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Soils & Soil Physical Properties

Unit 2.1 | Part 2 –11
Lecture 2: Soil Properties

Lecture 2: Soil Properties

Pre-Assessment Questions

1. What are the mineral parts of the soil that create soil texture?


2. What are some of the factors affecting soil structure?


3. What makes up the organic matter component of soil?


4. What factors affect soil permeability and water holding capacity?

A. Soil Properties

1. Texture

Non-technical definition: How the soil feels to the touch

Technical definition: The proportions of sand, silt and clay in the soil

a) Soil separates (mineral part of soil)
i. Sand particles are the largest in the soil, ranging in size from 0.05 to 2.00 mm. Soil
with high sand content feels gritty and doesn’t hold well in a ball.
ii. Silt particles are moderate size particles and range from 0.002 mm to 0.05. Soils high
in silt feel floury when dry and greasy when wet.
iii. Clay particles are the smallest in the soil, with sizes less than 0.02 mm

• Morphology: Most clay minerals consist of microscopic layers (see Baklava
  Demonstration in Supplemental Demonstrations and Examples). These are called
  phyllosilicate minerals. (Phyllo- is from Greek for leaf, as in phyllo dough used to
  make baklava.) Different types of clay have different kinds of layers and different
  properties.


• Properties of clays (see several demonstrations in Supplemental Demonstrations
  and Examples):

Sticky (adhesion—sticks to other things) (Target Demonstration)

Plastic (cohesion—sticks to itself )
(Ribbon Demonstration)

Shrink-swell (Slinky
Demonstration)

Large surface area, due to layers
and size (Block Demonstration)

Cation Exchange Capacity (CEC):
Clay particles have a net negative
charge, and so can attract positive
ions (cations), hold them, and then
release them to the soil water when
its cations have been lost through
leaching or plant uptake. Cations
such as potassium (K+), calcium
(Ca+2), magnesium (Mg+2), iron
(Fe+2 and Fe+3), and zinc (Zn+2) are
essential plant nutrients, so the
ability of soil to hold and release
these ions later is important for
plant growth and reproduction.

b) Texture Triangle (see t Figure 2.2, Soil
Texture Triangle)

t FIGURE 2.2 | SOIL TEXTURE TRIANGLE

Percent Silt

Percent Clay

Percent Sand