5.1 Introduction to soil systems

what is a soil profile

a way in which a soil system may be illustrated that has a layered structure

what are some examples of soil system storages

organic matter, organisms, nutrients, minerals, air, water

what are some transfers of material within the soil

biological mixing, leaching

what are some inputs of organic material in soil

leaf litter, inorganic matter from parent material, precipitation

what are some outputs in soil

uptake by plants, soil erosion

what are some transformations in soil systems

decomposition, weathering, nutrient cycling

what are some ways in which sand, clay and loam soils differ

mineral and nutrient content, drainage, water holding capacity, air spaces, biota and potential to hold organic matter

what does the soil texture triangle do

illustrate the differences in composition of soils

what does soil act as for water that passes through it

a filter that can alter the chemistry of the water

what do soils do with heat

store and transfer it, affecting atmospheric temperature which in turn can affect the interactions between soil and atmospheric moisture

what sphere is soil in

lithosphere

what are the four spheres of the Earth

atmosphere, hydrosphere, lithosphere, biosphere

what is the pedosphere

the soil sphere

what is soil made up of

mineral particles mainly from the underlying rock, organic remains that have come from the plants and animals, water within spaces between soil grains, air also within the soil grains

what is the ratio of solids and pore spaces in most soil

1:1

what are the different fractions of soil

rock particles, humus, water, air, soil organisms

what are the constituents of rock particles

insoluble gravel, sand, silt, clay, chalk. soluble mineral salts, compounds of nitrogen, phosphorous, potassium, sulphur, magnesim

what are the constituents of humus

plant and animal matter in the process of decomposition

what are the constituents of water in soil

water either seeping down from precipitation or moving up from underground sources by capillary action

what is the air in soil

mainly oxygen and nitrogen

what are the soil organisms

soil invertebrates, microorganisms and large animals

what is the function of rock particles in soil

provides the skeleton of the soil, derived from the underlying rock or from rock particles transported to the environment

what is the function of humus in soil

gives the soil a dark colour, as it breaks down, it returns mineral nutrients back to the soil, absorbs and holds on to a large amount of water

what is the function of water in soil

dissolved mineral salts move through the soil and so become available to plants, rapid downward movement of water causes leaching of minerals, rapid upward movement can cause salinisation, large volumes of water in the soil can cause waterlogging leading to anoxic conditions and acidification

what is the function of air in soil

well aerated soils provide oxygen for the respiration of soil organisms and plant roots

what is the function of soil organisms in soil

large particles of dead organic matter are broken down by soil invertebrates like worms, smaller particles are decomposed by soil microorganisms, thus recycling mineral nutrients, larger burrowing soil animals help to mix and aerate the soil

what is a horizons in soil

the different levels of soil, often visible

what is the top layer of soil

leaf litter

what is the second horizon in the soil

mineral horizon at the surface showing organic matter enrichment

what is the third horizon in soil

subsurface horizon showing depletion of organic matter, clay, iron, and aluminium compounds

what is the fourth horizon in soil

subsoil horizon showing enrichment of clay material, iron, aluminium or organic compounds

what is the fifth horizon in soil

horizons of loosened or unconsolidated material

what is the final horizon in soil

hard bedrock

give a descriptioO (1st) horizon

many soils contain an uppermost layer of newly added organic material that comes from organisms that die and end up on top of the soil

describe the A (2nd) horizon

upper layer, where humus builds up, humus forms from partially decomposed organic matter and is often mixed with fine mineral particles waterlogging reduces the number of soil organisms which results in a build up of organic matter that can eventually lead to the formation of peat soils

what is the final horizon in soil

hard bedrock

describe the B (3rd) soil horizon

the layer where soluble minerals and organic matter tend to be deposited from the layer above, in particular clay and iron salts can be deposited in this horizon

describe the C (4th) horizon

mainly weathered rock from which the soil forms

describe the R (final) horizon

parent material

do all soils contain all three a, b, c horizons

no, sometimes only two can be distinguished and sometimes there may not be distinct layering

what are the three parts of the mineral portion of soil

sand, silt, clay

what is the particle diameter of clays

< 0.002 mm

what is the particle diameter of silt

0.002 - 0.05 mm

what is the particle diameter of sand

0.05 - 2 mm

what are the main texture characteristics of sandy soils

gritty and fall apart easily

what are the main texture characteristics of silty soils

feel slippery like wet talcum powder and hold together better than sandy soils

what are the main texture characteristics of clay soils

feel sticky and can be rolled up into a ball easily

when is a soil known as a loam

when a fairly equal proportion of each soil size is present

what are two methods of getting a clearer picture of soil proportions of soil particles

drying out a soil sample, passing it through a series of sieves of decreasing mesh size and separating the soil into its portion of each particles, place a soil sample in a jam jar, fill it with water, shake vigorously and leave to settle out, the heaviest particles will settle firs and the finer particles will settle last

why is soil texture an important property of a soil

it determines the soils fertility and primary productivity

why are loam soils ideal for agriculture

the sand particles ensure good drainage and a good air supply to the roots, the clay retains water and supplies nutrients, the silt particles help to hold the sand and clay particles together and they can be worked easily

what is soil porosity

the amount of space between particles

what is soil permeability

the ease at which gases and liquids can pass through the soil

why do soils with a very fine clay texture have a combined large pore space

they have many micropores

why do clay soils have a low permeability

water molecules easily fill the micropores and adhere to the clay surface, trapping the water as a film around the clay particles

why do sandy soils have good drainage

they have relatively large pores which are too large for waters adhesive properties to work

why do clay soils often have a high mineral content but low fertility

it can lock dissolved minerals between its pores but this makes it hard for plant roots to access

why do clay soils tend to me more acidic

as the soil absorbs more water, clay particles begin to fill with positive hydrogen ions, this binds soil water tightly to the clay particles and makes the soil more acidic, it also reduces the amount of other positive ions that can bind, allowing potassium, magnesium and ammonium to be lost through leaching

other than the loss of important nutrients, what happens in clay soils as soil pH decreases

ions of aluminium and iron start to become more available to plants, both of these are plant toxins

why does acidification of soils have a major impact on forestry in northern Europe

acid rain caused by industrial pollution has made the soil more acidic, thi sin turn has meant more available aluminium and iron ions in the soil causing damage to evergreen forestry through needle death

is fertile soil a renewable resource

no

what is the natural soil renewal rate

1 tonne per hectare per year

what are the main nutrients in fertile soils

nitrates, phosphates, potassium