Soil Salinization

Soil Salinization Definition

All soils contain salts, but an excess salt concentration can disrupt ionic balances in soil and can have cascading negative effects on plant nutrient uptake and soil structure.

You’re probably familiar with the chemical formula for table salt, or NaCl (sodium chloride). This and all other salts are molecules formed by an ionic bond between a positively and negatively charged ion. Most salts readily dissolve in water due to their ionic bonds.

When dissolved in water, NaCl ions split to become mobilized as Na⁺ and Cl^-. Plants can then uptake the released chlorine atom, which is an essential micronutrient for plant growth. Soil salinization occurs when salts and water are out of balance, causing nutrients held in salts to become locked up and unavailable to plants.

Fig. 1 - Maranjab Desert in Iran shows signs of soil salinization. Water pools on the surface and leaves behind rings of salt when it evaporates.

Major Causes of Soil Salinization

Because salts are water-soluble, they can enter into soil environments through groundwater, flooding, or irrigation.² Salts may accumulate in soil for a variety of reasons, all of which relate to some disruption in water and water-soluble salt dynamics.

Natural Causes of Soil Salinization

Soil salinization is most common in arid and semi-arid climates, as well as coastal areas.

Climate

High temperatures and low rainfall create conditions where evaporation and transpiration exceed precipitation. Through capillary action, water containing salts deep in the soil is pulled up to the dry topsoil. As this water evaporates from the soil, the once dissolved salts are left behind in their undissolved salt form. With no water to dissolve the salts or carry them away through leaching, they begin to accumulate in the topsoil.

Topography

Topography can contribute to soil salinization through its influences on water accumulation. Low-lying areas such as river flood plains are susceptible to flooding. This type of topography promotes the temporary accumulation of water during floods, and when the water dissipates, salts are left behind in the soil. Similarly, mild slopes that create shallow pool areas for water accumulate salts as water is evaporated out.

Proximity to Salt Water

Coastal areas are very prone to soil salinization due to flooding. Salty or brackish water floods can deposit high concentrations of salt in coastal soils, making them difficult to use in agriculture.

Fig. 2 - Types of salts found in seawater, all of which are important to the soil ecosystem when supplied in their manageable concentrations.

Human-Induced Causes of Soil Salinization

Humans have a long history of altering landscapes for agriculture or other land uses. These changes can often impact salt concentrations at much faster rates than natural causes.

Land Cover Change

When a vegetated area is cleared for an alternate land cover type, such as a field for agriculture or a golf course, the hydrological balance of the area is disrupted. Excess water begins to accumulate when the roots of plants once responsible for uptaking this water are removed. As the groundwater level rises, salts buried deep in the soil and parent material are brought up to the surface. Without proper drainage, the salts remain and buildup in the topsoil.

Agriculture

Agricultural practices like irrigation and the application of synthetic fertilizers cause soil salinization. Over time, soil salinization can have detrimental effects on plants and soil structural properties, which disrupts agriculture and contributes to food shortages. Because soil is a finite natural resource, a great deal of agricultural research is concerned with preventing and restoring soils from being salinized.

Soil Salinization and Agriculture

Estimates by several studies suggest that more than 20% of all arable land is negatively affected by soil salinization.¹

The Effects of Agriculture on Soil Salinization

Agriculture and irrigation are the major causes of soil salinization across the globe.

Irrigation

Irrigation is the primary way that agricultural practices cause soil salinization. Similar to the removal of vegetation, irrigation can cause groundwater levels to rise above natural levels, bringing once buried salts up to the topsoil. Heightened water levels also prevent the removal of salts through drainage leaching.

Fig. 3 - A flooded field where salts will accumulate in the topsoil as the irrigated water evaporates.

In addition, rain water typically contains low amounts of dissolved salts, but irrigated water can contain much higher salt concentrations. Without a drainage system in place, an irrigated field will suffer from the accumulation of these salts as water evaporates out.

Synthetic Fertilizers

Agriculture can also contribute to soil salinization through the use of fertilizers. Synthetic fertilizers are applied in the form of plant minerals held up in salts. Water then dissolves the salts, unlocking the minerals for plant use. However, these fertilizers are often applied in excess, causing a variety of pollution and land degradation effects.

Soil Compaction

Soil can become compacted by farm equipment or grazing animals. When soil particles are over compacted, water can't percolate down and instead pools on the surface. As this water evaporates, salt is left on the soil surface.

Soil Salinity Effects on Agriculture

Soil salinization has negative impacts on plant health and soil structure, and it can bring about many concomitant socioeconomic problems.

Plant Health

Plants growing in soils with high a concentration of salts can suffer from sodium, chloride, and boron toxicities. These can serve as essential nutrients when supplied in the correct amounts, however, an excess can "burn" plant roots and cause the tips of leaves to turn brown.

As plant roots uptake water through osmosis, dissolved salts enter the plant. When there's a high concentration of salt in the soil, the osmotic potential of plant roots is reduced. In this case, the soil has a higher osmotic potential than the plant root because water molecules are attracted to the soil's salt. Water is then pulled into the soil and unavailable to the plant, causing dehydration and loss of crops.

Soil Degradation

Soil salinization contributes to soil degradation by making some soil aggregates more susceptible to breaking apart, particularly those with high clay content.³ When not held up in water stable aggregates, soil particles and nutrients are more prone to loss by erosion.

This process of breaking apart aggregates also decreases the porosity of soil, leaving less pore space for water to infiltrate down and drain out salts. Pools of water can then form on the surface, making soil microbes contend with anaerobic conditions and further stressing plant roots.

Socioeconomic Impacts

The socioeconomic impacts of soil salinization are felt most by subsistence farmers, who directly rely on their crops for access to nutrition. However, soil salinization can have widespread and even global impacts, particularly in arid and coastal regions.

Crop loss due to soil salinization is a concern for many countries, as it can disrupt supply chains and lower a country's GDP. In addition, measures to prevent or reverse soil salinization can be expensive. Many agricultural development projects are aimed at implementing water drainage systems to flush out salts, but they often require a great deal of funding and labor.

Soil Salinization Examples

Soil salinization is a pressing issue in global agriculture. Solutions for preventing excess accumulation of salts look different for each unique landscape. Let's look at some examples of soil salinization:

The Nile River Delta

The Nile River Delta has served as Egypt's cradle of agriculture for thousands of years. Every year, the Nile River swells with summer rains, which flood and irrigate the nearby fields.

Fig. 4 - The Nile River and its surrounding agricultural lands are irrigated with river and groundwater during dry periods.

In centuries past, these floodwaters were crucial to flushing out accumulated salts from the rich agricultural soils surrounding the river. However, Egypt is now facing soil salinization issues due to river dams that have increased local water tables. When the river floods in the summer, the flood water isn't able to drain downward and cannot leach out excess salts. Today, upwards of 40% of all land in the Nile River Delta is suffering from soil salinization due to inadequate drainage.

The Southwestern United States

States in the Southwest have adapted their agricultural practices to high desert temperatures and low annual rainfall with irrigation. Soil salinization occurs naturally in arid climates, but irrigation increases the rate at which salts can accumulate in the topsoil. Many farmers in southwestern states have implemented drainage systems to help flush some of these salts away. Crops are also being adapted to become more tolerant of salinized soils.

By breeding new varieties of important crops to the region, salt-tolerant varieties are being discovered. Microbes with symbiotic relationships with plant roots that influence salt uptake are also being investigated. In addition, genetically modified crops are being developed by removing or adding certain genes that control the uptake of salts in the root zone.

With ongoing research, there are likely to be new ways that humans can adapt agriculture to the pressing issue of soil salinization.