Quick Facts...
- An estimated 980,000 acres of irrigated land in Colorado are affected
by salts.
- Crop losses may occur with irrigation water containing as little as
700 to 850 mg/L TDS (total dissolved solids) or EC>1.2 dS/m.
- Salt-affected soils may inhibit seed germination, retard plant growth,
and cause irrigation difficulties.
- Saline soils cannot be reclaimed by chemical amendments, conditioners
or fertilizers.
- Saline soils are often reclaimed by leaching salts from the plant
root zone.
Soils high in salt and/or sodium may limit crop yields. Salt-affected
soils may contain an excess of water-soluble salts (saline soils), exchangeable
sodium (sodic soils) or both an excess of salts and exchangeable sodium
(saline-sodic soils). Periodic soil testing and treatment, combined
with proper management procedures, can improve the conditions in salt-affected
soils that contribute to poor plant growth.
Salinity problems are caused from the accumulation of soluble salts in
the root zone. These excess salts reduce plant growth and vigor by altering
water uptake and causing ion-specific toxicities or imbalances. Establishing
good drainage is generally the cure for these problems, but salinity problems
are often more complex. Proper management procedures, combined with periodic
soil tests, are needed to prolong the productivity of salt-affected soils.
This fact sheet describes techniques for managing saline soils. Management
for sodic soils may differ and is described in fact sheet 0.504,
Managing Sodic Soils. You also may want to review fact sheet
0.521, Diagnosing
Saline and Sodic Soil Problems to determine if you have a saline
soil, sodic soil or perhaps another problem in your field.
Salt Sources
Saline soils are found throughout Colorado. These salts originate from
the natural weathering of minerals or from fossil salt deposits left from
ancient sea beds. Salts accumulate in the soil of arid climates as irrigation
water or groundwater seepage evaporates, leaving minerals behind. Irrigation
water often contains salts picked up as water moves across the landscape,
or the salts may come from human-induced sources such as municipal runoff
or water treatment. As water is diverted in a basin, salt levels increase
as the water is consumed by transpiration or evaporation.
| Table 1. Common salt compounds. |
| Salts are ionic crystalline compounds consisting
of a cation and an anion. |
| Salt compound |
Cation (+) |
Anion (-) |
Common name |
| NaCl |
sodium |
chloride |
halite (table salt) |
| Na2SO4 |
sodium |
sulfate |
Glauber’s salt |
| MgSO4 |
magnesium |
sulfate |
epsom salts |
| NaHCO3 |
sodium |
bicarbonate |
baking soda |
| Na2CO3 |
sodium |
carbonate |
sal soda |
| CaSO4 |
calcium |
sulfate |
gypsum |
| CaCO3 |
calcium |
carbonate |
calcite (lime) |
Measuring Soil Salinity
Saline soils contain large amounts of water soluble salts that inhibit
seed germination and plant growth. The salts are white, chemically neutral,
and include chlorides, sulfates, carbonates and sometimes nitrates of
calcium, magnesium, sodium and potassium (Table 1).
Salinity is measured by passing an electrical current through a soil
solution extracted from a saturated soil sample. The ability of the solution
to carry a current is called electrical conductivity (EC). EC is measured
in deciSiemens per meter (dS/m), which is the numerical equivalent to
the old measure of millimhos per centimeter (Table 2). The lower the salt
content of the soil, the lower the dS/m rating and the less the effect
on plant growth.
Yields of most crops are not significantly affected where salt levels
are 0 to 2 dS/m. Generally, a level of 2 to 4 dS/m affects some crops.
Levels of 4 to 5 dS/m affect many crops and above 8 dS/m affect all but
the very tolerant crops (Table 4).
| Table 2. Terms, units and conversions. |
| Symbol |
Meaning |
Units |
| Total Salinity |
| TDS |
Total dissolved solids |
mg/La; ppmb |
| EC |
Electrical conductivity
|
dS/mc; mmho/cmd;
µmho/cme |
| Conversions |
| 1 dS/m = 1 mmho/cm = 1000 µmho/cm |
| 1 mg/L = 1 ppm |
amg/L = milligrams
per liter; bppm = parts per million; cdS/m =
deciSiemens per meter at 25° C;
dmmho/cm = millimhos per centimeter at 25° C; eµmho/cm
= micromhos per centimeter at 25°C |
Treatment of Saline Soil
Saline soils cannot be reclaimed by chemical amendments, conditioners
or fertilizers. A field can only be reclaimed by removing salts from the
plant root zone. In some cases, selecting salt-tolerant crops may be needed
in addition to managing soils.
There are three ways to manage saline soils. First, salts can be moved
below the root zone by applying more water than the plant needs. This
method is called the leaching requirement method. The second method,
where soil moisture conditions dictate, combines the leaching requirement
method with artificial drainage. Third, salts can be moved away
from the root zone to locations in the soil, other than below the root
zone, where they are not harmful. This third method is called managed
accumulation.
Leaching Requirement
For most surface irrigation systems in Colorado (furrow and flood), irrigation
inefficiency (or over-irrigation) generally is adequate to satisfy the
leaching requirement. However, poor irrigation uniformity often results
in salt accumulation in parts of a field or bed. Surface irrigators should
compare leaching requirement values to measurements of irrigation efficiency
to determine if additional irrigation is needed. Adding more water to
satisfy a leaching requirement reduces irrigation efficiency and may result
in the loss of nutrients or pesticides and further dissolution of salts
from the soil profile.
Leaching is accomplished on a limited basis at key times during the growing
season, particularly when a grower may have high quality water available.
Surface water in most areas of the state tend to have lower salinity than
shallow, alluvial groundwater. Deep groundwater may have an even lower
salinity than either shallow groundwater or surface water. In situations
where a grower has multiple water sources of varying quality, consider
planned leaching events at key salinity stress periods for a given crop.
Most crops are highly sensitive to salinity stress in the germination
and seedling stages. Once the crop growns past these stages, it can often
tolerate and grow well in higher salinity conditions. Planned periodic
leaching events might include a post-harvest irrigation to push salts
below the root zone to prepare the soil (especially the seedbed/surface
zone) for the following spring. Fall is the best time for a large, planned
leaching event because nutrients have been drawn down. However, since
each case is site-specific, examine the condition of the soil, groundwater,
drainage, and irrigation system for a given field before developing a
sound leaching plan.
Leaching Plus Artificial Drainage
Where shallow water tables limit the use of leaching, artificial drainage
may be needed. Cut drainage ditches in fields below the water table level
to channel away drainage water and allow the salts to leach out. Drainage
tile or plastic drainpipe can also be buried in fields for this purpose.
Proper design and construction of a drainage system is critical and should
be performed by a trained professional, such as your local USDA-Natural
Resources Conservation Service (NRCS).
With all artificial drainage systems you must also consider disposal
of the drainage water. Restrictions on the discharge of drain water to
streams may apply in certain situations and should be investigated through
the Colorado Department of Public Health and Environment. In the case
of regulated discharge, treatment or collection and evaporation of the
water on site may be required and may add significant costs.
The advantage of artificial drainage is that it provides the ability
to use high quality, low salinity irrigation water (if available to a
grower) to completely remove salts from the soil. However, artificial
drainage systems will not work where there is no saturated condition in
the soil. Water will not collect in a drain if the soil around it is not
saturated.
| Table 3. Estimated water application needed to leach
salts. |
|
Percent Salt Reduction
|
Amount of Water Required
|
|
50%
|
6 inches
|
|
80%
|
12 inches
|
|
90%
|
24 inches
|
| Example: If a soil’s electrical conductivity
is 8 mmhos/cm, and you want to reduce it to 4 mmhos/cm. This represents
a 50 percent reduction in salts. Therefore, 6 inches of water would
be required. |
After drainage appears adequate, the leaching process can begin. Table
3 shows how much water is required to leach salts. Actual salt reduction
depends upon water quality, soil texture and drainage.
 |
|
Good uniformity: salts accumulate
in the center of the bed and away from plants.
|
 |
|
Poor uniformity: salts accumulate
toward edge of bed near one row.
|
| Figure 1. Salt management in double-row bed system. |
| |
 |
|
Uniform, healthy plants with
alternate furrow irrigation (salt accumulates in the dry furrows).
|
 |
|
Irregular growth due to variable
accumulation of salt (plants may overcome this situation if roots
can grow out of the saline area).
|
| Figure 2. Salt management in single-row bed systems. |
Managed Accumulation
In addition to leaching salt below the root zone, salts can also be moved
to areas away from the primary root zone with certain crop bedding and
surface irrigation systems. Figures 1 and 2 illustrate several ways to
manage salt accumulation in this manner. The goal is to ensure the zones
of salt accumulation stay away from germinating seeds and plant roots.
Irrigation uniformity is essential with this method. Without uniform distribution
of water, salts will build up in areas where the germinating seeds and
seedling plants will experience growth reduction and possibly death.
Double-row bed systems require uniform wetting toward the middle of the
bed. This leaves the sides and shoulders of the bed relatively free from
injurious levels of salinity. Without uniform applications of water (one
furrow receiving more or less than another), salts accumulate closer to
one side of the bed. Periodic leaching of salts down from the soil surface
and below the root zone may still be required to ensure the beds are not
eventually salted out.
Alternate furrow irrigation may be desired for single-row bed systems.
This is accomplished by irrigating every other furrow and leaving alternating
furrows dry. Salts are pushed across the bed from the irrigated side of
the furrow to the dry side. Care is needed to ensure enough water is applied
to wet all the way across the bed to prevent build up in the planted area.
This method of salinity management can still result in plant injury if
large amounts of natural rainfall fill the normally dry furrows and push
salts back across the bed toward the plants. This phenomenon also occurs
if the normally dry furrows are accidentally irrigated.
Sprinkler Irrigation
Sprinkler-irrigated fields with poor water quality present a challenge
because it is difficult to apply enough water to leach the salts and you
cannot effectively utilize row or bed configurations to manage accumulation.
Growers should monitor the soil EC and irrigation water salinity. Where
adequate irrigation water exists above crop requirements, a leaching fraction
(or percent of additional water needed above crop requirements) can be
calculated for sprinkler irrigated fields using this equation:
In this equation, EC max is the maximum soil EC wanted in the root zone.
(See Table 4.)
Apply this leaching fraction to coincide with periods of low soil N and
residual pesticide. Again, fall is an optimal time to move salts below
the root zone.
| Table 4. Potential yield reduction from saline soils
for selected crops. |
| |
Relative yield decrease %
|
| |
0
|
10
|
25
|
50
|
| Field crops |
(ECe)
|
| Barley |
8.0
|
10.0
|
13.0
|
18.0
|
| Sugarbeets* |
7.0
|
8.7
|
11.0
|
15.0
|
| Wheat |
6.0
|
7.4
|
9.5
|
13.0
|
| Sorghum |
4.0
|
5.1
|
7.2
|
11.0
|
| Soybean |
5.0
|
5.5
|
6.2
|
7.5
|
| Corn |
1.7
|
2.5
|
3.8
|
5.9
|
| Bean |
1.0
|
1.5
|
2.3
|
3.3
|
| |
| Forages |
|
| Tall wheatgrass |
7.5
|
9.9
|
13.3
|
19.4
|
| Wheatgrass |
7.5
|
9.0
|
11.0
|
15.0
|
| Crested wheatgrass |
3.5
|
6.0
|
9.8
|
16.0
|
| Tall fescue |
3.9
|
5.8
|
8.6
|
13.3
|
| Orchardgrass |
1.5
|
3.1
|
5.5
|
9.6
|
| Alfalfa |
2.0
|
3.4
|
5.4
|
8.8
|
| Meadow foxtail |
1.5
|
2.5
|
4.1
|
6.7
|
| Cloveralsike, red, ladino, strawberry |
1.5
|
2.3
|
3.6
|
5.7
|
| Bluegrass and other turf ** |
|
|
|
|
| |
| Vegetables |
|
| Broccoli |
2.8
|
3.9
|
5.5
|
8.2
|
| Cucumber |
2.5
|
3.3
|
4.4
|
6.3
|
| Cantaloupe |
2.2
|
3.6
|
5.7
|
9.1
|
| Spinach |
2.0
|
3.3
|
5.3
|
8.6
|
| Cabbage |
1.8
|
2.8
|
4.4
|
7.0
|
| Potato |
1.7
|
2.5
|
3.8
|
5.9
|
| Sweet corn |
1.7
|
2.5
|
3.8
|
5.9
|
| Lettuce |
1.3
|
2.1
|
3.2
|
5.2
|
| Onion |
1.2
|
1.8
|
2.8
|
4.3
|
| Carrot |
1.0
|
1.7
|
2.8
|
4.6
|
*Sensitive during germination
and emergence, ECe should not exceed 3dS/m at this time.
Excerpted from R. S. Ayers and D.W. Westcot, 1976, Water Quality for
Agriculture, Irrigation and Drainage Paper 29, FAO, Rome. Crop salt
tolerance data in the table were developed, almost entirely, by the
U.S. Salinity Laboratory, Riverside, CA.
**For specifics on turfgrass species, see Colorado State University
Extension fact sheet 7.227,
Growing Turf on Salt-Affected Sites. |
Crop Tolerance to Soil Salinity
Excessive soil salinity reduces the yield of many crops. This ranges
from a slight crop loss to complete crop failure, depending on the type
of crop and the severity of the salinity problem.
Although several treatments and management practices can reduce salt
levels in the soil, there are some situations where it is either impossible
or too costly to attain desirably low soil salinity levels. In some cases,
the only viable management option is to plant salt-tolerant crops. Sensitive
crops, such as pinto beans, cannot be managed profitably in saline soils.
Table 4 shows the relative salt tolerance of field, forage, and vegetable
crops. The table shows the approximate soil salt content (expressed as
the electrical conductivity of a saturated paste extract (ECe) in dS/m
at 25 degrees C) where 0, 10, 25, and 50 percent yield decreases may be
expected. Actual yield reductions will vary depending upon the crop variety
and the climatic conditions during the growing season.
Fruit crops may show greater yield variation because a large number of
rootstocks and varieties are available. Also, stage of plant growth has
a bearing on salt tolerance. Plants are usually most sensitive to salt
during the emergence and early seedling stages. Tolerance usually increases
as the crop develops.
The salt tolerance values apply only from the late seedling stage through
maturity, during the period of most rapid plant growth. Crops in each
class are generally ranked in order of decreasing salt tolerance.
Other Management Options
Residue Management
Crop residue at the soil surface reduces evaporative water losses, thereby
limiting the upward movement of salt (from shallow, saline groundwater)
into the root zone. Evaporation and thus, salt accumulation, tends to
be greater in bare soils. Fields need to have 30 percent to 50 percent
residue cover to significantly reduce evaporation. Under crop residue,
soils remain wetter, allowing fall or winter precipitation to be more
effective in leaching salts, particularly from the surface soil layers
where damage to crop seedlings is most likely to occur.
Plastic mulches used with drip irrigation effectivly reduce salt concentration
from evaporation. Sub-surface drip irrigation pushes salts to the edge
of the soil wetting front, reducing harmful effects on seedlings and plant
roots.
Pre-plant Irrigation
As mentioned before, most crop plants are more susceptible to salt injury
during germination or in the early seedling stages. An early-season application
of good quality water, designed to fill the root zone and leach salts
from the upper 6 to 12 inches of soil, may provide good enough conditions
for the crop to grow through its most injury-prone stages
Irrigation Frequency Management
Salts are most efficiently leached from the soil profile under higher
frequency irrigation (shorter irrigation intervals). Keeping soil moisture
levels higher between irrigation events effectively dilutes salt concentrations
in the root zone, thereby reducing the salinity hazard.
Most surface irrigation systems (flood or furrow systems) cannot be controlled
to apply less than 3 or 4 inches of water per application and are not
generally suited to this method of salinity control. Sprinkler systems,
particularly center-pivot and linear-move systems configured with low
energy precision application (LEPA) nozzle packages or properly spaced
drop nozzles, and drip irrigation systems provide the best control to
allow this type of salinity management.
Summary
Under irrigated conditions in arid and semi-arid climates, the build-up
of salinity in soils is inevitable. The severity and rapidity of build-up
depends on a number of interacting factors such as the amount of dissolved
salt in the irrigation water and the local climate. However, with proper
management of soil moisture, irrigation system uniformity and efficiency,
local drainage, and the right choice of crops, soil salinity can be managed
to prolong field productivity. |
