Prevention and Remediation of Saline Soils

Interactions between Soil, Water and Salinization:

We have to make the water and salt balance to determine the amount of water including its dissolved salts, that will enter a soil profile. The water coming from rainfall and/or irrigation will fall on the soil surface. From there it will partly transpire, partly infiltrate and the remaining water will be removed as surface runoff.

In the root zone, the plant roots will take up the water but not the salts. Thus the remaining soil water will have a higher salinity. When the salinity of the water and soil get too high, plants can’t grow properly. The main soil characteristics that determine the flow of water in the soil are, the soil texture, infiltration rate and the hydraulic conductivity.

Let’s have a closer look at these parameters. The soil consists of mineral particles of widely varying sizes with pores in between. The size distribution of these particles defines the soil's texture. Texture refers to the particle-size distribution of the 'fine earth' of the soil. These are particles less than 2 mm in diameter and they consist out of sand, silt or clay. The textural class of a soil is determined by the relative proportions of sand, silt and clay particles. Usually these textural classes are presented in a texture triangle as you can see here in the graph.

A soil consists not only of soil particles but also the pores in between these particles. These pores can be completely filled by air or partly or completely filled with water. The porosity of a soil is the fraction of the volume of soil pores compared to the total volume of the soil. The porosity of a soil therefore indicates the potential ability for water to move through the soil. Before the water enter the soil it has to infiltrate from the soil surface into the soil. The infiltration rate is the rate at which water enters the soil. The infiltration rate not only depends on the soil texture, but is also a function of the time. In dry soils, the initial infiltration rate is high, but as the soils becomes wet, the infiltration rate reduces quickly and after some time reaches a nearly constant value. This rate of infiltration depends on the soil as you can see in the graph, but the general pattern is the same for all types of soils.

Another important characteristic is the hydraulic conductivity, or permeability, it determines how easily water can flow through the soil. The hydraulic conductivity depends largely on the texture of the soil, but is also affected by the density and viscosity of the groundwater. The denser the soil, the more difficult the flow; and the looser the soil, the easier the flow. The same applies to the density of the groundwater.

Finally I want to talk a little about sodicity and how it affects the water flow in soil. You have already heard about the sodicity: it is one of the classifications of salinization, and refers to the presence of sodium ions on the soil particles and in the soil solution. When too much sodium is present, the soil aggregates become unstable and are likely to disperse or fall apart. This lack of stability results in a reduction of the soil permeability and therefore makes it more difficult for water to flow through the soil. Sodicity also disturbs the nutrient equilibrium, causing toxicity to plants.

Prevention and Remediation of Salinization:

Irrigation water, even if of excellent quality is a major source of soluble salts. But not only irrigation water can be a source of salts. A second source of salinization in irrigated areas is capillary rise from the groundwater table, where water moves from deeper soil layers upwards into the root zone. As groundwater is often somewhat saline, even a small amount of capillary rise can increase the soil salinity in the root zone. And near the coast even rainfall can be a source of salts, because it can pick up some salt from sea-spray. You have also learned that the crop takes up the water in the soil, but the salts  remain behind in the soil. Without drainage these salts will accumulate in the root zone and can reach levels that interfere with soil productivity and the crop growth. We call this process salinization. To prevent or remediate salinization, the extra applied salts need to be washed out. This is done by ‘leaching’, the application of extra water to flush the soil and to remove salts.

For leaching to be effective you need to apply enough water and there is a needs for good drainage so there is some place for the extra water and salts to go out. The amount of extra water needed is based on the ‘leaching requirement’ which can be calculated using the water and salt balance. This extra water applied for leaching is not taken up by the crop but percolates through the root zone, carrying some of the extra salts out of the root zone via natural or a installed drainage system. Done effectively, leaching can keep, or return, the water salt balance to a range that is acceptable for crop growth. This in turn keeps or restores the soil’s ability to be productive and alive.

Prevention and Remediation Measures:

There are many available approaches to contain the salinity threat. A blend of engineering reclamation and biological approaches can be adopted to address salinity and water logging problems. These include:

1. Direct leaching of salts.

2. Planting salt tolerant varieties.

3. Domestication of native wild halophytes (plants growing in soils with high salinity) for use in

        agro-pastoral systems.

4. Phytoremediation.

5. Chemical amelioration.

6. Use of organic amendments.

Sources of Salts:

All water contains salts: (i) irrigation water, even if the quality is good enough for you to drink it (ii) groundwater, and (iii) even rainfall especially in coastal areas. Thus application of irrigation water means an input of salts. If soil salinization is to be avoided, these salts have to be leached out of the root zone by water percolating to the subsoil. This percolation water will cause the water table to rise and so it needs to be drained off because a second source of salinization in irrigated areas is capillary rise from the groundwater table. As groundwater is often somewhat saline, even a small amount of capillary rise can increase the soil salinity in the root zone.

Examples of Combating Salinization:

While salt affected soils occur in more than 100 countries and affect 10-16% of all irrigated land, the most widely affected countries are Pakistan, China, India and the US. Some other countries with significant areas of soil salinity include Egypt, Iraq, Iran, Kazakhstan, Mexico, Syria, Sudan and Turkey. Some countries have taken steps to combat salinization quite readily when the problem was realized, other countries have been slower to take action.

• Pakistan: In Pakistan, engineering solutions have included large-scale Salinity Control and Reclamation Projects (SCARPs), which covered 8 million ha at an estimated cost of US$2 billion. Two big drainage water disposal projects were also undertaken. Measures to address the saline soil problem included leaching of salts by excess irrigation, use of chemicals (such as gypsum and acids), the addition of organic matter, and biological measures such as salt-tolerant plants, grasses, and shrubs.

• North America: Improvements in on-farm water and crop management have been developed and practiced to reduce salinity issues. Changes in land use and management practices have reduced the risk of salinization and helped to improve soil health and agri-environmental sustainability.

• Egypt: Since the days of the Pharaohs until the 19th century, basin irrigation has been practiced in Egypt. For this ancient method of irrigation, based on the natural regime of the Nile, the natural drainage capacity was sufficient to protect the area against the twin problems of waterlogging and salinity. In the 19th century, new crops, i.e. cotton and sugarcane, were introduced that required water when the Nile’s water levels were low. Barrages were constructed in the River Nile and with the completion of the Aswan High Dam in 1968 the Nile’s season floods finally eliminated and allowed all agricultural lands to be brought under perennial irrigation. The elimination of the seasonal fluctuation in the River Nile reduced the natural drainage capacity. Together with the increased percolation from irrigation this gradually resulted in waterlogging and salinity problems in large areas. To overcome these problems the Egyptian Government embarked upon an ambitious programme to install subsurface drainage systems in all agricultural lands (3.4 Mha). Additionally, the use of organic amendments in Egypt has shown that the mixed application of farmyard manure and gypsum (1:1) significantly reduces soil salinity and sodicity. Recently, phytoremediation or plant based reclamation has also been introduced in the Near East region. 

• Iraq: Like in Egypt, irrigated agricultural has been practiced since time immemorial in Ancient Mesopotamia, but the risk of salinization was often not properly understood or treated and exploded like a time bomb over and over again upon the agricultural scene. With time, the soil became toxic and would no longer support crops. By about 2300 B.C., agricultural production in Mesopotamia was reduced to a tiny fraction of what it had been. Many fields were abandoned as essentially useless. Mesopotamian cuneiform tablets tell of crop damage due to salts. By 1950, 60% of Iraq’s agricultural land was considered seriously affected by salinity issues, and more than 20% had been abandoned (FAO, 2011). Only toward the end of the 20th century were surface and sub-surface drainage systems systematically installed to bring the fertile lands between the Rivers Euphrates and Tigris back into cultivation.

• Iran, Syria and other Gulf countries: Crop-based management, and fertilizers are used to combat salinization. In Iran, Haloxylon aphyllum,Haloxylon persicum, Petropyrum euphratica and Tamarix aphylla are potential species for saline environments. Also in Iran, Atriplex has been shown to be a potential fodder shrub in the arid lands which could bring annual income as high as US$ 200 ha-1. Breeding of salt tolerant crop varieties (e.g. wheat, barley, alfalfa, sorghum etc.) is also a recognized management response for saline environments. However, most results have been obtained in controlled environments, with few real field results so far.

• Sudan: Good responses for control of sodicity have been obtained through phytoremediation. The production of H+ proton in the rhizosphere during N-fixation from legumes such as the hyacinth bean (Dolichos lablab L.) removed as much Na+ as gypsum application. This indicates the importance of this technology in calcite dissolution of calcareous salt affected soils.