Soil Water Crop Relationship

Soil Constituents
• Mineral Material: Sand, clay and silt
• Organic matter
• Water
• Air
2Chapter-1

Proportions
of
Soil Constituents
45%
5%
30%
20%
MINERALS
OM
Water
Air
3Chapter-1

Mineral Components
• Except in the case of organic soils, most of a soil’s solid
framework consists of mineral particles.
• They are variable in size and composition. They can vary
from small rock particles to colloids.
• The mineral can be raw quartz and other primary
materials – coarse fractions which have not changed from
parent material.
• They can also be silicate clays and iron oxides formed by
the breakdown and weathering of less resistant minerals
as soil formation progressed. These are called secondary
minerals. 4Chapter-1

Mineral Constituents
USDA ISS
ROCKS > 2 mm > 2 mm
SAND 0.05 to 2 mm 0.02 to 2 mm
SILT 0.002 to 0.05 mm 0.002 to 0.02 mm
CLAY < 0.002 mm < 0.002 mm
5Chapter-1

Sand Component
Visible to the Naked Eye and Vary in Size.
They are Gritty when rubbed between Fingers.
Sand Particles do not Adhere to one another and are
therefore not Sticky.
6Chapter-1

Silt and Clay Components
Silt Particles are smaller than sand. The silt particles are too
small to be seen without a microscope. It feels smooth but
not sticky, even when wet.
Clays are the smallest class of mineral particles. They adhere
together to form a sticky mass when wet and form hard
clods when dry.
7Chapter-1

Colloidal Material
The smaller particles (< 0.001 mm) of clay and similar sized
organic particles) have colloidal properties and can be seen
with an electronic microscope.
The colloidal particles have a very large area per unit weight
so there are enough surface charges to which water and ions
can be attracted. These charges make them adhere
together. Humus improves the water holding capacity of the
soil.
8Chapter-1

Soil Water
Quantity of water in a soil as determined by its moisture content
does not give a true indication of the soil ‘wetness’.
A clay soil, which on handling feels dry, can be at the same moisture
content as a sandy soil, which feels wet.
A plant will have less difficulty extracting water from a sandy soil
than from a clay soil at the same moisture content.
There is need for a soil ‘wetness’ which reflects the ease or difficulty
of extraction of water from the soil by the plant.
The Concept of Soil Water Potential is therefore used in
Soil/Plant/Water Relations.
9Chapter-1

Mechanism of Soil Water Movement
The flow of water in any hydraulic system, including the soil-
plant-water system, takes place from a state of higher to one of
lower potential energy.
The steepness of the potential gradient from one point in the
system reflects the ease with which water will flow down the
potential gradient between the points.
Soil-water system is mainly made up of three components:
10Chapter-1

i) Gravitational Potential: Reflects gravitational forces on the soil
water.
ii) Pressure Potential: This is positive when greater than
atmospheric pressure, and negative when below atmospheric.
A negative pressure potential (or tension, or suction) is also known
as the matric potential.
It is characteristic of soil water above a free water surface.
iii) Osmotic Potential: reflects the effect of solutes in soil water, in
the presence of a semi-permeable membrane
The total potential of soil water at a point is the sum of all the
components of potential, which are acting. Note that the movement
of water in the soil is slow, so kinetic energy is neglected. 11Chapter-1

Classes of Soil Water
Water present in the soil may be classified under
three heads:
1. Hygroscopic water: When an oven dried sample
is kept open in the atmosphere, it absorbs some
amount of water from the atmosphere. This is
known as hygroscopic water, and is not capable of
movement by the action of gravity or capillary
forces.
2. Capillary water: Capillary water is that part, in
excess of hygroscopic water, which exists in the
pore space of the soil by molecular attraction.
3. Gravitational water: Gravitational water is that
part in excess of hygroscopic and capillary water
which will move out of the soil if favorable
drainage is provided.
12Water Requirement of Crops

Available Water in the Soil
Saturated
Field Capacity
Wilting Point
Available Water
•Excess water
100% available
•Readily Available Water
•Little reserve available
and plants stressed
0% Available
Oven dry •No water available
13Chapter-1

Availability of Soil Water
Soil moisture is always being subjected to pressure gradients and vapour pressure
differences that cause it to move. Certain moisture contents are of particular
significance, often called soil moisture constants, with regards to irrigation and
agriculture engineering, viz:
Saturation capacity: When all the pores of the soil are filled with water, the soil is said
to be under saturation capacity or maximum water-holding capacity. The tension of
water at saturation capacity is almost zero and it is equal to free water surface.
Field capacity: The field capacity of soil is the moisture content after the drainage of
gravitational water has become very slow and the moisture content has become
relatively stable.
This situation usually exists for one to three days after the soil has been thoroughly
wetted by rain or irrigation.
At field capacity, the large soil pores are filled with air, the micro pores are filled with
water and any further drainage is slow.
The field capacity is the upper limit of available moisture range in soil moisture and
plant relations.

Moisture equivalent:
This is an artificial moisture property of the soil and is used as an index of the
natural properties.
It is the percentage of moisture retained in a small sample of wet soil 1 cm
deep when subjected to a centrifugal force 1000 times as great as gravity,
usually for a period of 30 minutes.
Moisture equivalent is used as a single factor to which the properties of soil
can be related within reasonable limits.
The moisture equivalent roughly equals field capacity for a medium textured
soil. The relation between these are as follows:
Moisture equivalent ≈ Field capacity
= 1.8 to 2 Permanent wilting point
= 2.7 Hygroscopic coefficient

Permanent Wilting Point
Permanent wilting point or the wilting coefficient is that water content at
which plants can no longer extract sufficient water from the soil for its growth.
It is at the lower end of the available moisture range.
If the plant does not get sufficient water to meet its needs, it will wilt
permanently.
Temporary wilting may sometime take place during the hot windy day, but it
will recover in the cooler portion of the day.
A plant is considered to be permanently wilted when it will not recover after
being placed in a saturated atmosphere.
Recent studies indicate that the wilting point is closely indicated by the
moisture retained against a tension of 15 atm.
As an approximation, the permanent wilting % can be estimated by dividing
the field capacity by a factor varying from 2.0 to 2.4, depending upon the
amount of silt in the soil. For most of the soils, the wilting coefficient is about
150% of the hygroscopic water.

Available Moisture:
The difference in water content of the soil between field capacity and
permanent wilting is known as available water or available moisture.
Readily Available Moisture:
It is that portion of the available moisture that is most easily extracted by
plants, and is approximately 75% of the available moisture.

Soil Moisture Deficiency:
Soil moisture deficiency or field moisture deficiency is the water required to bring the
soil moisture content of the soil to its field capacity.
Depth of water stored in root zone.
In order to estimate the depth of water stored in the root zone of soil containing water
up to field capacity, let,
d = depth of root zone (in metres) ; Fc = field capacity (expressed as ratio);
γ = unit weight of soil; and γw = unit weight of water.
Considering unit area (1 sq. metre) of soil area;
This depth of water will be available for evapo-transpiration.
w
c
c
dF
dF
γ
γ
γ
..
areaunitinstoredwaterofDepth
..areaunitinretainedwaterofWeight
=∴
=∴
[ ]tCoefficienWilting-CapacityField
.
depthmoistureAvailable
w
d
γ
γ
=
( )d1
areaunitinretainedwaterofWeight
areaunitofsoilofWeight
areaunitinretainedwaterofWeight
××
=
=
γ
cF

Water requirements of a crop
Every crop requires a certain quantity of water after a certain fixed interval,
throughout its period of growth.
If the natural rain is sufficient and timely so as to satisfy both these
requirements, no irrigation water is required for raising that crop.
In England, for example, the natural rainfall satisfies both these requirements
for practically all crops, and, therefore, irrigation is not significantly needed in
England.
But in a tropical country like Pakistan, the natural rainfall is either insufficient,
or the water does not fall after fixed intervals, as required by the crops.
Since the magnitude as well as the frequency of the rainfall varies throughout
a tropical country, certain crop may require irrigation in certain part of the
country, and the same crop may not require any irrigation in some other part
of the country.

Limiting Soil Moisture Conditions

Depth and Frequency of Irrigation

The area where irrigation is a must for agriculture is called the
arid region, while the area in which inferior crops can be grown
without irrigation is called a semi-arid region.
The term Water requirements of a crop means “the total quantity
and the way in which a crop requires water, from the time it is
sown to the time it is harvested”.
water requirement varies with the crop as well as with the place.
In other words, “different crops will have different water
requirements and the same crop may have different water
requirements at different places of the same country depending
upon the climate, type of soil, method of cultivation, and useful
rainfall, etc”.

Effective Rooting Depth (mm) of Some Crops
Crops Effective Rooting Depth
Fruits 750
Lucerne 1200
Cotton 900
Maize, small grains,
wheat
600
Most Vegetables 300
Source: Hudson’s Field Engineering
24Chapter-1

Example 3.10, p/57, Punmia
Find the field capacity of a soil for the following data:
Root zone depth = 2 m
Existing water content = 5%
Dry density of soil = 1.5 g/cm3
Water applied to the soil = 500 m3
Water loss due to evaporation etc = 10%
Area of plot = 1000 m2

Example 3.11 , p/58, Punmia
After how many days will you supply water to soil (clay loam) in order
to ensure efficient irrigation of the given crop, if:
Field capacity of the soil = 27%
Permanent wilting point = 14%
Density of soil = 1.5 g/ cm3
Efficient depth of root zone = 75 cm
Daily consumptive use of water for the given crop = 11 mm
Solution:
Available moisture = Field capacity – Permanent wilting point
= 27 – 14 = 13%
Let, the Readily available moisture be 80% of the Available moisture.
Therefore, Readily available moisture = 0.8 x 13% = 10.4%

Suitability of Soil for Irrigation
The soil should be carefully studied with regard to the fol1owing:
(a) Size of soil particles
(b) Compactness
(c) Depth
(d) Organic matter content
(e) Position of water table.
All the above aspects influence the depth of available water that the
irrigator can store in the root zone of soil in a single application of
water and hence influence the required frequency of watering.

Crops
The principal crops grown in Pakistan can be grouped as:
1.Food grains, and
2.Non-food grains
Based upon their nature and use, the various types of crops are
given below:
1. Food grains
(a)Cereals : Wheat, Rice, Barley, etc.
(b)Millets : Great millet (Jowar), Spiked millet (Bajra),
Small millets (Chari)
(c)Pulses : Gram, Peas, Tur, Moong, Arhar, Soyabean, etc.

2. Non-food grains
(a)Oilseeds : Groundnut, Sunflower, Coconut, etc.
(b)Fibres : Cotton, Jute
(c)Vegetables : Cauliflower, Brinjal, Potato, Cabbage, etc.
(d)Fodder crops : Barseem, Clover, etc.
(e)Fruit crops : Mango, Apple, Guava, etc
(f)Plantation crops : Sugercane, Rubber, etc.
(g)Spices : Black pepper, Chilles, Ginger, etc.

Crop Classification
(1)According to irrigation requirements
(i) Dry crops: do not require irrigation water, e.g. castoorseed.
(ii) Wet crops: require irrigation water, e.g. cotton, wheat, etc.
(2) According to crop season
(i) Summer crops
(a) Hot weather crops: grown in summer season when
there is no/less rainfall, e.g. fodder crops.
(b) Kharif crops: cultivated from April to September, e.g.
rice, maize, pulses, cotton, etc.
(ii) Rabi (winter) crops: cultivated from October to March (i.e.
from Autumn to Spring), e.g. gram, wheat, peas, potato, vegetables, etc.
(iii) Perennial crops: These require irrigation and take full year
for maturity, e.g. suger cane, fruits, etc. 33Water Requirement of Crops

Some Definitions
Cash crops: Crops, other than food grains, which yield high
monetary return, such as sugar cane, spices, fruits, vegetables,
etc.
Seasonal crops: Crops planted and harvested in the same season.
Two Seasonal crops: Crops planted in one season and matured in
the next same season.

Crops Rotation
• When the same crop is grown again and again in the same field, the fertility
of land gets reduced as the soil becomes deficient in plant foods favorable to
that particular crop.
• In order to enhance the fertility of the land and to make the soil regain its
original structure, it is often found necessary and helpful to give some rest to
the land.
• This can be achieved either by allowing the land to lie fallow without any
cultivation for some time, or to grow crops which do not mainly require
those salts or foods which were mainly required by the earlier grown crop.
• This method of growing different crops in rotation, one after the other, in the
same field, is called Rotation of Crops.
• A cash crop may be followed by a fodder crop, which, in turn, may be
followed by a soil-renovating crop. The cultivators, who are found of
sowing cash crops always, should be educated and made to understand the
advantages of sowing crops in rotation. 35Water Requirement of Crops

• The rotation of crops will help in extracting different foods from the soil,
and thus avoiding the general deficiency of any one particular type.
• Moreover, if only one type of crop is grown in the same field, numerous
insects and pests (of similar nature) will get developed. The crop rotation
will also help in checking such growths.
• Crop rotation will thus help in increasing the fertility of soil and
reducing the diseases and wastage due to insects, and hence increasing
the overall crop yield.
• In general, the following rotations of crops may be adopted depending
upon the soil conditions:
(i) Wheat-Juar-Gram.
(ii) Rice-Gram
(iii) Cotton-Wheat-Gram or Sugarcane
(iv) Cotton-Juar-Gram

Soil Water Crop Relationship

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Soil Water Crop Relationship