Unit 1 SoilProperties

7/31/2019 Unit 1 SoilProperties

1/16

CIVE 425:

Unit 1: Soil-Water Relationships

Week 1: Understanding soil

Soil as a medium for plant growth,

Some important physical properties,

Soil classification for plant growth.

Week 2-3: Soil-water relations

Water retention, soil-water holding capacity

Water availability for plant growth

Water movement and uptake by plants.


2/16

The Soil

Loose surface of the earth asdistinguished from the solid

bedrock.

Material that supports and

nourishes growing plant life.

Soil is a loose physical mixture of

mineral solids consisting of sand,

silt and clay; organic matter

derived from decaying plant and

animal life; water and air.


3/16

Typical Soil Profile

Profile: a vertical section of a soil,with distinct horizontal layers or

horizons.

Weathering of parent rock results

in unconsolidated material

consisting of sand, silt and clay.

Decaying plant and animal life at

the surface contributes organic

matter to the top horizon.

The top A horizon consisting ofmineral solids (sand, silt and clay)

and organic matter is called the

soil.


4/16

Soil Profile through Bedrock


5/16

Desirable Soil Soil MassHas coarser particles (sand, silt)covered with clay/organic matterthat act as a binder,

Individual particles then would bebound together to formaggregates; and the aggregatescombine together to form the totalsoil mass.

With such structural arrangement,

we get:1) a large amount of pore space,

2) Non-uniform pore space;primary (small) and secondary(large size) pore space.

This is very important since inprimary pore space, due to highcapillary pressure, water will be

retained.

Whereas in large pores water willdrain, and its place taken by air.


6/16

Soil Composition

A loam soil, with good structure,will have 50% solids and by 50%

pore space, by volume.

Of solids, 45-47% will be inorganic

or mineral solids (sand, silt and

clay) and 3-5% organic matter(partially decaying and

resynthesized plant and animal

residues).

For ideal plant growth, the pore

space will be half filled with waterand half with air.

All these soil constituents exist in

intimately mixed conditions.


7/16


8/16

Defining Mineral &

Organic Fractions

Primarily based on physicalsize.

Sands:

Coarse 2.0 to 0.5 mm

Med 0.5 to 0.25 mm

Fine 0.25 to 0.0.05 mm

Silt: 0.05 to 0.002 mm

Clay: less than 0.002 mm.

Organic matter has the same

size as clay and exhibits the

same properties large

surface area, large porosity,

small pore size, binding agent.


9/16

Grain Size

Distribution

Knowing the % of sand, silt and clay, the soil

textural name is assigned by using the U.S.D.A

Classification figure.

Lab exercise to determine relativepercentage of sand, silt and clay

in a soil sample.

Coarser particles, consisting of sand

and gravel are separated by

mechanical Sieve Analysis,

Finer particles consisting of silt and

clay are separated by suspending the

soil sample in a solution and

determining the speed at which

individual particles settle

(Hydrometer Analysis) The rate at

which particles settle is related to ther

mass and the size.

The two result sets can be displayed as

the Grain-Size Distribution Curve on a

semi-log paper.


10/16

Field Method for

Estimating Soil

Texture


11/16

Soil Structure

Arrangement of individual soilparticles into aggregates of

desirable shape and size.

Factors that affect soil structure:

Amount and type of clay

Amount of organic matter

Freezing and thawing

Wetting and drying

Action of burrowing organisms

Growth of root systems of

plants

Most important: management

practices including tillage,

cultivation, mulching and

liming.


12/16

Defining Soil

Structure

Particle and Bulk Density

Very important as it influencesthe amount and nature of

pore space,

A property that can be

modified through proper

management practices (unlike

texture),

Most physical changes

imposed by farmers including

plowing, cultivating, liming

and mulching are structural in

nature.

No direct way of

measuring/determining

structure,

Some soil properties are used

as indicators of structure


13/16

Soil Density

Expressing soil densityBulk density, d = dry wt/volume = Wd/V

Particle density dp = Wd/Vs

o

Since total Volume V = 2Vso It follows that dp = 2 d

o Since Particle density of inorganic soils = 2.65 g/cm3

o Bulk density of most field soils = 1.33 g/cm3


14/16

Some Loose Ends !!!

In addition to the physicalproperties, soils have chemical

properties that affect soil-waterrelationships.

Most important is the Cation

Exchange Capacity (meq/L).

Because clay and humus carrynegative charge at their

surface, they can attract and

hold +ve ions (adsorption).

These adsorbed cations can

be exchanged with cations

absorbed in the soil solution.


15/16

Composition of Soil Minerals

Sand is mostly Quartz (Sio2) and

some primary minerals e.g micas

(iron and aluminum silicates) and

feldspars (aluminum silicates).

Clay has less quartz and more

secondary silicates. All mineral constituents weigh about

the same, i-e 2.65 g/cm3


16/16

Soil Profile Depth Around the Globe

Unit 1 SoilProperties

Documents

Transcript of Unit 1 SoilProperties