People and soils

What are zonal soils?

Fully developed soils

What is soil?

All of the material between the ground surface and bedrock

How are soils formed?

Combination of minerals from weathered bedrock and organic matter from vegetation

Soil and climate

Soil formation closely linked with climate- climate determines the rate of weathering of the bedrock, the type of vegetation that grows there and how quickly the vegetation decomposes

Development of zonal soils

Mature soils that have developed from the interaction between climate, vegetation and parent rock across a significant period of time

O horizon

Loose, partially decayed organic matter

A horizon

Topsoil with high humus content

E horizon

Pale layer of silt and sand, from which clay, iron and other minerals have been leached

B horizon

Zone in which minerals leached from above accumulate

Where do podzols occur?

In cool temperate climates in the northern hemisphere, where there is more precipitation than evapotranspiration

Where are podzols found?

Under coniferous woodland or heather moorland

O horizon in podzols

Layer of needles from the coniferous trees or heather leaf litter

A horizon in podzols

Narrow and acidic, nutrients leached by water

Acidity in podzols

Water travelling through soil is more able to dissolve certain minerals such as iron and aluminium compounds. Leaching of these minerals creates a pale E horizon formed mainly of quartz sand and silt

B horizon in podzols

Minerals accumulate, forming a reddish-brown layer, in very well developed podzols a hard pan forms- hard, continuous layer of deposited iron

Podzols and agriculture

Not good for arable farming. Acidity and lack of nutrients means few crops can grow. Hard pan prevents water draining, making soil vulnerable to waterlogging. Waterlogging makes ploughing difficult as machinery sinks into the ground.

Arable farming requires lime treatment and artificial fertilisation

Forestry

Coniferous trees- forestry is a common activity. Low soil temperatures mean it can take up to 100 years for a tree to reach maturity and be ready to harvest

Where are latosols found?

Under tropical rainforests

Depth of latosols

High temperature and humidity encourages fast chemical weathering of bedrock, soils 30-40m deep.

O / A horizon in latosols

Thick because of year round plant growth. As soon as leaf litter decomposes to humus, its nutrients are absorbed by vegetation rather than staying in the soil- fertile A horizon is very thin

Soil moisture

Rainfall higher than evapotranspiration, causing a soil moisture surplus. Lots of leaching- silicate minerals leached from the B horizon, less soluble iron and aluminium compounds are left behind. Iron compounds give the B horizon a red colour

Mineral build up in latosols

Forms a laterite horizon, which acts like clay

Latosols and agriculture

Poor for agriculture due to their low nutrient content. Once rainforest trees are removed, soil has no protection from heavy rainfall- leaching increases and the latosols becomes even more nutrient poor.

Adaptations to latosols

Traditional agriculture using slash and burn clears one small area for growing crops at a time. Burning vegetation to clear the plot adds nutrients to the soil. After one growing season, a new plot is cleared and the first is left to recover.

Expansion of farming in areas with latosols

Large areas of land cleared for agriculture, soil isn’t given a chance to recover, leading to permanent soil degradation.

Why is the laterite horizon good for building?

It is soft when moist but hard when dry

Clearing vegetation

Rain falls directly onto soil instead of being intercepted by plants, roots that were binding the soil together are removed. Vegetation slows down wind, so removing it exposes ground surface to strongest winds.

Topography

Soil more likely to be washed down a steep slope

Climate

High rainfall increases water erosion. Low rainfall and high temperatures mean soil is dry, making it more vulnerable to wind erosion

Land use

Ploughing loosens soil and exposes it to wind and rain, making it more vulnerable to erosion

Why is soil erosion a major threat to agriculture?

It removes nutrients and reduces the soil’s ability to hold water. In the last 40 years, almost 1/3 of the world’s arable land has become unproductive due to soil erosion

Crop rotation

Instead of leaving fields bare after a main crop is harvested, a cover crop is planted which helps maintain soil

Windbreaks

Hedges or trees can be planted around fields as barriers against wind erosion

Terracing

Steps can be cut into a steep hillside to slow down the movement of water down the slope

Contour ploughing

Ploughing across a slope instead of downhill, stoping water from flowing downhill as quickly

Mulching

Covering soil with a layer of plant material protects soil from wind and rain, and slows down runoff

Causes of waterlogging

• Soils with few airspaces fill quickly

• Hard pans hinder drainage

• Higher precipitation than evapotranspiration

• Too much irrigation

Problems with waterlogging

• Roots surrounded by water, rots them / limits worth

• Water decreases soil temperature, reducing crop growth

• Crops can be out competed by weeds

• Land hard to plough

Solutions to waterlogging

• Avoid over watering

• Drain soil using underground pipes or ditches around fields

• Change composition of soil, e.g. add sand to clay soil

Causes of salinisation

• High temperatures- water drawn to surface- evaporates and leaves salts behind

• Dry climates, not enough rainfall to leach salts

• Irrigation water has salts, evaporated to leave salt

• Some fertilisers contain salt, applying too much means some salts are left in soil

Problems with salinisation

• Salt stops crops absorbing water

• Some salts are toxic to plants

• Water flows from low to high salinity- water can flow from plant roots to soil, dehydrating plants

Solutions to salinisation

• Avoid waterlogging

• Only use as much water for irrigation as is needed

• Add appropriate amount and type of fertiliser

Causes of structural deterioration

• Use of heavy machinery, trampling- compaction

• Removal of vegetation - roots bind soil

• Salinisation in clay soils- salt causes clay to clump

Problems with structural deterioration

• Roots cannot grow because of compaction

• Loss of pore spaces- reduced capacity for water so plants dry out

• Land is hard to plough

Solutions to structural deterioration

• Avoid compaction by regularly moving livestock

• Maintain vegetation cover

• Change structure of soil by adding sand to clay soil

• Avoid salinisation