SALINITY MANAGEMENT

OVERVIEW OF SALINITY CHALLENGE

A salinity problem exists if salt accumulates in the crop root zone to a concentration that causes a loss in yield. In irrigated areas, these salts often originate from a saline, high water table or from salts in the applied water. Yield reductions occur when the salts accumulate in the root zone to such an extent that the crop is no longer able to extract sufficient water from the salty soil solution, resulting in a water stress for a significant period of time. If water uptake is appreciably reduced, the plant slows its rate of growth. The plant symptoms are similar in appearance to those of drought, such as wilting, or a darker, bluish-green colour and sometimes thicker, waxier leaves. Symptoms vary with the growth stage, being more noticeable if the salts affect the plant during the early stages of growth. In some cases, mild salt effects may go entirely unnoticed because of a uniform reduction in growth across an entire field.

Salts that contribute to a salinity problem are water soluble and readily transported by water. A portion of the salts that accumulate from prior irrigations can be moved (leached) below the rooting depth if more irrigation water infiltrates the soil than is used by the crop during the crop season. Leaching is the key to controlling a water quality-related salinity problem. Over a period of time, salt removal by leaching must equal or exceed the salt additions from the applied water to prevent salt building up to a damaging concentration. The amount of leaching required is dependent upon the irrigation water quality and the salinity tolerance of the crop grown.

Salt content of the root zone varies with depth. It varies from approximately that of the irrigation water near the soil surface to many times that of the applied water at the bottom of the rooting depth. Salt concentration increases with depth due to plants extracting water but leaving salts behind in a greatly reduced volume of soil water. Each subsequent irrigation pushes (leaches) the salts deeper into the root zone where they continue to accumulate until leached. The lower rooting depth salinity will depend upon the leaching that has occurred.

Following an irrigation, the most readily available water is in the upper root zone - a low salinity area. As the crop uses water, the upper root zone becomes depleted and the zone of most readily available water changes toward the deeper parts as the time interval between irrigations is extended. These lower depths are usually more salty. The

crop does not respond to the extremes of low or high salinity in the rooting depth but integrates water availability and takes water from wherever it is most readily available. Irrigation timing is thus important in maintaining a high soil-water availability and reducing the problems caused when the crop must draw a significant portion of its water from the less available, higher salinity soil-water deeper in the root zone. For good crop production, equal importance must be given to maintaining a high soil-water availability and to leaching accumulated salts from the rooting depth before the salt concentration exceeds the tolerance of the plant.

For crops irrigated infrequently, as is normal when using surface methods and conventional irrigation management, crop yield is best correlated with the average root zone salinity, but for crops irrigated on a daily, or near daily basis (localized or drip irrigation) crop yields are better correlated with the water-uptake weighted root zone salinity (Rhoades 1982). The differences are not great but may become important in the higher range of salinity. In this paper, discussions are based on crop response to the average root zone salinity.

In irrigated agriculture, many salinity problems are associated with or strongly influenced by a shallow water table (within 2 metres of the surface). Salts accumulate in this water table and frequently become an important additional source of salt that moves upward into the crop root zone. Control of an existing shallow water table is thus essential to salinity control and to successful long-term irrigated agriculture. Higher salinity water requires appreciable extra water for leaching, which adds greatly to a potential water table (drainage) problem and makes long-term irrigated agriculture nearly impossible to achieve without adequate drainage. If drainage is adequate, salinity control becomes simply good management to ensure that the crop is adequately supplied with water at all times and that enough leaching water is applied to control salts within the tolerance of the crop.

CAUSES OF SALINITY

Salt Sources

Saline soils are found in many parts of Pakistan. 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

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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.

Salinity occurs due to increased rates of leakage and groundwater recharge causing the watertable to rise. Rising watertables can bring salts into the plant root zone which affects both plant growth and soil structure. The salt remains behind in the soil when water is taken up by plants or lost to evaporation.

Recharge rates in irrigation areas can be much higher than dryland areas due to leakage from both rainfall and irrigation. This causes potentially very high salinisation rates. Watertables within two metres of the soil surface indicate the potential for salts to accumulate at the soil surface.

Inefficient irrigation and drainage systems are a major cause of excess leakage and increase the risk of salinity and waterlogging in irrigation areas. Poor water distribution on paddocks results in some areas being under-irrigated, causing salts to accumulate (where watertables are high) and other areas being over-irrigated and waterlogged. Groundwater mounds can develop under irrigation areas as a result of leakage from inefficient systems and restrictive layers. This puts pressure on the regional groundwater system forcing saline groundwater into waterways. Irrigating with saline water adds salt to the soil and increases the need for applying more irrigation water to leach salts past the plant root zone.

Continual under-irrigation also increases salinity as salts contained in the irrigation water need to be flushed or leached periodically to prevent them accumulating to levels that limit productivity. Inappropriate matching of crop, soil type and irrigation method can also cause excessive leakage. For example, irrigating high water-use crops using inappropriate irrigation methods should not be carried out on permeable soils (high sand content). Other factors which influence leakage rates include soil type (Figure 2), climate and the amount (or removal) of deep-rooted perennial vegetation. Replacing deep-rooted perennial pasture with irrigated annual crops reduces the level of evapotranspiration as rates are low following cultivation and during fallow periods. As a result, more water will infiltrate the soil profile and enter the watertable.

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THE EFFECT OF SALT ON PLANTS AND SOIL

The effect of salt on plants

As salts accumulate in saline discharge areas they can reach levels that affect plants in a number of ways. This leads to poor plant health, a loss of productive species and dominance of salt-tolerant species.

Osmotic effect

Under normal conditions, plants readily obtain water from the soil by osmosis (movement of water from a lower salt concentration outside the plant to a higher salt concentration in the plant). As soil salinity increases this balance shifts making it more difficult for plants to extract water.

Toxic effect

Plant growth can be directly affected by high levels of toxic ions such as sodium and chloride. Excess sodium accumulation in leaves can cause leaf burn, necrotic (dead) patches and even defoliation. Plants affected by chloride toxicity exhibit similar foliar symptoms, such as leaf bronzing and necrotic spots in some species. Defoliation can occur in some woody species.

Ionic imbalance

An excess of some salts can cause an imbalance in the ideal ratio of salts in solution and reduce the ability of plants to take up nutrients. For example, relatively high levels of calcium can inhibit the uptake of iron (‘lime induced chlorosis’), and high sodium can exclude potassium.

The effect of salt on soil

Highly saline soils often become highly sodic. The ion imbalance and effect on the soil will depend on the type of salt present. Sodium and magnesium ions can destroy soil structure whereas calcium carbonate may improve soil structure (due to calcium) and increase soil pH (due to carbonate). Highly saline soils may have dark greasy patches where organic matter has been destabilised. On very salty sites a complete loss of groundcover and visible salt crystals often occur on the soil surface making it vulnerable to erosion.

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MEASURING 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).

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)

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. 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).

Reporting the concentrations of ions and molecules

When reporting the concentrations of elemental ions and dissolved molecules, a commercial laboratory typically will report the results in milligrams per liter or milliequivalents per liter, or both. The former unit is useful when you need to evaluate

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the total mass or the "mass concentration" (i.e., milligrams per liter) of particular constituents. The latter type of measure — milliequivalents per liter — is the preferred reporting method if you need to check the quality of a water analysis or to calculate certain water quality parameters that involve electrochemistry. (One example is the SAR, described in detail later in this tutorial.)

A laboratory also may report concentrations of constituents in parts per million (ppm) or parts per billion (ppb). The ppm unit can be thought of as milligrams of solute per million milligrams of solution (water), or as milligrams of solute per kilogram of solution:

ppm = mg solute / 106 milligrams solution = mg/liter

= mg solute / kg solution

Similarly, parts per billion (ppb) is defined as follows:

ppm = μg solute / 109 milligrams solution = μg/liter

= μg solute / kg solution

Equating the units of mass per kilogram and mass per liter is appropriate for a dilute solution such as irrigation water. That's because, except for highly saline water, the density of water is very close to 1.00 kilograms per liter. (For brines and other waters of extremely high salinity, it is necessary to account for the higher-than-unity solution density. However, brines are not considered in detail here, as they are never suitable as irrigation water.)

Concentration data reported in milligrams per liter (mg/L) can be converted to milliequivalents per liter (meq/L), and vice versa. Simply use the following formula:

mg/L = meq/L × equivalent weight

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SALINITY CONTROL AND SALINITY ADAPTATION MEASURES

Leaching for Salinity Management

Soluble salts that accumulate in soils must be leached below the crop root zone to maintain productivity. Leaching is the basic management tool for controlling salinity. Water is applied in excess of the total amount used by the crop and lost to evaporation. The strategy is to keep the salts in solution and flush them below the root zone. The amount of water needed is referred to as the leaching requirement or the leaching fraction. Excess water may be applied with every irrigation to provide the water needed for leaching. However, the time interval between leachings does not appear to be critical provided that crop tolerances are not exceeded. Hence, leaching can be accomplished with each irrigation, every few irrigations, once yearly, or even longer depending on the severity of the salinity problem and salt tolerance of the crop. An occasional or annual leaching event where water is ponded on the surface is an easy and effective method for controlling soil salinity. In some areas, normal rainfall provides adequate leaching.

Determining Required Leaching Fraction

The leaching fraction is commonly calculated using the following relationship:

LF = ECiw/ ECe (1)

where

LF = leaching fraction, the fraction of applied irrigation water that must be leached through the root zone

ECiw =electric conductivity of the irrigation water

ECe = the electric conductivity of the soil in the root zone

Equation (1) can be used to determine the leaching fraction necessary to maintain the root zone at a targeted salinity level. If the amount of water available for leaching is fixed, then the equation can be used to calculate the salinity level that will be maintained in the root zone with that amount of leaching. Please note that equation (1) simplifies a complicated soil water process. ECe should be checked periodically and the amount of leaching adjusted accordingly. Based on this equation,

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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 necessary. 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. After drainage appears adequate, the leaching process can begin.

Seed Placement

Obtaining a satisfactory stand is often a problem when furrow irrigating with saline water. Growers sometimes compensate for poor germination by planting two or three times as much seed as normally would be required. However, planting procedures can be adjusted to lower the salinity in the soil around the germinating seeds. Good salinity control is often achieved with a combination of suitable practices, bed shapes and irrigation water management. In furrow-irrigated soils, planting seeds in the center of a single-row, raised bed places the seeds exactly where salts are expected to concentrate (Figure 3). This situation can be avoided using “salt ridges.” With a double-row raised planting bed, the seeds are placed near the shoulders and away from the area of greatest salt accumulation. Alternate-furrow irrigation may help in some cases. If alternate furrows are irrigated, salts often can be moved beyond the single seed row to the non-irrigated side of the planting bed. Salts will still accumulate, but accumulation at the center of the bed will be reduced.

With either single- or double- row plantings, increasing the depth of the water in the furrow can improve germination in saline soils. Another practice is to use sloping beds, with the seeds planted on the sloping side just above the water line (Fig. 3b). Seed and plant placement is also important with the use of drip irrigation. Typical wetting patterns of drip emitters and micro-sprinklers are shown in Figure 4. Salts tend to move out and upward, and will accumulate in the areas shown.

Other Salinity Management Techniques

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Techniques for controlling salinity that require relatively minor changes are more frequent irrigations, selection of more salt-tolerant crops, additional leaching, pre-plant irrigation, bed forming and seed placement. Alternatives that require significant changes in management are changing the irrigation method, altering the water supply, land-leveling, modifying the soil profile, and installing subsurface drainage.

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Residue Management

The common saying “salt loves bare soils” refers to the fact that exposed soils have higher evaporation rates than those covered by residues. Residues left on the soil surface reduce evaporation. Thus, less salts will accumulate and rainfall will be more effective in providing for leaching.

More Frequent Irrigations

Salt concentrations increase in the soil as water is extracted by the crop. Typically, salt concentrations are lowest following an irrigation and higher just before the next irrigation. Increasing irrigation frequency maintains a more constant moisture content in the soil. Thus, more of the salts are then kept in solution which aids the leaching process. Surge flow irrigation is often effective at reducing the minimum depth of irrigation that can be applied with furrow irrigation systems. Thus, a larger number of irrigations are possible using the same amount of water. With proper placement, drip irrigation is very effective at flushing salts, and water can be applied almost continuously. Center pivots equipped with LEPA water applicators offer similar efficiencies and control as drip irrigation at less than half

Preplant Irrigation

Salts often accumulate near the soil surface during fallow periods, particularly when water tables are high or when off-season rainfall is below normal. Under these conditions, seed germination and seedling growth can be seriously reduced unless the soil is leached before planting.

Changing Surface Irrigation Method

Surface irrigation methods, such as flood, basin, furrow and border are usually not sufficiently flexible to permit changes in frequency of irrigation or depth of water applied per irrigation. For example, with furrow irrigation it may not be possible to reduce the depth of water applied below 3-4 inches. As a result, irrigating more frequently might improve water availability to the crop but might also waste water. Converting to surge flow irrigation may be the solution for many furrow systems. Otherwise a sprinkler or drip irrigation system may be required.

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Chemical Amendments

In sodic soils (or sodium affected soils), sodium ions have become attached to and adsorbed onto the soil particles. This causes a breakdown in soil structure and results in soil sealing or “cementing,” making it difficult for water to infiltrate. irrigation water will increase during the entire time water is transported through irrigation canals or stored in reservoirs. Replacing irrigation ditches with pipe systems will help stabilize salinity levels. In addition, pipe systems, including gated pipe and lay-flat tubing, reduce water lost to canal seepage and increase the amount of water available for leaching. Chemical amendments are used in order to help facilitate the displacement of these sodium ions. Amendments are composed of sulphur in its elemental form or related compounds such as sulfuric acid and gypsum. Gypsum also contains calcium which is an important element in correcting these conditions. Some chemical amendments render the natural calcium in the soil more soluble. As a result, calcium replaces the adsorbed sodium which helps restore the infiltration capacity of the soil. Polymers are also beginning to be used for treating sodic soils.

It is important to note that use of amendments does not eliminate the need for leaching. Excess water must still be applied to leach out the displaced sodium. Chemical amendments are only effective on sodium- affected soils. Amendments are ineffective for saline soil conditions and often will increase the existing salinity problem.

Adaptation to Salinity

Plants adapted to saline waters are halophytes. These plants have a mechanism to resist the internal water loss to the external environmental salinity or they adjust themselves inside to match the salt water they live in. This reduces plants metabolisms to either to salt avoidance or salt tolerance.

Plants in terrestrial situations with high salinity must use a biochemical pathway to exclude the salt and take up the water at their roots. Others take up the salty water and then separate the salt by biochemical pathways and sequester it inside.

Excretive plants secrete accumulated salt electrolytes from trichome (leaf hairs) gland pores onto the leaf surface. They avoid or exclude salts to reduce their salt content.

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Succulent plants include salt and tolerate it by increasing the water content in large cell vacuoles thus minimizing salt toxicity.

These pathways are expensive but allowed the plants to occupy a niche. Other plants lack these specialized adaptations. Halophytes are salt tolerant hydrophytes

Mangroves are unique amongst land plants in their ability to tolerate a wide range of salinities. They do this by one of three adaptations:

Secreting salts from their leaves. This occurs through salt glands. The salt can often be seen sitting on the surface. If you lick a mangrove leaf you can taste the salt. The salt is removed from the leaves during rain. (Grey Mangrove, River Mangrove)

Excluding salt. These mangroves have special tissue in their roots which prevent larger salt molecules from entering but allow smaller water molecules to pass through. This can prevent 80% of the salt entering. (Most of the NSW Mangroves do this to some extent)

Salt storage. Salt is stored in leaves which then fall off the plant taking the salt with them (Milky Mangrove).

Salt excretion on mangrove leaves

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Bio-saline agriculture

Biosaline agriculture is a relatively new way of dealing with salinity in agriculture. It develops cropping systems for saline environments, using the capacity of certain plants to grow under saline conditions in combination with the use of saline soil- and water-resources and improved soil and water management. As salinity is influencing our environments more and more, there is un urgency in developing cropping systems that can produce in saline conditions.

Halophytes and salt tolerant plants

The species used by biosaline agriculture are salt tolerant plants – which support higher salinity levels of soil and water - and halophytes which even produce better under saline circumstances.

Saline water resources

Globally 98% of the water volume is seawater, 1% is fresh water and 1% is brackish water. Agricultural use is currently responsible for 70% of all fresh water consumption. With great differences between the various regions. Competition between the various users of fresh water is growing fast. Industry and megalopolises will ask for and get a larger share. The growing population of the world needs more food and agricultural crops for energy and industrial uses. It is highly unlikely however that there will be more water available for agriculture. Solutions are twofold: (1) increase the efficiency of the current water use and (2) learn to use other water resources: brackish water and seawater.

A strong point of biosaline agriculture is that it does not depend on fresh water, but uses unconventional saline water resources: in some cases seawater can be used (salicornia, mangroves, seawater grasses), but brackish water offers a higher potential in salt tolerant species that can flourish on it.

Sources of saline water:

• Naturally available in the seas • Naturally available in groundwater or deeper aquifers • Drainage water from irrigated agriculture

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• Excess water from oil production, geo thermal plants etc • Waste water from industrial use or from intensive fish & shrimp farming

Biosaline technology is based on two major components: the knowledge of salt tolerant plants and halophytes in various saline environments and how to improve their productivity via plant breeding and agronomics.

The knowledge of salinity management in various saline environments including infrastructure for irrigation and drainage adapted to the needs of specific salt tolerant and halophytic crops.

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