INTRODUCTION
Saudi Arabia is an arid country with a total land area of 2.253x106
km2. The total cropped area in the Kingdom increased from 1.25 (1988)
to 1.51 (1992) million hectares (Ministry of Agriculture and
Water, 1992). Consequently, the demand for irrigation supplies showed manifold
increases from 1.75 billion m3 in 1975 to 22.93 billion m3
in 1992 (Dabbagh and Abderrahman, 1997). According to
an estimate, more than 80% of water demand in agriculture sector is currently
being met from non-renewable groundwater sources. To meet the growing demand
of water for domestic, industrial and agriculture sector, exploration of alternative
source of water especially for use in agriculture is important. Abu-Rizaiza
(1999) stated that the Saudi standards for effluent are stringent and unintentionally
impose unnecessary limitations on disposal and reuse of wastewater. He presented
wastewater reuse regulations and existing treatment facilities and argues the
case for a more discriminating set of standards which would allow a variety
of reuses for present wastewater production at different sites.
Quality of irrigation water is determined by its chemical composition and the conditions of use. Because all the waters, surface or sub-surface, contain soluble salts which increases the concentration of the soil solution upon irrigation. The main objective of this review paper is to highlight irrigation water quality criteria and the management strategies for optimizing the use of available water resources for sustainable irrigated agriculture in the Kingdom of Saudi Arabia.
Climate of Saudi Arabia: The Arabian Peninsula is located in an arid
belt extending from Northern Africa through Arabian Peninsula, Iran and Mongolia.
The yearly potential evaporation (Hofuf-3359 mm) is much greater than the yearly
mean rainfall (Hofuf-69.6 mm). High evaporative conditions along with short
irrigation supplies determine the hydrology, land development and vegetation
of the area. High temperature during summer is the most significant climatic
factors of Saudi Arabia. An extreme maximum air-temperature of 51.3°C was
recorded at Hofuf in June 1983. However, in general, the maximum daily air-temperature
often exceeds 45°C and the relative humidity is also very low in summer.
The diurnal variation of the air-temperature is strikingly high and causes the
apparent diurnal variations of relative humidity. Though the overall air-temperature
variation have been observed from -2.6 to 51.3°C but night frosts are rare
(Lin, 1984).
SOIL TYPES OF SAUDI ARABIA
According to the Land Management Department, Ministry of
Agriculture and Water (1985), there are three extensive areas of sand dunes
covering about 40% of the Kingdom’s land. The land has been classified
as below:
The land suitable for agriculture consists mainly of alluvial soil developed
by the weathering of silt stone, shale and sandstone and thoroughly mixed by
the action of wind and water, or deposited in layer of varying textures. Soil
suitable for cultivation contains loamy sand or sandy loam textural classes
with predominantly coarse soil. The soil is usually calcareous and in some cases
contains gypsum. Different soil types have been developed due to the difference
in soil forming processes such as high temperature, aridity and wind erosion.
The chemical weathering of the soil occurs at a slow rate due to low rainfall,
although some areas contain high level of soluble salts. In general, the soils
are low in organic matter (less than 1%), available phosphorous and available
nitrogen and adequate in potassium. The dominant soil types include Entisols,
Inceptisols and Aridisols. The high level of CaCO3 and the coarse
texture of soils advocate adoption of special fertility management practices
for increased land productivity. The chemical characteristics of soils from
some major agricultural production areas in the Kingdom are summarized below
(Table 1).
WATER RESOURCES
There are no perennial streams or rivers in Saudi Arabia, therefore, the main water sources are underground water aquifers, rainfall and seawater desalination. A brief detail of these is summarized below:
Groundwater: The quantity and quality of groundwater depend on the geological
formation of underlying strata, the size of aquifer and the site location. The
groundwater is classified into four hydro-geological zones i.e. confined aquifers,
main aquifers, free flowing aquifers and the springs. Agriculture largely draws
water from the main aquifers. There are in all nine main aquifers (Noory,
1983; Water Atlas of Kingdom of Saudi Arabia, 1985).
The flow rate and water salinity of these aquifers are presented in Table
2.
Mean chemical composition of some selected groundwater is given in Table
3.
| Table 2: |
Salient
features of major aquifers in Saudi Arabia |
 |
| Source: Noory (1983) |
Rainfall: Rainfall which occurs normally between October and April in
most of the Kingdom is highly unpredictable, sporadic and the seasonal variations
are also very high. The mean annual rainfall is 100 mm or less, except in the
mountainous part of the northern region (Asir) where in excess of 500 mm are
not uncommon over some small areas (Lin, 1984).
Desalination: Water demand for various uses increased tremendously over
the last decade due to extensive rural and urban development. To meet the growing
water needs, construction of desalination plants for freshwater supply was inevitable.
The quantity of water produced by the sea water desalination plants in various
regions of Kingdom of Saudi Arabia ranged from 989.31 million cubic meters in
2002 to 1.033 billion cubic meters in 2006 according to Saline
Water Conversion Corporation, Kingdom of Saudi Arabia (2006).
Other resources: The treated and un-treated wastewater is another potential source of supplemental irrigation for expansion of irrigated agriculture, landscape development and establishment of windbreaks along the main highways, oil refineries and potential agriculture farms to minimize sand encroachment.
CRITERIA FOR IRRIGATION WATER QUALITY EVALUATION
Total water salinity: The total water salinity is expressed as Total
Dissolved Solids (TDS) in mg L-1, or the electrical conductivity
in dS m-1. It is the total water salinity which is responsible for
the build up of salts in soils after irrigation. The soil salinity increases
in direct proportion to the salinity of irrigation water and total depth of
water applied. The reduction in plant growth and crop's yield is mainly caused
by the increase in osmotic potential of soil solution which takes place due
to addition of excessive salts through irrigation water. This increase in osmotic
potential reduces the availability of water to plants which results in stunted
growth and significant yield losses.
The reduction in plant growth and yield is almost proportional to the concentration
of the salts in soil solution in and around root zone. Since 70-90% of water
uptake by plants takes place in the upper active root zone (50%), hence the
effect of soil salinity on plant growth and performance might be less than as
predicted by earlier investigations which were carried out either in sand culture
or under concept of uniform soil salinity in plant root zone. Bernstein
and Francois (1963), Rhoades et al. (1973)
and Rhoades (1974) stated that fairly high slat concentration
in the lower root zone and with much lower salt concentration in the upper root
zone did not reduce crops yield significantly. Therefore, the previous salt
tolerance limits of crops (US Salinity Lab. Staff, 1954)
can be revised and raised substantially for utilization of more mineralized
irrigation waters.
Sodium Adsorption Ratio (SAR): The relative proportion of sodium (Na+) to calcium (Ca+2) and magnesium (Mg+2) is expressed as Sodium-Adsorption-Ratio (SAR), or the adj.SAR or the adj.RNa ratio. These criteria evaluate the sodium hazards of the irrigation water on becoming soil solution after irrigation. The effects of sodium are manifold such as specific nature of plant toxicity, impaired soil drainage and plant nutrition imbalance. The mechanism involved is that the high sodium contents of irrigation water increases the exchangeable sodium contents on soil exchange complex and disperses the soil more rapidly. The dispersed soil particles seal the soil macro-pores and reduce the soil permeability. Under these conditions, the water availability to plant is also reduced appreciably.
| • |
The
Sodium Adsorption Ratio (SAR) is calculated as below: |
SAR = Na/[(Ca + Mg)/2]1/2
|
| • |
The
Adjusted Sodium Adsorption Ratio (adj.SAR) is calculated as below (Ayers
and Westcot, 1985): |
adj.SAR = SARiw [1 + (8.4-pHc]
|
The adj.SAR takes into account the effect of Na and CO3+HCO3
of irrigation water on soil properties. This concept was developed by Bower
and Massland (1963) and has been found very useful for predicting the effect
of sodium hazard of irrigation water on soil properties.
An alternate procedure discussed in the following paragraphs, takes a new look at the older SAR equation and adjusts the calcium concentration of the irrigation water to expected equilibrium value following an irrigation and concludes the effects of carbon dioxide (CO2), bicarbonate (HCO3) and the salinity (ECw) upon the calcium initially present in the applied water but now a part of the soil water. The procedure assumes a soil source of calcium- from soil lime (CaCO3) or other soil minerals such as silicates- and no precipitation of magnesium.
The new term for this is the Adjusted Sodium Adsorption Ratio (adj.RNa)
and can be used to predict more correctly potential infiltration problems due
to relatively high sodium (or low calcium) in irrigation supplies (Suarez,
1981; Rhoades, 1984) and can be substituted for
simple SAR. The equation for calculation of adj.RNa of surface soil
is very similar to the older SAR equation and is (Suarez,
1981):
adj. RNa = Na/[(Cax + Mg)/2]1/2
|
Unlike salinity, the effect of sodium hazard of irrigation water on plant growth
and yield is indirect. The concept regarding sodium hazard from irrigation waters
developed by Bower and Massland (1963) is being used
to predict the effect of sodium hazard on soil properties which in turn affect
plant growth and yield. It has been established that if exchangeable sodium
percentage (ESP) of soil exceeds 15, the soil permeability decreases correspondingly.
In addition to the above, excess of soluble sodium in soil solution directly
affects plant growth and yield of some sodium sensitive plants such as trees,
crops and woody perennial plants.
Residual Sodium Carbonate (RSC): The residual-sodium-carbonate (Na2CO3) in waters is determined as the excess of CO3-2 and HCO3¯ over that of Ca+2 and Mg+2 concentration in the irrigation water when expressed in milli-equivalents per liter. It gives an over view of the soil permeability problems irrigated with high sodium waters than the normal irrigation waters.
Boron concentration: The boron (B) concentration is expressed in mg L-1 in the irrigation waters to determine its toxic limits. The concentration of boron in excess of 1 mg L-1 will cause crop damage (Ayers and Westcot, 1985).
Toxic effects of specific ions: Specific ion toxicity normally results
when certain ions are taken up with the soil-water and accumulate in the leaves
during water transpiration to an extent that results in damage to plants. The
magnitude of damage depends upon time, concentration, crop sensitivity and crop
water use and if damage is severe enough, crop yield is reduced. The usual toxic
ions in irrigation water are chloride, sodium and boron. The toxic symptoms
appear in the form of leaf burn, scorch, dead tissues along the side of the
leaf, drying of leaves, yellowing of leaf and spotting on the leaf etc.
Chloride: The most common toxicity is from chloride in the irrigation water. Chloride is not absorbed or held back by soils, therefore, it moves readily with the soil-water, is taken up by the crops, moves in the transpiration stream and accumulates in the leaves. If the chloride concentration in the leaves exceeds the tolerance of the crop, injury symptoms develop such as leaf burn or drying of the leaf tissue. The limits for chloride toxicity for some fruit crop cultivars are given by Ayers and Westcot (1985).
Sodium (Na): Sodium toxicity is not as easily diagnosed as chloride toxicity, but clear cases of the former have been recorded as a result of relatively high sodium concentrations in the water (high Na or SAR). Typical toxicity symptoms are leaf burn, scorch and dead tissue along the outside edges of leaves in contrast to symptoms of chloride toxicity which normally occur initially at the extreme leaf tip. Sensitive crops include deciduous fruits, nuts, citrus, avocados and beans, but there are many others. For tree crops, sodium in the leaf tissue in excess of 0.25-0.50 percent (dry weight basis) is often associated with sodium toxicity (Ayers and Westcot, 1985).
Boron: Boron, unlike sodium, is as essential element for plant growth. It is required in relatively small amounts. However, if present in amounts appreciably greater than needed, it becomes toxic. For example, for some crops, if the permissible limit is 0.2 mg L-1 boron in irrigation water, then the concentration of 1-2 mg L-1 may be toxic (Ayers and Westcot, 1985).
Soil salinity development (SSD) was calculated according to FAO
(1985) and the ESP was predicted according to USDA, 1954 as below:
where, SAR is the SAR of the soil solution resulting from irrigation with groundwater.
The salinity and sodicity hazards of the irrigation waters were determined according
to the classification given by USDA Handbook No. 60, 1954.
REVIEW OF IRRIGATION WATER CLASSIFICATION SCHEMES
Irrigation water quality classification is important for optimizing the use of available water resources. The irrigation water quality depends on a number of factors for its successful application and beneficial uses. These factors include soil type, crop selection, climatic conditions, irrigation methods adopted, drainage conditions of the area, fertilizer use, farm management practices followed and irrigation supplies.
A number of irrigation water quality classification schemes have been proposed
by many researchers dealing with irrigation management. Some have proposed classification
schemes based on two or more factors, usually total salinity and the relative
amount of sodium. In some instances boron and residual carbonate have been included.
Scofield (1933) and Eaton (1942)
indicated that waters with a sodium percentage of 70 or more were generally
unsuitable for irrigation. Scofield (1936) published
a table of permissible limits for irrigation waters which include consideration
of chloride and sulfates in addition to salinity and sodium percentage (Table
4).
Wilcox and Magistad (1943) adopted more simplified classification as given in Table 5.
Wilcox (1948) suggested a different classification as
diagramed in Fig. 1. The main difference in this scheme and
previous classifications is that waters having a low salt concentration in conjunction
with a high sodium percentage (60-90%) could be rated as excellent.
Wilcox (1948) also presented a table of permissible
limits similar to that given by Scofield (1933) except
that it did not contain limits for Cl and SO4.
| Table 4: |
Permissible
limits for classes of irrigation water (Scofield,
1936) |
 |
|
| Table 5: |
Standards
for irrigation water (Wilcox and Magistad, 1943) |
 |
Class 1: Excellent to good, suitable for most plants under
most conditions. Class 2: Good to injurious, probably harmful to more sensitive
crops. Class 3: Injurious to unsatisfactory, probably harmful to most crops
and unsatisfactory for all the most tolerant crops. If a water falls in
class 3 on any basis, i.e., electrical conductivity, salt content, it should
be classed as unsuitable under most conditions. Where the salt present are
mostly sulfates, the value for salt content in each class can be raised
50% |
|
| Fig. 1: |
Water
classification according to Wilcox (1948) [Redrawn] |
|
| Fig. 3: |
Water
classification Scheme by USDA (1954) |
| Table 6: |
Permissible
limits of boron for several classes of irrigation waters (Scofield,
1936) |
 |
|
In a study of Utah waters, Thorne and Thorne (1951)
presented a diagram similar to Wilcox's as given in Fig. 2.
The diagram recognized two main criteria i.e., electrical conductivity with
a rating number, 1 to 5 and the sodium effects with a letter, A to E. This diagram
gives a rating to water having a low electrical conductivity (above 60%).
The US Salinity Laboratory (1954) published a semi-logarithmic
diagram in Handbook 60, that rates waters with respect to total salinity (EC)
and sodium hazard and the sodium adsorption ratio (SAR) as presented in Fig.
3. This classification scheme has been used extensively since its publication.
The permissible limits for boron are shown in Table 6 (Scofield,
1936).
With respect to residual sodium carbonate, Eaton (1950)
said that waters with more than 2.5 meq L-1 of residual sodium carbonate
are not suitable for irrigation purposes. Waters containing 1.25 to 2.5 meq
L-1 are marginal and those containing less than 1.25 meq L-1
are probably safe. Wilcox (1948) discussed four factors
affecting irrigation water quality; salinity, sodium, boron and sodium carbonate.
His limits for electrical conductivity are the same as that of the Salinity
laboratory classification, but he presented another sodium diagram, Fig.
4, for classifying water with respect to the sodium hazard, S1 to S4.
Doneen (1954) presented a classification scheme based
on effective salinity expressed in meq L-1. He defined effective
salinity as the total salinity less that of CaCO3, Ca(HCO3)2,
MgCO3, Mg(HCO3)2 and CaSO4 subtract
in that order. Doneen's tentative classification of irrigation water based on
effective salinity and soil conditions is given in Table 7.
Christiansen and Olsen (1973) proposed a procedure for
evaluating the quality of water for irrigation that delineated seven factors
that should be considered: EC, Na%, SAR, Na2CO3, Cl, effective
salinity (ES) and boron. This evaluation for irrigation water with some modification
of the limiting values is given in Table 8.
| Table 7: |
Tentative classification of effective salinity of irrigation
water (Doneen, 1954) |
 |
Ayers and Westcot (1985) suggested the
following water quality standards for agriculture use (Table
9).
| Table 9: |
Guidelines for Interpretation of water quality for Irrigation
(FAO, 1985) |
 |
1: ECw means Electrical conductivity of irrigation
water at 25°C. 2: SAR means sodium adsorption ratio. 3: NO3-N
means nitrate nitrogen reported in terms of elemental nitrogen |
| Table 9: |
Continue |
 |
|
Tipton and Kalmbach Inc. Engineers (1965) proposed the
following irrigation water standards for upper Rachna Doab area in Pakistan:
Tipton and Kalmbach Inc. (1969) proposed the following
water quality standards for the generally permeable sandy soil of the Lower
Thal Doab area in Pakistan.
Hunting Technical Services and Macdonal and Partners (1964)
suggested that irrigation water having total salt contents less than 750 ppm
may be directly used for irrigation. Water in the range of 750 to 1500 ppm has
to be mixed with canal water (TDS = 150-200 mg L-1).
Hamid et al. (1966) reported that water with
electrical conductivity less than 1500 micromhos/cm is safe for irrigation.
Water having electrical conductivity between 1500 to 3000 micromhos/cm requires
mixing with canal water before being used for irrigation. Water with electrical
conductivity exceeding 3000 micromohs cm-1 requires corresponding
higher dilution with canal water and can be used only to irrigate salt tolerant
crops.
Hunting Technical Services proposed the following standards for irrigation water (1964):
They further established that if the TDS and SAR exceed the safe limits, the
water still can be rendered usable by mixing with canal supplies in the following
proportions:
The Land Reclamation Directorate Lahore, Pakistan proposed the following classification
scheme for irrigation water:
The limit of critical values as prescribed above would eliminate most of the
sodium hazards potentially present in the groups of water listed above, In case
it is intended to use water of inferior quality, the judicious use of gypsum
will be required constantly.
The Mona Reclamation and Experimental Project (MREP) Bhalwal Sargodha, Pakistan proposed the following scheme for the use of poor quality waters:
PROBLEMS OF EFFICIENT WATER USE
Water losses: Loss of irrigation water through evaporation from irrigation
systems (for example: open irrigation canals, sub-irrigation channels and drains
in Al-Ahsa Oasis in particular and around the world as a whole) and the deep
percolation due to over irrigation of open field crops by flood irrigation is
a major problem. This water loss is defined as that portion of applied water
which is not used beneficially and is also not accounted towards agricultural
production. For an example, take the case of Al-Ahsa Oasis, the water losses
from irrigation canals were estimated using Class A-pan evaporation data in
the area and were 3.75 to 4.53x106 m3 per year according
to Lin (1984) (Fig. 5). If the total
surface area for all the drainage canals is also considered then the total evaporation
losses of water will be two times to that of irrigation canals. Correspondingly,
based on the consumptive use of different crops, the total crop water requirements
ranged from 26.65 to 137.82x106 m3 for the period from
1974-83 (Fig. 5).
Al-Ajaji (1985) found mean actual deep percolation (ADP)
losses up to 58% of the actual irrigation water applied and is the main source
of excessive drainage flow in Al-Ahsa drainage canals while conducting a basin
irrigation evaluation study in alfalfa field (Fig. 6).
|
| Fig. 5: |
Class
A-Pan Evaporation in Al-Ahsa Oasis (Lin, 1984) |
|
| Fig. 6: |
Deep
percolation losses from alfalfa field in Al-Ahsa Oasis. ADP as % of total
volume applied (Source: Al-Ajaji, 1985) |
This was also very well supported by the high negative correlation between
Actual Deep Percolation (ADP) and application efficiency of low quarter (AELQ
r = 0.86). He further concluded that ADP is related mainly to basin length (L),
average depth applied (ADA) and the ratio between soil moisture deficit (SMD)
and management allowed deficit (MAD) by the following equation:
ADP = 69.20-0.77 L = 7.15 ADA-78.13 SMD/MAD
|
Because this equation has high statistical significance and a high coefficient of determination value of 99.9 and 0.91, respectively.
RESALINIZATION OF AGRICULTURAL LANDS
Resalinization of agricultural lands is a serious problem in an arid environment
due to extremely high evaporative conditions, low rainfall and the use of marginal
quality water for crop irrigation. This could be the result of many factors
such as climate, geology and configuration of the terrain which determine soil
properties, land drainage, soil, water and crop management practices adopted
(El-Khatib, 1974). In an arid climate, where evapo-transpiration
exceeds rainfall, the magnitude of soil resalinization process is of very high
order. The problem of resalinization is world wide (United
Nations, 1977). The countries of the Middle and the Near East suffer from
this problem to a greater extent. There are signs of soil resalination in the
Middle East countries but no statistics is available. However, according to
Vidal (1951), the arable land in Al-Ahsa Oasis was estimated
around 16,000 hectares. But during the last 15 years, this area decreased to
8,000 hectares because of resalinization due to over-irrigation and poor field
drainage conditions (HIDA, 1984).
INADEQUATE LAND DRAINAGE
Poor soil permeability is a one of the major factors limiting the use of marginal
quality water for agricultural expansion. Because it will reduce water infiltration
rate of soil and create a temporary water ponds (a perched water table condition)
below the soil surface. This will not only restrict salt leaching but also is
a main source of soil resalinization. Furthermore, high evaporative conditions
will increase salt movement from lower soil depths to the surface layer by capillary
action (HIDA, 1984).
UNSCIENTIFIC USE OF IRRIGATION WATER
Efficient use of irrigation water depends on soil type, water quality, irrigation methods adopted, planting methods, irrigation scheduling, amount and intensity of rainfall, crop selection and crop water requirements. Inadequate information and lack of awareness among the farming community on the use of water causes over or under irrigation of crops in some cases. This has resulted in land degradation especially when the depth to hard layer (clay pan) is shallow. Such typical situation of shallow hard layer along with soil resalinization is clearly evidenced in the Al-Ahsa Oasis, Kingdom of Saudi Arabia. The once potentially irrigated agricultural land has decreased from 25 thousand hectares to around 10 thousand hectares in the whole oasis due to resalinization due to poor drainage conditions.
WATER USE EFFICIENCY AND NITROGEN REQUIREMENTS OF CROPS
Estimation of water use efficiency (WUE) and crop water requirements is a key
element for efficient water management. The WUE of wheat ranged between 2.67
and 12.24 kg/ha/mm (well water) and 4.29 and 12.67 kg/ha/mm (aquaculture effluent).
I t was found that application of 150-225 kg N ha-1 for well water
irrigation and 75-150 kg N ha-1 for aquaculture effluent containing
40 mg N L-1 would be sufficient to obtain optimum grain yield and
higher WUE of wheat in Saudi Arabia (Al-Jaloud et al.,
1993a, b). However, a very little work is accomplished
on consumptive use of water for different crops under the hot climatic conditions
of Saudi Arabia (Aziz et al., 1983). Al-Ghamdi
et al. (1991) conducted a trial on the effect of intervals of irrigation
on yield, yield components and WUE of sunflower on a sandy-loam soil. The mean
values for WUE were 5.19, 5.09 and 3.95 (kg seed/ha/mm) for 1986-87 crop season
and 5.79, 5.33 and 3.87 (kg seed/ha/mm) for 1987-88 crop season corresponding
to I-1 (40% depletion of available water), I-2 (60% depletion of available water)
and I-3 (80% depletion of available water) treatments, respectively. In conclusion,
an irrigation interval of 10-days (equivalent to 60% depletion of available
water) is optimum for reasonable sunflower production in Al-Ahsa, Saudi Arabia.
PREVIOUS RESEARCH ON BIO-SALINE AGRICULTURE
Al-Zarah (2008) analyzed 101 well water samples from
Al-Ahsa Oasis. The EC of groundwater ranged between 1.23 and 5.05 dS m-1.
Sodium was the most abundant cation followed by Ca, Mg and K in descending order.
Chloride was the most abundant anion followed by SO4 and HCO3
in groundwater of Al-Ahsa Oasis. A significant correlation was found between
Na and Cl (R2 = 0.936). Thermodynamics calculation revealed that
an appreciable amount of Ca and Mg is associated with Cl and SO4
ions. The SAR and ESP values are within the permissible limits according to
US Salinity Lab. (1954). The NO3 concentration
is within safe limits for drinking purpose according to WHO
(1984) standards. The saturation indices (SI) indicated that groundwater
is under-saturated (negative SI) with respect to certain minerals (for example:
calcite, dolomite, gypsum, anhydrite, halite, pyrite, fluorite and aragonite)
and oversaturated (positive SI) with respect to some other minerals (For example:
goethite, siderite and hematite). The negative saturation index (SI) reveals
that most of minerals are in un-saturated state and will dissolve more Ca and
Mg into the soil solution after irrigation. A good relationship exists between
Cl and other ions (Na, Ca and Mg) as well as between SO4 and Ca and
Mg ion of groundwater. The salinity and sodicity hazards of groundwater of Al-Ahsa
Oasis were classified as C3S1 and C4S2 i.e., high salinity with medium sodicity
problems. The predicted soil salinity suggested that 15-20% leaching requirement
should be applied to keep soil salinity within permissible limits. Also, cultivation
of slight to moderate salt and sodium tolerant crops is recommended for optimal
agricultural production and efficient water use.
SALINE IRRIGATION
Presently, due to inadequate freshwater irrigation supplies, the use of marginal
quality waters is inevitable for agricultural expansion. Hussain
et al. (1994) found that the survival period of trees decreased significantly
with increase in soil salinity resulting from irrigation water salinity (Table
10). The survival period of Prosopis juliflora was significantly more than
Casuarina equisetifolia and Eucalyptus camaldulensis. Also, the tree biomass
decreased significantly with increase in soil salinity (Table
11). Soil salinity and sodicity increased significantly with an increase
in irrigation water salinity and sodicity (Table 12, 13).
Prosopis juliflora tolerated soil salinity (Ece) up to 38.3 dS m-1
with irrigation water salinity of 13.5 dS m-1, Casuarina equisetifolia
up to 27.6 dS m-1 with irrigation water salinity of 6.6 dS m-1
and Eucalyptus camaldulensis up to 15.2 dS m-1 with irrigation water
salinity of 2.12 dS m-1 for proper establishment provided 15% extra
water is applied as leaching requirement to control soil salinity. The experiment
proved the sequence in salt tolerance for different trees as prosopis >casuarina
>eucalyptus.
Hussain et al. (1995) reported mean greenmatter
yield of 56.72 g (Hassawi), 54.29 g (Supreme) and 59.91 g (CUF-101) per pot
with total water salinity of 7.8 dS m-1. Nabulsi
et al. (1994) stated that drainage water salinity reduced straw yield
from 666-460 g m2, plant height from 0.88-0.73 m, tillers per plant
from 8.2-5.3, kernel yield from 546-201 g m-2, spikes from 336-251
m-2 and 100 kernel weight from 4.2-2.5 g. Generally, the application
of 150 and 200 kg N ha-1 significantly improved crop performance
and yield under salinity stress.
| Table 10: |
Effect
of saline water on survival period (weeks) of windbreak trees |
 |
| Data in a column followed by the same letter do
not differ significantly by LSD (p = 0.05). SP: Survival Period. Source:
Hussain et al. (1994) |
| Table 11: |
Effect
of saline water of tree biomass yield |
 |
| Data in a column followed by the same letter do not differ
significantly by LSD (p = 0.05). Source: Hussain et
al. (1994) |
| Table 12: |
Effect
of water salinity on soil salinity |
 |
| Data within a column followed by the same letter do not differ
significantly by LSD (p =0.05). Source: Hussain et
al. (1994) |
| Table 13: |
Effect
of sar of waters on SAR of soils |
 |
| Data within a column followed by the same letter do not differ
significantly by LSD (p = 0.05). Source: Hussain et
al. (1994) |
Helalia et al. (1996) found that alfalfa produced good growth and drymatter yield under both the canal
(3.2-3.4 dS m-1) and mixed canal and drainage (6.4-8.6 dS m-1)
irrigation waters. Whereas, irrigation with drainage water (10.2-15.6 dS m-1)
reduced the yield significantly especially at biweekly irrigation frequency
and at low nitrogen and phosphorus levels.
AQUACULTURE EFFLUENT
A field experiment was carried out on sandy-clay-loam soil on the use of aquaculture
effluent as a supplemental source of nitrogen fertilize to wheat crop (Al-Jaloud
et al., 1993a,b). The mean ranges for different
crop growth parameters under various fertilizer treatments were plant height
between 51.9 and 74.8 cm (well water) and between 60.7 and 79.0 cm (aquaculture
effluent); greenmatter yield between 6.9 and 22.8 Mg ha-1 (well water)
and 9.6 and 25.1 Mg ha-1 (aquaculture effluent); drymatter yield
between 1.71 and 4.53 Mg ha-1 (well water) and 2.28 and 4.89 Mg ha-1
(aquaculture effluent); total biomass between 5.5 and 18.7 Mg ha-1
(well water) and 7.1 and 18.8 Mg ha-1 (aquaculture effluent); grain
yield between 1.68 and 7.70 Mg ha-1 (well water) and 2.70 and 7.97
Mg ha-1 (aquaculture effluent), straw yield between 3.8 and 11.0
Mg ha-1 (well water) and between 4.4 and 10.9 Mg ha-1
(aquaculture effluent); and the number of tillers per plant from 2.06 to 4.63
(well water) and from 3.19 to 5.81 (aquaculture effluent). Overall, the results
obtained with 25 to 50% nitrogen application under aquaculture effluent irrigation
were comparable with those obtained with 75 and 100% nitrogen application under
well water irrigation. In conclusion, a 50% saving in nitrogen application as
an inorganic fertilizer can easily be achieved if crops are irrigated with aquaculture
effluent containing around 40 mg N L-1.
Hussain and Al-Jaloud (1995) observed that water use
efficiency (WUE) for grain yield in 1991-1992 was 2.67-12.24 kg/ha/mm (well
water) and 4.29-12.67 kg/ha/mm (aquaculture effluent). Whereas, the WUE based
on grain yield in 1992-1993 was 1.22 kg/ha/mm (well water) and 3.40-9.21 kg/ha/mm
(aquaculture effluent). The WUE, obtained in T-4 and T-5 irrigated with well
water and receiving 75 and 100% nitrogen requirements, respectively was comparable
with T-4 and T-5 irrigated with aquaculture effluent and receiving 25 and 50%
nitrogen requirements, respectively. It was, therefore, concluded that application
of 150-225 kg N ha-1 for well water irrigation and 75 and 150 kg
N ha-1 for aquaculture effluent containing 40 mg L-1 would
be sufficient to obtain higher WUE of wheat in Saudi Arabia.
Al-Jaloud et al. (1996) in a field experiment
on canola found the mean plant height range of 1.20-1.40 m (well water) and
1.40-1.57 m (aquaculture effluent) in different fertilizer treatments. Mean
biomass yield for canola ranged between 14.60-17.84 Mg ha-1 (well
water) and 12.70-20.74 Mg ha-1 (aquaculture effluent). The mean seed
yield for canola varied from 2.65-3.44 Mg ha-1 (well water) and 3.02-3.74
Mg ha-1 (aquaculture effluent): and for rapeseed from 2.73-3.26 Mg
ha-1 (well water) and from 2.62-3.29 Mg ha-1 (aquaculture
effluent). The mean oil contents for canola were 30.92-34.53% (aquaculture effluent)
and 32.47-35.78% (well water): and for rapeseed from 30.15-34.53% (well water)
and 33.50-35.96% (aquaculture effluent). Application of 175 kg N ha-1
with 50 kg P ha-1 showed significant effect on crop yield under both
types of irrigation water. Based on the results of this study, it appears that
cultivars of rapeseed recently introduced from Canada have an excellent potential
as oilseed crops in Saudi Arabia.
USE OF TREATED MUNICIPAL WASTEWATER
Hussain et al. (1996) investigated the yield
and Nitrogen Use Efficiency (NUE) of wheat under field conditions with two types
of irrigation waters with and without nitrogen application on a sandy-loam to
sandy soil. Depending upon different nitrogen treatments, the mean crop yield
ranges in 1992-93 were: grain yield 6.19-6.87 Mg ha-1 and biomass
15.41-16.34 Mg ha-1 receiving treated effluent. The mean crop yield
ranged in 1993-94 were: grain yield 0.46-3.23 Mg ha-1 (well water)
and 5.20-6.54 Mg ha-1 (treated effluent): and biomass 1.84-10.80
Mg ha-1 (well water) and 16.00-19.29 Mg ha-1 (treated
effluent). The NUE for grain yield in 1992-93 was between 16.70-50.23 kg kg-1
N (well water) and 20.65-91.56 kg kg-1 N (treated effluent). Whereas,
the NUE in 1993-94 varied between 10.49-32.13 kg grain kg-1 N (well
water) and 21.30-72.93 kg grain kg-1 N (treated effluent). The NUE
for total biomass in 192-93 varied between 46.54-130.32 kg kg-1 N
(well water) and 53.66-158.77 kg kg-1 N (treated effluent). Similarly,
the NUE in 1993-94 varied between 35.99-102.10 kg biomassg-1 N (well
water) and 59.27-161.89 kg biomass kg-1 N (treated effluent). It
was concluded that a higher grain yield and NUE of wheat crop can be achieved
with low application of nitrogen if the crop is irrigated with treated effluent
containing nitrogen in the range of 20 mg L-1.
Alshammary et al. (2008) conducted a field experiment
to evaluate the growth response, proline and mineral content of four warm-season
turfgrasses to saline water irrigation. Four salinity treatments were imposed
on sandy soil by irrigation with waters at 2.0, 6.25, 12.5 and 18.8 dS m-1.
The local Bermuda grass, Tifgreen Bermuda grass, Nagisa zoysia grass and salt
grass experienced a 25% shoot growth reduction at 7.9, 20.5, 10.2 and 26.0 dS
m-1, respectively. Although shoot Na+ and Cl¯ contents
increased linearly with increasing salinity for all species, the extent of increase
ranked as: local Bermuda grass >Nagisa >Tifgreen >Salt grass. Sodium
and Cl exclusion likely contributed to the superior salinity tolerance of salt
grass and Tifgreen.
| Table 14: |
Effect
of irrigation water salinity on fresh biomass and fresh root weight of
plants (kg plant-1) |
 |
| Values in a row followed by the same letter are not significantly
different by LSD0.05 (p = 0.05). Source: Alshammary
et al. (2008) |
The experiment demonstrated that at 2.5 dS m-1 irrigation water
salinity, Tifgreen, local Bermuda grass and Nagisa all performed very well in
Saudi Arabia (Table 14). At 6.25 to 12.5 dS m-1
salinity, Tifgreen exhibited better turf quality than local Bermuda grass and
Nagisa and salt grass. However, at the highest salinity (18.8 dS m-1),
only salt grass and Tifgreen showed acceptable turf quality which was significantly
higher than the local bermuda grass and Nagisa.
Alshammary (2008) conducted a field experiment to determine
the growth characteristics and mineral composition of two local halophytes (Atriplex
halimus and Salvadora persica) under saline irrigation at Kind Abdulaziz City
for Science and Technology (KACST), Research Station Al-Muzahmyia, Riyadh. The
experiment treatments were one soil ( sandy), four irrigation waters of different
salinities (2000, 8000, 12000 and 16000 mg L-1 TDS), two halophytes
(Salvadora persica and Atriplex halimus) and one irrigation level (irrigation
at 50% depletion of moisture at field capacity). Mean fresh biomass yield and
fresh plant root weight of A. halimus increased while that of S. persica decreased
significantly with increasing irrigation water salinity in all the treatments.
Soil salinity increased significantly with increasing water salinity (Table
15). A positive correlation (r = 0.987) existed between the irrigation water
salinity and the soil salinity resulting from saline irrigation. The plant tissue
protein contents increased in A. halimus, but decreased in S. persica with increasing
irrigation water salinity. The Na ion uptake by plant roots was significantly
less than K in A. halimus compared to S. persica which indicated adjustment
of plants to high soil salinity and high Na ion concentration for better growth.
The order of increasing salt tolerance was A. halimus >S. persica under the
existing plant growing conditions.
Al-Rehaili (1997) suggested that practice of municipal
wastewater treatment and reuse in the Kingdom of Saudi Arabia is in urgent need
of improvement in all aspects.
He recommended immediate expansion of wastewater
collection, treatment capacity and reuse, establish process selection and design
guidelines. Considering the Kingdom’s local requirements, establish rational
and enforceable quality control criteria for effluent disposal and reuse.
Hussain and Alshammary (2008) carried a greenhouse
experiment to determine the effect of water salinity on the survival and growth
of landscape trees and soil properties. The survival period of trees decreased
significantly with increase in soil salinity resulted from irrigation water
salinity. The survival period of Acacia nilotica and Prosopis juliflora
was significantly more than Eucalyptus camaldulensis and Parkinsonia
aculeate under different water salinity levels and soil types. The total
biomass decreased significantly with increase in soil salinity. Soil salinity
and sodicity increased significantly with increasing irrigation water salinity
and sodicity. Prosopis juliflora tolerated soil salinity (Ece)
up to 39.5 dS m-1 and Acacia nilotica up to 44.9 (Ece)
when irrigated with water salinity of 12.80 dS m-1; Parkinsonia
aculeate up to 29.26 (ECe) when irrigated with water salinity
of 6.45 dS m-1 and Eucalyptus camaldulensis up to 34.3 (ECe)
when irrigated with water salinity of 6.45 dS m-1. Tree survival
and proper establishment is possible provided management practices such as leaching
requirement (at least 15%), proper selection of trees, right irrigation water
salinity and proper planting methods are followed. The salt accumulation was
significantly more in light than in heavy soil. A strong correlation (r2)
was observed between soil salinity and plant biomass which indicated significant
decrease in biomass with increasing soil salinity resulting from saline irrigation.
The experiment proved the sequence in salt tolerance for different landscape
trees as Prosopis and Acacia >Parkinsonia >Eucalyptus.
The study findings were converted to an extension fact sheet for use in saline irrigation management as below:
PLANT GROWTH AND SOIL PROPERTIES
Effects of wastewater on plant growth and soil properties were studied in a
pot experiment (Al-Jaloud et al., 1995). Mean
biomass ranged between 159 and 210 g per pot for maize and between 165 and 212
g per pot for sorghum in different eater salinity treatments. Mean drymatter
yield ranged betwee n 28.9 and 38.3 g per pot for maize (corn) crop and between
34.9 and 50.4 g per pot for sorghum. The crop yield showed significant increase
with increase in water salinity. This was presumably due to the nutrients present
in wastewater, especially the nitrogen. Plant yield decreased slightly at water
salinity level of 2330 mg L-1 (TDS), indicating that higher water
salinity can neutralize the beneficial effects of nutrients in wastewater. Soil
salinity and sodicity increased significantly with corresponding increase in
water salinity and sodicity (r values of 0.98 for maize and 0.98 for sorghum
with respect to soil salinity and 0.96 for maize and for 0.95 sorghum with respect
to SAR of soils). The interaction between crop and water treatments was significant
for soil salinity (LSD0.05 = 0.48) and SAR of soil (LSD0.05
= 2.55). Overall, the soil salinity and sodicity was significantly more in sorghum
than maize. The results showed that wastewater can successfully be used to grow
corn ad sorghum as forage crops, provided 15 to 20% excess water is applied
to meet leaching requirements to maintain soil salinity within acceptable limits
for optimal agricultural production.
In the second part of the above mentioned experiment, it was found that the
ranges for different minerals in corn plant were 0.67-0.89% (Ca), 0.38=0.58%
(Mg), 0.09-1.29 5 (Na), 0.81-1.87% (N), 1.81-2.27% (K), 0.1200.16% (P), 190-257
mg kg-1 (Fe), 3.5-5.6 mg kg-1 (Cu), 37.1-44.5 mg kg-1
(Mn), 21.6-33.6 mg kg-1 (Zn), 1.40-1.84 mg kg-1 (Mo),
11.0-45.7 mg kg-1 (Pb) and 2.5-10.8 mg kg-1 (Ni). Whereas
for sorghum plants, the ranges were: 0.56-0.68% (Ca), 0.19-0.32% (Mg), 0.02-0.27%
(Na), 0.69-1.53% (N), 1.40-1.89% (K), 0.10-0.14% (P), 190-320 mg kg-1
(Fe), 3.8-6.0 mg kg-1 (Cu), 29.20-37.6 mg kg-1 (Mn), 21.1-29.9
mg kg-1 (Zn), 2.2-3.7 mg kg-1 (Mo), 12.3-59.0 mg kg-1
(Pb) and 2.5-15.2 mg kg-1 (Ni). however, heavy metal such as Co and
Cd were below detection limits at mg kg-1 level. The concentration
of Ca, N, K, P, Cu and Mn in corn plants was in deficient range except for Mg,
Fe, Zn and Al. The concentration of Ca, N, P, K, Cu, Mn, Mg and Zn in sorghum
plants was in the deficient range except for Fe and Al. The analyses of regression
indicated a strong interaction between Pb, Ni, Ca and Fe in corn and sorghum
plants, In conclusions, wastewater irrigation did not increase mineral concentrations
of either macro/micro-elements or heavy metals in corn and sorghum plants to
hazardous limits according to the established standards and could safely be
used for crop irrigation.
CROP IRRIGATION
Some research conducted in Saudi Arabia has shown a lot of potential for the
reuse of saline water for crop irrigation without economical yield losses. Al-Tahir
et al. (1989) and Fallatah and Hussain (1988).
Al-Tahir et al. (1989) also observed that highly
saline drainage water could be used efficiently by mixing it with normal irrigation
water (Table 16, 17).
Similarly, Al-Mashhady et al. (1983) investigated
on the use of highly saline water for wheat and barley irrigation. They found
that yields of both wheat and barley crops decreased significantly with increase
in irrigation water salinity. The yield losses were highly uneconomical (Table
18).
| Table 16: |
Effect
of irrigation water salinity on greenmatter and drymatter yield of faba
bean (Vicia faba L.) cultivars |
 |
| aLSD: Least significant difference F-32
and F-21(Egyptian cultivars), FSLH (local cultivar). (Source: Fallatah
and Hussain, 1988) |
| Table 17: |
Effect
of drainage and normal irrigation waters on dryweight (mg) of different
plant parts of faba bean cultivars |
 |
| (Source: Al-Tahir et al.,
1989) |
WATER USE IN SAUDI ARABIA
The total water consumed in the country was 20,406.1x106 m3
in 1996 (1415 H.). Out of this 14,481.1x106 m3 was used
by agriculture, 1,925x106 m3 for domestic and industrial
purposes (Ministry of Planning, 1998). It indicated that
91% of the total used accounted for agriculture and the remaining 9% was utilized
for domestic and industrial purposes. Estimated water consumption in the kingdom
is shown in Table 19.
SALT TOLERANT PLANTS
There are many salt tolerant plants which showed promising results. The research done on some plants is summarized below:
Atriplex: It is a salt and drought resistant plant. For example, A.
hastata survived in the presence of 25.6% of NaCl (Flowers
et al., 1977). Atriplex is also a drought resistant plant and is
recommended as a forage crop in area where irrigation water is in short supply
and the rainfall is less than 250 mm per year (Brown, 1977).
Similarly, Hyder et al. (1989) studied the performance
of atriplex species at different salinity levels in various locations in the
Kingdom. The initial soil salinity (ECe) was 80 dS m-1
and 40 dS m-1 in Al-Hofuf anf Al-Qateef, respectively. Plant characteristics
are summarized from Al-Qateef region in Table 20.
| Table 20: |
Yield
biomass, diameter, protein and ash contents of atriplex species grown
at al-qatif for plant of 6 months age |
 |
| Source: Hyder et al. (1989) |
Salicornia: Salicornia plants grow in a salty environment such as coastal
marshes. The plants are succulent with fleshy stem and small leaves and have
a good potential for forage and oil from the seed. A study conducted by Charnock
(1988) at the Environmental Research Laboratory, University of Arizona,
USA showed that salicornia can be grown in some coastal lands of the United
Arab Emirates with salinity reaching up to 30% which is about 10 times more
than the seawater and can be irrigated with seawater as a fodder crop. It was
also calculated that one hectare of salicornia crop could feed up to 20 goats
or sheep where as one hectare of forage crop irrigated with good quality can
raise 35 goats or sheep (Ministry of Agriculture and Water,
1992). It was also suggested that several trials should be made to grow
salicornia at different sites in the Kingdom (North of Umm Lojj).
Kochia: It is another salt tolerant and drought resistant plant. It
is a bushy herb and may be used as a green or dry fodder for livestock (Zahran,
1990). A field experiment was conducted by Zahran (1986)
on germination, growth and vegetative yield of Kochia in a salt affected land
in the western regions of Saudi Arabia near Bahra (Site midway between Makka
and Jeddah). The initial soil salinity (ECe) was 2.6 dS m-1.
Saline artesian water having salinity of 8.3 dS m-1 was used for
irrigation. The yield was about 55.3 ton ha-1.
MANAGEMENT STRATEGIES FOR SALINE WATER IRRIGATION
Saline irrigation management means to maximize water use and to minimize deleterious
effects on crop production. Many strategies have been developed to use saline
water for irrigation without considerable loss in crop yields (Rhoades,
1984; Ayers and Westcot, 1985; Stromberg,
1980). Some of the management practices to be adopted under saline irrigation
are:
| Table 21: |
Concentration factors (F) for predicting leaching requirement
(LR) and crop water requirements |
 |
| Ayers and Westcot (1985) |
Leaching requirements: All irrigation waters, surface or sub-surface,
contain salts in different amounts and proportions. Accumulation of salts in
soils is a common phenomenon under long term irrigation even with less saline
water. Therefore, application of leaching requirements is important for sustained
irrigated agriculture. Furthermore, Ayers and Westcot (1985)
proposed the method to calculate leaching requirements (LR) based on a water
uptake distribution of 40:30:20:10% for the first through fourth quarters of
root zone will be referred to as the tabular method (Table 21).
Provision of adequate drainage: Oftenly poor drainage is the root cause of resalinization of arable agricultural lands under irrigation and sometimes causes sizable crop yield losses. Also the presence of hard layer (clay pan) beneath the crop root zone could cause perched water table and create soil-water conditions unfavorable for normal plant growth. Therefore, provision of adequate drainage is a key factor for successful agriculture using saline water irrigation.
Adoption of improved irrigation practices: Over-irrigation, in some
cases, is another source to keep soil salinity within acceptable limits for
optimum production. However, depending upon many factors such as irrigation
water quality, crop to be planted, initial soil salinity, water availability
and the soil type, water requirements for reclamation of salt affected soil
can be determined. Al-Jaloud (1994) and Hussain
et al. (1988) developed leaching curves for the reclamation of salt
affected soils in Al-Qasseem and Al-Ahsa regions, respectively to determine
actual water requirements needs to bring soil salinity within acceptable limits.
Besides this, adoption of sprinkler and semi-drip irrigation systems could minimize
salt accumulation from saline irrigation.
Water requirements for land reclamation: Estimation of water requirement
for reclamation of salt affected soils is important for proper planning, management
and economical use of water resources. Because saline wastewater can easily
be used for initial reclamation of highly salt affected soils instead of freshwater.
A number of studies conducted on sandy salt affected soils showed that a 20
to 30 cm depth of water is required to reclaim the surface 0 to 30 cm depth
of soil, a 60 to 90 cm depth of water is required to reclaim the subsurface
30 to 60 cm depth of soil and a 50 to 60 cm depth of water is required to reclaim
the whole profile 0 to 60 cm depth of soil for a given salt affected soil and
method of water application (Hussain et al. 1988).
The study also provided a useful tool in the form of a leaching curve to evaluate
water requirement for the reclamation of some salt affected soils in Al-Ahsa,
Kingdom of Saudi Arabia. Whereas, Abdelhadi and Hussain
(1987) showed that soil salinity was reduced from 75.50 to 16.75 dS m-1,
SAR from 20.50 to 12.50 with 20 cm depth of water application. However, soil
salinity and SAR (sodicity) of soil reached safe limits by applying 60 cm depth
of water in a three month period. The gypsum contents decreased significantly
with increase in irrigation water application. The study indicated that a highly
salt-affected soil can be reclaimed with 60 cm depth of water for 0-30 cm surface
soil and with 100 cm depth of water for 0-90 cm profile in a 3 to 5 month period.
Use of mulch and soil conditioners: Application of mulches and high
moisture absorbing materials (synthetic polymers) increased significantly the
water use efficiency, enhanced productivity of sandy soils and saved water up
to 15 to 28% under normal irrigation practices (Hussain
et al., 1986; Al-jaloud, 1988).
Crop selection: Selection of suitable crop for cultivation is important
especially when saline irrigation is to be practiced. The yield reduction of
crops depends on water salinity (EC) and soil salinity (ECe). The
equation used is:
Where:
| Y |
= |
Relative crop yield (percent) |
| a |
= |
Salinity threshold value |
| b |
= |
Yield loss per unit increase in salinity |
| ECe |
= |
Salinity of soil saturation extract (dS m-1) |
As an example, if barley is grown and irrigated with saline water having EC
10 = 6400 mg L-1 and a is 8 dS m-1 and b is 5%, the yield
reduction will be only 10% (Maas and Hoffman, 1977).
Planting practices: Salts in soils move in the direction of waterflow
and exposed evaporative surfaces. Therefore, the patron of salt accumulation
is different in furrow irrigation and sloping beds plantation. salt concentration
in the center of the bed regularly becomes 5 to 10 times greater than the original
concentration in the plough layer (Bernstein, 1964).
It is recommended that plantation should be done in the lower 2/3 of the furrow
or to irrigate only alternate furrows or two rows in a wide bed. Since most
of the plants are sensitive at germination stage than at later stages of growth,
hence frequent irrigation should be done at germination stage.
Fertility management: All fertilizers contains salts and contribute
to soil salinity. Therefore, special methods must be followed for fertilization.
According to Ayers and Westcot (1985), spreading of 50
kg N ha-1 in the form of (NH4)2SO4
is not expected to cause salinity problems. For example, if the soil fertility
is adequate but the soil salinity is a limiting factor, then any increase in
fertilizer application will not improve crop yield or modify crop salt tolerant
limits. However, correction of most limiting factors for yield reduction should
be done to optimize production.
Pre-planting irrigation: There is a tendency for salts to accumulate during non cropping season on the surface of soils and the magnitude is high in arid climatic conditions. It is recommended that heavy pre-planting irrigation should be applied to leach surface salt in order to improve crop germination and stand.
Cultural practices: Leaching is the principal method of toxic ions control. The cultural practices are considered an aid in the management of irrigation water at the farm level for its success. Because cultural practices which offer better control and distribution of water include land grading, profile modification and artificial drainage if natural drainage is inadequate. These steps are complementary to those previously discussed for improved salinity and toxicity control.
The severity of a toxicity problem will increase as the crop withdraws soil-water and the soil dries between irrigations. The ions become concentrated in the smaller volume of soil water. As the upper soil dries, the crop must withdraw more and more of its water needs from the deeper soil where salinity and toxic ions are usually in greater concentration. Increasing the frequency of irrigation supplies a greater proportion of the water needs from the upper soil as well as diluting the deeper soil water and should reduce the impact of both salinity and toxic ions.
Sodium toxicity (high SAR) from applied water is generally countered by the
use of a soil or water amendment such as gypsum. In general, where salinity
of water is relatively low (ECw <0.5 dS m-1), the beneficial
response to a water-applied amendment is much greater than if salinity is high
because it is far easier to change the sodium to calcium ratio of a relatively
low salinity than one of a high saline water or to a high ESP soil.
Blending of fresh and saline water supplies: If an alternative water supply is available, but not fully adequate in quantity or quality, a blend of waters may offer an overall improvement in quality and reduce the potential toxicity problem. This phenomenon of blending is specially effective for a sodium toxicity problem since proportions of monovalent (Na+) and divalent (Ca++) cations absorbed on the soil depend on the concentration, with dilution favouring adsorption of the divalent cations (calcium and magnesium) rather than the monovalent sodium.
CONCLUSION AND FUTURE STUDIES
The use of saline water as a supplemental source of irrigation is inevitable in arid and semi-arid countries due to inadequate freshwater for irrigation. In order to alleviate the use of saline water for crop production, studies in the following areas need priority.
| • |
Biosaline
research on field and vegetable crops for breeding and screening salt
tolerant and drought resistant cultivars |
| • |
Identify
plant growth factors limiting production |
| • |
Classification
of promising crop cultivars for different regions of Saudi Arabia |
The comprehensive review has shown a lot of potential on the use of marginal quality groundwater for irrigation and reuse of wastewater after primary treatment for agricultural expansion provided certain soil, water and crop management practiced are considered under local climatic conditions.
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