaf evergreen rainforest.

Rendzinas will be discussed in the next lesson.

Here are the answers to the questions given last week to review the lesson:

1. The factors that influence the formation of soil are parent material, climate, topography, vegetation and other organisms and time.

2. Climate affects soil formation (i) directly through its weathering effects and (ii) indirectly as a result of its influence on plant life.

3. The components of soil are mineral fraction, water, air, living organisms and organic fraction.

4. Humus is important in the soil as it forms a source of food for plants; helps to maintain bacterial life in the soil; helps to break down pesticides in the soil; improves the soil's ability to retain moisture.

The following questions should help you to recall some of the major points in today's lesson:

• What is soil order?
• Name the soil orders.
• To which soil orders do latosols and rendzinas belong?
• Under what climatic condition are latosols found?
• What soil-forming process is responsible for the development of latosols?
• What contributes to the thin humus layer of this soil type?
• What is responsible for the reddish colour of this soil type?

Marjorie Henry teaches at Glenmiur High School.