2.1
Definition and Objective of Irrigation
Irrigation is the artificial supply
of water for plant growth when the natural water supply is inadequate.
Irrigation has been used to control soil and air temperature, insect pest and
excess leaching of soluble salts in the salts in the soil.
The objective of irrigation is to
add the amount of water needed when the plant requires it. Anything less
reduces vegetative growth, and gives poor quality and quantity of yield. Some
factors to be considered in the amount and frequency of irrigation are plant
rooting depth, minimum water potential to be maintained in the root zone, plant
use or consumptive use and availability of adequate irrigation water at any
time it might be required.
2.2 Purpose
of Irrigation
Irrigation may benefit crop growth
in several deferent ways. The reason for irrigation may be any one of these or
combination of several of them.
1. To
supply plant with water as needed to obtain optimum yield and the quality of
harvested product.
2. It serves as an insurance against
drought.
3. It
aids the movement of plants nutrients, plant nutrients are usually absorbed in
solution form to leach undesirable salts, that is to wash them out in solution
example preventing salinity build up.
4. To
control the environment of growing plants and easy application of fertilizer
and pesticides.
Where and when to irrigate depends
on, if the soil is suitable but rainfall is too low for crop production, or
where rainfall is sufficient in quality but badly distributed so that the land
alternates between uncontrolled flooding and draught. Again rainfall may be usually
sufficient in quality and reasonably distributed but not reliable. In this case,
supplement in irrigation water can increase yield and profit (Joe, 2003)
2.3 Toxicities
of Elements
Seven elements referred to as micronutrients
are Cu, Fe, Zn, Mo, B, Cl, Mn. They are also called trace elements or minor
elements they are called trace elements because although they are essential but
are required in very small amount, slightly high concentration is toxic to
plant (Brady and Weil 1999) Table 1.
Table
2.1 Recommended Maximum Concentration of Trace Elements and Heavy Metals in Irrigation
Water.
Elements
|
For water use continuously on all soil (mg/Liter
|
For use up to 20years on time texture soils at pH
6.0 to 8.5 (mg/Liter)
|
Aluminum
|
5.0
|
20
|
Arsenic
|
0.10
|
20
|
Beryllium
|
0.10
|
0.50
|
Boron
|
0.75
|
20.10.0
|
Cadmium
|
0.010
|
0.050
|
Chromium
|
0.10
|
10
|
Cobalt
|
0.050
|
3.0
|
Copper
|
0.20
|
5.00
|
Fluorine
|
1.0
|
15.00
|
Iron
|
5.0
|
0.0
|
Lead
|
5.0
|
10.0
|
Littium
|
2.5
|
2.52
|
Manganese
|
0.20
|
10.0
|
Molybdenum
|
0.010
|
0.050
|
Nickel
|
0.20
|
2.0
|
Selenium
|
0.020
|
0.020
|
Vanadium
|
0.10
|
1.0
|
Zinc
|
2.0
|
10.0
|
Source:
Hoffman and Ayers (1980)
2.4 Sodium
Absorption Ration (SAR)
Sodium absorption ration (SAR) is a
ratio of the sodium versus calcium and magnesium in the irrigation water
(Hoffman et al.1980). The day portion
of soils consists of very small plate like structure stacked liked decks of
cards. Water in soil moves and it enters soils, by flowing between the “stacks”.
The plates are held together primarily by calcium ion and to a lesser degree by
magnesium ions. Replacement of the calcium ion between the plates with sodium
ions tends to force the plates apart and infact to break with sodium ions tends
to force the plates apart and infact to break up the “stack” or deck.
As the stack brake apart, or
disperses, the rate at which water enters the soil (the infiltration) decrease.
In some cases this rate may become very close to Zero. This makes production of
certain crops impractical. The effect does not occur in soils that have no clay
and the size of the effect depends on the amount (and type) of clay in the soils.
The effects are also directly
related to the absolute abundance of all the ions as the EC of water increase a
given SAE becomes less harmful. The mathematical relationship between EC and
the SAR that will result in no reduction in infiltration is SAR (EC x
0.0071)-2-4754 where EC is expressed as us/Cm8 (Hoffman et al. 1980).
The sodium hazard varies with soil
mineralogy. That is generally sanctities, vermiculites, unites, kaolinites and
sesquioxides are of increasing tolerance to sodium hazard because their degree
of expansion is low. If water contain high level of CO3 and Hco3
ions, they will further accentuate the sodium problems because these ions will
precipitate the Ca2+ and MG2+ ions present in the water
and thereby further increase, relatively the sodium Na+ present in the water
and is directly the SAR (Stewart et al
1990). The sodium absorption ratio is give by.
Na+ .
Square
Ca++Mg2+
2
Where
Na+ = concentration of sodium in the water.`
Ca2+
= Total Calcium
Mg2+
= total magnesium
SAR adj
Na+ .
Square
Ca++Mg2+ 1+8.4-PHc
2
Where:
PHC = (PK12 – PK 1c)
+ P(HCO-3) + P(Ca2+ + Mg2+)
(PK12 – PK1c)
Conc of Ca2+Mg2+Na+(Mg/L)
P(HCo3) – Conc of co2 + Hco3
P(Ca2+ + Mg2+) = C3onc
of Ca2+ + Mg2+
Table
2.2 classification of sodicity in irrigation in water
Class
|
Interpretation
|
SAR
(Meg/L)
|
1
|
Low
|
0-10
|
2
|
Medium
|
10-18
|
3
|
High
|
18-26
|
4
|
Very
high
|
>26
|
Source: Ayers and West cost (1976)
2.5 Sodium
Toxicity
Most tree crops such as deciduous
fruits and nuts and other woody type species of plants ate particularly
sensitive to low concentration of sodium. Most annual crops, with some
exception are not as sensitive but may be affected by higher concentrations use
of irrigation water high in sodium usually result in a soil that has high sodium content. But it
may take several irrigations to cause the change. The crop takes up sodium with
the water and it is concentrated in the leaves as water is lost by
transpiration damage (Toxicity) can result if sodium accumulates to
concentration that exceeds the tolerance of crop leaf burn, scorch and dead
tissues along the outside edge of leaves are typical symptoms.
2.6 Boron
Hazard
Boron is one of the essential elements
for plant growth but is needed in relatively small amount. Excessive boron
appears to affect a wide variety of crop while a chloride toxicities are most
common with three crops and woody perennials.
Boron is taken up by the crop and is
accumulated in the leaves and other parts of the plant. Toxicity symptoms
typically show first on older leaf tips and edges as either a yellowing or
spotting in some case is followed by drying which progresses from the tip along
the leaf edges and towards the center between the veins (Interveinal) a
gummosis or exudates affected tree such as almonds.
Many sensitive crops show toxicity
symptom when boron concentration in leaf blades exceeds 250 to 300 ppm (dry
weight). Some crops, however, are sensitive but do not accumulate boron in leaf
blade, stone fruits (peaches, plums, almonds etc) and pane fruits pear, apple
and others) even through being damaged by boron, may not accumulate boron in
leaf tissue to the extent that leaf analysis a reliable text 9Hoffman et al 1980)
Table
2.3 Classification of irrigation water based on boron concentration in relation
to plant tolerance (mg/l)
Classification
|
Sensitive
plant
|
Semi
tolerant plant
|
Tolerant
plants
|
Excellent
|
0.1-0.3
|
Below
0.3-0.6
|
Below
1-0
|
Good
|
0.4-0.6
|
0.7-1.3
|
1.0-2.0
|
Fair
|
0.7-1.0
|
1.4-2.0
|
2.1-3.6
|
poor
|
1.1-1.3
|
2.1-2.5
|
3.1-38
|
Unsuitable
above 1.3
|
Above
1.3
|
Above
25
|
Above
3.8
|
Source:
Ayers and West cost (1976)
2.7 Bicarbonate
hazard
Bicarbonate even at very low
concentration has been a problem primarily when fruit crops or nursery crops
are sprinkler irrigated during periods of very low relative humidity (R/H<30%)
and high evaporation. Under these conditions, white deposits are formed on
fruits or leaves which are not washed off by later irrigation. The deposit
reduces the marketability of fruits and nursery plants. Toxicity is not
involved but as the water on the leaves partially or completely evaporates
between rotations of the sprinkles, the salts are concentrated and Co2
is lost to the atmosphere.
Co-2 and Hco-3
are not in themselves toxic but their presence in irrigation water increase the
Na+ hazard since they bring about the precipitation of Ca2+
and Mg+ from the irrigation water and thus increase the Na+
hazard. Apart form assessing the Hco-3 hazard along with the Na+
hazard in the adjusted SAR, HCo3 hazard can be assessed by the use
of the Residual Sodium Carbonate (RSC) which is given by the formula shown
below
RSC
= (– Ca2+ + Mg2+) meg/L …………………(equ 3)
RSC
= residual sodium carbonate
= Carbonate ion
HCo3-
= Hydrogen Carbonate
Ca2+
= Calcium ion
Mg2+
= Magnesium ion
Table 2.4 Interpretation for RSC value
RSC
|
Class
|
<1.25
|
Safe
|
1.25-2.5
|
Marginal
|
>2.5
|
Not
Suitable
|
Source:
Hoffman and Ayers (1980)
2.8
Salinity Hazard
Salinity hazard refers to total soluble salt in a
water sample it is an important criterion for assessing the suitability of
water for irrigation. Salt concentration of a few tenths or few percent can
hinder plant growth. Salt affects plants by increasing the osmotic pressure of
water making the plant exert more energy to absorb water and it this process
continues the plant will wilt even in the presence of water. Salt content are
measured in micro-ohms per meter. Based on salinity irrigation water can be
classified in to four (Hoffman et al
1980).
Table
2.5 water classification based on salinity
Class
|
Interpretation
|
(EC)Mh.M
|
i.
|
Low
|
100-250
|
ii.
|
Medium
|
250-750
|
iii.
|
High
|
750-2250
|
iv.
|
Very
high
|
>2250
|
Note:
whether given water is suitable for growth depends on the plant or crop in
question, in this regard certain crops are more tolerant to salinity than
others.
2.9 Nutrient
Concentration (N, P, K, Ca, Mg, S)
Accumulations of one or more
nutrients in excess amounts, which may be attributed to the irrigation water,
inhibit the uptake of other essential nutrients, this can alter the plants
mineral nutritional characteristics, crops grown on soils having any imbalance
of calcium and magnesium may also exhibit toxic symptoms. Sulphate salts affect
structure of crop by limiting the uptake of calcium and increasing the
absorption of sodium and potassium resulting in a disturbance in the cation
balance within the plant. The bicarbonate ion in soil solution harms the
mineral nutrition of the plant through its affect on the uptake and metabolism
of nutrients. High concentration of potassium may introduce a magnesium
deficiency and iron chlorosis. An Imbalance of magnesium and potassium may lead
to high calcium levels. Excessive vegetative growth, lodging and delayed crop
maturity may result from excessive nitrogen in irrigation water (Hoffman et al. 1980). Most tree crops and other
wood perennial plants are sensitive to low concentration of chloride while most
annual crops are not though less sensitive crops may be affected at higher
concentration, chloride is not adsorbed by soils but moves readily with soil
water. It is taken up by the roots and moves upward to accumulate in the
leaves. The toxicity symptom for chloride is a leaf burn or drying of leaf tissue
which typically occurs first at the extreme leaf tip of older leaves and
progresses back along the edges as severity increase. Excessive leaf burn is
often accompanied by abnormal early leaf drop and defoliation.
Table 2.6: Laboratory Determination of (N, P, K, Ca, Mg, S)
Needed
to Evaluate common irrigation water quality problems.
Water
Parameters
|
Symbol
|
Usual
range in Irrigation water Me/L
|
Calcium
|
Ca++
|
0-20
|
Magnesium
|
Ma++
|
0-5
|
Potassium
|
K+
|
0-2
|
Sulphate
|
SO-4
|
0-20
|
Nitrate-Nitrogen
|
NO3-N
|
0-10
|
Phosphate-phosphorus
|
PO4-P
|
0-2
|
Source:
Ayers and Hoffman (1980).
Note:
NO3-N means the labor analyze for NO3 but will report the
NO3-N in terms of chemically equivalent nitrogen.
The total nitrogen available to the
plant will be the sum of equivalent elemental nitrogen. The same reporting
method is used for
phosphorus.
Table 6 shows the permissible values of the plant nutrients in irrigation
water.
2.10 Water
pH
Water pH is a measure of the acidity
or alkalinity of water. It is of interest as an indicator but is seldom of any
real importance itself. The main use of pH is a quick evaluation of the
possibility that the water may be abnormal. If an abnormal value is fond, this
should be a warming that the water needs further evaluation. The pH ranges from
1-14, with pH 1-6.9 being acidic and 7.1-14 being alkaline, while pH 7 is
neutral. A change in pH, as from pH 7 to pH 8 represents 9 to 10 fold decrease
in acidity or a 10-f 10 fold increase in alkalinity. The normal range of pH for
irrigation water is from pH 6.5 to pH 8.4. Within this range most crops will
perform well. Irrigation water having pH value outside this range may still be
satisfactory but other problems of nutrition or toxicity would become a suspect
(Stewart and Nelson 1990).
TABLE 2.7: WATER pH VALUE AND THEIR INTERPRETATION
<4.5
|
Extremely
acid
|
4.5
- 50
|
Very
strongly acid
|
5.1
– 5.5
|
Strong
acid
|
5.6
– 6.0
|
Moderately
acid
|
6.1
– 6.5
|
Slightly
acid
|
6.6
– 7.5
|
Neutral
|
7.6
– 7.8
|
Slightly
alkaline
|
7.9
– 8.4
|
Moderately
alkaline
|
8.4
– 9.0
|
Strongly
alkaline
|
>9.0
|
Very
strongly alkaline
|