Soil, Water & Plants
Salinity is a generic term used to describe elevated concentrations of soluble
salts in our soils and water. Comprised primarily of the most easily dissolved
ions ~ sodium and chloride, and to a lesser extent calcium, magnesium, sulfate,
and potassium ~ salinity in our environment adversely impacts water quality,
soil structure, and plant growth. Although minimal accumulations (some in trace
amounts) are required for normal biological function, excess salinization is
fast becoming one of the leading constraints on the availability of freshwater,
the expansion of arable land, and the productivity of affected farms.
links to many of the topics addressed below, refer to the Books & Articles page.
Manifestations of Salinity
Salinity Measurements & Classifications
Impact of Salinity on Plant Growth
Reclamation & Management Techniques
Causes & Manifestations of
The principal causes of
salinity in our environment are ultimately linked to the redistribution of water
and soluble salts, both above and below ground. The problem of salinization is
most acute in the arid and semi-arid regions where both natural processes
(evaporation and plant water consumption) and human interventions (land
clearing, resource management, and irrigation) play their part in the buildup of
salts in our soils and water.
EVOLUTIONARY Processes ~
Primary salinity occurs most often when salts are brought to the
soilís upper layers by upward capillary pressures (due to evaporation) and
underground water movements, where salts carried in downward or lateral flows
tend to concentrate in soil moisture and become insoluble as a result of
evaporation. Sea salt deposits from wind/rain along the coasts and the
weathering of rocks/minerals further inland have also contributed to salinity
over the millennia. Likewise over time, saltwater migrations caused by
hydraulic forces, water gradient changes, and tidal influences have resulted in
the formation of coastal salt marshes and the salinization of freshwater
Climatic changes (i.e. temperature and moisture) in the dry inland regions are
partly responsible for increased environmental salinity and desertification. On
some low-lying arid lands, naturally-occurring brine deposits, restrictive
drainage, and shallow water tables may also contribute to salinization and the
creation of salt flats (or seeps). Saline seeps form when water on higher lands
percolates down until reaching a sloped layer of relatively low permeability in
the soil or bedrock. Water then begins its lateral flow to nearby (or even
distant) low-lying areas where it forms a perched table or remains restricted
within the substrate. Without suitable drainage, these saline waters tend to
rise to the surface and evaporate leaving behind salt deposits.
In the past, ample seasonal rainfall and vegetative cover, healthy soil
structure/composition, and unimpeded nutrient cycling were sufficient to prevent
excess salinity in our soils and water. With adequate natural drainage and good
soil permeability, salts tend to accumulate within the deep soil substrate and
water tables below the aquifer or return to the ocean via rivers and underground
HUMAN INTERVENTION ~ SECONDARY SALINITY
Within the last century, our soils and water have been subjected to
unprecedented contamination and degradation: the normal balance between the
leaching and accrual of salts has been seriously disturbed by human activities.
Disrupting this dynamic equilibrium (of water and salt circulation), the
destruction of critical watersheds and indiscriminate clearing of native
vegetation for agricultural, industrial, and urban development is now beginning
to impose significant constraints on the earthís biomass productivity.
The removal of deep-rooted vegetation (i.e. perennial grasses, shrubs and trees)
and the subsequent cultivation of large-scale, shallow-rooting annual
monocultures in low-lying basins, on alluvial flood plains and marginal lands is
perhaps the principle cause of secondary salinity and land degradation. In many
regions, increased salinization and rising water tables (waterlogging) can be
directly attributed to land clearing, irrigated farming, and overgrazing.
Certain salts found in chemical fertilizers and some organic soil amendments
(i.e. sulfates, nitrates, and sodium/calcium bicarbonates) often accrue in toxic
proportions on agricultural lands and other habitats subject to their
discharge. Similarly, the widespread use of pesticides and herbicides has
effectively sterilized large tracts of land and contaminated important sources
of freshwater. In addition, the overgrazing and compactation of lands by
livestock has contributed to salinity and sodicity on our prairies and
savannas. Road salts, brine spills, and the improper disposal of saline waste
into our rivers and wetlands by the mining and energy industries are also
threatening many diverse habitats.
Irrigation and Drainage
Much of the secondary
salinization associated with irrigation in dry regions is due to poor water
management, both on- and off-farm: the damming of rivers, exhaustive pumping of
freshwater aquifers, and relative inefficiencies of irrigation/drainage
practices including the application of brackish/saline waters. Since all
surface and groundwater contains salts to varying degrees, irrigation is often
seen as the primary culprit for bringing salts into the field.
Salinity in the root zone increases when evapotranspiration rates (the combined
effect of soil moisture evaporation and water consumption/transpiration by
plants) exceed those of freshwater allocation. Under irrigated conditions, the
most common technique for leaching salts from the root zone requires the
application of freshwater in amounts greater than losses due to
evapotranspiration. However, without sufficient drainage, salts lodged in the
sub-soils become soluble and rise to the surface with the water table. As a
consequence, surface and sub-surface waterlogging has become another major
productivity constraint associated with irrigation in many dryland regions.
Some of the direct causes of secondary salinity associated with water use
inefficiencies are (1) increased evaporation from the interruption of surface
water flows by dams and reservoirs; (2) seepage from unlined and poorly
maintained canals; (3) the untenable rate of aquifer and groundwater pumping;
(4) irrigation practices (with increasingly saline water) that are subject to
poor infiltration, surface evaporation, and runoff; and (5) the inadequacy of
natural or man-made drainage systems.
In certain areas, the intensive pumping of freshwater aquifers has resulted in
saltwater intrusions known as Ďupconingí: a situation where saline water, below
the surface of the table, is drawn into wells in a conical shape. Without
proper soil and water management, large-scale irrigation and industrial farming
methods tend to accelerate the rate of nutrient depletion, secondary
salinization, and soil erosion.
Lessons from the Past
A number of ancient civilizations that cultivated the major river
deltas of the world were eventually forced to abandon their lands to salinity,
erosion, and waterlogging. Historically, the inefficiencies of dryland
irrigation and the lack of adequate drainage have resulted in rising water
tables, increasing soil salinization, and the retirement of once productive
In Mesopotamia, the ancient Sumerian civilization irrigated the surrounding
lowland valleys of the Tigres and Euphrates Rivers (known as the Fertile
Crescent) for thousands of years before the land succumbed to silt and
salinity. The desert was transformed into productive farms through irrigation
until the increasingly salt-rich waters and rising water tables choked the high
levels of wheat production upon which the civilization was founded. Improved
canal and drainage management, land fallowing, and the transition to more
salt-tolerant barley varieties are thought to have postponed the eventual
decline in productivity.
On a smaller scale, the Hohokam people, who once populated the lush arid valleys
of the Gila and Salt Rivers in the Sonoran Desert, are thought to have suffered
a very similar fate. Among their ruins is the most extensive network of
irrigation canals in all of the Americas, able to deliver sufficient irrigation
water for up to 10,000 acres. However, the elevated salt content of redirected
water flows, shallow water tables, unlined canals, and high rates of evaporation
rendered the land uncultivable within a few centuries.
Today, the productivity of most irrigated lands is now being threatened by
increased salinization. The
Australia, West Africa,
Northern India and Pakistan, and the inland sea regions of Asia and Europe are
current examples of irrigation-induced salinity and waterlogging that are having
devastating impacts on commercial crops, pasture lands, and the availability of
freshwater. Most disconcerting is that many of these lands have been abandoned
after only a few decades of cultivation.
Salinity Measurements &
Since plants absorb most
salts through their roots, their concentrations within the root zone provide the
most relevant indices for measuring and classifying salinity; sea spray and
sprinkler irrigation can also result in significant salt absorption by the
plant's foliage. Samples taken from different layers of the soil profile are
useful in forecasting the impact of salt levels on plant germination at the
surface and optimum growth to a maximum root depth.
By far, the most common method of measuring soil salinity is to
determine its level of electrical conductivity (EC) using a
saturated paste extract (soil mixed with distilled water). Expressed as
decisiemens per meter (dS/m), increases in EC values are directly correlated to
increased concentrations of soluble salt ions, predominantly sodium and
chloride. These dS/m calculations quantify the capacity of soil moisture to
conduct electrical impulses with a resistance of 1 ohm, and are thus equivalent
to millimhos per centimeter (mmhos/cm).
The exchangeable sodium percentage (ESP) is defined as the
proportion of adsorbed exchangeable sodium (which binds with clay) relative to
the total cation exchange capacity of the soil. As ESP values increase, soil
quality diminishes and plant growth becomes restricted. Closely related to the
ESP, the sodium adsorption ratio (SAR) measures the relative
preponderance of sodium relative to calcium and magnesium, specific elements
that moderate the adverse effects of sodium (SAR and ESP are often used
interchangeably). pH tests, which determine acidity or alkalinity, can
be used as an indicator of the soilís capacity to deliver nutrients to the
plant: acidity or low pH increases the toxic effect of trace metals (i.e.
aluminum and manganese) while alkalinity or high pH tends to reduce the
availability of essential micro-nutrients (i.e. iron, magnesium, zinc, copper
Although EC assessments may be used, the salinity of irrigation water is
frequently calculated by the weight of its inorganic particulates or
concentration of total dissolved solids (TDS). These TDS
measurements are often expressed in the numerically equivalent milligrams per
liter (mg/l) or parts per million (ppm). When comparing EC and TDS values, note
that one dS/m is roughly equal to 650-700 ppm, and closer to 800 ppm at
relatively high levels of salinity (seawater concentrations and above).
Salt-affected soils are commonly classified as either saline, sodic
or a combination of the two in accordance with the above measurement techniques.
Saline soils predominate, primarily loose and sandy with high rates of
surface evaporation and deep percolation depending on their texture (fine or
coarse). They generally contain significant amounts of water-soluble salts
(i.e. sodium, chloride, calcium, magnesium, and sulfate) as measured by an EC
greater than 4 dS/m, an ESP less than 15%, and a pH less than 8.5 (acidity).
Naturally-forming saline soils can be found along the coasts and in arid regions
with minimal amounts of clay and organic matter.
Sodic soils are, for the most part, dense and clogged with low air, water,
and gas permeability. These soils are characterized by a low soluble salt
content (EC less than 4 dS/m), poor soil structure (ESP greater than 15%), and
alkalinity (pH greater than 8.5). Sodicity is usually associated with high
concentrations of insoluble sodium carbonate and bicarbonate which bind to
clay/organic particles, cause swelling in the soil matrix, and form a dense
crust (claypan) near the surface. Sodic soils have become some of the most
unproductive and difficult to reclaim, affecting once fertile irrigated river
basins and grazing prairies.
Saline-sodic soils, prevalent in the arid and semi-arid regions, often
represent a transitional stage between salinity and sodicity with an EC greater
than 4 dS/m, ESP greater than 15%, and pH less than 8.5. Without physical
manipulation, calcium amendments, and proper water infiltration, these soils are
often not amenable to normal leaching especially as sodicity tends to prevail in
the upper layers.
Lands deemed suitable
for modern commercial farming are largely determined by their elemental
(nutrient) composition, permeability, and structure (tilth). These qualities
are directly influenced by the salinity of the soil profile and irrigation water
(EC or TDS), sodium imbalances relative to other elements (ESP or SAR), and the
soluble nutrient availability or toxicity (pH). In contrast to saline soils,
the high alkalinity of sodic soils further limits agricultural productivity as
sodium assumes a greater proportion of the total salt concentration, and the
resulting negative charge causes clay/organic particles to swell. This
expansion in the soil matrix disturbs hydraulic conductivity, reduces leaching
capacity, and diminishes the solubility of other essential nutrients needed by
plants (i.e. iron, manganese, zinc, cobalt, copper, boron, and phosphorus).
When using EC measurements to characterize salinity, water is generally
considered non-saline at less than 0.7 dS/m, brackish between
0.7-2.0 dS/m and saline (to varying degrees) above 2 dS/m; rain or
distilled water has an EC value of 0.02-0.05 dS/m whereas seawater, at the other
extreme, averages between 45-60 dS/m. Water salinity can also be easily
measured and classified according to TDS calculations.
potable, all crops
livestock, most crops
few halophytes, algae
These methods for
calculating the levels of soluble salts in our soils and water offer static
absolute values which can be useful as points of reference for comparing the
relative growth and yield potential of one crop, species, or population to
another. However, by themselves, these values offer no indication of their
significance and are virtually meaningless without considering the myriad of
interrelated factors associated with actual plant salt-tolerance. Therefore, a
combination of indices (i.e. EC, ESP, SAR, and pH) can only serve as a
convenient, albeit rough, guideline for determining salt-induced constraints
which inhibit plant growth and reduce yields.
Impact of Salinity on Plant Growth
Elevated levels of
soluble salts in soil moisture (>4 dS/m) inhibit the germination and growth of
most commercial crops, significantly reducing biomass production, economic
yields, and overall plant survival. Soil and water salinity is directly
correlated with salt accumulation in plant cells/tissue and decreased shoot/leaf
production, resulting in lower rates of photosynthesis and water consumption.
The symptoms of excessive salinity are similar to those associated with water
deficiencies including root desiccation (cell dehydration), increased
succulence, and diminished chlorophyll production. Plant stress (low immune
resistance), stunted growth, leaf burn, and necrosis become obvious as the level
of salinity increases.
Maximum salinity thresholds are generally defined as the amount of salt in the
root zone that a plant can endure without impairing growth. Other important
thresholds indicate the highest level of salt-tolerance accompanied by a
reduction in yield or biomass (usually between 10-50%) while zero yield
thresholds specify levels of salinity at which the plant can no longer survive.
A continuum exists between the extremes in salt-tolerance as demonstrated by the
diverse spectrum of terrestrial plants, from those that thrive in seawater and
higher salinities to those that are unable to tolerate even minimal
concentrations without significant decline.
Virtually all of our domesticated plants have been selected and bred from
freshwater ancestors that originated along the equator, and can ultimately be
traced back to the sea. Certain crops (i.e. asparagus, cotton, rapeseed, beets,
sorghum, barley, and rye) are considered 'moderately' salt-tolerant with maximum
thresholds between 5-10 dS/m, which for practical purposes can be seen as the
zone whereby plants cross into the halo-sphere. Beyond this point, generally
only those plants classified as halophytes are able to grow and produce normal
yields. In modern agriculture, crops classified as salt-sensitive generally
have maximum thresholds of 1-3 dS/m and zero yields at 8-16 dS/m while the more
salt-tolerant have maximum thresholds of 5-10 dS/m and zero yields at 16-24 dS/m.
At 8 dS/m, most commercial rice varieties experience a 50% yield decline while
barley growth by and large remains unaffected.
The major causes of growth and yield decline associated with salinity are
osmotic stress, specific ion toxicity, and nutrient deficiency.
This is the principle
cause of reduced plant growth and yield as salinity mounts. When separated by a
permeable membrane, osmotic pressures force water to move from lower to higher
salt concentrations. As the osmotic potential of soil moisture increases with
sodium and chloride accumulations, and exceeds that of the root cells, the
plant, unable to replenish the water lost through transpiration, will then begin
to desiccate. Dehydration due to changes in the soilís osmotic potential tends
to have a more immediate and detrimental impact on plant growth than subsequent
toxicities or deficiencies, although all three are interrelated.
Osmotic stress is somewhat analogous to water stress experienced under drought
conditions whereby plants are unable to absorb enough water to maintain vigor
and growth. Under saline conditions, plants absorb increasing amounts of salts
from the soil in order to adjust their osmotic potential and maintain cell
turgor and water absorption; whereas without salinity, plants rely primarily on
their production of organic solutes in order to cope with dehydration. The
impact of osmotic stress on germination is most considerable as the seed expends
most of its energy absorbing moisture and can easily poison itself by taking up
a disproportionate amount of salt.
Specific Ion Toxicity
Specific ion toxicity occurs in plants once sodium, chloride and, in
some cases, boron begin to penetrate cell membranes and concentrate in the
cytoplasm (cell sap). Ensuing ion imbalances tend to increase the carbohydrate
requirements of the roots in order to maintain proper metabolic function and a
balanced nutrient uptake. Carbohydrate deficiencies in the shoots/leaves
inhibit plant growth as the overall rates of respiration, transpiration, protein
and photosynthesis decline. Owing to this loss of vigor at both the cellular
and whole-plant level, leaves begin to lose their chlorophyll, turning yellow
then brown. Under certain conditions, slightly elevated concentrations of boron
can be toxic imposing immediate and severe restrictions on vegetative growth.
These are generally due
to specific ion imbalances that interfere with the uptake of vital
macro-nutrients (i.e. nitrogen and phosphorus) and inhibit their absorption by
the roots; consequently, plants suffer from malnourishment. Elevated soil
salinity reduces essential biological processes (i.e. flora and fauna) and
hinders the uptake of important nutrients from organic materials by limiting
their availability. Nutrient deficiencies in plants experiencing salt-stress
are often exacerbated by disturbed root function and stunted development as well
as the increased competition of sodium and chloride relative to other elements
such as potassium.
Although the causes of plant decline are not always distinguishable in their
effects, they can be summarized as a plantís increased demand for energy within
its root system in order to maintain adequate water and soluble nutrient uptake,
diverting the energy needed for proper cell function and development,
whole-plant adjustments, and optimum growth/yield.
Salt-tolerance refers to the plantís physiological ability to adjust
to salt stress and endure elevated concentrations in the rooting medium. Plant
salt-tolerance thresholds reported in the field are usually based on biomass or
yield reductions under saline conditions relative to those under non-saline
conditions. These measurements can be helpful when comparing the relative
tolerances of potential crops and, in most cases, are only applicable once
plants have been established in the field.
Individual plant salt-tolerance thresholds can vary widely in response to a host
of environmental and human factors that influence actual salinity in the root
zone and the plantís ability to adjust. The following are some of the more
important considerations when evaluating the potential growth and yield of crops
in salt-affected environments.
Stages of Plant Growth
Virtually all plants
exhibit the greatest sensitivity to salts during germination and seedling
development, when osmotic stress suppresses the proper hydration of the seed and
initial growth. Certain crops, such as rice and barley, have shown increased
sensitivity at other stages of growth (i.e. when producing seed), however, in
general, salt-tolerance thresholds tend to increase as plants mature. The life
cycle variability of plant salt-tolerance, from germination to maturity, is an
important consideration as is genetic and ecotype variation within a species.
It is important to remember that most threshold measurements and field
assessments refer to established and more mature plants.
Ambient factors (i.e.
temperature, humidity, light, and wind) significantly affect the plantís ability
to tolerate salts. Hot and dry environments tend to increase the severity of
salt-stress on plants: high temperatures tend to enhance sodium ion activity in
soils while aridity hastens evaporation at the surface and increases plant water
consumption/transpiration. Under these conditions, a plantís water potential is
effectively reduced, inducing further salt accumulation in its cells and
tissue. Other daily and seasonal factors such as wind, rain, and sun exposure
also play a role in determining actual salinity thresholds.
Soil and Water
Although topsoils tend
to accumulate the highest concentrations of salt due to evaporation, the
variability of salt concentrations within the soil profile (to the maximum depth
of the root zone) should be taken into consideration. Horizontal variability
within the field is also a factor, normally characterized by gradual transitions
from lower to higher salinity or abrupt patchiness in certain low-lying areas.
Soil texture (i.e. sand, loam, and clay), fertility, and permeability are
important characteristics that affect actual soil salinity and influence the
plant's physiological adjustments. Other considerations that should be taken
into account include inconsistencies in irrigation water, surface distribution,
methods (i.e. mechanization, chemical fertilizers, and irrigation) can affect
field salt-tolerance to a significant extent. For example, the frequency and
method of irrigation influences leaching and salinity levels in the root zone.
Fertilizer, pesticide, and herbicide residues often cause imbalances in soil and
water salt composition while intensive plowing and mechanical harvesting lead to
structural degradation and erosion.
Reclamation & Management
The reclamation of
salt-affected lands refers to human activities that attempt to remove excess
salts and restore productivity to our soils. The prevention and management of
salt accumulation throughout the root zone is by far the most salient issue in
maintaining healthy and productive fields. However, many of these techniques
are costly and relatively inefficient: the removal or leaching of salts and
their disposal or drainage is oftentimes only a temporary measure that ignores
the fundamental causes of salinity.
For saline soils, proper leaching and drainage are key interventions for
reducing salt accumulations. Sodic and compacted saline soils are often the
most difficult to reclaim, usually requiring a combination of treatments as a
result of their elemental imbalances and poor leaching qualities. Listed below
are some of the more prevalent methods of reclamation and management.
Many salt-affected soils
require some sort of physical manipulation before other reclamation techniques
can be implemented. Surface crusts, commonly found in the arid regions, must
first be removed (scraped) or flushed with water, properly disposed of, and
replaced with non-saline topsoil. Under certain circumstances, deep plowing
with soil inversion can be employed to bring relatively less saline subsoils to
the surface and the more saline topsoil to the bottom for faster and more
effective leaching in the root zone. This practice breaks up dense claypans,
improves water/air infiltration, and allows for the more rapid reclamation of
deeply disturbed soils. On sodic and compacted soils with low permeability in
the upper layers of the profile, techniques, such as auger hole drilling, can be
implemented for better water infiltration and revegetation. On their own,
physical soil manipulations may provide immediate short-term benefits while, at
the same time, exacerbate the underlying causes of salt imbalances.
The prevention and
management of salinity in the root zone often requires vast amounts of
increasingly scarce and expensive freshwater. In order for leaching to be
successful, greater quantities of salt must be removed than are added by
evapotranspiration and the application of water. On well-drained soils, excess
irrigation or flooding (ponding) is the most widespread method of flushing salts
below the critical layers of the profile. Leaching is most cost-effective when
lands are level and water tables are at their annual lows just before the onset
of the rainy season.
Whenever possible, water should have a low salt content and be applied slowly
and evenly in amounts equal to or less than the infiltration capacity of the
soil. Without adequate sub-surface drainage or free return to the sea, leaching
may be effective in the short-term removal of salts from the root zone, however,
a rising water table and subsequent waterlogging will eventually reduce and
cease agricultural production. Salinity constraints have become most acute in
dryland (as opposed to rainfed) agriculture where deep percolation water and
soluble salts filter back to groundwater sources, resulting in a cycle of
irrigation and leaching with increasingly saline water.
Many of the productivity
constraints associated with secondary salinity and waterlogging occur as a
result of poor drainage and the removal of deep-rooting vegetation. When the
amount of percolation water exceeds the carrying capacity of natural drainage
systems, engineered solutions for managing the water table can be effective in
preventing further salinization and rising water tables. In such cases,
horizontal and vertical systems (i.e. gravity-fed tile drainage and tube-well
pumping) can be employed to prevent lateral seepage and drain saline water that
has leached down through the soil profile or risen up to the root zone.
Unfortunately, good drainage infrastructure is expensive to establish and
maintain; these costs must include proper management and disposal so as not to
affect surrounding fields and water sources.
An innovative technique, known as
Integrated Farm Drainage Management
(IFDM), may prove to be a sustainable commercial method of restoring
productivity to salt-affected soils and water. By cultivating economically
useful halophytes and other salt-tolerant plants, IFDM fosters the multiple use
of irrigation water for a progressive succession of increasingly salt-tolerant
crops. Ultimately, highly saline wastewater is drained into solar evaporation
ponds that could be used for algae, seaweed, and aquaculture production or
energy generation before harvesting and disposing of crystallized salts.
Sodic soils tend to be
the most difficult to manage and rehabilitate because their structure and
composition has been significantly compromised as a result of high
concentrations of sodium ions trapped within the clay/organic particles. In
situations where water infiltration is poor, deep tillage is often necessary to
break up the restrictive layers before amendments are added. The direct
application of gypsum, commonly used to replace exchangeable sodium with
calcium, must be followed by regular flooding in order to leach the newly
soluble sodium ions below the root zone. The application of calcium-rich
amendments, broadcast evenly and tilled into the soil, is generally a slow and
costly method of reclaiming sodic and saline-sodic soils. Other similar
treatments that provide soluble calcium and reduce alkalinity include the
addition of sulfuric acid or elemental sulfur.
Organic farming and
permaculture offer alternative techniques for managing salinity in the root
zone, improving soil structure/fertility, and reducing surface evaporation.
Although plastic is the most prevalent method of retaining soil moisture,
traditional agro-forestry (i.e. alley, hedgerow, and inter-cropping) and organic
farming practices hold tremendous promise for revitalizing soils and sustaining
long-term agriculture in salt-affected areas. Halophytic grasses, shrubs, and
trees cultivated for shade, wind/rain protection, and mulch (leaf litter and
crop residues) have demonstrated their capacity for preventing excessive salt
accumulation that results from evaporation and erosion.
Mulching with decomposing vegetative matter has long been used to retain soil
moisture and create a favorable environment in which micro-organisms can carry
out essential biological processes that contribute to fertility. The planting
of green manure crops (i.e. leguminous trees and groundcover) is another
relatively low-cost technique that can easily be integrated into alternate
(fallow) or multi-cropping (polyculture) systems for a continuous supply of
mulch and fertilizer. When applied properly, organic amendments (i.e. manure,
biosolids, and compost) will, over time, improve soil structure, permeability,
and fertility creating more favorable conditions for plant growth and more
effective salt management. Other mulching methods involving non-leaching
inorganic materials (i.e. shell, gravel, and volcanic rock) have also been shown
to improve surface moisture retention and prevent topsoil erosion.
Halophytes and other
salt-tolerant perennials have demonstrated great potential for rehabilitating
saline lands and controlling soil erosion while, at the same time, offering both
commercial and environmental benefits. Extensive root penetration,
nitrogen-fixation, and the creation of humus improve soil quality and water
infiltration making fields more conducive to salt leaching. The cultivation of
extreme halophytes, that accumulate significant amounts of salt in their cells
and tissue, may actually result in the net removal (harvesting) of salts on
certain rainfed lands.
Under waterlogged conditions, the cultivation of deep-rooting, water-hungry
halophytes has been shown to reduce saline water tables and reclaim certain
inundated lands. Deep-rooting trees and shrubs, with their continuous demand
for water, help reduce excess soil moisture and push salts below the root zone
of most other plants. In addition, salt-tolerant vegetative bands around
agricultural fields can be strategically planted to filter drainage waste
(chemical residues) and intercept lateral flows of saline water. There are a
number of options available for revegetating salt-affected lands with halophytic
perennials including alley- and inter-cropping systems, agro-forestry
plantations, and rehabilitation/conservation schemes.
The use of reverse
osmosis and other pressure-driven membrane processes, that reduce the total
dissolved solids in surface and groundwater used for irrigation, can be helpful
in managing elevated salt concentrations on irrigated lands, particularly when
combined with efficient field distribution systems. Although expensive on a
large scale, the removal of salts from irrigation water may prove cost-effective
in extending the productive life of individual fields or particular trouble
spots. The appropriate disposal of brine slurry resulting from most
desalination processes must again be considered with regard to both its economic
and environmental costs.
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