Soils and Studying Living Organisms and Their Environment

7/30/2019 Soils and Studying Living Organisms and Their Environment

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Studying Living Organisms and

their Environment


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Observable Characteristics of Organisms

Organisms can be grouped or classifiedaccording to their colour, texture, appendagesand means of locomotion, shape, size andbehaviour.

Keys are used for these classification (recall:dichotomous keys, spider and number keys)


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Some definitions


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Biotic/abiotic

Biotic factors are the living components of anecosystem that have an influence on anorganism living in it.

Abiotic, or physical, factors are the non-livingcomponents of an ecosystem that have aninfluence on an organism living in it.


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Environment/habitat

Environment - The sum of allexternal conditions affectingthe life, development andsurvival of an organism.

Habitat the location wherean organism can befound/where it lives.


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Population/community

Population a group of organisms of the samespecies living in the same

habitat

Community a

collection of variousspecies living in the samehabitat


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The Soil The major importance of soil

relates to its function inproviding nutrients and asupport medium for green

plants. Soil has 4 major components:

Mineral fraction; small rockparticles and mineral salts

Organic matter; remains of living organisms and theirproducts

Water Air


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The Soil

The soil is also a habitat to a host of micro-organisms and small animals such asearthworms and beetle larvae.

These too are considered part of the soil. Each of the components contribute to the

characteristics of a soil.


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Soil breakdown


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Soil is important because

it provides a medium forplant growth

it provides green plants

with water and mineral saltsfor photosynthesis it provides food and

shelter for a large range of organisms that live both inand on it.


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Soil food web


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Major Components of soil


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Mineral Fraction

The bulk of the soil is made up of rockparticles of varying sizes.

These are the result of weathering, whichcauses the rock to break into smaller particles.

These particles give the soil its texture, andtherefore many of its properties.

Particle size may be fine (clay), to coarse(sand) to even larger particles (gravel).


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Particle sizes classification

Type of mineral particle Size range

Gravel Larger than 2.0mm

Sand 0.02 2.0 mmsilt 0.002 0.02mm

Clay Less than 0.002 mm


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Types of soil

Soil can be classified in many ways. How it was formed (alluvial or sedentary)

Depth it is found at (top soil or sub soil) Its composition (loam, clay, sand)


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Mineral Fraction

The mineral fraction gives the soil its structureand is important for anchoring plants as wellas providing some mineral elements.

The mineral salts in the soil are also inorganic. They provide plants with compounds such as

nitrates which are essential.


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Soil composition by particle size


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Organic Matter

The organic fraction of the soil is derived fromdead and decaying remains of plants andanimals as well as their waste products.

When these materials are fully decomposedand no longer recognizable, it is called humus.

The breakdown of humus by soil microbesreleases minerals that plants require.


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Organic Matter

Humus also helps to holdclumps of inorganic particlesin soil together to formcrumbs.

Crumb structure affects thephysical properties of thesoil.

A good crumb structureimproves aeration anddrainage as well as water holding capacity.


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Water Content


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Water Content

It is important for soil to have a fair amount of water, since plants require a constant uptakeof water.

Water is important for support, as a mediumfor the uptake of and transport of mineralsalts and manufactured food.

Water takes part in many metabolic processessuch as photosynthesis and helps to cool theplants in transpiration.


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Water Content

The soil needs to be able to hold enoughwater for these purposes, but at the sametime water must not be allowed to clog up the

air spaces (water-logging). A good soil for plants is one which retains

sufficient water and also drains readily.Whether a particular soil type meets theserequirements depends on its crumb structure.


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Air content


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Air in the Soil

An important constituent of soil is air since itprovides oxygen for all the living organisms includingthe roots of plants which are anchored in it.

Oxygen is needed for respiration during which energyis released from food molecules to do work.

The size of air spaces, and hence the air content of asoil, is directly related to particle size.


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Air in the Soil

Small particles pack more tightly together andleave little space for air. Soils depend onaerobic bacteria to break down organicmolecules to inorganic ones which provideplants with minerals.

Water-logged soils that lack air are poor in

nutrients.


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Soil Organisms Numerous organisms inhabit the

soil. They depend on it for shelterand nutrients.

Certain animals such as beetles,ants and earthworms tunnelthrough the soil and by their modeof feeding help to mix the soil withuseful organic material and create

passages that aerate the soil andimprove its drainage.


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Soil Organisms

Their excreta helps to addnutrients to the soil as well.

Micro-organisms living in thesoil break down the humusand the dead organic matterfrom which it originatesmaking the nutrientsavailable to plants.


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Investigating soil properties

Three important soil properties can beinvestigated, these are:

Air content Water content Water-holding capacity Humus content

Refer to handout


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Air content

To find out what percentage of a soil is madeup of air, a known volume of soil is mixed witha known volume of water

It is assumed that the air in the soil will bedisplaced by the water

The difference between the final volume of

the and the sum of the two volumes of soiland water is assumed to be equal to the airdisplaced


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Air Content

A major source of error in this investigation isthe treatment of the soil

If the soil is pulverized or ground before the

investigation, the soil will hold les air thannormal

Large pebbles and stones ought to be

removed from the samples and the very largepieces of soil broken up but there should beno further treatment


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Water Content

Water in soil is held primarily around the soilparticles and in the spaces between them

The water content is usually expressed as apercentage of the total mass of the soil.

A known volume of soil is dried to a constantweight in an oven and the weight loss is usedto calculate the percentage water content


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Water Content The temperature of the oven must not exceed 105

degrees Celsius as the organic matter may begin toburn

The weight loss will include the loss of organic matter

and be inaccurate If an oven is not present, the sample can be air-dried

to a constant weight over a long period of time However, the film around the soil particles will still

be present but can still be used to compare differenttypes of soil


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Water-holding capacity

For a particular volume of soil, the water-holdingcapacity refers to the amount of water the soil willretain or hold on to after it has been supplied with

water and allowed to drain A known volume of water is added, when the water

stops dripping, the soil will be at capacity This capacity can be measured by comparing the

volume of water added to the soil with the volumedrained through.


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Use of Natural and ArtificialFertilizers

Soil may lose the ability to provide nutrientsand support plants and other organismsbecause of erosion and leaching or overuse

Erosion refers to the physical removal of theexposed top soil by wind and rain

Leaching is the washing away of suluble saltsfrom the soil

The soil becomes deficient in mineral utrientsand unable to support plant growth


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Use of natural and artificialfertilizers in soils

Erosion and leaching arecaused by activities suchas

Deforestation Poor farming methods Overgrazing

These all result in the loss of humus from the soil.


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Use of natural and artificialfertilizers in soil

Loss of humus reduces the crumb structure of the soil and accelerates erosion

A particular problem is caused by modern

agricultural practices Intensive agriculture uses up soil nutrients

rapidly and since the crops are harvested theydo not get a chance to die, rot and replenishthe soil

The soil becomes poor in humus


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Natural Fertilizers Adding compost or manure to the

soil is one way to replace humusand eventually, mineral nutrients

There are other advantages tousing natural fertilizers

They will improve soil structure byimproving the crumb of the soil

Drainage and aeration improve as aresult

In addition, manure will also add

micro-organisms to the soil They also tend to be non-polluting


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Natural fertilizers Some of the disadvantages using

natural fertilizers is that it is bulky,time consuming and difficult tospread and it is not available inlarge enough quantities to satisfythe demands of large scaleagriculture

In addition, the nutrients are notreadily available

It takes time for it to break down


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Artificial fertilizers

These are often used in place of natural fertilizers,especially in large farming areas because theyprovide a ready supply of important nutrients

They are relatively inexpensive and fast acting They are concentrated compounds of the major

mineral elements found in the soil They usually contain varying proportions of nitrate,

phosphate and potassium


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Artificial fertilizers BUT they must be used with care They should preferably be

combined with a program to re-supply the soil with organic humus

Exclusive use of chemicals leads tothe loss of microorganisms whichdie when the soil water becomestoo concentrated

The humus content then

decreases, which in turn results inthe deterioration of the soilscrumb structure and porosity.


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Plant effects are obvious Less obvious but also hazardous to the

balance of the ecosystem is the over-fertilization or eutrophication of lakes andrivers

This results in when unabsorbed nitrates andother minerals are washed by rain from soil,eventually entering the water system


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Algae are richly nourishedand grow excessively

They then die anddecompose

Decomposers require oxygen The oxygen supply in the

water is depleted to such anextent that there is notenough oxygen left for otheraquatic life


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Together with semi-decomposed organicremains of algae they help form a foul mud

Rivers and lakes may have an ordinary surfaceappearance but beneath that surface theymay be at hazard of dying


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The Eutrophication Process


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Eutrophication


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Fertilizers and pH Sometimes soils become too acidic Since most plants flourish in soils that have a pH that

is about neutral, it is desirable to increase the pH of acidic soils

Calcium oxide (lime), calcium carbonate or calciumhydroxide are usually added to acid soils to increasepH

This has three side benefits It adds minerals It improves granule structure (drainage of soil) It stimulates the growth of useful nitrogen-fixing bacteria

and decomposers

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Soils and Studying Living Organisms and Their Environment