1ESC 590.Air WaterTemp pH

7/30/2019 1ESC 590.Air WaterTemp pH

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Soil Temperature: ProcessesSoil Temperature: Processes

I. Importance:

Affects physical, biological and chemical

processes occurring in soil.

II. Processes Affected

1. Microbial Activity


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Soil Temperature: ProcessesSoil Temperature: Processes

2. Seed Germination

Germination of seeds stop between 0-5oC

3. Root growth

4. Physical Weathering


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Factors Affecting SoilFactors Affecting Soil

TemperatureTemperature

1. Energy Received

30 to 45% of heat is reflected back

3% is used for photosynthesis

Remainder is used to evaporate water

3 to 5% is stored as heat in soil and plant

cover


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Absorbs heat is lost by

1. Radiation into atmosphere

2. Heating of air above soil 3. Evaporation of water

4. Heating of soil

2. Slope and Gradient


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3. Soil Cover

Color affects heat absorbed.

Dark colored soil absorbs about 80% of

heat

Light color soil absorbs only about 30%


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4. Water Content

Mineral soil require small amount of heat to

raise their temp. The Heat capacity of soil is the heat

required to raise 1 gram of soil 1oC

Specific heat of water is 1.0 cal/gram

The heat capacity of soil is 1/5 that ofwater, i.e. specific heat of soil is 0.2cal/gram


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Thus moisture content is important in

determining soil temperature

Drainage is thus an important influence on

soil temperature.


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Control of Soil TemperatureControl of Soil Temperature

IV.Control Of Soil Temperature

1. Removal of Excess Water

2. Use of mulches and various shading

devices


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I. pH ConceptI. pH Concept

Water neutral pH 7

HOH H+

+ OH

-

At 25oC 1 liter of water weighs 997 gm

1 mole of water weighs 18 gm

Therefore 1 liter of water contains 55.4 moles ofwater


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In a liter of water 55.339,999,8 moles exist

as H2O

0.000,000,1 is in H+ form and 0.000,000,1

is in the OH - form


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pH = -log [H+] or

pH = 1/[H+]

If [H+] = 10-7 moles/L

pH = -log [10-7] = 7


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III. Development o f Soil AcidityIII. Development o f Soil Acidity

1.1. Strongly Acid Soil.Strongly Acid Soil.

Much H+ under very acid soils because

Al becomes soluble and is present in theform of Al3+ or Al hydroxyl cations.

These become preferentially absorbed in

preference to H+ by the permanentcharges on soil colloids.


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The adsorbed Al is in equilibrium with Al3+ions in the soil solution. H+ released as Al 3+hydrolysis results in the soil acidity instrongly acid soils

Adsorbed H+ ions is the second major

source of H+ concentration under theseconditions.


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2.2. Moderately Acid Soils.Moderately Acid Soils.

Al compounds and H + ions account for H+

ions in these soils but the mechanism isdifferent .

These soils also have higher percent base

saturation and pH values. Al3+ is converted to aluminum ions by

reactions such as:


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Al3+.6H20 Al (OH)

2.5H

2O + H+

Al(OH)2+.5H2O Al( OH)2+.4H

20 + H+

Some Al hydroxy ions are absorbed as

exchangeable cations


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In moderately acid soils absorbed H+ ions

makes a contribution to the soil solution H+

concentration. As pH rises, some H+ held strongly by clay

are now subject to release.

These are associated with pH -dependentgroups.


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3.3. Neutral to Alkaline Soils.Neutral to Alkaline Soils.

Soils that are neutral and Alkaline are no

longer dominated by H+ and Al3+ ions. Permanent charge sites are now occupied

by exchangeable bases and both Al and H

are largely replaced by cations such as Ca2+,Mg2+, K+.


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H+ ion is released more into soil solution and

react with OH- ions to form H2O.

Overall pH in soil is a balance between Al 3+and H+ in soil and OH- produces by basic

cations.

The ion which predominates determine thesoil pH. The right balance yields a pH of 7

pH is between 6.5 and 7


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5.5. Calcareous SoilsCalcareous Soils

Contain CaCO3which is relatively

insoluble.

Calcareous soils are 100% base saturated

and pH is controlled by the hydrolysis of

CaCO3as follows:


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Greater dissociation of Ca(OH)2 and production of OH-

as compared to H+ results in pH in 7-8.5 (maximum) range.

MICELLE SOIL SOLUTION MICELLE

- H+ Ca2+ -

- H+ + CaCO3 H2O + CO2 -

CaCO3 + 2H2OCa(OH)2 + H2CO3


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6. Sodic Soils6. Sodic Soils These are soils are dominated by sodium.

Occurs when soil is 15% or more saturated

with Na or Na2(CO

3).

Hydrolysis of Na2(CO

3) release NaOH.

Organic matter is highly dispersed in these soils. Soils contain small amounts of Ca2+ and Mg2+ but

larger amounts of Na+.


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MICELLE SOIL SOLUTION MICELLE

- H+ Na+ -

- H+ + Na2(CO3 ) H2O + CO2 + Na+ -

Na2(CO3) + 2H2O Na(OH) + H2CO3

pH of these soils is maybe between 8.5 and 10.


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Energy Concept - WaterEnergy Concept - Water

Potential.Potential.

Free Energy :

Free Energy - Summation of all forms ofenergy available to do work, e.g. potential,

electrical and mechanical (kinetic).

Substances have a tendency to move from a

state of higher to one of lower free energy.


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Energy Concept - Water Potential.Energy Concept - Water Potential.

Water moves from soil saturated with water

(high free energy) to dry soil (low free

energy).

Absolute free energy is not as important as

differences in energy levels from onecontiguous site to another.


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Factors Affecting FreeFactors Affecting Free

Energy :Energy :

1. Adhesion - attraction to soil solid

(matrix)

This provides matric force responsible for

capillarity and reduces the free energy of

the adsorbed water and those held bycohesion.


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Factors Affecting Free EnergyFactors Affecting Free Energy

2. Attraction of ions and other solutes for

H2O results in osmotic forces. This also

tends to reduce free energy of H2O.

3. Gravity tends to pull water downwards.

Free energy at given elevation higherthan at lower elevation.


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Total Water Potential:Total Water Potential:

This is the difference in free energybetween two contiguous sites.

It ultimately determines soil water behavior.

Total soil water potential is in effect thesum of the potential resulting from variousforces acting on soil H

2O and is described

by the relation below:


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Total Water Potential:Total Water Potential:

t = g + m + o

- Where:

t= total soil water potential

g= gravitational potential

m= matric potentialo= osmotic potential


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Gravitational PotentialGravitational Potential

This is the component due to the position of

the soil water in a gravitational field.

The gravitational potential is important in

saturated soils and is shown by the tendency

of water to flow to a lower elevation.


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Matric Potential:Matric Potential:

This is the result of the adhesive andcohesive forces associated with the particlenetwork of the soil or the soil matrix.

The potential is expressed relative to purewater; thus, as soils dry and the energy

content of water decrease, the matricpotential decreases


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Matric Potential:Matric Potential:

The matric potential is the controlling factor

in water movement in unsaturated soils.

It is also important in movement of water

from soil into plant roots and microbes.


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Osmotic Potential:Osmotic Potential:

This is due mainly to the attraction of water

molecules for ions produced by soluble salt.

Normally in leached soils the osmotic

potential is small and is a minor factors in

water absorption.

The osmotic potential of saline soils, by

contrast, reduces the ease that water moves

into plant roots and microbes.


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Plant - Soil Water Relations :Plant - Soil Water Relations :

1. Maximum rententive Capacity

Matric potential - 0.

2. Field Capacity : Following rain orirrigation water moves rapidly down dueto gravity or hydraulic gradient.

The point at which rapid movementbecomes negligible is called the fieldcapacity.


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At this time water has moved out of themacropores and have been replaced by air.

Micropores are still filled with water and willsupply with water.

The matric tension will vary slightly from soilto soil but is generally between 0.1 - 0.3 bars.

SMT at field capacity generally set at 1/3atm

(equivalent to 11ft high of water). At field capacity SMT is low and plants root

can easily absorb water.


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3. Permanent Wilting Percentage:

As plants absorb water they lose most of it

at leaf surface through evapo-transpiration. Water also lost by evaporation.

Loss occur simultaneously.

As soil dries, plants regain vigor at night.


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Ultimately, the rate of water supply is so

slow that plants will remain wilted both day

and night. Although not dead, the plants are in apermanent wilted condition and will die ifwater is not added.

Matric potential at this time will be about15 bars (kpa) for most crop.


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Soil moisture content at this point is called

the permanent wilting percentage.

Water remaining in soil is found in the

smallest of micropores.

A considerable amount of water is not

available to plants.


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4. Hygroscopic Coefficient : If water is

kept at an atmosphere that is essentially

completely saturated with water vapor (48%relative humidity), it will lose liquid held

even in the smallest micropores.

The remaining water will be associated withthe surfaces of soil particles, particular

colloids, as adsorbed moisture.


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It is held so tightly that it is considerednonliquid and can only move in vaporphase.

Water content at this point is termedhygroscopic coefficient.

Tension at this point is 31 bars.

Soils high in colloidal materials hold morewater under this condition than sandy soils.


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1. Gravitational Water: Water in excess offield capacity (0.1 - 0.3 bars).

Under saturated conditions water inmacropores have positive potentialdetermined by distance below surface ofsaturated zone.

This water will flow freely from regions ofhigher pressure to lower pressure (higherelevation to lower elevation).


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Physical ClassificationPhysical Classification

The water that "freely flows or drains out of

soil is called gravitational water.

1. Exist in micro pores. 2. Is either free or under very low tension.

3. Moves freely through macropore space

in response to very small water pressurediffusion or gravitation.


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Physical Classification of Soil WaterPhysical Classification of Soil Water

2. Capillary - Water held in capillary pore

(0.1 - 31 bars).

3. Hygroscopic water - Water held in

tension values greater than 31 bars.

Bi l i l Cl ifi i f


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BiologicalBiologicalClassification ofClassification of

Soil WaterSoil Water

1. Available water :

Water retained in soil between field

capacity (0.1 - 0.3 bars) and permanentwilting percentage (15 bars) is said to be

usable by plants and said available.

2. Unavailable water : Water held at tension greater than 15 bars.


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Soil Water DeterminationSoil Water Determination

1. Gravimetric.

a. Per Cent By Weight

- Pw= X 100

b. Per Cent by volume- P

v= P

wx D

b


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Soil AerationSoil Aeration

Soil Aeration : Soil aeration is the mechanism ofgas exchange in soils that prevents O2 deficiencyand CO2 toxicity.

Well-aerated soil : This is a soil in which gasexchange between the soil air and the atmosphereis sufficiently rapid to prevent a deficiency of O2

or CO2 toxicity and thereby permits normalfunctioning of plant roots and aerobic organisms.


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Conditions for Satisfactory :

1. Sufficient spaces free of solids and

water should be present.

2. Ample opportunity for easy movement

of air.


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Soil Atmosphere Vs Atmosphere :

Atmosphere = 79% N, 21%, O2, 0.03% CO

2

Soil Atmosphere = 10-100% CO2concentration

Slightly less O2concentration

N remains about the same.

O2can drop to 5% or even zero in subsoils.


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Under actual field conditions two conditions mayresult in poor aeration of soil.

1. Moisture content excessively high.

2. Gaseous exchange not sufficiently rapid.

1. Excess Moisture: Waterlogging

poorly grained, fine-textured soils small macropores.

ell-drained soil - compaction.


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1. Low-lying areas - water tends to stand.

Consequences : Root growth hampered.

Prevention: Rapid removal of excesswater either by land drainage or controlled

runoff.

Artificial drainage of heavy soils.


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2) Gaseous Interchange : Dependent on two

factors:

a. Rate of biochemical reactions.

b. Actual rate at which gas is moving into and

out of soil.

- a. More rapid oxygen use leads to carbon

dioxide.Factors : - Temperature, Organic residues


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b. Air Exchange :

Two mechanisms: (i) Mass flow (ii) Diffusion.

(i) Mass flow due to pressure difference between

atmosphere and soil air. Very small thus not very important in determining the

total exchange that occurs.

(ii) Diffusion : Most gaseous exchange occurs by

diffusion. Gas tends to move in direction determined by partial

pressure.


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Heavy-texture top soils, especially thosewith poor structure, and in compact subsoils, rate of oxygen movement is very

slow.

Such soils also allow only slow oxygen

penetration and thus prevent rapid escape ofcarbon dioxide.


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Factors Affecting AerationFactors Affecting Aeration

a. Air space available, biochemical ratesand gaseous exchange.

Total porosity determined by bulk density.

This in turn is related to texture andstructure and soil organic matter.

Also macropore to micropores is important.

In poor drained soils high proportion of soilis occupied by water.


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(ii) Carbon dioxide content related to biological

activity in soil.

Microbial decomposition of organic residues

accounts for major portion of carbon dioxide

evolved.

Incorporation of large quantities of organic matter,

manure, sewage sludge will alter soil aircomposition considerably if soil moisture and

temperature is adequate.


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Respiration by higher plants and contribution oftheir roots to organic mass by sloughage are alsosignificant processes.

b. Subsoil Vs Topsoil : Subsoils more deficient in oxygen than topsoil.

Total pore space as well as average size of pores isgenerally less in deeper horizons.

Oxygen percent in soil air decreases with depth,the rate of decrease is much rapid in heavy soils.


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c. Soil heterogeneity : Considerable variationexists in the aeration status of soil.

Thus poorly aerated zones may be found in anotherwise well drained soil.

d. Seasonal differences : This has marked effecton in the composition of soil air.

Most of this variation is accounted for by soil

moisture and soil temperature differences. High soil moisture tends to favor low oxygen and

high carbon dioxide levels in soil air e.g. in winterand spring.

Eff t f S il A tiEff t f S il A ti


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Effects of Soil Aeration onEffects of Soil Aeration on

Biological ActivitiesBiological Activities

a. Effects on higher plants :

High plants adversely affected in at least fourways by poor aeration.

(i) The growth of the plant, particularly the roots,is curtailed.

(ii) The absorption of nutrients is decreased.

(iii)The absorption of water is decreased.

(iv) The formation of toxic inorganic compounds.

Eff t f S il A tiEff t f S il A ti


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Effects of Soil Aeration onEffects of Soil Aeration on

Biological ActivitiesBiological Activities

b. Effect on Microbes:

Slow decay of organic matter in surveying

areas. Transformation of nutrients.

Class of microbes.

Reduced compounds Mn2+, Fe2+ leadingto toxicity.

Eff t f S il A ti Bi l i lEffects of Soil Aeration on Biological


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Effects of Soil Aeration on BiologicalEffects of Soil Aeration on Biological

ActivitiesActivities

c. Other Effects

Anaerobic decomposition of organic matter much

slower than that occurring when oxygen is

available.

C6H

12O

6----------> 3CO

2+ 3CH

4

Organic acid production ------> toxicity.

C2H

4affects plant roots.

A not subject to nitrification.

Eff t f S il A ti Bi l i lEffects of Soil Aeration on Biological


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Effects of Soil Aeration on BiologicalEffects of Soil Aeration on Biological

ActivitiesActivities

Carbon CO2 CH4

N NO3- N2, NH4+

Sulfur SO42- H2S, S2-

Fe Fe 3+ (ferric ) Fe2+(ferrous)

Mn Mn 4+ Mn2+


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Documents

1ESC 590.Air WaterTemp pH