BIOSPHERE

Soils:– What is a soil?– What does soil contain?– What factors affect soil formation?

Soil Profiles

SOIL SOIL

MINERALS FROM WEATHERED ROCKS

WATERAIR

INORGANIC MATERIAL ORGANIC MATERIAL

DEAD AND LIVING ORGANISMS

What is soil?

WHAT ARE SOILS?

INORGANIC MATERIAL = 45% OF SOIL VOLUME

WATER = 25% OF SOIL VOLUME

AIR = 25% OF SOIL VOLUME

ORGANIC MATERIAL = 5% OF SOIL VOLUME

SOIL FORMATION AND SOIL PROFILES

Soil Profile:

A vertical section through the soilFrom surface to the bedrockInformation comes from boringthe soil with an augerLayers are called horizons

Decomposition and Humification

Capillary action

Translocation

Weathering

Leaching

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Soils are complex and dynamic systems, in which many processes are taking place.

Soil Forming Factors

SOIL PROCESSES

Leaching

Wherever rainfall exceeds evaporation and there is free downward movement of water through the soil pore system, soluble minerals are leached or removed from the soil profile.

Continual leaching tends to impoverish the upper mineral horizons.

A soil with small soil crumbs and high porosity leading to free drainage and

active leaching

Capillary action

Where evaporation exceeds precipitation, moisture moves upwards within the soil profile by capillary action. It is therefore in the reverse direction to leaching.

In Britain precipitation generally exceeds evaporation. As a result capillary action in British soils rarely occurs, apart from in very sandy soils.

Decomposition and Humification

Decomposition is the breakdown of plant derived material into its simpler organic constituents. This is accomplished by enzymes, earthworms, mites and other organisms.

Humification is the breakdown of plant remains- leading to the formation of different types of humus. It is probably the most important biological process taking place in soils.

MULL humus develops under deciduous woodland, where base-rich plant remains are actively broken down by a prolific soil biota.

MODER humus is intermediate between mor and mull.

MOR humus usually develops beneath coniferous woodland or heather moorland, under cool, wet climatic conditions. Breakdown is slow due to the absence of soil biota.

Weathering

Physical weathering involves continual breakdown of rocks into smaller and smaller particles.

Chemical weathering involves alteration of the chemical composition of rock minerals.

This refers to the breakdown and decomposition of rocks and minerals by factors including air, water, sun and frost.

Translocation

The movement of material in solution or suspension from one horizon to another is referred to as translocation.

The upper mineral horizon losing the material is the ELUVIAL or E horizon. This is where maximum leaching or eluviation takes place.

The lower horizon gaining the material is the ILLUVIAL horizon. This is the zone of maximum accumulation.

NEW TERMS

• Eluviation Downward movement of material where rainfall

exceeds evaporation.

• Illuviation Accumulation of material in one layer of soil which

have been leached from a layer above.

SOIL FORMATION AND SOIL PROFILES

Soil Profile:

Horizons can be used to work out the type of soilThere are usually four horizons in a model profile

1. O horizon – the surface, organic layer of decaying vegetation (where present)

– LITTER – leaves, pine needles etc.– FERMENTATION LAYER – organic material begins to

decompose here– HUMUS – decomposed remnants of plants and animals

along with their waste products

SOIL FORMATION AND SOIL PROFILES

Soil Profile:

2. The A Horizon – Main top layer– Consists of organic and inorganic materials– Usually nutrient rich– Often referred to as topsoil

3. The B Horizon– Known as the subsoil– Coarser in texture: shows importance of weathering– Nutrients may be leached into the B horizon from the A– Leaching – removal of soluble minerals and humus

downwards by rainwater

SOIL FORMATION AND SOIL PROFILES

Soil Profile:

4. The C Horizon– Zone of regolith– Large particles sit on top of underlying bedrock

Soil profiles vary in depth– Can be up to 50m– Average in the UK is 1.5m

SOIL FORMATION AND SOIL PROFILES

Depending on the variations within these variations it is possible to group soils into a classification system

These zones include soils with certain common characteristics. Closely linked with:

– CLIMATE– VEGETATION

SOIL FORMING FACTORSSOIL FORMING FACTORS

1. PARENT MATERIAL – the rock underneath, or other source material (e.g. from glacial, alluvial, volcanic deposition)

2. VEGETATION – e.g. Coniferous, DeciduousLIVING ORGANISMS – worms and other creatures help to mix and improve the soil. Plants help stabilise soils and concentrate nutrients at surface of the soil

3. CLIMATE – Temperature and Rainfall4. RELIEF – steepness of slopes, aspect of slope,

height above sea level;5. TIME – 100,000 YEARS!!!6. NATURAL EVENTS – e.g. floods, glaciation,

volcanic eruptions.HUMAN ACTIVITY – farming, deforestation

WHAT ARE SOILS?ACTIVITIES:

It is important to know the main factors which affect soil Formation.

Use the Core Textbook to complete the following activities in the:

Soil Formation and Soil Profiles section:

Questions 1 – 6 on page 174

SOIL FORMATION

STAGES IN SOIL FORMATION:

1. Weathering of parent rock over a considerable time period2. Water, gas, living organisms and decaying organic matter

added3. Rate of weathering of parent rock depends greatly on

climate (faster in hotter regions)4. Where rainfall is heavier minerals are transported

downwards in a process called leaching5. Leached soils tend to be acidic6. Type of vegetation in an area is linked to precipitation –

this provides the humus area – more found in forest areas

TOPSOIL

SUBSOIL

REGOLITH

(Weathered parent

material)

…and in more detail

Homework-Biosphere Textbook Questions

• Page 174• ‘Soil Types, Case Studies and

Catenas’– Questions 1 and 2 (all of)

• Q 2 a) use a whole jotter page for each soil profile.

                                                                                                                          

•Brown earth is the soil that develops under deciduous forest.•It is found mainly on the west coasts of continents, in the cool temperate zones, 40° to 60° north and south of the equator. •These areas include north-western Europe (including the British Isles), the north-west of the United States, southern Chile, Tasmania, and South Island, New Zealand. •Brown earths have the second highest primary productivity of all soils, second only to tropical soils, the difference between them resulting from the winter season when deciduous trees are virtually dormant and the colder weather inhibits growth. •Brown earths are fertile and can develop to about 2 m (about 6 ft) in depth, because of the action of tree roots breaking up the parent rock. •If cleared, this soil can be productive for agriculture.

BROWN EARTHS FOREST SOILS

BROWN EARTH SOILS

DESCRIPTION:– Horizons less distinct– Mildly acidic due to a mull humus – A-Horizon is dark brown– B-Horizon is light brown– A-Horizon has a loamy texture– B-Horizon has a light texture– Generally free draining soil

EXPLANATION:Climate:

– Precipitation is greater than evaporation only in summer

– Leads to moderate leaching– Warm summers

BROWN EARTH SOILS

EXPLANATION:Vegetation and Soil Organisms:

– Mixed broad leaf and much plant litter– Broken down easily by active soil biota and warmer climate– Greater abundance of earthworms, insects etc.– Quicker breakdown of material than podsol soil– Dark brown horizon as humus replaces material leached

out– Leaching is less pronounced because evaporation and

precipitation are more equally balanced– Colour lightens in the B horizon as humus becomes less

abundant– Deep roots can penetrate the C horizon and extract

minerals to ensure greater recycling

•Podzol soils are mainly found in the taiga (boreal forest) biome at latitudes greater than 60° in Eurasia and North America, and at higher altitudes in temperate latitudes. •They are also found in Britain in many moorland areas. Podzols form under a harsh, cold climate where growth is slow during the winter months and snow accumulates and stays on the ground for long periods. •The vegetation consists largely of coniferous trees, which are specially adapted to the climatic conditions, or moor and heathland plants in Britain. •Productivity is low due to the climatic conditions. The soils are impoverished and the climate is unsuitable for agriculture, but is suitable for commercial forestry.

Leaching is the key process at work in Podzols.

PODZOLS

PODSOL (PODZOL) SOILS

DESCRIPTION:

– Horizons well defined– Mor humus– Thin black layer with decaying litter

needles– A Horizon very dark, then ash grey– B Horizon – red / dark brown– Iron pan can develop– A Horizon has a sandy texture– B Horizon has a denser texture (clayey)– Iron pan can impede drainage and cause

waterlogging

PODSOL (PODZOL) SOILS

EXPLANATION:Climate:

– Colder climate – found at higher latitudes or altitudes– Lowers the rate at which litter is decomposed– Excess of precipitation over evaporation coupled with;– Spring snowmelt and rainfall contribute to leaching– Minerals are eluviated from the A horizon– This leads to colouring and iron-pan formation in the B

horizon– Aluminium, iron and clay are re-deposited (illuviated)

Vegetation:– Coniferous vegetation – needles and cones– These are acidic in nature with a waxy coating– This leads to slow breakdown of organic matter– And an acidic humus layer

PODSOL (PODZOL) SOILS

EXPLANATION:

Biota:– Clearly defined horizons show a lack of soil biota– Lack of mixing of soil by earthworms– Reflects low winter temperatures

SOILS

PAST PAPER QUESTION

For Brown Earth Forest Soil or a Podsol Soil:

a) Draw a soil profile and annotate it to show the main characteristics of the soil.

b) Explain the processes which have created this soil profile.

12 marks

Attempt b) 6 marks

•Tundra Gley occurs in the extreme northern parts of Alaska, Greenland, Russia, and Canada to the north of the taiga. The climate is very harsh, and the temperatures rarely rise above 0° C, so the ground is permanently frozen. •Precipitation accumulates as snow for most of the year. In the short summer temperatures rise sufficiently for a few centimetres at the surface of the soil to thaw. •Plant growth is limited by the cold temperatures and lack of available water; lichens, mosses, and low shrubs predominate. •The cycles of freezing and thawing with the changing seasons causes the weathering of rock by frost shattering. Frost heave can occur, whereby broken rock fragments are brought to the surface. No clear horizons are developed.•Over time soils develop layers or horizons. Each horizon has distinct physical and chemical characteristics. These develop as a result of many different processes, such as water moving through the soil, carrying with it dissolved salts and small particles. The activity of soil organisms, particularly earthworms mixes the soil and prevents the development of clear horizons.

•In Scotland, Gley soils are common in valley bottoms where the impermeable geology causes waterlogging of the soil.

GLEYS

GLEY SOILS

DESCRIPTION:– A horizon contains black, acidic humus– B horizon is bluish in colour with clayey mud– Fragments of weathered material often found in

the B horizon– Frozen subsoil or permafrost layer throughout

the year

GLEY SOILS

EXPLANATION:Climate:

– Cool to cold – Tundra areas

Vegetation and Soil Organisms:– Limited species can survive under conditions– Very slow breakdown of plant materials

Relief and Drainage:– Often flat areas with poor drainage– Poor drainage is the key feature for this soil type– Soil becomes waterlogged or gleyed– Waterlogged soil lacks oxygen

Soils

Answering a Soil question:

1. Include a well annotated soil profile – labelling the humus layer, the A horizon, hard pan if present, B horizon, C horizon, roots and leaching if present

2. What is the climate like? How does this affect soil?3. Vegetation? What type of litter does this create?4. PH of the soil?5. Organic matter? High or low – why?6. Horizons – distinct or not?7. Why does leaching occur if it does?8. Colour?9. Texture?