Soil

Soil is composed of:Soil is composed of:

rock particles

humus

mineral salts water air organisms

Rock particlesRock particlesvary in size

Gravel Sand Silt Clay

Three types of Soil:Three types of Soil:

Clay Sandy

LoamBEST BEST TYPE TYPE

ThreeThree types of soil:- types of soil:-

2 Sand : 1 Clay2 Sand : 1 Clay

Clay Soil:Clay Soil:

heavy soil difficult to dig as clay sticks

Clay Soil:Clay Soil: when dry it forms hard clods little air present

Clay Sandy

Loam

Clay Soil:Clay Soil:retains water soil becomes waterlogged

A waterlogged soil has few air spaces as these become filled with

water

1. amount of air

Particle size affects the Particle size affects the following properties of soil:-following properties of soil:-

2. the rate and amount of drainage

Particle size affects the Particle size affects the following properties of soil:-following properties of soil:-

3. the rise of water in the

soil (capillarity)

Capillary waterCapillary water

The smaller the particle size, the higher water rises.

Question: [APRIL, 2010] Question: [APRIL, 2010] Paper 2BPaper 2B

List TWO properties of soil that are affected by soil texture. (4)

The distance between soil particles The amount of air between soil

crumbs/drainage The rise of water in the soil

(capillarity)Any TWO

Clay Soil is made lighter by Clay Soil is made lighter by adding:- adding:-

i) humus

How do these work?

ii) lime

Adding Adding limelime

Clumping of soil particles Clumping of soil particles introduces air in soilintroduces air in soil

A clay soil before

treatment.

lime

humus

A clay soil aftertreatment.

Good crumb structure Good crumb structure [texture] of soil [texture] of soil

What is ‘humus’? What is ‘humus’?

black or dark brown material that remains after decomposition

Dark soil: rich in humusDark soil: rich in humus

0-45cm

45-90cm

Bedrock

Upper topsoil layer is dark. Why?

Lower soil layer is rich in salts. Why?

Lower soil layer is rich in salts. Why?

Question: [APRIL, 2010] Paper 2BQuestion: [APRIL, 2010] Paper 2B

Explain how humus:i) Increases the soil’s water content. (2)

Absorbing large amounts of water or reducing evaporation from the soil.

Question: [APRIL, 2010] Paper 2BQuestion: [APRIL, 2010] Paper 2B

b. Explain how humus:ii) Improves the texture of a clay soil. (2)

Humus helps soil particles stick together into larger crumbs.

Sandy Soil:Sandy Soil:

light soil water drains quickly

As water drains quickly TWO

disadvantages result:

1. Soil dries up quickly1. Soil dries up quickly

Why is this a disadvantage for the farmer?

Water is expensive!!

2. S2. Salts are leached easilyalts are leached easily

LeachingLeaching::

salts dissolve & salts dissolve &

are lost are lost

Sandy Soil is improved by:Sandy Soil is improved by:adding humus as this:- 1.holds more water2.provides salts

Soil is composed of:Soil is composed of:

rock particles

humus


HumusHumusa jam-like substance made up of

decayed plants and animals

is found in the topsoil – blackish in colour

HumusHumus if there is not enough oxygen,

materials accumulate to form half decayed materials called peat

peat decays into humus if placed in well aerated soil

HumusHumushelps to clump soil particles into

crumbs

Humus improves soil by:-Humus improves soil by:-

providing mineral salts from decay by bacteria and fungi

providing air spacesretaining water improves crumb structure

(prevents soil from being blown away)

Loam SoilLoam Soil

contains twice as much sand as contains twice as much sand as clayclay

Clay [1 part]

Sand [2 parts]silt

How can you How can you separate the soil separate the soil

particles as shown particles as shown in the chart?in the chart?

add water mix leave to

settle

ExperimentExperimentTo find the components of soil.

To find the organic content of soil.

ExperimentExperiment

percentage of organic matter =

loss in weight

weight of original dry sample

100

Original weight: 105 gFinal weight: 100 g Loss in weight: 5 g

5

100 4.8%105

rock particles

humus


SOILSOIL

WaterWater is found as thin films around soil

particles

water moves upwards towards the surface by capillarity

To find the water content of soil.


Precaution when heating soil:Precaution when heating soil:

Temperature must NOT be more than 100C.

WHY?

loss in weight

original wet sample

100

Original wet weight: 200 gFinal weight: 185 g Loss in weight: 15 g

15

100 7.5%200

Percentage of water in soil:Percentage of water in soil:

SOILSOIL rock particles

humus


1. rock erosion2. decay of dead organisms

TWO sources of salts in soil:TWO sources of salts in soil:

Salinisation is a Soil ProblemSalinisation is a Soil Problem

often in arid and semi-arid areas (natural mineral salt conc. high)– The little precipitation that

falls is quickly evaporated– Leaves behind salts

salt concentrations get to levels toxic to plants

gradual accumulation of salt in the soil, usually due to improper irrigation techniques

Many organisms bring about Many organisms bring about decay:decay:

To compare the drainage of two soil samples.


SOILSOIL

rock particles

humus mineral salts

water air organisms

1. roots for respiration

Air is used by:Air is used by:2. decomposers to

decay material into humus

How can a farmer

introduce air into the soil?

A good soil has a lot of air A good soil has a lot of air spaces between the crumbsspaces between the crumbs

By ploughing

To estimate the volume of air in a soil.


Conclusion: Air is important in soil for roots to respire

and for decomposers to remain alive.


volume of air in soilpercentage of air =

volume of soil sample100

rock particles

humus mineral salts

water air organisms

SOILSOIL

Bacteria & FungiBacteria & Fungi

make salts available to

plants

Earthworms improve soil by:Earthworms improve soil by:1. their burrows they mix and loosen the soil

and so help to drain and aerate it

2. providing salts due to their wastes and by pulling leaves into the soil

Earthworms improve soil by:Earthworms improve soil by:3. making the soil alkaline as it passes

through their intestine – bacterial growth is favoured by this pH

Earthworms improve soil by:Earthworms improve soil by:4. binding soil particles together by the

secretions from the intestines

5. making the soil finer – young roots can emerge easier

Harmful OrganismsHarmful OrganismsNematodes can be harmful by:- 1.feeding on bacteria2.being ectoparasites and pierce

root3.being endoparasites and damage

roots and crops

Harmful OrganismsHarmful Organisms

various insect larvae eat the roots of plants e.g. wireworms (beetle larvae)

Soil fertility can be increased Soil fertility can be increased by:by:

1. Adding fertilisers2. Crop rotation3. Ploughing4. Keeping a neutral pH

Two types of Soil FertiliserTwo types of Soil Fertiliser

1. Natural fertilisers

2. Artificial / Chemical fertilisers

Natural FertilisersNatural Fertilisers

Animal manureCompost

Why is the temperature at the Why is the temperature at the centre of the compost pile centre of the compost pile higher than ambient higher than ambient temperature?temperature?

Heat released by microbes is difficult to be lost to the

surroundings – far from surface.

Coolair in

Coolair in

Warm air out

Advantages of Natural Advantages of Natural FertilisersFertilisers

provide a wide variety of saltsmaintain the crumb structurelong-lasting

Disadvantages of Natural Disadvantages of Natural FertilisersFertilisers

Salts are released slowly – decay is slow

Bad smellPests can grow in it

Artificial FertilisersArtificial FertilisersManufactured from chemical

compounds

Advantages of Artificial Advantages of Artificial FertilisersFertilisers

Salts are very soluble and are quickly available to the plant

No pests in itNo unpleasant smell

Disadvantages of Artificial Disadvantages of Artificial FertilisersFertilisers

salts are leached quickly destroy the crumb structure (soil

becomes a powder)

Disadvantages of Artificial Disadvantages of Artificial FertilisersFertilisers

have less types of salts than manureno water-holding capacity

Describe what may happen to a soil when Describe what may happen to a soil when only chemicalonly chemical

fertilizers are used over a long time period. fertilizers are used over a long time period. loss of soil humus/organic matter content;loss of soil texture/crumb structure;impaired soil drainage/waterlogging;loss of aeration;

Good crumb

structure

Badcrumb

structure

Question: [SEP, 2011] Paper 2Question: [SEP, 2011] Paper 2

List TWO types of natural fertiliser. (2)

1. Compost / peat2. Animal manure

Question: [MAY, 2011] Paper 1Question: [MAY, 2011] Paper 1Give the biological explanation of the message in the following poster. (3)

Decomposers, consisting of bacteria and fungi, bring about the decay of the dead plant material. The decaying material is called compost and is vital for the growth of plants as mineral ions are released into the soil. Plants use nitrates, phosphates and other mineral salts to keep healthy.



Crop rotationCrop rotation means the planned order of specific

crops planted on the same field rotation may vary from 2 or 3 year or

longer period

e.g.- wheat in autumn- root crop in summer- barley or oats in spring- leguminous plants (clover, peas)

if the same crop is planted: certain salts are depleted

Benefits of Crop Benefits of Crop rotationrotation

1. maintains crumb structure

2. supplies nitrates

3. reduces infection to plants as a pest cannot continue its life cycle if a different crop is planted



Beneficial effects of ploughing: makes soil porous so water drains

better

introduces air into the soil

Beneficial effects of ploughing: incorporates the residue from the

previous crop into the soil

reduces the prevalence of weeds in the fields

THE ENDTHE END

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Soil