Download physical properties of soil

PHYSICAL PROPERTIES OF SOIL
INTRO TO SOILS
CH 4
SOIL TEXTURE
Describes the proportion of soil particle sizes:
Sand
Silt
Clay
Soil Texture influences other traits such as:
Water Holding Capacity
Aeration
EFFECT OF PARTICLE SIZE
Smaller particles –
larger internal
surface area
EFFECT OF PARTICLE SIZE
More pores –
micropores
Larger pores –
macropores
SOIL SEPARATES
SAND
2.00 – 0.05 mm
SILT
0.05 – 0.002 mm
CLAY
< 0.002 mm
TEXTURAL CLASSIFICATION
Twelve
Textural
Classes
Textural Triangle
SOIL DENSITY AND PERMEABILITY
DENSITY:
MASS per VOLUME
D=M
V
SOIL DENSITY AND PERMEABILITY
Two densities in Soil:
Particle Density – PD
Bulk Density - BD
SOIL DENSITY AND PERMEABILITY
PD average soils ~ 2.65 gm/cu cm
BD average range from 1.0 – 1.8 gm/cu cm
Depends on amount of pore space
e.g.
e.g.
BD = wt. dry soil = _g_
vol. dry soil
cu cm
BD = 650 g = 1.3 g/cu cm
500 cu cm
SOIL POROSITY
Usually expressed as a percentage; e.g. 50%
1)
2)
Two means determining porosity:
Calculate ratio water volume to total core
volume
Calculate from bulk density and particle density
SOIL POROSITY
Examples
Water Volume to Core Volume
Porosity = wet weight (g) – dry weight (g)
soil volume (cu cm)
x 100%
SOIL POROSITY
Examples
An oven-dry soil core, volume 500 cu cm, weighs
650g. When wet, it weighs 900g. Find it’s
% porosity.
Porosity = 900g – 650g
x 100% =
250g
x100% = 50%
500 cu cm
500 cu cm
Note: the unit cancellation is made possible by
the metric system which defines 1 cu cm of
water as weighing 1g.
SOIL POROSITY
Examples
Bulk Density to Particle Density
Defines the percentage of the soil that is solid
matter
The percent solid matter is subtracted from
100% to give percent porosity:
Porosity = 100% - (BD/PD x 100%)
SOIL POROSITY
Examples
An undisturbed oven-dry soil, BD of 1.3 g/cu cm,
consists of average mineral composition
(PD 2.65 g/cu cm). Find its % Porosity:
Porosity = 100% - (1.3 g/cu cm/2.65 g/cu cm x 100%)
Porosity = 100% - (0.49 x 100%) = 100% - 49% = 51%
SOIL POROSITY
Question:
Which has greater porosity, Sand or Clay?
Answer:
Clay at about 50%; Sand is lower at about 30%
Why?
TEXTURE AND SOIL PORES
 Why, then, does water move through sandy soil
very rapidly, but moves s l o w l y into clay ?
This is explained by the physical
property Permeability
PERMEABILITY
Permeability – the ease with which air, water, and
roots move through soil
- Depends on number, size, and continuity of pores
- Liken to a maze
Fine-textured soils would be
impermeable if not for:
Soil Structure
STRUCTURE
Structure – the way
soil particles clump
together into large
units called
aggregates or
peds
STRUCTURE
Structure can alter the effects of texture
e.g. a fine-textured silty clay with good structure can
be permeable!
STRUCTURE
Structure is classified by three groups of traits:
1) Type – refers to shape of aggregates
-
e.g. Granular, Platy, Blocky, Prismatic, Columnar
2)
Class – refers to size of peds
e.g. very fine, fine, medium, coarse, very coarse
3)
Grade – refers to strength and distinction of peds
e.g. weak/not visible vs. strong/easily distinguished
FORMATION OF SOIL STRUCTURE
Two-step formation:
1) Individual soil particles loosely aggregate
2) Weak aggregates are cemented to strengthen
- clay
- iron oxides
- organic matter
- microorganism gums
ADDNL. SOIL PHYSICAL PROPERTIES
Soil Consistence – the behavior of soil when
pressure is applied; measured at three different
moisture levels: Wet, Moist, Dry (fig 4-15, p. 59)
Soil Tilth – ease of tillage, seedbed preparation,
and seedling/root movement
Compaction – results from pressure applied at the
soil surface
ADDNL. SOIL PHYSICAL PROPERTIES
Puddling, Clods, and Crusts:
Puddling occurs when pressure is applied to
very wet soils (esp. plowing)
Crusts occur when bare soil is struck by
raindrops; disperses soil then dries to a
hardened crust
ADDNL. SOIL PHYSICAL PROPERTIES
Improving Tilth:
Best accomplished by improving structure
- tilth relates to texture, structure, permeability,
and consistence; however,
texture and consistence cannot be altered easily
therefore, improve tilth by improving structure
and avoiding compaction
ADDNL. SOIL PHYSICAL PROPERTIES
Soil Channels – continuous macropores leading
from surface to deep subsoil
Soil Pans – any layer of hardened soil; includes:
- claypans
- fragipans (clays)
- plinthite (tropics)
- caliche (Ca cemented)
ADDNL. SOIL PHYSICAL PROPERTIES
Soil Temperature (varies w/color, texture, O.M.)
Soil Color
Munsell soil color chart
Hue, Value, Chroma
e.g. 10YR 3/6