Physical properties of the soil

1. The physical properties
of soil

2. • Soil color
• Soil Texture
• Soil structure
• Soil density
• Soil moisture
• Soil temperature

3. Significance of physical properties of soils
The soil physical properties profoundly influence how
soil functions in an ecosystem and how they can be
best managed
It influence plant growth by controlling penetration of
plant roots, drainage, aeration, retention of moisture
and availability of plant nutrients
The success and failure of agricultural and engineering
projects is often hinged on the physical properties of
the soil

4. 1. Soil Color
• - it is the property of the soil that depend on the
wavelength of light it reflects or emits.
• Colors are derived from the color of Fe oxides and
organic matter that coat the surfaces of soil particles
high OM – Dark color
High Fe oxides- Red to yellow
Measurable variables in color
1. Hue- most dominant spectral color
2. Value – degree of darkness or lightness (zero is black)
3. Chroma – intensity or brightness (zero is neutral
gray)

6. Hue - The dominant color
• 10 R is red; 2.5YR has some yellow, 7.5YR are tans
and browns, 2.5 Y is yellow, G is green).
• As soils age they oxidize and change from yellow to
brown to red (e.g., 2.5Y to 10YR to 7.5YR to 5 YR to
10R).
Value - The relative darkness or lightness
• from 1 (dark) to 8 (light). The value is often a
function of the amount of humic organic
material in the soil. Darker soils have more
organic material. Very black horizons may be
buried charcoal or accumulations of MnO2

7. . Whiter horizons may be the result of leaching as in an E
horizon, or the accumulation of carbonate or gypsum.
Chroma - The strength or intensity
the color from 0 (least with none of the hue) to 8 (most
vivid). This is indicative of the amount pigmenting material
present, but it is strongly influenced by the texture of the
soil.
• Example: a soil with color 10YR5/6 is 10YR hue, 5 value,
and 6 chroma,

8. Interpreting soil color variations
A- horizon is usually darker than B horizon due to
OM
Dry soils generally have lighter color than moist
soil
Bright colors throughout the profile indicates well
drainage
Gray, bluish green denotes anaerobic condition
Red soils are typical of tropical and subtropical
areas while dark gray and brown are typical of
temperate regions

9. 2. Soil Texture
• It is defined as the relative proportion of sand, silt
and clay in the mineral matter fraction of the soil
• Most permanent physical property
Soil textural class
1. Sandy soils/coarse textured soils/light soils
2. Loamy soils/medium textured
3. Clayey soils/fine textured/heavy soils

10. Name Diameter (mm)
ISSS
Diameter (mm)
USDA
Number of
particles/gram
Surface area
(mm²/g)
Coarse gravel
Fine gravel
Coarse sand
Medium sand
Fine Sand
Very fine sand
Silt
clay
15
2-15
0.2-2
-
0.02-0.2
-
0.002-0.02
≤0.002
-
2-1
1-0.5
0.5-0.25
0.25-0.1
0.1-0.05
0.05-0.002
≤0.002
-
90
722
5777
46,213
722,074
5,776,674
50,260,853,860
-
11.3
22.4
45.4
90.7
226.9
453.7
11,342.5
Table 1. Some characteristics of soil separates

11. Table 2. Generalized influence of soil separates on
some properties and behavior of soils
Property/ Behavior Rating Associated with Soil Separates
Sand Silt Clay
Water holding capacity
Aeration
Drainage rate
Soil OM
Decomposition of OM
Warming up in spring
Compactibility
Susceptibility to wind erosion
Susceptibility to water erosion
Shrink-swell potential
Sealing of ponds/dams
Suitability to tillage after rain
Pollutant leaching potential
Ability to store nutrients
Resistance to pH change
Low
Good
High
Low
Rapid
Rapid
Low
moderate
Low
Very low
Poor
Good
High
Poor
low
Medium to high
Medium
Slow to medium
Medium to high
Medium
Moderate
Medium
high
High
Low
Poor
Medium
Medium
Medium to high
medium
High
Poor
Very slow
High to medium
Slow
Slow
High
Low
Low if aggregated
Moderate to V high
Good
Poor
Low
High
High

12. Importance of soil texture
• The effect of soil texture can be beneficial or
detrimental to plants, usually high clay in the subsoil
is desirable
1. Relative resistance to root penetration – high silt
and clay may retard root growth
2. water holding capacity
sandy – low WHC
clay – high WHC
3. Soil Fertility – clayey is more fertile than sandy
4. Soil aeration – heavy soil (aggregated – high,
Non-aggregated – low)
- sandy- high

13. • Methods of determining soil texture
1. Feel and Roll
2. Sieve
3. Hydrometer
4. pipet

15. • Sand – feels gritty and rough
• Silt – feels floury or powdery
• Clay – feels sticky and plastic

16. Sieve method

17. 3. Hydrometer method

19. 3. Hydrometer method- based on Stoke’s
law
V = kd²
where: k – constant that is relative to
the acceleration due to
gravity and density and
viscosity of water
d – Diameter of particle

20. Stoke’s law
V= h/t; and V = d²g(Ds-Df)/18η
where: g = gravitational force = 9.81 Newtons/kg
η = viscosity of water at 20°C= 1/1000 N-sec/m²
Ds = Density of solid particles, 2.65 x 10³ kg/m³
Df = Density of fluid (water) = 1 x 10³ kg/m³

21. • Therefore:
d² x 9.81 Newtons/kg (2.65 x 10³ kg/m³- 1 x 10³ kg/m³)
V =
18 X 1/1000 N-sec/m²
9.81 N/kg (1.65x 10³ kg/m³) x d²
V =
0.018Ns/m²
V = 16.19 x 10³ N/m³ x d²
0.018Ns/m2

22. (9 x 10⁵) . d²
V=
sm
V = K d²
Calculate the settling time (sec) of silt particles at a
depth of 10 cm. The diameter of silt is 2 x 10⁻⁶ m
Note: V = K d² ; h/t = K d²
Therefore:
t = h/ K d²

23. t = 0.1m/ (9 x 10⁵/sm) (2 x 10⁻⁶ m)²
t = 27,777 sec or 7.72 hrs

24. Using the textural triangle, find the soil texture if
% clay = 30%, % silt = 25% and % sand= 45%

26. • seatwork
Calculate the settling time of smallest
sand particle (0.05 mm) at a depth of 10cm.

27. 3. SOIL STRUCTURE
- refers to the arrangement of soil particles and their
aggregates into certain well defined patterns
Types of soil structure
1. Spheroidal/Granular – soil separates combine to
form small, rounded and loose or porous
aggregates. Most surface soils have this kind of soil
structure.
- This enhances aeration and drainage. Soils rich in
organic matter and calcium assumes this form of
structure

28. • 2. Platy – The soil separates assumes the form of
sheets one on top of the other lying horizontally .
Soils of this structure have poor drainage and root
penetration.
• 3. Blocky – Soil separates combine together to form
cube-like blocks. This kind of structure is generally
found in the subsoil and has something to do with
good soil drainage, aeration and root penetration
a) angular blocky – with distinct and sharp
angles
b) sub-angular blocky – with somewhat rounded
edges

29. • 4. Prism like – soil separates assume a post-like
appearance standing upright
a) columnar – with rounded tops
b) prismatic - with flat tops
• 5. Structureless – soil separates do not assume
any definite form
a) single grained – typical to most sandy soils
b) massive – typical to most lowland rice
soils

31. Factors affecting aggregation
• 1. climate – alternate wetting and drying due to
rainfall causes the soil to expand and contract
allowing the particles to group or orient themselves
into aggregates
• 2. vegetation – aside from the effect of OM on
aggregation, the physical effect of plant roots assists
in the process of aggregate formation
• 3. microbial activity- microorganisms excrete
substances as by-product of metabolism and these
are very useful as cementing materials. Fungi and
other filamentous organisms produce mucilaginous
substances for aggregate formation

32. Implication of desirable and undesirable
structure
• 1. Desirable structural condition – it is the one with
high proportion of medium sized particles, a low bulk
density and an appreciable large amount of large
pores
a) it implies that it is highly permeable to water
b)have satisfactory water infiltration and
retaining capacities
c) readily penetrated by plant roots
d) resist compaction of farm implements

33. • 2. Undesirable structure – with high bulk
density, a few large pores and low content of
water -stable aggregates
a) it implies that permeability of air
and water is slow
b) there is resistance to root
extension
c) anaerobic conditions prevail

34. Density and Pore Spaces
Particle Density (g/cc)– weight per unit volume
of soil particles not including the pore spaces.
 the particle density of most mineral soils
varies from 2.6 to 2.75 g/cc
 the particle density of OM varies from 1.2
to 1.7 g/cc
PD = weight of soil/volume of solids

35. • Particle density increases as the amount of
heavy minerals such as magnetite, limonite,
hematite increases while PD decreases as the
amount of OM increases.
• It is usually determined using the Pycnometer
method

36. • Bulk Density (Apparent density)– ovendry weight of
a unit volume of soil including the pore spaces
BD = ODW/total volume of soil
 The bulk density of uncultivated soil varies from
1.0 to 1.6 g/cc.
 BD increases with compaction
 It helps in estimating the weight of soils per unit
area

37. Porosity – refers to the percentage of soil
volume which is occupied by pore spaces.
% pore space = 100 – (BD/PD x 100)
 Porosity varies with texture, shape of
individual particles, soil structure, amount of
OM an degree of compaction

38. • 1. Calculate the weight of 1 hectare furrow
slice to a depth of 15 cm if the soil BD is 1.4
g/cc. What is the percentage porosity if PD is
2.65g/cc.
2. Fresh soil sample weighs 300 g and has a
volume of 150 cc. If the ovendry weight is
200g, what is BD?

39. Soil Moisture
• Most variable soil component
hydrology – study of the water cycle.

40. The occurrence of water
• Integral part of living cell
• It occurs in great depth in earths crust
• 99% of all the earths water occurs in the
troposphere. It reaches an average height of
15 km
• The average depth to which the water occurs
in the ground is about 3 km and in some
locations about 8 km.

42. Forces holding water in the soil
1. Adhesion – attraction between soil and water
2. Cohesion – attraction between water
molecules

43. • Expression of forces holding water in the soil
1. Height of a unit column of water in cm (h)
2. Bar or atmosphere (atm)
1 bar = 1 atm
1000 cm = 1 bar = 1 atm
3. pF – log of free energy difference measured
on a gravity scale
pF = log h

44. Moisture
constants
Bars/atm Depth (cm) pF
Ovendryness 10,000 10,000,000 7
Hygroscopic
coefficient
- 31,623 4.5
Wilting point 14.8 or 15 15,340 4.18
Field
capacity
1/3 341 2.53
saturation 0.001 1 0

45. Soil moisture measurement
1. Gravimetric method/ovendrying method –
ovendrying a moist soil sample at 105°C for
8-24 hrs.
%water = weight of water/ ODW of soil x 100
2. Tensiometer method – suction tensiometer
3. Gypsum block

46. Soil Moisture Constants
1. Saturation – a soil whose pores are completely
filled with water.
- the water in the soil is at zero tension
2. Field Capacity (1/3 atm)– Percentage of water
after the soil has been saturated and allowed
to drain for 2-3 days.
- macropores is without water but micropores
are filled
- water being used by plants

47.  it is approximately 1/3bar or a pF of 2.53
 soil is best tilled at a moisture of pF 2.8 to
4.4 because the soil is friable and maintains
its small aggregates
 organic soil can be tilled even wetter than pF
2.8 because they have stable aggregates

48. 3. Wilting point –the moisture condition at which
the ease of release of water to the plant roots
is just barely too small to counterbalance the
transpiration losses.
- the wilting point is also called as wilting
coefficient or permanent wilting percentage.
- it is about 15 bars or a pF of 4.18

49. 4. Hygroscopic Coefficient – the moisture
tension in which the soil water is in
equilibrium with an atmosphere of 98 percent
water vapor saturation ( 98% RH)
- this corresponds to pF of 4.5 or 30.5 atm
- note that 100 % Relative humidity has zero
tension, hence soil is saturated with water.

50. 5. Oven dry – soil is oven dry when it has
reached equilibrium with the vapor pressure
of an oven at 105oC
- the oven dry condition corresponds to a
relative humidity of zero or a pF near 7

51. Biological Classification of Soil water
1. Superfluous water – free or drainage water
2. Available water – moisture held between FC
and PWP
3. Unavailable water – moisture held at the
PWP

52. • Physical classification of soil moisture
1. Free (drainage water) –loosely held, < 0.1
atm
2. Capillary water – held between FC and
hygroscopic coefficient
- functions as soil solution and not all are
available to plants
3.Hygroscopic water (vapor water) – held at
hygroscopic coefficient, 31 to 10,000 atm

53. Classification of soil moisture
Kinds of Water pF
Water of constitution and
interlayer water
Hygroscopic water
Capillary water
Gravitational water
Groundwater
Above pF 7
7-4.5
4.5-2.5
2.5 – 0
Tension free

54. Water in relation to plant growth – water
makes up 80% of the weight of green plants
1. Absorption of water – moisture enters plant
roots thru osmosis
osmosis – movement of water through a
semi-permeable membrane cased by
unequal concentration on the two sides.

55. 2. Movement of nutrients in soil moisture –
nutrients dissolved in water usually move with
it
3. Water requirement of crops – water lost
through evaporation and transpiration
(evapotranspiration)
Consumptive use – inches of water lost by
evapotranspiration in the production of a crop

56. Crop Kg of water transpired
per kg dry plant tissue
Corn
Beans
sorghum
349
736
277

57. 4. Water-use efficiency – any practice that
promotes plant growth and photosynthesis to
increase dry matter production increases
water use efficiency

58. 1. After a large soaking rain, a soil was sampled as it dried. The
following weights were obtained:
a) immediately after the rain – 300 g
b) 2 days after the rain - 270g
c)5 days after the rain - 250g
d) when plant growing wilts - 230g
e) when the soil is airdry - 220g
f) when the soil is ovendry - 200g
Calculate the the moisture content (%) at saturation, field
capacity, 5 days, PWP and airdryness. Also find the available
moisture at FC, 5 days, and gravitational water.

59. 2. A sample of soil at field capacity weighs 130 g.
After airdrying it weighs 105 g and after
ovendrying it weighs 100 g. What are the
percentages of capillary and hygroscopic
water.

60. 3. A sample of soil weighs 200 g and contains
15% water by weight. Its volume is 150 cc and
its Particle density is 2.65 g/cc. Calculate the
percentage of the pore space (by volume) that
is filled with water after 30 g of water was
added to the original sample.

61. 4.A soil has the following characteristics:
Hygroscopic coefficient - 8%
Field capacity - 34%
PWP - 14%
Bulk Density - 1.3 g/cc
Considering the first 20 cm of the soil in the field,
calculate the following in centimeters of water
a)Total available water
b) amount of water needed to bring the moisture
content from hygroscopic coefficient to 60%of TAW

62. 5. The following information about a soil were
obtained in the laboratory
BD – 1.19 g/cc
1/3 atm percentage - 40%
15 atm percentage- 25%
If you were asked to irrigate the first 30 cm depth of
the soil
a) calculate the amount of water (in cm) that you
must apply if the field moisture content is 30%
B) If the field is grown to corn and is now wilting
permanently, how much water will you apply to the
soilto bring the MC to FC?