1.Introduction ~ According to the depth at which it is found , soil is classified

topsoil and subsoil .

a. Topsoil

~It is the surface layer of soil and is only about a few

centimeters deep .

~It contains humus and many mineral salts which are

require for crop growth .

b. Subsoil

~It is composed of large soil particles and stones .

~It is less fertile .

Top Soil

2.Components of soil

a. soil particles

~ They bind with many ions are important for crop growth.

Type of soil particles Average diameter(mm)

Gravel larger than 2

Coarse sand 2.0 - 0.2

Fine sand 0.2 - 0.02

Silt 0.2 - 0.002

Clay smaller than 2

b. Mineral Salts

~ Many salts are the essential nutrients for healthy plant

growth .

~ They affect the pH value of soil which affects plants

growth .

~ Most crops grow best in the slightly acidic soil (pH6-6.5).

c. Soil water

~ Soil water is absorbed by the root hairs of the crops to

support their life .

d. Soil air

~ The oxygen in soil air supports the aerobic respiration of

the roots of crops .

~ In water-logged soil there is a shortage of oxygen that :

I. hinders the healthy growth of roots .

II. slow down the decomposition of humus to form mineral

salts .

III. enhances the undesirable activities of the denitrifying

bacteria that remove the useful nitrates from soil .

e. Humus

I. Introduction

~ Humus is the dark brown finely divided organic matter

on the surface of top soil.

~ They derive from the excretory wastes and dead remains

of living organisms .

II. Importance of humus

1. Humus is decomposed by bacteria into nitrates and other

mineral salt which are essential for healthy growth of

plants .

2. Humus improve the texture of soil :

a. It sticks the fine particles of clay soil into larger

aggregates so that the aeration and drainage of

the soil are improved .

b. It sticks the loosely packed sand particles of sandy

soil closer together so that the retentivity of the soil

is improved .

3. Humus acts as a sponge at the surface of the soil to :

a. absorb water .

b. reduce the evaporation of water from soil and thus

reduce the danger of soil particles to be blown away

by wind.

Humus add to Clay and Sandy soil

F. Micro-organisms

i. Beneficial bacteria

The saprophytic bacteria(decomposer) decompose the

excretory products and the dead remains into humus

which further broken down into soluble salt such as

ammonium compounds and nitrates .

ii. Harmful bacteria

The denitrifying bacteria change the useful nitrates

into gaseous nitrogen which cannot be used by plants.

Sandy soil clay soil

1.size of soil particles large small

2.air space(aeration) large small

3.drainage(permeability) rapid slow

a.mineral content low,mineral salt high, mineral salt

easily leached out strongly attached

b.fertility Infertile soil Fertile soil

4.Retentivity Poor , dry soil Good,wet soil

5.Capillarity Low High , easily get water

from water-table at

greater depth .

6. Soil texture large soil particles small soil particles are

are loosely packed strongly stuck together by

together,they eroded the surface tension of the

by wind easily water film covering their

surface .When dry they

form hard clods which

hinder root growth and

animal movement .

7.Activity of Low, low humus High , high humus content

micro-organism content

8.a)specific heat Low High

b)Temp. change High,temperature Low,temperature is

extremes rather constant

c)Thermal cond. Warm Cold in wet clay

9.Relative weight Light as the soil is dry Heavy as the soil is wet

and has large air space and has small air space

10. Digging and Easy, dry particles are Difficult , wet particles

ploughing easily separated . strongly stick together

~Neither sandy soil nor clay soil is an ideal soil for cultivation.

~The loam soil is the most productive soil in agriculture since

it has properties intermediately between that of the sandy

and clay soil .

A. sandy soil

~ Though a sandy soil is well aerated , its rapid drainage

causes the low retention water(dry) and the serious leaching

of mineral salts(infertile) .

It can be improved by adding :

I. Clay to make a loam with greater retention of water and

mineral slats.

II. Humus

1. To increase the fertility of the soil .

2. To reduce evaporation and to protect it from the

leaching action of heavy rain .

B. clay soil

~ A clay soil is easily water logged and difficult to dig and

plough . It can be improved by :

I. Adding slaked lime which joins the small clay particles

into bigger aggregates (soil crumbs) to promote aeration

and drainage .

II. Adding humus to improve texture , aeration and drainage

III. Building drainage channels

~ to improve drainage and aeration

~ to prevent water logging .

Definition :soil erosion is the removal of the top-soil by the wearing action of rain and wind .

Reason :a. Deforestation

b. Over-grazing

c. Poor farming methods

A. Deforestation :

• The removal of trees exposes the top-soil which is then no longer protected by the leafy canopy of the forest .

• The thin layer of top-soil will be easily washed away by heavy rain .

B. over-grazing

• Too large a population of animals rearing on a piece of grassland will quickly turn it into barren land because of over-grazing .

C. Poor farming methods

•After harvesting several crops without adding humus will greatly reduce the retentivity of soil so that the soil will dry up quickly and easily be blown away .

• The soil structure is deteriorated by using chemical fertilizer instead of natural fertilizers . It is because chemical fertilizer contain no humus .

•Long time agriculture can bring out the nutrient from soil .

•Erosion and soil loss is the end member of a variety of physical, vegetative and nutrient factors that lead to soil degradation .

•Using heavy machinery on soil rapidly compacts it into a hard layer which can hardly absorb any water . Eventually the soil become dry and easily be blown away .

•Soil fertility is a complex issue and nutrient loss is not necessarily nor always a consequence of erosion.

Soil conservation

• Soil conservation is to take steps to prevent soil erosion .

A. Crop rotation

i. Definition Crop rotation is growing of different crops on a

field from year to year to prevent the soil from

lacking of one particular type of material due to

the withdrawal by one type of crop .

ii . method

1st and 2nd year :

~ Crop with a high nutritional demand can be followed by the crop which have lower nutritional demands .

~ long-rooted plants crop can be followed by short-rooted crops so that the minerals are removal from different soil level .

3rd year : ~ Leguminous plant(e.g. pea or bean) can be grown .

~ Their root nodules contain nitrogen-fixing bacteria which change the atmospheric nitrogen content to soil. ~ When the residues of these leguminous plants are ploughed in , they increase the nitrogen content of the soil .

Wheat Barley Oat Pea / bean (high nutritional ( low nutritional ( increase nitrogen demand ; deep- demands ; short- content of soil ; rooted ) rooted ) leguminous plants)

[1st year] [2nd year] [3rd year]

iii . Advantages of crop rotation i. Several crops can be obtained from a single field, i.e. maximum use of land . ii. Depletion of a particular type of minerals is avoided .

iii. The nutrients content of the field can be increased by growing leguminous plants .

iv. The spread of certain pests and crop diseases can be prevented .

B. Other methods

~ Steep slope are grown with tree to provide canopy to

protect the top-soil from washing away by heavy rain .

~ Adequate number of animals is reared on the grassland

to avoid the over-grazing of grasses .

~ Gypsum can reduce erosion on heavy clay soils by

improving soil structure and increasing water infiltration.

~Lime and other calcium sources. Agricultural lime, chemically described as the oxide, hydroxide, or carbonate of calcium and/or magnesium, is used to correct soil pH.

• Use of pesticide to the kill the pest .

• Common weed control practices : herbicide (weed -killer)applications and cultivation .

• Herbicides kill the weeds which are unwanted plants that compete with the crops for the nutrients , sunlight , water and space in the fields .

• Reduce the disease of plants .

• Control of pest and weeds helps to increase crop production .

• provide a favor condition for the plant growth.

• Pesticides kill the pests which are animals (insect) that consume the crops in the fields .

•The amount aphid and thrips populations decreased .

•Prevent the pest population from reduces yield.

• cause crop and environmental damage.

• Cause environmental contamination, and adverse side effects .

• Some weed control may tie up soil nitrogen .

• Pesticide contain toxic substance .

• Pesticides , particular DDT , is very poisonous and cannot be easily degraded by natural process .

• The toxic chemicals such as pesticide and herbicide may be leached into the river . It can toxic to human by pass through the food chain . It would also kill the aquatic animals and plants .

• It also cause water pollution .

As a result ,The application of DDT has the following side effects :

A. universal poison .

B. Accumulation of DDT concentration along food chain.

C. Development of resistant population of pests .

A. universal poison :

DDT is non-selective poison that kills not only the pest ,but

also the organism in the field . i.e. animals .

B. Accumulation of DDT concentration along food chain . ~ DDT sprayed into crop fields may be absorbed by the surrounding plants . Though the initial concentration in plant is very small , DDT will be accumulated in the herbivores when they eat the contaminated plants .

~ when secondary consumer eat the contaminated herbivores, they also accumulate DDT to a higher concentration .

~ So that DDT is concentrated along the food chain to higher levels in the final predators . As a result , predators are eliminated .

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Losses through respirationand excretion

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blomass

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Secondary carnivores

(predatory bird)

Primary carnivores

(blowfish)

herbivores

(mud small)

plant

(aquatic plant)

C. Development of resistant population of pests .

~ The prolonged use of DDT causes immune populations

of the pests to emerge .

~ Therefore greater does of insecticides are required to

kill these resistant strains . It aggravates the problem

of pesticide pollution .

• Crops absorb mineral salt from soil . Continuous harvesting crops remove the mineral salt from soil .

• Adding fertilizers can replace for the loss and thus maintain the yields of crops .

• conductance during periods of water shortage.

• Repair the nutrient (e.g. : nitrogen )loss in agriculture and drained by water .

~ Provide the nutrient to plant to make it growth health

~ Nitrogen in the fertilizer is necessary for plants to produce chlorophyll .

A. Introduction

~They are mainly composed of nitrogenous , phosphorus

and potassium(NPK) compounds .

~ They are water soluble and can be absorbed by plants

immediately i.e. quick action .

~ They contain no humus so that they cannot improve

the soil texture .

B. Undesirable effects of excessive use

Excessive use of fertilizer would cause pollution :

i. Eutrophication

~ The leaching of excessive chemical fertilizers to river

or lake will cause eutrophication (nutritional enrich-

ment of water that promotes the growth of aquatic

plants) .

~ As a result , algal bloom (excessive growth algae) will

occur and cause the following effects :

1 . Blue-green algae unsuitable for fish food and may

be harmful to them .

2 . Algae reduce the penetration of light to the submerged

plants .

3. On a cloudy day or at night they will compete with fish

for dissolved oxygen .

4. Decomposition of the dead algae further deplete the

dissolved oxygen . As a result , fish and submerged

plants may be killed .

ii. Disease

~ Drink water with high nitrate content may cause

anaemia in child .

Fertilizer

3. Prevention of the undesirable effects

a. use adequate amount of chemical fertilizers

b. use natural (organic) fertilizers

They are farmyard manure derived from the faces of

animals .

i. Advantage

~ cheap

~ contain humus which improves soil textures .

ii. Disadvantage

~ They are slow in action because of :

1. They are less soluble in soil ,they cannot be

absorbed easily .

2. It takes time for decomposition before their

nutrients become available to plants .

Agricultural areas have the potential Agricultural areas have the potential to pollute water in many ways. Runoff to pollute water in many ways. Runoff from farms carries sediment, nutrients, from farms carries sediment, nutrients,

salts, pesticides, and fertilizers. Both salts, pesticides, and fertilizers. Both groundwater and surface water can be groundwater and surface water can be

contaminated with these pollutantscontaminated with these pollutants..

Agrochemicals (insecticides, herbicides and Agrochemicals (insecticides, herbicides and fungicides) are widely used on farms in the fungicides) are widely used on farms in the

UK to improve crop yields by killing insects, UK to improve crop yields by killing insects, weeds and fungi which might cause crop weeds and fungi which might cause crop

damage or impair crop growth. Scientists have damage or impair crop growth. Scientists have been studying what happens to these chemicals been studying what happens to these chemicals once they have been sprayed onto fields to see once they have been sprayed onto fields to see if any drain through the soil and pollute rivers if any drain through the soil and pollute rivers

and streamsand streams.

The majority of pesticide (generally more than 99%) stayed in the soil and degraded there. Most of the pesticide movement occurred immediately after significant rainstorms (>10 mm in 24 hours). However, out of 123 events involving pesticides moving from soil to water, 90 events had a mean concentration of pesticide that exceeded the EC Drinking Water Directive limit for individual pesticides in drinking water (0.1 micrograms/litre ).

Twenty five events showed peak concentrations over 10 micrograms/litre and 48 events had a mean concentration of 1-10 micrograms/litre. The remaining 50 events were below 1 micrograms/litre. The highest recorded pesticide concentration in streamwater was 68 micrograms/litre. Even pesticides traditionally thought to be strongly soil-adsorbed (e.g. deltamethrin) were found in the stream.

solutionSolutions involve finding sustainable ways for the agricultural area to reduce both its dependence on

pollutants and the amount of pollutants it produces, and to properly recycle or dispose of pollutants before they

contaminate soil, water, or air.

Organic farming avoids the use of pesticides and fertilizers. Efficient irrigation can slow salt buildup in soil

and drainage water. Avoiding large concentrations of animals can reduce nutrient pollution, and their waste can be used as fertilizer for crops. Certain cropping methods

can reduce the erosion of sediment from the land.