Soil salinization: Causes, effects and mitigation strategies

Introduction

Soil is a dynamic natural body developed as a result of pedogenic processes during and after the weathering of rocks, consisting of minerals and organic constituents, possessing definite chemical, physical, mineralogical, and biological properties, having a variable depth over the surface of the earth and providing a medium for plant growth for land plants.

Soils containing excess neutral soluble salts dominated by chlorides and sulfates so as to affect plant growth are known as saline soils.

Understanding the salinity

The soluble salts that accumulate in soil consist principally of sodium, calcium, and magnesium as cations and chlorides and sulfate as anions. The concentration of potassium, carbonates, bi-carbonates, and nitrate is comparatively low. The total concentration of salts is estimated in the soil solution either from weight after evaporation or from the electrical conductivity.

CATIONS ANIONS
K+ 0.2 CO32- 0
Na+ 102.0 CI‚Äď 78.0
Ca2+ 31.5 SO24- 90.0
Mg2 37.2  HCO33- 4.5

 

    CO2/3-= Formula of the carbonate ion, SO2/4-= Formula of sulfate

 

Classification of salt-affected soil

Salt-affected soils have been classified into two main categories:

1.   Saline soils

When the soil contains excess sodium salts while the clay complex still contains a preponderance that is greater in number of exchangeable calcium, the soil is known as saline soil or Solonchak.

2.   Alkali soils

Soils containing high exchangeable sodium percentage and those influenced by sodium salts capable of alkaline hydrolysis.

Formation

The primary source of salts in soil is from rock weathering. In the humid and per humid regions with adequate rainfall most of the soluble salts are leached down below the ground. Fluctuating depth of groundwater or the water table leads to salinity in the soil. Soil salinization in the coastal area is due to the accumulation of salts from inundated seawater.

Impact on plant growth

  • Saline soils are harmful to plants on account of the excessive concentration of soluble salts in soil solution.
  • Soluble salts produce primary harmful effects by increasing the concentration of the soil solution.
  • The primary effect of excess salts is to retard the growth of plants by limiting the absorption of water.
  • They cause injury to the plant cells as it is toxic and lead to the death of plants.
  • Generally, Na2CO3 (Sodium carbonate) is the most harmful salt for plants.
  • The presence of large amounts of soluble salts in the soil retards germination mainly because water cannot easily enter the germinating seed due to high osmotic pressure.

Electrical Conductivity

Effects on Crops and Plants

Total soluble salt (%)

<2  All crops grow well <0.2
2-4 Only very sensitive crops are affected 0.2-0.4
4-8 Many crops were affected and their yield reduced 0.4-0.6
8-16 Only salt-tolerant crops are not affected 0.6-1.0
 >16 All except a very few tolerant crops are affected >1.0

Effects on the ecosystem

Much of the natural vegetation of salt-affected areas have been destroyed or damaged. This has resulted in humungous changes to the landscape and biodiversity which includes the destruction of remaining natural habitat in many agricultural areas and the fragmentation of many wildlife corridors.

Effect on water resources

  • If the level of salts in the soil is too high, water may flow from the plant roots back to the soil causing dehydration in plants.
  • Salinity in soil penetrates ground water damaging the quality of water for drinking and irrigation.
  • This phenomenon damages the economic, social, and environmental growth of both rural and urban communities.
  • Salt changes the ecological health of streams and estuaries.
  • Salt also helps fine materials like clay to flocculate thus allowing more sunlight to penetrate rivers thus disturbing algae ecosystems.

Soil salinization and global food security

In spite of technological innovations in agriculture, food security globally is being challenged by salinization.

Soil salinization alone has rendered huge lands unproductive. Soil salinization is a global problem. It reduces net cultivable area, and has serious implications for agricultural productivity and quality, the choice of cultivable crops, etc.

Estimates suggest global economic losses due to soil salinization around US $27.3 billion per year.

Monitoring and assessing the soil salinity level

The process of accumulation of salts leading to the formation of saline soils is known as salinization. Salts usually present in saline soils are chlorides, sulfates, etc sodium forms less than 50% of the total cations present in the soil solution. The reaction of saline soils is slightly to moderately alkaline and varies between pH 7.5 and 8.5. Exchangeable sodium percentage (ESP) is very low, being less than 15% of the total cation exchange capacity (C.E.C.).

The distinctive feature of these soils is the high salt concentration. When the total soluble salt content is more than 0.1%, it is high enough to interfere with the normal growth of most plant species.

Mitigation strategies

It is defined as the reduction of something severe, hence mitigating soil salinization refers to the reduction of salt level in soil by using various ways.

a.   Irrigation management techniques

  • The danger of the development of salinization wherever irrigation has been pointed out.
  • Irrigation is usually practiced in valleys where the land is more level or flat.
  • The deterioration of land under perennial irrigation(water collected from rivers flowing throughout the year) is associated more often with the existence or creation of a high water table.
  • It is when the high water table persists over a long time that the accumulated salts begin to show their effect.
  • A high water table depicts the rock and surrounding ground materials joining the upper soil layer due to the excess amount of water beneath it.
  • As the chief cause of the trouble is the high water table, it is very necessary to ensure free and efficient drainage. Water is available in plenty, but there is a tendency for farmers to use it in excess which in the course of time raises the water table and brings the underground water down.
  • Excess water should not be allowed to accumulate. It must be removed by providing suitable drains. All irrigation schemes should, therefore, provide efficient drains for the removal of excess water from whatever source it originates.
  • Irrigation water should be used sparingly and judiciously. Flooding the land with large quantities of water must be avoided. Evaporation should be checked as far as possible by mulching or by providing proper shade.

b.   Soil reclamation and restoration

  • The method of reclamation depends on the type of alkalinity, i.e. whether the soil is saline or not.
  • In the case of saline soils, reclamation consists mainly of removing the excess salts.
  • This can be done either by scrapping the salts from the surface, or washing them down into the lower layers beyond the root zone, preferably completely out of the solum.
  • Another way is by growing salt-tolerant plants, or by a combination of two or more of these methods.
  • Scraping helps to remove salts that have formed an encrustation on the surface, but it is never very helpful in complete reclamation.

The method of removing the salts by leaching them with water into the lower levels and from there into a stream is the most efficacious method of reclamation. The water used for the purpose must be low in sodium salts. Rainwater may also be utilized for the purpose. It is suitably impounded on the land for varying periods during which it dissolves the salts and takes them down.

If the soil is not free draining, effective reclamation agents. The acidity developed during the course of artificial drains is opened or tile drains laid underground to help wash out the salts.

c.   Genetic and crop breeding approach

  • The growing of salt-tolerant crops with the view of removing salts is a proposal. Although these plants remove fairly substantial quantities of salts from the soil, comparatively larger quantities are still left behind.
  • Salt-tolerant crops of economic importance may, however, be grown in rotation with other crops. Both these methods, viz., scraping and growing salt tolerant crops.

d.   Sustainable agriculture

  • Water management and cropping systems are very important for crop production on saline soils.
  • Proper water management for raising crops is also essential to control further development of salinization.
  • For optimum crop production in saline areas, the selection of crops and crop varieties and, the adoption of best-suited cultural and fertilizer practices are essential.
  • Rice is considered a satisfactory crop during the initial years of reclamation.
  • The practice of ponding water for rice dilutes salt concentration in soil and causes leaching of salts under saturated conditions.
  • After rice, growing a legume suitable for the region is recommended.
  • Cultivation of salt-tolerant crops is another management practice. Some of the crops categorized as high, medium, and low salt-tolerant are listed below.

Examples of some high salt-tolerant crops

Rice, sugarcane, oats, berseem, senji, fenugreek, barley

Examples of some Medium salt-tolerant crops

Castor, cotton, sorghum, pearl millets, maize, mustard, wheat.

Examples of some low salt-tolerant crops

Pulses, pea, gram, lime seed.

Future challenges

Salinization is a serious problem challenging food security in the world particularly in developing countries. It is a dynamic process caused by several natural and human-induced processes. And quite often, the socio-economic and political considerations become extremely important in accelerating the process of salinization.

Many times such factors are beyond the control of individual farmers and call for their attention. It becomes the responsibility of the respective governments to take appropriate policy decisions and corrective measures in order to keep a check on soil salinization and also restore the sols already affected by salts.

Summary

  • The process of accumulation of salts leading to the formation of saline soils is known as salinization.
  • The reaction of saline soils is slightly to moderately alkaline and varies btw pH 7.5 and 8.5.
  • During the process of rock weathering and soil formation, soluble salts are formed.
  • The fluctuating depth of groundwater or the water table leads to salinity in the soil.
  • Saline soil refers to the soil that consists of salts that impair its productivity.
  • Removal of excessive salt to a desired level in the rooting zone is the basic principle of reclamation of saline soils.
  • Varieties of crops differ in their salt tolerance
  • Salinization leads to food scarcity and food security threats.

References

Website reference

https://www.frontiersin.org/articles/10.3389/fsufs.2020.533781/full

Book references

D. Biswas and S. K. Mukherjee, Soil Science, McGraw Hill Education Private Limited, New Delhi, 2013.

Dr. J. A. Daji, A textbook of soil science, Media Promoters and Publishers PVT.LTD, Bombay, 2005.

Written By: ‚Äč‚ÄčAnkita Gangopadhaya