Soil Salinity

SALINITY

The salinity of soil refers to the amount of salts in the soil.  It can be estimated by measuring the electrical conductivity (EC) of an extracted soil solution. Worldwide, the major factor in the development of saline soils is a lack of precipitation. Most naturally saline soils are found in (semi)arid regions and climates of the globe.

SOIL AFFECTED AREA :

Normally, the salinization of agricultural land affects a considerable area of irrigation projects, on the order of 20 to 30%.  A regional distribution of the 3,230,000 km² of saline land world wide is shown in the following table derived from the FAO/UNESCO Soil Map of the World.

Region	Area (106ha)
Australia	84.7
Africa	69.5
Latin America	59.4
Near and Middle East	53.1
Europe	20.7
Asia and Far East	19.5
Northern America	16.0

o                  CAUSES OF SOIL SALINITY :

Salt-affected soils are caused by excess accumulation of salts, typically most pronounced at the soil surface. Salts can be transported to the soil surface by capillary transport from a salt laden water table and then accumulate due to evaporation. They can also be concentrated in soils due to human activity, for example the use of potassium as fertilizer, which can form sylvite, a naturally occurring salt. As soil salinity increases, salt effects can result in degradation of soils and vegetation.

Salinization is a process that results from:

Ø               High levels of salt in the soils.

Ø               Landscape features that allow salts to become mobile.

Ø               Climatic trends that favoUr accumulation.

Ø               Human activities such as land clearing, aquaculture activities and the salting of icy roads.

o                  NATURAL OCCURENCE :

Salt is a natural element of soils and water. The ions responsible for salinization are: Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cl^-.

As the Na⁺ (sodium) predominates, soils can become sodic. Sodic soils present particular challenges because they tend to have very poor structure which limits or prevents water infiltration and drainage.

o                  DRY LAND SALINITY :

Salinity in drylands can occur when the water table is between two to three metres from the surface of the soil. The salts from the groundwater are raised by capillary action to the surface of the soil. This occurs when groundwater is saline (which is true in many areas), and is favored by land use practices allowing more rainwater to enter the aquifer than it could accommodate.

For example, the clearing of trees for agriculture is a major reason for dryland salinity in some areas.

o                  SALINITY DUE TO IRRIGATION :

Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and eventually begin to accumulate.

Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must be leached out of the plant root zone by applying additional water. Salination from irrigation water is also greatly increased by poor drainage and use of saline water for irrigating agricultural crops.

o                  EFFECT OF SALINITY ON PLANT :

Salinity can affect plant growth in several ways, directly and indirectly:

Ø                 DIRECT SOIL SALINITY DAMAGE :

§                  DECREASED WATER UPTAKE : High salts concentration results in high osmotic potential of the soil solution, so the plant has to use more energy to absorb water. Under extreme salinity conditions, plants may be unable to absorb water and will wilt, even when the surrounding soil is saturated. 

§                   ION-SPECIFIC TOXICITY : When a plant absorbs water containing ions of harmful salts (e.g. Sodium, Chloride, excess of Boron etc.), visual symptoms might appear, such as stunted plant growth, small leaves, marginal necrosis of  leaves or fruit distortions.

Ø                 INDIRECT SOIL SALINITY :

§                  INTERFERENCE WITH UPTAKE OF ESSENTIAL NUTRIENTS : An imbalance in the salts content may result in a harmful competition between elements. This condition is called "antagonism", i.e. an excess of one ion limits the uptake of another ion. For example, excess of chloride reduces the uptake of nitrate, excess of phosphor reduces the uptake of manganese, and excess of potassium limits the uptake of calcium.

§                  SODIUM EFFECTS ON SOIL STRUCTURE : In saline soils, sodium replaces calcium and magnesium, which are adsorbed to the surface of clay particles in the soil. Thus, aggregation of soil particles is reduced, and the soil will tend to disperse. When wet, a sodic soil tends to seal, its permeability is dramatically reduced, and thus water infiltration capacity is reduced as well. When dry, a sodic soil becomes hard has the tendency to crack. This may result in damages to roots.

Salinity by itself actually improves soil structure and eliminates to some degree the negative effect of sodium ions, but of course, salinity cannot be increased without affecting plants growth.

o                  EFFECT OF SALINITY ON SOIL PHYSICAL PROPERTIES  :   

Soil water salinity can affect soil physical properties by causing fine particles to bind together into aggregates. This process is known as flocculation and is beneficial in terms of soil aeration, root penetration, and root growth. Although increasing soil solution salinity has a positive effect on soil aggregation and stabilization, at high levels salinity can have negative and potentially lethal effects on plants. As a result, salinity cannot be increased to maintain soil structure without considering potential impacts on plant health.

o                  MORPHOLOGICAL SYMPTOMS :

Most of the parameters like low tillering, spikelet sterility, less florets per panicle, low 1000 grain weight and leaf scorching, are affected uniformly under both sodicity and salinity, however it is not a thumb rule.

Major symptoms are : 

§       White leaf tip followed by tip burning (salinity)

§       Leaf browning & death (sodicity)

§       Stunted plant growth

§       Spikelet sterility

§       Low harvest index

§       Poor root growth

§       Patchy growth in field

o                  TYPES OF SALT-AFFECTED SOIL :

Salt buildup can result in three types of soils:

I.      SALINE SOIL :

Saline soils contain enough soluble salts to injure plants. They are characterized by white or light brown crusts on the surface. Salts generally found in saline soils include NaCl (table salt), CaCl2, gypsum (CaSO4), magnesium sulfate, potassium chloride and sodium sulfate. The calcium and magnesium salts are at a high enough concentration to offset the negative soil effects of the sodium salts. The pH of saline soils is generally below 8.5.

II.      SALINE-SODIC SOIL :

Saline-sodic soils are like saline soils, except that they have significantly higher concentrations of sodium salts relative to calcium and magnesium salts. The pH is generally below 8.5. Water moves through these soils much as it does in saline soils, although the steps for correcting saline-sodic soil are different. Simply leaching the salts from this soil will convert it from saline-sodic to sodic soils.

III.       SODIC SOIL :

Sodic soils are low in soluble salts but relatively high in exchangeable sodium. Sodic soils are unsuitable for many plants because of their high sodium concentration, which may cause plant rooting problems, and because of their high pH, which generally ranges from 8.5 to 12.0.

These high sodium levels disrupt both the chemical and physical composition of soil clays. As a result, the soil surface has low permeability to air, rain and irrigation water. The soil is sticky when wet but forms hard clods and crusts upon drying. This phenomenon may not occur in very sandy soils because they lack clay content.

o      MEASURES TAKEN FOR ANTI-SALINIZATION :

If steps are taken early, to correct the soil it will be easier and less expensive, and it will have less negative impact on soils and plants.

§                    SELECTION OF CROPS SUITING THE CONDITION :

Ø                SOIL TYPE-  water infiltration capacity, how much air does the soil contain ,how much water will be needed to wash the soil in order to avoid salinity build up should be known. It is better avoid planting a salt sensitive crop in a soil which is not well drained.

Ø               THE MICROCLIMATE CONDITIONS IN THE FIELD - parameters such as wind direction and solar radiation should be measured because they may affect water consumption of the crop.

Ø               TYPE OF IRRIGATION SYSTEM & ITS DISTRIBUTION - Each type of irrigation system has its own water distribution pattern, depending also on the soil properties. Make sure the emitters are set in the appropriate spacing, to allow uniform irrigation depending on your soil type.

§                    USE OF APPROPRAITE FERTILIZER :

The fertilizers type and their quantities should coincide with to the requirements of the crop and with nutrients which are already in the soil. There are fertilizers which contain salts which are not taken up by plants in large amounts, such as chlorides. These salts tend to accumulate in the soil.

§                    INTERVALS BETWEEN IRRIGATION :

Irrigation regimen and intervals must be appropriate to the soil conditions and to growth stage of the crop. Frequent and shallow (superficial) applications result in salt accumulation in the root zone, while larger applications, in longer intervals, will flush the salts below the root zone.

§                    PERIODIC TESTS OF SOIL :

A practical approach in order to prevent salinity buildup early enough is sampling the soil 5 times over a growing period of 8 months (a test every 6 weeks or so). It is recommended to do at least one water analysis as well. The tests will indicate any change in soil content, allowing you to adjust the fertilization and irrigation regimen as needed.

o                  MEASURES TAKEN FOR DE-SALINIZATION :

Salt-affected soils can be corrected by:

§       Improving drainage

§       Reducing evaporation

§       Leaching

§       Application of chemical treatments

§       A combination of these methods

§                         REDUCING EVAPORATION :

Applying residue or mulch to the soil can help lower evaporation rates

§                    IMPROVING DRAINAGE :

In soils with poor drainage, deep tillage can be used to break up the soil surface as well as claypans and hardpans, which are layers of clay or other hard soils that restrict the downward flow of water. Tilling helps the water move downward through the soil.

While deep tillage will help temporarily, the parts of the soil not permanently broken up may reseal.

§                                       LEACHING :

Leaching can be used to reduce the salts in soils. Enough low-salt water should be added to the soil surface to dissolve the salts and move them below the root zone. The water must be relatively free of salts (1,500 - 2,000 ppm total salts), particularly sodium salts. A water test can determine the level of salts in your water.

Leaching works well on saline soils that have good structure and internal drainage. Highly saline soils should be leached using several applications, so that the water can drain well.

§                                      CHEMICAL TREATMENTS :

Before leaching saline-sodic and sodic soils, it must first treated with chemicals, to reduce the exchangeable sodium content. To remove or exchange with the sodium, add calcium in a soluble form such as gypsum. The laboratory analysis can determine how much calcium to add.

After the calcium treatment, the sodium can then be leached through the soil along with the other soluble salts. Gypsum is the most common amendment used to correct saline-sodic or sodic soils that have no calcium source such as gypsum or free carbonates.

Water-Logging

Water-logging is one of the major problems of land degradation. An irrigated area is said to be waterlogged when the surplus water stagnates due to poor drainage or when the shallow water table rises to an extent that soil pores in the root zone of a crop become saturated, resulting in restriction of the normal circulation of the air, decline in the level of oxygen and increase in the level of carbon dioxide.

Water-logging is often accompanied by soil salinity as waterlogged soils prevent leaching of the salts imported by the irrigation water.

o                  EFFECTS OF WATERLOGGING :

Waterlogging occurs when the soil is saturated with water. Heavy soils are most likely to waterlog. They have limited pore space through which water and air can move only very slowly. If the soil is saturated for too long oxygen is used up. Then roots stop growing and absorbing nutrients, stomata close preventing photosynthesis and soil denitrification commences. Because plants cannot absorb their nitrogen from the soil, they have to extract it from older leaves to support the growth of new leaves. During this extraction, old leaves become ‘nitrogen deficient’ and yellow during a period of waterlogging. Generally there is not enough nitrogen available in old leaves to support new tiller growth, so tillering does not occur.

Wheat deteriorates rapidly in waterlogged soils if temperatures are high; seedlings die within as little as 2 days. Later stages are more tolerant but can still lose a high proportion of their leaf area and yield. Waterlogging is avoided by ensuring that any water drains through the soil before it has time to stagnate. Wheat growing acceptably in mildly saline soils will not survive if waterlogging occurs.

The growth of most crops is affected when groundwater is shallow enough to maintain the soil profile in the root zone wetter than field capacity. This excess water and the resulting continuously wet root zone can lead to some serious and fatal diseases of the root and stem. Working the soil when overly wet can destroy soil structure and thus restrict root growth and drainage further. The chemistry and microbiology of waterlogged soils is changed due to the absence of oxygen.

o      TOLERANCE LEVELS OF CROP TO WATERLOGGING :

Crops vary in their tolerances to waterlogging and a high water table. The table below presents the different tolerances of some crops.

Ø     HIGH TOLERANCE :  Sugarcane, Potatoes, Rice, Willow, Plum, Broad beans, Strawberries, some Grasses etc.

Ø   MEDIUM TOLERANCE : Sugarbeet, Wheat, Oats, Citrus, Bananas, Apple, Barley, Peas, Cotton,  Pears, Blackberries, Onion etc.

Ø   SENSITIVE : Maize, Tobacco, Peaches, Cherries, Olives, Peas, Beans, Date palm etc.

  

o      CAUSES OF WATERLOGGING :

§              The soil is inadequately drained.

§             The seedbed is above the  level of the drainage channels.

§             The field is not level.

§             Too much irrigation water has been applied which cannot drain sufficiently quickly.

§             Rainfall has been heavy.

§             The soil is naturally heavy with poor structure and inadequate pore space.

§             After heavy rainfall waterlogging can occur even in light soils because crusting seals the soil surface and prevents air from entering.

§             Not restricting irrigation supplies during period of no demands.

§             High sub-soil water table conditions

o                  MINIMIZING THE EFFECTS OF WATERLOGGING :

§                    NITROGEN :

Apply nitrogen after a period of waterlogging. It will make nitrate readily available and accelerate plant recovery.

§                    WEEDS :

Keep the field free of weeds to reduce competition for oxygen in the root zone.

§                    CULTIVATION :

Consider a light cultivation if crusting occurs after intense rainfall. This will help aerate the saturated soil.

§                    RAISED BEDS :

If the soil is prone to waterlogging, consider adopting the raised bed system with its intrinsically good drainage.

§                    LEVELING :

Level the field, improve the drainage channels and put them closer together.

§                    GREEN MANURE :

Next season, grow and then incorporate a green manure crop to improve soil organic matter and pore space. Alternatively incorporate farmyard manure or crop residues.

§                    DEEP CULTIVATION :

Use deep cultivation to increase soil pore space and break up any hard pans that might have developed. Pore space should be around 10% to avoid waterlogging.

§                    IRRIGATION SCHEDULE :

Adjust the irrigation timetable to allow for rainfall events.