Salinity Stress in plants
Increased amount of soluble salt in soil solution.
Different types of salts, e.g. NaCl, Na2SO4, NaNO3, MgSO4,
MgCl2, K2SO4, CaCO3, etc., are present in saline soil in which NaCl causes serious problems for higher plants.
Soil Salinity Affects:
- Soil physico-chemical properties
- Biological soil properties
- Soil fertility
Source of Salinity
- Primary minerals
- From deeper soil layers (soil solution)
- evaporation > rainfall
- Irrigation water
- Coastal soils
- High doses of chemical fertilizers
Anions – SO42-, Cl-, CO32-,NO3-, PO43-
Cations – Ca2+, Mg2+, K+, Na+; Al3+, Fe3+
Reason for Soil Salinity
- A saline soil possesses high concentration of Na+, K+, Ca2+, Mg2+ and Cl− salts, which comes from weathering of minerals, irrigation water or evaporation of shallow groundwater.
- Due to insufficient precipitation, ions could not leach from the soil profile resulting salts to accumulate in the soil.
- Rainfall contains seawater salts, mainly sodium chloride (10 mg/kg) that would affect the land by deposition of 10 kg/ha of salt during each 100 mm of rainfall per year.
- In arid and semiarid regions of the world, soils are becoming saline due to poor irrigation management.
- Soil salinity is regularly increasing, and it has been estimated that 20% of total cultivated and 33% of irrigated agricultural lands worldwide are salt affected.
- If it happens continuously, the cultivable land would be 50% salinized by the year 2050.
Causes of salinity
Inherent soil salinity (weathering of rocks, parent material)
Brackish and saline irrigation water
Sea water intrusion into coastal lands as well as into the aquifer due to over extraction and overuse of fresh water
Restricted drainage and a rising water-table
Surface evaporation and plant transpiration
Sea w\ater sprays, condensed vapors which fall onto the soil as rainfall
Wind borne salts yielding saline fields
Overuse of fertilizers (chemical and farm manures)
Use of soil amendments (lime and gypsum)
Use of sewage sludge and/or treated sewage effluent
Dumping of industrial brine onto the soil
a. furrow irrigated
b sprinkler irrigated
c. sea water intrusion
Methods of Determination
- Measurement of electrical resistance
- Measurement of electrical conductivity
Determination of single ions
- by titration
- by advanced methods
Quantitative determination of total salinity by soil ethanol extract conductivity measurement
- Weight 10 g of soil into plastic flask
- Add 50 mL of 50% ethanol
- Let it shake for 45 min
- Filtration of soil suspension
- Measurement of soil ethanol extract conductivity by conductometer
Evaluation – conductivity of ethanol extract
- < 30 µS.cm-l : most agricultural soils, with normal (lower) intensity of fertilization and liming, with a minimum load of soil salts
- 30 – 60 µS.cm-l : mineral-rich soil with moderate to high intensity fertilizing and liming, without the negative effects of salinity
- 60 – 120 µS.cm-l: soils with a high degree of fertilization and liming in mineral-rich substrates (as well as highly acidic soil) with an increased load of soil salts (in loamy, clay soils without adverse effects)
- > 120 µS.cm-l : high load of soil salts with possible negative effects on plant growth and development (especially in drought conditions).
- 0-2 mS.cm-l :
- 2-4 mS.cm-l :
highly sensitive plants may be affected (bean, lime)
- 4-8 mS.cm-l:
most of sensitive plants can give lower yields
- 8-16 mS.cm-l :
only resistant plants gives satisfactory yields (wheat, olives)
- > 16 mS.cm-l :
only highly tolerant plants give satisfactory yields (barley, sugar beet, some palms)
- 0-4 mS.cm-l :
- 4-8 mS.cm-l :
- 8 -15 mS.cm-l:
- > 15 mS.cm-l :
Quanlitative determination of selected ions
1.Weight 10 g of soil
2.Add 50 mL of overboiled distilled water
3.Let it shake for 3 min
4.Filtrate the soil suspension into the beaker
CO32- : Take 2-3 mL of filtrate into test tube, add one drop of phenolphtalein, bring to a boil over burner
Pink colour → the carbonates are present
HCO3–: After the pink colour disappearance add one drop of methyl orange (into the same extract)
Yellow colour → hydrogen carbonates are present
Cl–: Move 2-3 mL of filtrate into test tube, add 3-5 drops of HNO3 and 3-5 drops of 5% AgNO3
White precipitate → chlorides are present
SO42-: Take 2-3 mL of filtrate into test tube, add 3 drops of 10% HCl and 3-5 drops of 10% BaCl2
White precipitate → sulphates are present (insoluble BaSO4 )
- Fresh water contains about 0.1% salt.
- Ca is predominant cation and carbonate is the principal anion.
- Main salts in sea water are the chlorides, sulphates and carbonates of Na, K, Ca and Mg.
- Na is the most dominant cation and Cl is the predominant anion.
Only some halophilic organisms can tolerate such high degree of salinity.
Measuring the Salinity of Water
- There are two main methods of determining the salt content of water: Total Dissolved Salts (or Solids) and Electrical Conductivity.
- Total Dissolved Salts (TDS) is measured by evaporating a known volume of water to dryness, then weighing the solid residue remaining.
- Electrical conductivity (EC)
- It is the measure of a material’s ability (e.g. water sample) to allow the transport of an electric charge.
- The more dissolved salt in the water, the stronger the current flow and the higher the EC. Measurements of EC can be used to give an estimate of TDS.
- Measurement of TDS is tedious and cannot be carried out in the field. EC measurement is much quicker and simpler and is very useful for field measurement.
- Soils are classified as saline when the ECe is 4 dS/m or more, which is equivalent to approximately 40 mM NaCl and generates an osmotic pressure of approximately 0.2 MPa. This definition of salinity derives from the ECe that significantly reduces the yield of most crops.
- Soil water flows from higher osmotic potential (low salt concentration) to lower osmotic potential (high salt concentration).
- A soil solution with low osmotic potential due to the higher concentration of soluble salts compared to the plant cells, will not allow plant roots to extract water from soil (figure), causing drought-like symptoms in the plants. That process is called “osmotic effect”
Classification of saline soils
- Natural salinity occurs where the rainfall is low and the salt remains in the subsoil. Salt can move in and out of the root zone with seasonal rainfall, giving rise to the term ‘transient salinity’.
- Neither agronomic nor engineering practices can address this form of salinity, and genetic improvement provides the only way to increase productivity of plants on these lands.
- Sodicity occurs in soils in which Na+ makes up a high percentage of the cations bound to clay particles. This causes loss of soil structure, and the soil becomes waterlogged when wet, and hard as it dries.
- Sodic soils affect plant growth because roots cannot penetrate layers that are hypoxic or hard.
- A soil is defined as sodic if the exchangeable sodium percentage (ESP) is 6% or greater than it.
- Some sodic soils are also saline or have serious chemical constraints such as very high pH, boron toxicity, aluminium toxicity and micronutrient deficiency.
- Sodicity is a measure of sodium ions in soil water, relative to calcium and magnesium ions. It is expressed either as sodium adsorption ratio (SAR) or as the exchangeable sodium percentage (ESP). If the SAR of the soil equals or is greater than 13 (mmoles l−1)0.5, or the ESP equals or is greater than 15, the soil is termed sodic
Visual Indicators of Soil Sodicity
- Poorer vegetative growth than normal, with only a few plants surviving, or with many stunted plants or trees
- Variable heights of the plants
- Poor penetration of rainwater – surface ponding
- Raindrop splash action – surface sealing and crusting (hard setting)
- Plants exhibit a shallow rooting depth
- Soil is often black in color due to the formation of a Na-humic substances complex
- In contrast to saline soils, sodic soils have excessive levels of sodium (Na+) adsorbed at the cation exchange sites (Figure). Soil sodicity causes degradation of soil structure. That process is called soil dispersion.
- The forces that hold clay particles together are greatly weakened when excessive sodium is adsorbed at the negative charges of clay particles, forming sodium-clay particles.
Table Classification of salt-affected soils
In Pakistan, about 6.30 million hectares of land are salt-affected and of which
- 1.89 million hectare is saline,
- 1.85 million hectare is permeable saline-sodic,
- 1.02 million hectare is impermeable saline-sodic
- 0.028 million hectare is sodic in nature.
In contrast to dryland salinity, secondary salinity refers to the salinization of soil due to human activities such as irrigated agriculture.
Common forms of secondary salinity are:
1.Irrigation—irrigated areas, either as a result of rising groundwater tables (from excessive irrigation) or the use of brackish irrigation or poor-quality water
2.Dryland—non-irrigated landscapes, generally as a result of clearing vegetation and changes in land use
3.Sea water intrusion—coastal aquifer systems where sea water replaces groundwater that has been over-exploited
4.point source—large levels of salt in effluent from intensive agriculture and industrial wastewater.
Salinity Development in Soils – A Hypothetical Cycle
Damage Caused by Soil Salinity
- Loss of biodiversity and ecosystem disruption
- Declines in crop yields
- Abandonment or desertification of previously productive farmland
- Increasing numbers of dead and dying plants
- Increased risk of soil erosion due to loss of vegetation
- Contamination of drinking water
- Roads and building foundations are weakened by an accumulation of salts within the natural soil structure
- Lower soil biological activity due to rising saline water table
Visual Indicators of Soil Salinity
- A white salt crust
- Soil surface exhibits fluffy
- Salt stains on the dry soil surface
- Reduced or no seed germination
- Patchy crop establishment
- Reduced plant vigor
- Foliage damage – leaf burn
- Marked changes in leaf color and shape occur
- The occurrence of naturally growing halophytes – indicator plants, increases
- Trees are either dead or dying
- Affected area worsens after a rainfall
Classes of Soil Salinity and Plant Growth
- Electrical conductivity of the soil saturation extract (ECe) is the standard measure of salinity.
- General relationship of ECe and plant growth, as below.
- Non-saline (ECe ≤ 2 dS m−1): salinity effects mostly negligible
- Very slightly saline (ECe 2–4 dS m−1): yields of very sensitive crops may be restricted
- Slightly saline (ECe 4–8 dS m−1): yields of many crops are restricted
- Moderately saline (ECe 8–16 dS m−1): only salt tolerant crops exhibit satisfactory yields
- Strongly saline (ECe >16 dS m−1): only a few very salt tolerant crops show satisfactory yields
Socio-economic and environmental Impacts of Salinity
- Reduced crop productivity on saline land leads to poverty due to income loss
- Ecosystem fragmentation
- Poor vegetative growth and cover, lead to enhanced soil degradation (erosion)
- Storage capacity of water reservoirs is reduced due to eroded soil material
- Dust with high salt levels causes environmental issues
- Contamination of groundwater with high levels of salts occurs
- Soil salinity has a significant impact on food production in many countries of the world, including the USA, Australia, China, India and Pakistan. In the latter three countries, it greatly reduces the production of their staple crops, wheat and rice.
- High costs for soil reclamation
- Loss of good quality soil (organic matter and nutrients) requires more inputs, such as fertilizer – financial pressure on farmer