Soil Salinity and Sodicity

Salinity and sodicity are conditions of salt-affected soils in which excess soluble salts or exchangeable sodium impair plant growth and soil structure, especially in arid and irrigated lands.

Definition

Soil salinity is the accumulation of soluble salts sufficient to impair plant growth, measured by the electrical conductivity of the soil solution; sodicity is a high proportion of exchangeable sodium that disperses clay and degrades soil structure.

Scope

This topic covers the diagnosis of saline, sodic, and saline-sodic soils, the measures of electrical conductivity, sodium adsorption ratio, and exchangeable sodium percentage, the effects of salts and sodium on plants and soil structure, and the reclamation of salt-affected soils. It addresses a major form of chemical land degradation.

Core questions

• How are saline, sodic, and saline-sodic soils diagnosed?
• How do soluble salts harm plants?
• How does exchangeable sodium destroy soil structure?
• How are salt-affected soils reclaimed?

Key concepts

• Electrical conductivity (salinity)
• Sodium adsorption ratio (SAR)
• Exchangeable sodium percentage (ESP)
• Osmotic and specific-ion effects on plants
• Clay dispersion and structural collapse
• Leaching and gypsum reclamation

Key theories

Classification of salt-affected soils

Salt-affected soils are distinguished by electrical conductivity, sodium adsorption ratio, exchangeable sodium percentage, and pH into saline, sodic, and saline-sodic classes, each requiring different management.

Sodium-induced clay dispersion

When exchangeable sodium dominates the colloids and salt concentration is low, clay particles disperse rather than flocculate, collapsing structure, sealing pores, and drastically reducing permeability.

Mechanisms

Soluble salts lower the osmotic potential of the soil solution, making it harder for plants to take up water, and specific ions such as sodium, chloride, and boron can be directly toxic. Excess exchangeable sodium, when salts are leached, causes clay colloids to disperse instead of aggregate, plugging pores and collapsing structure so the soil becomes nearly impermeable. Reclamation removes salts by leaching and replaces exchangeable sodium with calcium, commonly supplied by gypsum, before leaching.

Clinical relevance

Salinity and sodicity reduce crop yields and degrade large areas of irrigated and arid land worldwide; diagnosing and managing them through drainage, leaching, gypsum application, and irrigation-water management is essential for sustaining agriculture and reclaiming degraded soils.

History

The systematic diagnosis and improvement of saline and alkali soils was codified in the 1954 USDA Handbook 60 from the U.S. Salinity Laboratory, which introduced the electrical conductivity, sodium adsorption ratio, and exchangeable sodium percentage criteria still in use, framing salt-affected soils as a manageable form of degradation.

Key figures

• Nyle C. Brady
• Ray R. Weil
• Donald L. Sparks

Seminal works

• usslab1954
• brady2016
• sparks2003

Frequently asked questions

What is the difference between a saline and a sodic soil?

A saline soil contains enough soluble salts to stress plants but usually retains good structure, whereas a sodic soil has a high proportion of exchangeable sodium that disperses clay and destroys structure; saline-sodic soils combine both problems and must be managed carefully to avoid collapsing the structure during reclamation.

Why is gypsum used to reclaim sodic soils?

Gypsum supplies calcium that replaces the exchangeable sodium on soil colloids; this lets the clay flocculate and the structure recover, and the displaced sodium can then be leached away with adequate drainage.