1. Ch. 4 continued
Soil Properties

2. Basic Soil Property Because it is not readily subject to change
Soil Texture (proportion of different-sized particles <2-mm)
Master Variable Because it affects so many other soil properties and variables
Basic Soil Property
Because it is not readily subject to change

3. Heavy or Light Soils These terms actually refer to texture, not weight
A heavy soil is a clayey soil
called ‘heavy’ because of the soil's ability to retain moisture and the difficulty in working the soil (and it’s heavy after rain or irrigation)
A light soil is a sandy soil
called ‘light’ because it is usually easy to work in the field and drains quickly
Actually, a clayey soil weighs less than a sandy soil when dry

4. Clay vs Silt vs Sand:
The surface area to volume ratio greatly increases as the particle size decreases and the shape changes from rounded to plate like
As the surface area increases, so does the ability to adsorb compounds and interact with the soil solution:
more chemical reactions,
more biological interactions,
more surface for water to cling to

5. Calculate: (1) total surface area of original cube (uncut) and
8cm x 8cm x 8cm
8cm
2 x 2 x 2
2 x 2 x 2
Calculate: (1) total surface area of original cube (uncut) and
(2) total surface area after cutting it into smaller cube of the given sizes
8cm
4 cm x 4cm x 4cm
8cm

6. Specific surface area
2 cm
2 cm
2 cm
Each cube is 2 cm on each side, the same mass of material would now be present as 64 smaller cubes
Surface area for one face of a small cube = 2 cm x 2 cm = 4 cm2
Surface area for each cube (6 faces, 4 cm2 per face) = 6 faces x 4 cm2/face = 24 cm2
Total surface area (64 cubes, 24 cm2/cube) = 64 cubes x 24 cm2/cube = 1536 cm2
Surface area for each face (8 cm x 8 cm) = 64 cm2
A cube has six faces
Total surface area (6 faces 64 cm2 per face) = 6 faces x 64 cm2/face = 384 cm2

7. Sand and silt are largely chemically inert, while clay is chemically active (charged)
Mainly because of size, shape, and chemical characteristics of clay
Cations (+) stick to clay particles because of negative charges on the clay

8. How particle size affects soil properties
Specific surface area
CEC
Sorption capacity
Stickiness
Plasticity
Cohesion
Porosity
Water holding cap.
All increase as size decreases
Colloidal Clay Silt Sand
clay

9. How particle size affects soil properties
Soil separates
Property
Clay
Silt
Sand
Size
Specific surface area
Water holding capacity
Stickiness/plasticity
Cation exchange capacity
Chemical sorption
Porosity amount
Pore size
Bulk Density
Puddling
Tillage
<0.002 mm
Very high
High
Small
Low
Hard mm
Moderate
Medium mm
Very low
Large
Easy

10. Field and Laboratory Analyses For Soil Texture (Box 4.1)
Feel method: determines the textural class in the field.
<2-mm soil sample is moistened, kneaded and 'ribboned' between the thumb and fore finger.
With practice, 'experts' can come within a few % of the separates.
For consistence, the same person should do it each time.

11. Laboratory methods are more precise Remove cementing agents that complicate the feel method (organic matter, carbonates, Fe oxides) Soil is mixed with water and the density of the water is measured or the amount of material in suspension is determined after varying time periods.

12. Hydrometer Method
Depends on the relationship between settling velocity and particle diameter (big particles fall faster)
Hydrometer measures the density of liquid: more suspended soil = higher density
As soil particles settle the density decreases
Particles settle according to Stoke's Law.
v = k d2
v = velocity of fall; k = constant (g, liquid properties, etc); d = particle diameter (assume spherical)

13. hydrometer readings are taken at various times to measure liquid density (each graduation = 1 g/L as soil colloids/liter)

14. 10,800 sec
lowest density
(less suspended soil)
40 sec
Highest density
(most suspended soil)

15. Example:
54% sand (40 s)
21% clay (3 h)
How much silt?
Silt = 100 – (21+54)
= 25%
Ck: = 100
What texture?
Sandy Clay Loam

16. Soil Texture Texture influences most other soil properties
Soil texture does not change in nature over a short period of time
Adding organic matter may improve characteristics of soils but not change texture
Large quantities of sand, silt, or clay must be added and thoroughly mixed before the texture significantly changes
Mixing media for potting soil, golf greens, etc. is generally the only time when textural modification is economically feasible

17. Particle Density (Dp) of Soil (or specific gravity)
Mass of a known volume of dry soil solids. Doesn't vary much (for mineral soils) because the minerals that dominate (quartz, feldspar, mica) are silicate minerals
Usually taken as 2650 kg/m3 (2.65 Mg/m3 or 2.65 g/cm3)
Often written as g/cm3 (SI units use kg, Mg and m).
Mg is 1 metric ton or 1000 kg.
Dp = mass of particle/volume of particle

18. Bulk Density of Soil (Db)
Mass of a volume of dry soil.
The volume includes spaces (but not water).
Db is less than particle density
Db ranges from Mg/m3
Common to assume 1.3 Mg/m3

20. Bulk density (“Db”) of soil depends on texture and soil organic matter content. Clays have lower Db values and sands have higher Db values. High organic matter content increases aggregation and porosity and thus decreases Db.
Maintenance of low Db is important for good soil fluid and gaseous exchange, root penetration, and plant growth.
Db increases with increasing depth, compaction, and traffic and decreases with increasing clay and organic matter content.
Manage soil to prevent compaction and decrease Db
Never till when wet!