People and soils

What are zonal soils simply?

Fully developed soils

What is soil?

All of the material between the ground surface and the bedrock.

How are soils formed?

From a combination of minerals from weathered bedrock and organic matter from vegetation.

What is soil formation closely linked with?

The climate of the area.

Climate determines the rate of weathering of the bedrock, the type of vegetation that grows there, and how quickly this vegetation decomposes.

What are zonal soils?

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

What do different parts of the world have?

Different dominant zonal soil types e.g. podzols and latosols.

How can soil types be identified?

By looking at the distinct layers in the soil profile, which are known as soil horizons.

There are 5 main horizons (though not all soils have 5)

What is the top soil horizon?

Loose, partially decayed organic matter.

What is the second top soil horizon?

Topsoil with high humus (organic matter) content.

What is the 3rd top soil horizon? O

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

What is the second bottom soil horizon?A

Zone in which minerals leached from above accumulate.

What is the bottom soil horizon of the soil profile? E

Mainly broken bedrock.

What is leaching? B

When water moves minerals and nutrients down through the soil profile.

What are podzols simply? C

Acidic and have distinct horizons.

Where do podzols occur?

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

Where are podzols usually found?

Under coniferous woodland or a heather moorland.

What is the O horizon?

A layer of needles from the coniferous trees or heather leaf litter, and below this is a narrow, acidic A horizon.

Nutrients are leached from the A horizon by water.

What happens because of the acidity of podzols?

Water travelling through the soil is more able to dissolve certain minerals such as iron and aluminium compounds.

Leaching of these minerals creates a pale E horizon formed of mainly quartz sand and silt.

What letters are the 5 layers of soil?

• O

• A

• E

• B

• C

Where do minerals accumulate in podzols?

In the B horizon, forming a reddish-brown layer.

In very well developed podzols, a ‘hard pan’ or ‘iron pan’ forms in the B horizon - this is a hard, continuous layer of deposited iron.

Why are podzols acidic?

Because the leaf litter from coniferous trees and heather is acidic, and they often form on acidic sandstones or clays.

What are podzols not good for?

Agriculture.

How do podzols impact livestock?

Some livestock can be grazed on areas with podzols.

How do podzols impact arable farming?

It is difficult;

• acidity and lack of nutrients means that few crops are able to grow.

• formation of a ‘hard pan’ can prevent water from draining away, making the soil vulnerable to waterlogging, damaging crops. Waterlogging also makes ploughing difficult because machinery sinks into the ground.

How does some arable farming take place on podzols?

This requires treatment with lime to reduce acidity, and extensive artificial fertilisation.

Why is forestry a common human activity on podzols?

Because coniferous trees typically grow successfully on podzols.

Low soil temperatures mean that it can take up to 100 years for a tree to reach maturity and be ready to harvest. 

What are latosols simply?

Deep and red

Where are latosols found?

Under tropical rainforests.

What do tropical conditions encourage for latosols?

High temperature and humidity in tropics encourage fast chemical weathering of the bedrock, forming soils up to 30-40m deep.

What does year-round plant growth mean for latosols?

The O horizon is quite thick.

However, as soon as leaf litter decomposes to humus, its nutrients are absorbed by vegetation rather than staying in the soil - means the fertile A horizon is very thin.

What is rainfall like in tropical rainforests?

Rainfall is higher than evapotranspiration, causing a soil moisture surplus. Means there is a lot of leaching - silicate minerals are leached from the B horizon, but less soluble iron and aluminium compounds are left behind.

Iron compounds give B horizon a red colour.

What can the build up of minerals in latosols lead to?

Can form a laterite horizon, which acts like clay. 

What can latosols influence?

Human activity.

What are latosols like for agriculture?

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

How have humans living in the rainforest adapted 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 one is left to recover fully. 

How has farming expanded in areas with latosols?

Large areas of land are cleared for agriculture, and soil isn’t given a chance to recover - this has led to permanent soil degradation.

What is the laterite horizon like?

Soft when its moist, but hard when it dries out - this makes it useful for building.

What is soil erosion?

The wearing away of soil by wind or water.

What factors make soil more vulnerable to erosion?

• Clearing vegetation - rain falls directly onto the soil instead of being intercepted by plants, and the roots that were binding the soil together are removed.

• Topography - e.g. soil is more likely to be washed down a steep slope.

• Climate - e.g. high rainfall causes water erosion. Low rainfall and high water temperatures mean soil is dry, which makes it more vulnerable to wind erosion.

• Land use - e.g. ploughing loosens soil.

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 a third of the world’s arable land has become unproductive due to soil erosion.

What management strategies are put in place to decrease soil erosion?

• Crop rotation

• Windbreaks

• Terracing

• Contour ploughing

• Mulching

What is crop rotation?

Instead of leaving fields bare after main crop is harvested, farmers can plant cover crops, which help to maintain the soil until a main crop is sown again.

What are windbreaks?

Hedges or trees can be planted around a field as barriers against wind erosion.

What is terracing?

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

What is countour ploughing?

Ploughing across the slope instead of downslope - this stops rainwater from slowing downhill as quickly.

What is mulching?

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