Quick Facts...
- Knowledge of irrigation water quality is critical to understanding
management for long-term productivity.
- Water with electrical conductivity (ECw) of only 1.15 dS/m
contains approximately 2,000 pounds of salt for every acre foot of water.
- In many areas of Colorado, irrigation water quality can influence
crop productivity more than soil fertility, hybrid, weed control and
other factors.
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|
Corn plant damaged by saline sprinkler
water.
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Salt-affected soils develop from a wide range of factors including: soil type,
field slope and drainage, irrigation system type and management, fertilizer
and manuring practices, and other soil and water management practices.
In Colorado, perhaps the most critical factor in predicting, managing,
and reducing salt-affected soils is the quality of irrigation water being
used. Besides affecting crop yield and soil physical conditions, irrigation
water quality can affect fertility needs, irrigation system performance
and longevity, and how the water can be applied. Therefore, knowledge
of irrigation water quality is critical to understanding what management
changes are necessary for long-term productivity.
Irrigation Water Quality Criteria
Soil scientists use the following categories to describe irrigation water effects
on crop production and soil quality:
- Salinity hazard - total soluble salt content
- Sodium hazard - relative proportion of sodium (Na+)
to calcium (Ca2+) and magnesium (Mg2+)
ions
- pH
- Alkalinity - carbonate and bicarbonate
- Specific ions: chloride (Cl), sulfate (SO42-), boron (B),
and nitrate-nitrogen (NO3-N).
Other potential irrigation water contaminants that may affect suitability
for agricultural use include heavy metals and microbial contaminants.
Salinity Hazard
The most influential water quality guideline on crop productivity is
the water salinity hazard as measured by electrical conductivity (ECw).
The primary effect of high ECw water on crop productivity is
the inability of the plant to compete with ions in the soil solution for
water (physiological drought). The higher the EC, the less water is available
to plants, even though the soil may appear wet. Because plants can only
transpire "pure" water, usable plant water in the soil solution decreases
dramatically as EC increases.
| Table 1. Suggested criteria for irrigation water
use based upon conductivity. |
| Classes of water |
Electrical Conductivity |
| |
(dS/m)* |
| Class 1, Excellent |
≤0.25 |
| Class 2, Good |
0.25 - 0.75 |
| Class 3, Permissible1 |
0.76 - 2.00 |
| Class 4, Doubtful2 |
2.01 - 3.00 |
| Class 5, Unsuitable2 |
≥3.00 |
*dS/m at 25ºC = mmhos/cm
1Leaching needed if used.
2Good drainage needed and sensitive plants will have difficulty obtaining
stands. |
| Table 2. Potential yield reduction from saline water
for selected irrigated crops.1 |
| |
% yield reduction
|
| Crop |
0%
|
10%
|
25%
|
50%
|
| |
ECw2
|
| Barley |
5.3
|
6.7
|
8.7
|
12
|
| Wheat |
4.0
|
4.9
|
6.4
|
8.7
|
| Sugarbeet3 |
4.7
|
5.8
|
7.5
|
10
|
| Alfalfa |
1.3
|
2.2
|
3.6
|
5.9
|
| Potato |
1.1
|
1.7
|
2.5
|
3.9
|
| Corn (grain) |
1.1
|
1.7
|
2.5
|
3.9
|
| Corn (silage) |
1.2
|
2.1
|
3.5
|
5.7
|
| Onion |
0.8
|
1.2
|
1.8
|
2.9
|
| Beans |
0.7
|
1.0
|
1.5
|
2.4
|
1Adapted from “Quality
of Water for Irrigation.” R.S. Ayers. Jour. of the Irrig. and
Drain. Div., ASCE. Vol 103, No. IR2, June 1977, p. 140.
2ECw = electrical conductivity of the irrigation
water in dS/m at 25oC.
3Sensitive during germination. ECw should not
exceed 3 dS/m for garden beets and sugarbeets. |
The amount of water transpired through a crop is directly related to
yield; therefore, irrigation water with high ECw reduces yield
potential (Table 2). Beyond effects on the immediate crop is the long
term impact of salt loading through the irrigation water. Water with an
ECw of only 1.15 dS/m contains approximately 2,000 pounds of
salt for every acre foot of water. You can use conversion factors in Table
3 to make this calculation for other water EC levels.
| Table 3. Conversion factors for irrigation water
quality laboratory reports. |
| Component |
To Convert
|
Multiply By
|
To Obtain
|
| Water nutrient or TDS |
mg/L
|
1.0
|
ppm
|
| Water salinity hazard |
1 dS/m
|
1.0
|
1 mmho/cm
|
| Water salinity hazard |
1 mmho/cm
|
1,000
|
1 µmho/cm
|
| Water salinity hazard |
ECw (dS/m) for ECw <5
dS/m
|
640
|
TDS (mg/L)
|
| Water salinity hazard |
EC (dS/m) for EC >5 dS/m
|
800
|
TDS (mg/L)
|
| Water NO3N, SO4-S,
B |
ppm
|
0.23
|
lb per acre inch of water applied
|
| Irrigation water |
acre inch
|
27,150
|
gallons of water
|
|
Definitions
|
| Abbrev. |
Meaning |
| mg/L |
milligrams per liter |
| meq/L |
milliequivalents per liter |
| ppm |
parts per million |
| dS/m |
deciSiemens per meter |
| µS/cm |
microSiemens per centimeter |
| mmho/cm |
millimhos per centimeter |
| TDS |
total dissolved solids |
Other terms that laboratories and literature sources use to report salinity
hazard are: salts, salinity, electrical conductivity (ECw), or total dissolved
solids (TDS). These terms are all comparable and all quantify the amount
of dissolved “salts” (or ions, charged particles) in a water
sample. However, TDS is a direct measurement of dissolved ions and EC
is an indirect measurement of ions by an electrode.
Although people frequently confuse the term “salinity” with
common table salt or sodium chloride (NaCl), EC measures salinity from
all the ions dissolved in a sample. This includes negatively charged ions
(e.g., Cl¯, NO¯3) and positively charged ions (e.g., Ca++, Na+).
Another common source of confusion is the variety of unit systems used
with ECw. The preferred unit is deciSiemens per meter (dS/m), however millimhos
per centimeter (mmho/cm) and micromhos per centimeter (µmho/cm)
are still frequently used. Conversions to help you change between unit
systems are provided in Table 3.
Sodium Hazard
While ECw is an assessment of all soluble salts in a sample, sodium hazard
is defined separately because of sodium's specific detrimental effects
on soil physical properties. The sodium hazard is typically expressed
as the sodium adsorption ratio (SAR). This index quantifies the proportion
of sodium (Na+) to calcium (Ca++) and magnesium
(Mg++) ions in a sample. Calcium will flocculate (hold together),
while sodium disperses (pushes apart) soil particles. This dispersed soil
will readily crust and have water infiltration and permeability problems.
General classifications of irrigation water based upon SAR values are
presented in Table 4.
 |
meq/L = mg/L divided by atomic weight of ion divided
by ionic charge (Na+=23.0 mg/meq,
Ca++=20.0 mg/meq, Mg++=12.15
mg/meq) |
| Table 4. General classification of water sodium hazard
based on SAR values. |
| SAR values |
Sodium hazard of water |
Comments |
| 1-9 |
Low |
Use on sodium sensitive crops must be cautioned. |
| 10-17 |
Medium |
Amendments (such as gypsum) and leaching needed. |
| 18-25 |
High |
Generally unsuitable for continuous use. |
| ≥26 |
Very High |
Generally unsuitable for use. |
However, many factors including soil texture, organic matter, crop type,
climate, irrigation system and management impact how sodium in irrigation
water affects soils. Additionally, at the same SAR, water with low ECw
(salinity) has a greater dispersion potential than water with high ECw.
Sodium in irrigation water can also cause toxicity problems for some crops,
especially when sprinkler applied. Crops vary in their susceptibility
to this type of damage as shown in Table 5.
| Table 5. Susceptibility ranges for crops
to foliar injury from saline sprinkler water. |
| |
Na or Cl concentration (mg/L) causing foliar
injury
|
| Na concentration |
<46 |
46-230 |
231-460 |
>460 |
| Cl concentration |
<175 |
175-350 |
351-700 |
>700 |
| |
Apricot |
Pepper |
Alfalfa |
Sugarbeet |
| Plum |
Potato |
Barley |
Sunflower |
| Tomato |
Corn |
Sorghum |
|
| Foliar injury is influenced
by cultural and environmental conditions. These data are presented
only as general guidelines for daytime irrigation. Source: Mass (1990)
Crop salt tolerance. In: Agricultural Assessment and Management Manual.
K.K. Tanji (ed.). ASCE, New York. pp. 262-304. |
pH and Alkalinity
The acidity or basicity of irrigation water is expressed as pH (<
7.0 acidic; > 7.0 basic). The normal pH range for irrigation water
is from 6.5 to 8.4. Abnormally low pH’s are not common in Colorado,
but may cause accelerated irrigation system corrosion where they occur.
High pH’s above 8.5 are often caused by high bicarbonate (HCO3-)
and carbonate (CO32-
) concentrations, known as alkalinity. High carbonates cause calcium and
magnesium ions to form insoluble minerals leaving sodium as the dominant
ion in solution. This alkaline water could intensify sodic soil conditions.
In these cases, a lab will calculate an adjusted SAR (SARADJ)to reflect
the increased sodium hazard.
Chloride
Chloride is a common ion in Colorado irrigation waters. Although chloride
is essential to plants in very low amounts, it can cause toxicity to sensitive
crops at high concentrations (Table 6). Like sodium, high chloride concen-trations
cause more problems when applied with sprinkler irrigation (Table 6).
Leaf burn under sprinkler from both sodium and chloride can be reduced
by night time irrigation or application on cool, cloudy days. Drop nozzles
and drag hoses are also recommended when applying any saline irrigation
water through a sprinkler system to avoid direct contact with leaf surfaces.
| Table 6. Chloride classification of irrigation water.
|
| Chloride (ppm) |
Effect on Crops |
| Below 70 |
Generally safe for all plants. |
| 70-140 |
Sensitive plants show injury. |
| 141-350 |
Moderately tolerant plants show injury. |
| Above 350 |
Can cause severe problems. |
| Chloride tolerance of selected crops.
Listing in order of increasing tolerance: (low tolerance) dry bean,
onion, carrot, lettuce, pepper, corn, potato, alfalfa, sudangrass,
zucchini squash, wheat, sorghum, sugar beet, barley (high tolerance).
Source: Mass (1990) Crop Salt Tolerance. Agricultural Salinity Assessment
and Management Manual. K.K. Tanji (ed.). ASCE, New York. pp 262-304.
|
Boron
Boron is another element that is essential in low amounts, but toxic
at higher concentrations (Table 7). In fact, toxicity can occur on sensitive
crops at concentrations less than 1.0 ppm. Colorado soils and irrigation
waters contain enough B that additional B fertilizer is not required in
most situations. Because B toxicity can occur at such low concentrations,
an irrigation water analysis is advised for ground water before applying
additional B to crops.
| Table 7. Boron sensitivity of selected Colorado plants
(B concentration, mg/ L*) |
|
Sensitive
|
Moderately Sensitive
1.1-2.0
|
Moderately Tolerant
2.1-4.0
|
Tolerant
4.1-6.0
|
|
0.5-0.75
|
0.76-1.0 |
|
Peach
|
Wheat
|
Carrot
|
Lettuce
|
Alfalfa
|
|
Onion
|
Barley
|
Potato
|
Cabbage
|
Sugar beet
|
|
|
Sunflower
|
Cucumber
|
Corn
|
Tomato
|
|
Dry Bean
|
|
Oats
|
|
Source: Mass (1987) Salt
tolerance of plants. CRC Handbook of Plant Science in Agriculture.
B.R. Cristie (ed.). CRC Press Inc.
*Maximum concentrations tolerated in soil water or saturation extract
without yield or vegetative growth reductions. Maximum concentrations
in the irrigation water are approximately equal to these values or
slightly less. |
Sulfate
The sulfate ion is a major contributor to salinity in many of Colorado
irrigation waters. However, toxicity is rarely a problem, except at very
high concentrations where high sulfate may interfere with uptake of other
nutrients. As with boron, sulfate in irrigation water has fertility benefits,
and irrigation water in Colorado often has enough sulfate for maximum
production for most crops. Exceptions are sandy fields with <1 percent
organic matter and <10 ppm SO4-S in
irrigation water.
Nitrogen
Nitrogen in irrigation water (N) is largely a fertility issue, and nitrate-nitrogen
(NO3-N) can be a significant N source
in the South Platte, San Luis Valley, and parts of the Arkansas River
basins. The nitrate ion often occurs at higher concentrations than ammonium
in irrigation water. Waters high in N can cause quality problems in crops
such as barley and sugar beets and excessive vegetative growth in some
vegetables. However, these problems can usually be overcome by good fertilizer
and irrigation management. Regardless of the crop, nitrate should be credited
toward the fertilizer rate especially when the concentration exceeds 10
ppm NO3-N (45 ppm NO3¯).
Table 3 provides conversions from ppm to pounds per acre inch.
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