CHAPTER 1. SOIL PHYSICAL PROPERTIES

SSC107-Fall 2000

Chapter 1, Page - 1 -

CHAPTER 1. SOIL PHYSICAL PROPERTIES

Contents:

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Soil physics

Soil texture

Soil surface area

Soil structure

Volume and mass relationships

Water content measurements

Units

SOIL PHYSICS: THE STUDY OF THE STATE AND TRANSPORT OF ALL FORM OF

MATTER AND ENERGY IN SOILS

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Why study soil physics ???

ATMOSPHERE

ROOT

ZONE

SOIL SURFACE

VADOSE ZONE

CROPS

STREAM

GROUNDWATER

SOILS:

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SOURCE OF NUTRIENTS AND WATER FOR CROP AND PLANT GROWTH

CONDUIT BETWEEN SOIL SURFACE AND GROUNDWATER

ACT AS FILTER AND A BUFFER

SSC107-Fall 2000

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Soils are extremely complex, hence we often simplify to study and understand soil physical

principles, e.g.,

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soil particles are spherical

soil pores are composed of capillary tubes

soil is homogeneous

soil particles

soil gas

soil water

SOIL SOLID PHASE IS CHARACTERIZED BY

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SOIL TEXTURE - SIZE DISTRIBUTION OF SOIL PARTICLES

CHEMICAL AND MINERALOGICAL PROPERTIES

SHAPE AND SURFACE AREA OF SOIL PARTICLES

SOIL STRUCTURE - ARRANGEMENT OF INDIVIDUAL SOIL PARTICLES

Soil texture:

SSC107-Fall 2000

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Has a large influence on water holding capacity, water conducting ability and chemical

soil properties

Soil Texture Classification:

Soil separate

equivalent diameter size (mm)

gravel

Sand

> 2 mm

0.05 - 2 mm

1 - 2 mm

0.5 - 1 mm

0.25 - 0.5 mm

0.1 - 0.25 mm

0.05 - 0.1 mm

0.002 - 0.05 mm

< 0.002 mm (< 2 micrometer)

very coarse

coarse

medium

fine

very fine

Silt

Clay

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How to measure soil particle size ???

1. Mechanical sieving, if size > 0.05 mm

2. Sedimentation - Stokes¡¯ law, if size < 0.05 mm

Hydrometer method

Pipette method

Soil is dispersed, and mixed with water (soil suspension);

Settling velocity of individual particles depends on particle diameter;

Forces acting on soil particle are gravitation, buoyancy and drag forces,

and all depend on particle size;

The larger particles settle first ¨¤ Stokes law

Since soils are a mixture of different size particles, soil¡¯s are classified using the so-called

soil textural triangle.

SSC107-Fall 2000

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Soil Textural Triangle

Soil mineralogical composition:

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Primary minerals: present in original rock from which soil is formed. These occur

predominantly in sand and silt fractions, and are weathering resistant (quartz,

feldspars);

Secondary minerals: formed by decomposition of primary minerals, and their

subsequent weathering and recomposition into new ones (clay minerals).

Humus or organic matter (decomposed organic materials)

Mineral type has large influence on soil behavior:

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Ion exchange, related to cation exchange capacity

Hydration and swelling; dehydration and shrinking

Flocculation and dispersion

Preferential flow, as through soil cracks

Barrier to flow, as by swelling clays

Chemical adsorption of contaminants and nutrients, both in liquid and gas phase

SSC107-Fall 2000

Chapter 1, Page - 5 -

Soil¡¯s specific surface area (s, m2/g):

Depends on shape of soil particle.

For example, if spherical (r = radius and ¦Ñ = density)

Surface area (a) = 4¦Ð r2

Mass (m) = ¦ÑV = ¦Ñ[4¦Ðr3/3]

Thus, specific surface area (s=a/m): s = 3/¦Ñr

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(inversely proportional to radius)

How to approximate surface area of a clay particle ???

Surface area of soil affects its physical and chemical properties and is largely

determined by amount of clay present in soil:

Specific surface area of soil particles

Effective

Particle

Diameter (cm)

Mass (g)

Gravel

2 x 10-1

Sand

5 x 10-3

Silt

2 x 10-4

a

Clay

2 x 10-4

a

Thickness = 10-7 cm

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1.13 x 10-2

1.77 x 10-7

1.13 x 10-11

8.48 x 10-15

Area

(cm2)

1.3 x 10-1

7.9 x 10-5

1.3 x 10-7

6.3 x 10-8

Compute the surface area of 1 gram of clay in m2 .

Specific Surface Area

(cm2 g-1)

11.1

444.4

11.1 x 104

7.4 x 106

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