1. PHYSICAL PROPERTIES OF SOIL
INTRO TO SOILS
CH 4

2. SOIL TEXTURE Describes the proportion of soil particle sizes: Sand
Silt
Clay
Soil Texture influences other traits such as:
Water Holding Capacity
Aeration

3. EFFECT OF PARTICLE SIZE
Smaller particles – larger internal surface area

4. EFFECT OF PARTICLE SIZE
More pores –
micropores
Larger pores –
macropores

5. SOIL SEPARATES SAND 2.00 – 0.05 mm SILT 0.05 – 0.002 mm CLAY

6. TEXTURAL CLASSIFICATION
Twelve
Textural
Classes
Textural Triangle

7. SOIL DENSITY AND PERMEABILITY
MASS per VOLUME
D = M V

8. SOIL DENSITY AND PERMEABILITY
Two densities in Soil:
Particle Density – PD
Bulk Density - BD

9. 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
BD = wt. dry soil = _g_
vol. dry soil cu cm
e.g. BD = 650 g = g/cu cm
500 cu cm
e.g.

10. SOIL POROSITY Usually expressed as a percentage; e.g. 50%
Two means determining porosity:
Calculate ratio water volume to total core volume
Calculate from bulk density and particle density

11. SOIL POROSITY Examples
Water Volume to Core Volume
Porosity = wet weight (g) – dry weight (g)
soil volume (cu cm) x 100%

12. 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 cu cm
Note: the unit cancellation is made possible by the metric system which defines 1 cu cm of water as weighing 1g.

13. 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%)

14. 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%

15. SOIL POROSITY Question: Which has greater porosity, Sand or Clay?
Answer:
Clay at about 50%; Sand is lower at about 30%
Why?

16. TEXTURE AND SOIL PORES

17. Why, then, does water move through sandy soil very rapidly, but moves s l o w l y into clay ?

18. This is explained by the physical property Permeability

19. 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

20. Fine-textured soils would be impermeable if not for:
Soil Structure

21. STRUCTURE
Structure – the way soil particles clump together into large units called aggregates or peds

22. STRUCTURE Structure can alter the effects of texture
e.g. a fine-textured silty clay with good structure can be permeable!

23. STRUCTURE Structure is classified by three groups of traits:
Type – refers to shape of aggregates
e.g. Granular, Platy, Blocky, Prismatic, Columnar
Class – refers to size of peds
e.g. very fine, fine, medium, coarse, very coarse
Grade – refers to strength and distinction of peds
e.g. weak/not visible vs. strong/easily distinguished

24. FORMATION OF SOIL STRUCTURE
Two-step formation:
Individual soil particles loosely aggregate
Weak aggregates are cemented to strengthen
- clay
- iron oxides
- organic matter
- microorganism gums

25. 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

26. 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

27. 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

28. 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)

29. 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