Ag Science: Soil formation and classification

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Reside definition

remains of a substance

sediment definition

small particles that settle to the bottom of liquid

Weathering definition

decomposition of rock without changing its location

erosion definition

breakdown of rock particles by wind, water or ice

Hydrolysis definition

chemical breakdown of substance when reacted with water

Haematite definition

red-black mineral containing iron

Carbonation definition

dissolving carbon dioxide in liquid

topography definiton

shape of surface of land

Humus definition

dark-coloured decomposed plant and animal material in soil

Leaching definition

soluble matter dissolves in water and filtered downwards

Ombrotrophic definition

bog and vegetation depending on atmosphere moisture for nutrients

Minerotrophic definition

fen or peatland supplied with dissolved minerals from ground or surface water

Calcareous definition

chalky mineral containing calcium carbonate

Illuviation defintition

accumulation or deposition of soil particles or minerals to another soil horizon by percolating water or leaching

Alluvium definition

particles deposited by rivers or marine

Water table definition

level below which soil is saturated

Mottling definition

spots of colour

Groundwater definition

water seeped through soil into pores and cracks in rock

Surface water definition

water on upper layer of soil that has not penetrated soil pores

Podzolisation definition

minerals leached from A horizon and forming impermeable iron pan in B horizon

Eluviation definition

transport of soil particles or minerals from upper layers to lower layers by precipitation

Three groups of rocks in ireland

sedimentary, igneous, metamorphic

Factors in soil formation

• parent material

• climate

• living organisms

• topography

• time

How are igneous rocks formed

• magma under crust turns to lava above crust

• lava or magma cools to form rock

• if magma cools quickly small crystals formed

• if magma cools slowly large crystals formed

Most common igneous rock crystals

quartz, feldspar and mica

How does crystal type affect soil

crystal properties determine rock characteristics and soil formed from weathered rock

Nature of quartz

acidic

Nature of feldspar

alkaline

Two most common igneous rock types in ireland

granite and basalt

Granite crystal description

contains lots of quartz and so is acidic and makes acidic soils

Basalt crystal description

contains small amount of quartz so less acidic, contains feldspar so smoother and fine grained rock and soils

Sedimentary rock formation

• layers of sediment deposited on sea and lake beds

• build up and compact

• solidify to form rock

• Different types of sediment deposited in different places

• Quartz becomes sand forms acidic sandstone

• clay forms shale

• fossils and shells form alkaline limestone

Metamorphic rock formation

• formed from igneous or sedimentary rock

• heat or pressure brings change in rock and mineral content

• if rock is acidic, acidic soil formed

• limestone forms marble

• shale forms slate

• granite forms gneiss

• sandstone forms quartzite

Weathering definition

decomposition of rock without changing its location

Types of weathering of rock

• physical

• chemical

• biological

Types of physical weathering

• heating and cooling

• freezing

• activity of roots

• activity of animals

• grinding action

heating and cooling description

• increased temperatures cause minerals to expand

• minerals expand at different rates, causes rock to shatter and disintegrate

• minerals contract as they cool, contributing to weathering

Freezing description

• water fills cracks

• freezes and turns to ice, expands

• rocks shatter and break

• new cracks fill with water and repeat

activity of roots description

• roots of trees penetrate rock

• roots growing put pressure on rock and cause breaking

Activity of animals definition

animals dig and burrow break down rock

Chemical weathering definition

chemical reactions bring change in minerals

Types of chemical weathering

• hydrolysis

• oxidation and reduction

• hydration

• solution and carbonation

oxidation and reduction description

• oxidation is loss of electrons

• reduction is the gain of electrons

• oxidation is addition of oxygen to a substance

• reduction is the reduction of oxygen from a substance

• oxidation of rock iron in dry conditions

• haematite (reddish-black mineral containing iron) formed

Hydrolysis definition

chemical breakdown of a substance when it reacts with water

Hydration description

• the addition of water to rock mineral

• frequently after oxidation produces iron oxide

• water molecule combines with iron and causes rock to expand and decompose

solution and carbonation description

• water is solvent

• dissolves minerals in rock as it perlocates through rock

• metal ions soluble in water

• water + CO2 = carbonic acid

• carbonic acid dissolves alkaline minerals in rock faster than water

• limestone particularly weathered by carbonic acid

Bioligical decompostion of rock

• microorganisms (bacteria, fungi and lichens) colonise bare rocks

• some utilise nitrogen and CO2 from atmosphere

• secrete acids that dissolve mineral nutrients eg. calcium and phosphorus

carbonation definition

dissolving carbon dioxide in liquid (water)

lichen definition

consists of an alga and a fungus

What is organic parent material

remains of plants which have partially dissolved, found in waterlogged conditions and lakes

What is produced from organic parent material

peat

What is a bog

an area where peat accumulates

Types of bog in ireland

• basin peats (fens)

• raised bogs

• blanket bogs

Depth of basin peats

2m

Depth of raised bogs

8-12m

Depth of blanket bogs

2.6m

Shape of basin peats

flat

Shape of raised bogs

dome

Shape of blanket bogs

sloped or flat, depends on landscape

pH of basin peats

6.0

pH of raised bogs

4.0-6.0

Location of basin peats

valleys/lakes

Location of raised bogs

lakes

Location of blanket bogs

mountains and lowlands on west coast

Formation of basin peats

• flat bogs develop in hollows in landscape (eg. lake)

• mineral-rich water supply

• slightly acidic

Formation of raised bog

• water supply of basin peat runs out

• more material accumulates

• becomes more acidic

Formation of blanket bogs

• high rainfall and low evaporation in anaerobic conditions

• mountain areas along west coast

• not as deep as basins and very acidic

Factors of soil formation

• parent material

• climate

• living organisms

• topography

• time

How does parent material influence soil formation

• 45% of soil is mineral matter

• acidity and texture of parent material influences that of the soil

• soils from granite contain quartz and is acidic, mica particles weathered into clay particles

• soils from limestone has calcium and magnesium raise pH, good for broadleaf trees

How does climate contribute to soil formation

• temperature and precipitation

• water contributes to weathering (freezing, solution, hydrolysis, hydration)

• Temperature contributes to heating and cooling (low temp=low evaporation so more water for chemical weathering)

How do living organisms contribute to soil formation

• provide humus after death

• soils lacking vegetation lack organic matter for structure and fertility

• soils under grasslands benefit from roots (bind soil and give humus) (grasses have high alkaline compounds so lowers acidity)

• soils under forests suffer acidification, esp. conifers (acidic needles make soil acidic and large roots dont provide as much humus)

Humus definition

dark coloured, decomposed plant and animal material in soil, nutrient rich and contributes to structure

topography definition

mapping/shape of the surface of the land

How does topography contribute to soil formation

• steep slopes: erosion carries soil from top of hills to valleys

• makes thin soils on hillsides and deep fertile soils in valleys

• water evaporated more rapidly from hillsides, water content in valleys means suitable for cultivation

How does time contribute to soil formation

• old soils: material for formation eroded or decomposed

• fertility from organic material in old soils

• young soils: high level of parent rock available, so good fertility

Horizons in a soil profile

• O

• A

• B

• C

• R

O horizon desription

• not always present due to absence of vegetation

• consists of parent material

A horizon description

• topsoil

• contains minerals and organic matter

• may lack minerals from leaching in some conditions

B horizon description

• subsoil

• lighter colour, except where leached minerals accumulate

C horizon description

• contains parent material and rocky in nature

R horizon description

• bedrock

• solid

Horizon A2 description

• maximum leaching

• lots of minerals in topsoil because of decomposition of organic matter and fertilisers

• heavy rainfall leaches minerals from A2

• pale colour

Horizon B2 description

• zone of accumulation

• minerals from A2 leached into subsoil and accumulate in B2

• darker colour

• may form iron-pan and waterlogging

leaching definition

the process in which soluble matter such as minerals dissolve in water filtering through soil and carried downwards

Great soil groups

• ombrotrophic peat

• minerotrophic peat

• rendzina

• lithosol

• alluvial

• groundwater gley

• surface water gley

• podzol

• brown podzolic

• luvisol

• brown earth

ombrotrophic peat description

rain fed peat

minerotrophic peat description

groundwater-fed peat

Rendzina description

shallow calceous soil (less than 40cm deep)

Lithosol description

shallow non-calcareous soil (less than 40m deep)

alluvial description

soil derived from alluvium, no distinct layer development

groundwater gley description

soil gleyed within 40cm of surface due to water table

surfacewater gley description

soil gleyed within 40cm of surface due to slowly permeable horizon