Soil water relationship,properties of soil, porosity, permeability,infiltration, surface tension, soil moisture content, problems.

1. 1
Soil water relationship
It is very much important to obtain the best use of water. The
characteristics of soil water relations :
• Well-drained soils in the field to retain water available for plants
and the flow or movement of water in soils
• The salinity and alkali conditions,
• Translocations and concentration of soluble salts due to the
movement and
• Evaporation of soil water.
Soil water relations depend on the properties of soil.
Presentation-3

2. 2
The properties of soil are
1. Soil Texture
2. Soil Structure
3. Real Specific gravity
4. Apparent Specific gravity
5. Pore Space
6. Infiltration
7. Intake
8. Permeability
9. Depth of soil
10.Plant Food Compounds
11.Excess Soluble Salts
12.Surface Tension
13.Tension Heads
14.Soil Moisture Tension
15.Soil Moisture Content
16.Classes and Availability of Soil Water
17.Filling the Available Soil Water
Reservoir
18.Representative Physical Properties
Presentation-3 (contd.)

3. 3
1. Soil texture:
Soil texture means the properties of soil which determines the
size of grains/particles. These particles range in size from fine
gravel to clay.
particles > 1 mm diameter : gravel
particles from 0.05 to 1.00 mm dia: sand
particles from 0.002 to 0.05 mm dia :silt
particles < 0.002 mm dia :clay
Presentation-3 (contd.)

4. 4
Most of the soil contains a mixture of sand, silt and clay.
If in mixture
 sand particles dominate : the soil is sand
 clay particles dominate : the soil is clay
 silts fall in between clays and sands
 Loams are medium textured soils having about
equal amounts of clay, silt and sand particles.
Presentation-3 (contd.)

5. 5
2. Soil structure
Structure is defined in terms of grade, class and type of aggregates.
The arrangement of the particles within the soil is said to be
structure of soil. The structure of soil indicates the aggregation of
primary soil particles (sand, silt and clay) into compound particles
or cluster of primary particles which are separated by the adjoining
aggregates by surfaces of weakness. Structure modifies the effect
of texture in regard to moisture and air relationships, availability of
nutrients, action of microorganisms and root growth. E.g. a highly
plastic clay (60% clay) is good for crop product if it has a well
developed granular structure which facilitates aeration and water
movement A soil is too wet is said to be puddled and has a poor
structure.. Favorable soil structure is the key to soil fertility.
Presentation-3 (contd.)

6. 6
The steps to maintain and improve the structure of irrigated soils
are as follows:-
 Plow below compacted layers not at the same depth each year,
 Allow as much time as practical for soil and air to interact before
preparing seed bed
 Return all possible organic matter to soil
 Follow a good crop rotation on legumes, cash crops and fibrous –
rooted crops
 Reduce cultivation and tillage operations to a minimum
Presentation-3 (contd.)

7. 7
3. Real specific gravity
The ratio of the density (weight per volume) of a single soil particle
to the density of a volume of water equal to the volume of the
particle of soil.
Density of a single soil particle
Real Specific Gravity, RS =
Density of volume of water equal
to the Volume of the particle of soil
Real Specific Gravity is dimensionless.
The Specific Gravity of the common soil forming minerals varies
from 2.50 to more than 5. The real specific gravity of soil is 2.65 .
Some irrigated soils which are formed largely of organic matter-
have real specific gravity of 1.5 to 2.0 depending on the amount of
mineral matter present.
Presentation-3 (contd.)

8. 8
4. Apparent specific gravity
The ratio of the weight of a given volume of dry soil including air space
to the weight of an equal volume of water
(Weight of a given volume of dry soil including
air space)
Apparent Specific Gravity, AS =
( Weight of an equal volume of water)
This ratio is also called ‘volume weight` or bulk density.
It is necessary to know the apparent specific gravity to
find out the water applied to irrigated soil
Presentation-3 (contd.)

9. 9
5. Pore space/Porosity
The ratio of the apparent specific gravity to the real specific
gravity gives the proportionate space occupied by the soil and
this ratio subtracted from unity gives the pore space. The pore
space is generally represented by the following equation:
The apparent specific gravity (As)
The proportionate space occupied by soil =
The real specific gravity (Rs)
Pore space
Presentation-3 (contd.)

10. 10
The apparent specific gravity
The Pore Space Porosity = 1 – x 100
The real specific gravity
This can be represented as follows
n = (1 – As / Rs) x 100 (1-3)
where,
n = The percentage pore spac
Rs = The Real specific gravity = 2.65 for most agricultural soils
As = The Apparent specific gravity
Presentation-3 (contd.)

11. 11
Porosity
The ratio of the volume of voids (air space and water – filled space)
to the total volume of soil plus water and air. It is equivalent to pore
space and much used in soil mechanics.
Volume of voids (air space and water -filled space)
Porosity =
The total volume of soil plus water and air
6. Infiltration
Percolation of water per unit time is infiltration. It is generally high
rate at the beginning of rain than few hours later. It is influenced
by soil properties and also moisture gradient.
Presentation-3 (contd.)

12. 12
7. Permeability
Permeability is the important soil property and is the ability to convey
water flow through the pore spaces with the given force. Hence,
permeability is the linear movement of water per unit time.
Thus, permeability is the velocity expressed as
Physical dimensions of length
Permeability =
8. Surface tension
The tension of the surface film of a liquid caused by the attraction
of the particles in the surface layer by the bulk of the liquid which
tends to minimize the surface is called surface tension.
Time
Presentation-3 (contd.)

13. 13
The surface tension of water causes suction on water within
the soil. The depth of water sucked due to surface tension is
termed as ‘suction head’ , it is also called ‘tension head’.
Suction head is generally expressed in terms of an
equivalent length of vertical column.
Presentation-3 (contd.)

14. 14
Tension heads/suction head
Consider tension head is ht at α height above the water surface
by an upward force due to surface tension in the water.
ht
α
Water surface
Water in container
2 r (d)
Downward pressure
Fd = π r2 htw
Upward pressure,
Fu = 2 πrT where,
Fd = the total downward pressure
Fu = the total upward pressure
r = radius of pipe
w = specific weight of water
T = surface tension
ht = (2 T) /(wr) (1-4)
Multiply by w in both sides of Eq-2 there results negative soil
pressure or suction p
p = w ht = 2 T/ r (1- 5)
Fu
Fd
Presentation-3 (contd.)

15. 15
The maximum height of rise of water in centimeters is
given approximately by the following equation:
ht = 0.75 /r and ht= 1.5/d (1- 6)
Thus the actual height that water can raise in a soil by capillary
action is usually less than the theoretical heights computed
from the equation(1-4) . Under the average conditions capillary
usually acts freely to 150 centimeters, frequently to 300
centimeters and rarely 10 meters.
Presentation-3 (contd.)

16. 16
Measurement of Soil moisture content
Drive or bore to desired depths with a soil augur or a
soil tube. Remove soil samples of the moist and
place it in cans provided with covers. Then take the
soil samples to a laboratory for weighing and drying.
Samples of 100 or more grams of moist soils are kept
in an oven having a temperature of 1050 C- 1100 C
until the soil is free from moisture.
Thus the yields the moisture percentage on the dry-weight
basis (Pw.)may be computed as
(i) Soil moisture content on the dry –weight basis (Pw)
Loss of weight of soil in drying
(Pw) = × 100
weight of the Water free soil
Presentation-3 (contd.)

17. 17
The measurement of soil moisture content directly is costly.
Volume of water
(iii) Moisture Content(Volume basis), Pv = ×100
Volume of space within the soil
Pv = Pw × AS (1-7)
AS = Apparent Specific gravity
(ii) Soil moisture content on the wet-weight basis (Pw)
Loss of weight of soil in drying
Pw = ×100
Weight of the moist soil
Presentation-3 (contd.)

18. 18
Field capacity range
normally 1/10 - ⅓
Permanent
wilting point
0 5 10 15 20
Soilmoisturecontent,%ofdry
weight
50403020100
Figure 1-1: Typical curves of soil moisture variation with tension
Soil moisture tension, atm
clay
Loam
Sand
Presentation-3 (contd.)

19. 19
Example
Weight of moist Soil = 100 gm, Weight of water free soil = 80 gm.
Estimate soil moisture content (i) dry weight basis and (ii) wet-
weight basis.
Solution:
Loss of weight in drying = (100-80) = 20 gm
20 gm
(i) Dry weight basis moisture content , Pw = × 100 = 25 % Ans
80 gm
20 gm
(ii) Soil moisture content on wet-weight basis, = ×100 = 20 %
100 gm
Presentation-3 (contd.)

20. 20
Moisture content of soil can also be represented as a depth d
obtained by multiplying the percentage volume Pv by the depth of
soil D
Pv× D
d = (1-8)
Eq1 -4 and Eq 1 -5 we get the following:
Pw × AS Pw
d = × D = × AS D (1-9)
100 100
100
Presentation-3 (contd.)