1. Physical Properties of Soil

2. Objectives After completing this topic, you should be able to:
Describe the concept of soil texture and its importance
Identify the texture of a sample of soil

3. Physical Properties of Soil
Soil characteristics: grower can see or feel
Neither chemical nor biological, but both affect them
Greatly affect how soils are used to grow plants or other activities

4. Soil Texture Most fundamental soil property
Determined by the proportion of soil particles
Sand (large)
Silt (medium)
Clay (small)
The size of soil particles, in turn, affects such soil traits as water-holding capacity and aeration

5. Effects of Particle Size
Soil particle size ffects two important features:
Specific surface area
Soil pores: number and size
Specific surface area is defined as the amount of surface area exposed by all the particles in a certain weight of soil. 
The smaller the soil particles, the greater the specific surface area
Soil surface area is important because reactions occur on the surface of soil particles, and because water is held as a film around soil particles
Thus the smaller the particles in a soil, the more water and nutrients the soil can retain

6. Soil Pores (A) small particles create many small pores.
Soil pore number and size depends on particle size
Macropores (aeration pores): large
Micropores: small
(A) small particles create many small pores.
(B) Pores are larger but fewer in number between large particles. Micropores usually hold water, macropores air.
Sometimes the larger micropores are distinguished as mesopores, medium-sized pores that hold readily plant-available water

7. Soil Separates
Soil scientists divide mineral particles into size groups called soil separates
This consists of three broad classes
Sand (divided into four subcategories)
Silt
Clay
These three together make up the fine earth fraction of soil used to determine texture
Larger particles, such as gravel, are considered to be coarse fragments, and are not considered in texture

8. The USDA system of soil separates
The USDA system of soil separates. The comparison shows the difference by setting a very coarse sand grain equal to 3 feet in diameter. Engineers use a different system
Separate
Diameter (mm)
Comparison
Feel
Very coarse sand
2.00–1.00
36″
Grains easily seen, sharp, gritty
Coarse sand
1.00–0.50
18″
Medium sand
0.50–0.25 9″
Fine sand
0.25–0.10
4½″
Gritty, each grain barely visible
Very fine sand
0.10–0.05
1¾″
Silt
0.05–0.002
7/16″
Grains invisible to eye, silky to touch
Clay
<0.002
1/32″
Sticky when wet, dry pellets hard, harsh

9. Comparing the size of soil separates
Comparing the size of soil separates. On this scale, very coarse sand would be 3 feet across

10. Sand Characteristics Largest of the soil separates
Composed mainly of weathered grains of quartz or other minerals
Particles range in size
Enough sand in a soil creates large pores, so sand improves water infiltration (rate at which water enters the soil) and aeration
Large amounts of sand lower the ability of the soil to retain water and nutrients

11. Silt Characteristics Medium-sized soil separate
Silt particles are silky or powdery to the touch, like talc
Best ability to hold large amounts of water in a form plants can use
Erodes readily in moving water and wind

12. Clay Characteristics Smallest of the soil separates
Consists of tiny, sheet-like crystals
Results from chemical reactions between weathered minerals to form tiny particles of new minerals
USDA size classification for stones in the soil.
Class
Diameter Range (mm)
Diameter Range (in.)
Gravel
2–75
1/12–3
Cobbles
75–250
3–10
Stones
250–600
10–24
Boulders
>600
>24

13. Textural Classification
Soil usually consists of more than one soil separate
All three separates are found in most soils
Actual percentages are called soil texture
12 textural classes are shown in the soil triangle
Another important textural name is loam, a soil in which sand, silt, and clay contribute equally to the soil’s properties.
Determining soil texture
Amount of sand, silt, and clay in a soil can be measured by mechanical analysis
Mechanical analysis is based on the fact that the larger a soil particle, the faster it sinks in water
E.g. it takes only 45 seconds for very fine sand to settle through 4 inches of water, but it takes about 8 hours for large clay particles

14. Question
Identify a soil that is 10% cay, 10% silt and 80% sand

15. Soil Triangle Each side of the triangle is a soil separate
Soil Triangle Each side of the triangle is a soil separate. The numbers are percentage of soil particles of that type.

16. Soil triangle redrawn to show fine-, medium-, and coarse-textured soils.

17. Characteristics of Textural Classes
Soils can generally be classified as fine, medium, or coarse
Indicative of a number of soil properties
Infiltration –water entering the soil
Percolation –water draining through soil
Water-holding capacity
Fine soils retain plant nutrients better than coarse soils.
This is true partly because the rapid percolation of water through coarse soil leaches out nutrients.
Also, clay particles have the best ability to retain nutrient chemicals.

18. Modifying soil texture
Impractical, except in small areas e.g. golf greens or potting soils
Very large quantities of sand are needed to loosen clay soils—enough that sand grains touch and there isn’t enough clay to fill all the gaps.

19. Soil Density and Permeability
Particle density (PD)
This is the density of solid particles only
Average 2.65 grams per cubic centimeter
PD varies according to the type of minerals in the parent material and the amount of organic matter in the soil.
Mineral
Density (grams/cm3)
Density (lbs/ft3)
Water
1.0
62.5
Quartz
2.65
166
Feldspars
2.5–2.7
156–169
Micas
2.7–3.0
169–188

20. BD = Weight dry soil/Volume dry soil = g/cm3
Bulk density
Actual density of a soil is less than the PD
 It is the mass of a volume of undisturbed oven-dry soil.
To measure BD, a core of soil of known volume is carefully removed from the field.
The soil core is then dried in an oven at 105°C until it reaches a constant weight
The example that follows is for a core of 500 cubic centimeters (cm3) that weighs 650 grams (g):
BD = Weight dry soil/Volume dry soil = g/cm3
Determine the bulk density of the soil given in the above example
Determine the volume of the particles in the soil given in the above example (Hint: use the average particle density of soils to solve this problem)