Biosalinity Awareness Project

...understanding the impact of salinization and implications for future agriculture

Home
Global Perspective
Soil, Water & Plants
Halophytes
Salt-Tolerant Plants
Biosaline Agriculture
Current Research
News & Interviews
Books & Articles
Seed Sources
Project Members
Contact

 

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.  For links to many of the topics addressed below, refer to the Books & Articles page.

Causes & Manifestations of Salinity

Salinity Measurements & Classifications

Impact of Salinity on Plant Growth

Reclamation & Management Techniques
 

Causes & Manifestations of Salinity

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

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 aquifers. 

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 water flows. 

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 lands. 

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
Southwestern US, 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 & Classifications

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.

MEASURING SALINITY

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 and cobalt).

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).

CLASSIFYING SALINITY

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.

Soils EC (dS/m) ESP (%) SAR pH
Saline >4 <15 <12 <8.5
Sodic <4 >15 >12 >8.5
Saline-Sodic >4 >15 >12 <8.5

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. 

Water

EC (dS/m)

TDS (mg/l)

Application

Fresh

<0.7

<500

potable, all crops

Brackish

0.7-2.0

500-1,500

livestock, most crops

Moderate

2-10

1,500-7,000

salt-tolerant crops

High/Severe

10-45

7,000-35,000

most halophytes

Seawater

45-60

35,000-45,000

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.

Osmotic Stress

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. 

Nutrient Deficiencies

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 VARIABILITY

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. 

Climatic Conditions

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 Variability

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, and infiltration.

Human Interventions

Modern cultivation 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 Techniques

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.

Physical Manipulation

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.

Leaching

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. 

Drainage

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.


Calcium Amendments

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 Amendments

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.

Revegetation

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.

Desalination

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. 
 

TOP OF PAGE
 
Copyright © Biosalinity Awareness Project All Rights Reserved