Tropical LatosolsLatosol – A Tropical Rainforest Soil

Exam Brief

You may be asked the following:

“Compare and contrast the characteristics of any two soil types which you have studied.”

That is why you must know Irish Brown Earth Soils and Latosols (tropical rainforest soil).

Exam Brief

• Typical Exam Questions:• Describe and explain the characteristics of any one soil type

studied by you.• With reference to one soil type studied by you, examine

how parent material, climate and organic matter influence the soil.

• Examine the general composition of any one soil type that you have studied.

Soil Type: Zonal

• Zonal soil type: developed in response to tropical and equatorial climates that are hot (average 27C), humid (88% humidity) and wet (up to 6,000mm per annum).

• Latosols:• Red, heavily leached infertile zonal soils.• Occupy 7.5% of total land area.• Cover large areas of South America, Africa and Asia.• Major obstacle to development of profitable agriculture.• Fragile soils which can be easily damaged creating a useless

laterite.

Soil Type: Zonal

• Latosols:• Support the richest vegetation on the planet – the

tropical rainforests. But how if latosols are not fertile?• The relationship between soil, climate and vegetation in

tropical regions causes a short nutrient cycle. Plants grow rapidly, otherwise they will not get the nutrients before they are leached by the high rainfall.

• If vegetation is removed the soils quickly become infertile and vulnerable to erosion.

Soil Profile of a Latosol

O Horizon

• Thin O Horizon (humus layer) due to intense bacterial activity which rapidly decomposes dead organic matter.

A Horizon

• Contains aluminium and iron oxides. • Sometimes iron and aluminium compounds build up in a hard layer

down the profile.

B Horizon

• Very deep.• Uniform in texture due to intense leaching in high temperatures.

C Horizon

• Consists of parent rock.

Factors affecting latosols

1. Climate

2. Relief

3. Parent material

4. Living Organisms

5. Time

Factors affecting latosols

1. Climate:• Latosols form in very hot, wet conditions in the tropics

and equatorial regions. High rainfall, humidity and temperatures cause deep chemical weathering and rapid leaching of minerals down through the soil.

• Average rainfall: 3,000mm.• Humidity: High – 88%• Average temperature: 27°C• Latosol soils are very deep – high temperatures and

permeability of soil mean heat and moisture reach great depths and rot the parent material into a deep soil.

Factors affecting latosols

2. Relief• Latosols form under the rainforest on flat land and

on slopes which allow tree growth:• Thicker on flat land.• Thinner and better drained where land is sloping.

Factors affecting latosols

3. Parent Material• Parent materials found under latosols in

Brazil vary from metamorphic to sedimentary rocks. • These different parent materials cause the

latosol to vary in colour from red to yellow.

Factors affecting latosols

4. Living Organisms• Hot and damp conditions of the forest floor are

perfect for fungi and bacteria to thrive and cause rapid humification. This provides plentiful nutrients but these nutrients are in high demand so they do not remain in the soil for long.

• This nutrient cycle in latosols is very short – a few days in some cases.

Factors affecting latosols

5. Time• Deep latosols result from the rapid weathering of

parent material and the fast breakdown of organic material by fungi, bacteria and other living things.

• Tropical regions were not affected by the last ice age and so have had many thousands of years to develop.

Characteristics of Latosols

Colour• Red or yellow – due to

aluminium and iron compounds left over from leaching.

Characteristics of Latosols

pH• Moderately acidic.• Rapid absorption of

nutrients by vegetation growing in soil prevents latosol becoming more acidic.• Once forest is cleared,

latosol acidity rises.

Characteristics of latosols

Humus Content• Low humus content.• Rapid breakdown of organic material

but equally rapid uptake of humus by plants.

• Any humus formed is quickly absorbed by plants.

Characteristics of latosols

Structure• Latosols lack a clearly defined

structure.• Structure is often poorly developed

due to intense chemical weathering of mineral grains which prevents well shaped peds forming.

• Where parent rock is granite, chemical weathering causes clay minerals to form giving the latosols a platy structure.

Characteristics of latosols

Texture• May be any texture – loamy / clay /

sandy.• This is due to a variety of parent

material.• Latosols formed on metamorphic or

igneous rock tend to have a more sandy texture.

Characteristics of latosols

Water content• Wet due to high rainfall in tropical

region.• Latosols are very permeable

(water passes through easily).• But, if forest cover is removed,

the soil dries out rapidly and becomes impermeable.

Processes Affecting Latosol Formation

1. Laterisation

2. Humification

Processes Affecting Latosol Formation

1. Laterisation• Definition: A combination of deep leaching and chemical

weathering by carbonation, oxidation and hydrolysis.• Dominant process in forming latosols.• Leaching and chemical weathering in the high

temperatures of the tropics combine to dissolve all minerals except iron and aluminium oxides. These minerals give the soil its distinctive red colour.

• Due to the constant high temperatures, these soil-forming processes have reached deep ground and formed soils up to 40m deep.

Processes Affecting Latosol Formation

2. Humificaiton• Hot, damp conditions = rapid humification.• This provides plentiful nutrients easily absorbed by

plant roots.• These nutrients stay close to the surface of the soil

because they are quickly absorbed by the plants of the rainforest.