The zonal soil of the tropical forest

Overview

Zonal soil, horizons of a latosol from the surface downwards and process of leaching in a latosol and what it removes.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a zonal soil?

   Model answer

   A mature, well-developed soil whose characteristics are determined chiefly by the climate and vegetation of the region in which it has formed, rather than by the local parent rock or relief. Because climate is the controlling factor, zonal soils occur in broad belts that correspond to the world's major climatic zones and biomes, which is why each biome has a characteristic zonal soil.

2. 2

   What is the zonal soil of the tropical forest called, and what does it look like in the field?

   Model answer

   It is called a latosol, and the term ferralsol is used for the same soil in international classifications. It is very deep, often 20-30 m or more, and strikingly red or reddish-orange in colour. There is a very thin surface layer of dark litter and humus, below which the profile is fairly uniform in appearance with few clear horizon boundaries, becoming mottled and then passing into weathered rock at great depth. The texture is clayey but surprisingly free-draining and crumbly, and the profile may contain a hard, iron-rich layer.

3. 3

   Describe the horizons of a latosol from the surface downwards.

   Model answer

   At the top is a thin litter layer, often only a few centimetres thick because decomposition is so rapid. Immediately beneath is a shallow, dark humus-rich A horizon, rarely more than 10-20 cm deep, containing a dense mat of tree roots and mycorrhizal fungi. Below that is a very deep, red or orange B horizon of weathered clay, enriched in iron and aluminium oxides, from which silica and soluble bases have been removed; this may include a hardened lateritic layer. Beneath it lies a mottled zone, and then a pale, deeply weathered C horizon of decomposing parent rock, with the unweathered bedrock far below.

4. 4

   Why is a latosol so deep?

   Model answer

   Because chemical weathering proceeds continuously and rapidly. High temperatures greatly speed up chemical reactions, heavy rainfall supplies abundant water to drive them, and organic acids from the forest floor assist. There is no cold season to halt the process and no dry season to interrupt it, and the surface has been stable for a very long time in geological terms, since these areas were not glaciated or scoured. The result is weathering that has penetrated tens of metres into the bedrock.

5. 5

   Explain the process of leaching in a latosol and what it removes.

   Model answer

   Rainfall greatly exceeds evapotranspiration, so there is a strong net downward movement of water through the profile. That percolating water dissolves the soluble constituents of the soil and carries them downwards out of reach of roots or into the groundwater. Soluble bases such as calcium, potassium, magnesium and sodium are removed first, which is why the soil is acidic and low in plant nutrients. Silica is also removed in solution over long periods, a process called desilication. What is left behind are the least soluble compounds, the oxides of iron and aluminium, which is why the residual soil is clay-rich and red.

6. 6

   Why is a latosol red?

   Model answer

   Because prolonged leaching has removed the more soluble material and left a residual concentration of iron and aluminium sesquioxides. Under the warm, well-drained and generally oxidising conditions of the profile the iron is oxidised to ferric compounds, which are red to orange, and these coat the clay particles and colour the whole soil mass. The intensity of the colour is therefore direct visual evidence of how far leaching and oxidation have gone.

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