SSC 100 - Soil Salinity and Sodicity

What is soil salinity? Which ions commonly contribute to salinity?

Accumulation of base cations (Ca, Mg, Na) with chloride and salts (sulfate, bicarbonate) to a point where plant growth and soil function are damaged

What natural factors cause soil salinity in arid and semi-arid regions?

Low precipitation and high temperatures

Explain how groundwater with high salt concentration leads to salinity problems.

In low-lying areas, the salty groundwater seeps upward through capillary rise, contaminating the soil with salt

What human-induced factors contribute to soil salinity?

Excessive groundwater pumping —> saltwater intrusion; improper irrigation/fertilizer management can lead to salt accumulation

Why is salinity a major problem for plants in terms of osmotic potential and water uptake?

forces plants to lower their osmotic potential to be able to take up water (costly —> reduces their water uptake)

What is soil sodicity?

when soil has an excessive accumulation of sodium cations (Na+) relative to other types of salt cations

Why does sodium (Na⁺) cause clay dispersion?

it has a large hydrated radius, so when it enters the interlayer space in clays, it pushes the layers apart

Explain why monovalent cations (e.g., Na⁺) cause dispersion while divalent cations (Ca²⁺, Mg²⁺) promote flocculation.

Monovalent cations have large hydrated radii and keep the colloids from being able to clump together; Divalent cations are tightly bound and have smaller hydrated size, so the colloids are still able to clump together (flocculate).

What physical soil problems result from sodicity?

massive soil structure, which leads to loss of porosity, decreased infiltration and increased runoff, and decreased seedling emergence

What visual indicators can help you identify sodic soils in the field?

Stunted plant growth, visible salt accumulation on the soil surface, subsurface columnar structure

What is electrical conductivity (EC)?

measure of the soil’s ability to conduct an electrical current

How is EC used to diagnose salinity?

salts in solution increase conductivity, so higher EC values indicate increased salinity/sodicity

What is the Sodium Adsorption Ratio (SAR)? What is the threshold SAR value for sodic soils?

measurement of the comparative concentration of Na+, Ca2+, and Mg2+ ions in soil; threshold is 13 (>13 indicates a sodic soil)

How does salinity impact nutrient uptake in plants?

Nutrients are taken up with water through the roots, so since salinity reduces water uptake, nutrient uptake is also reduced

Why do sodic soils have poor infiltration and high erosion risk?

Na+ causes dispersion, which reduces the ability for colloids to form —> leads to a lack soil structure and pore space, so water cannot infiltrate and the soil is easily eroded away

What are the essential steps to reclaim saline soils?

Reduce salt input and leach out and drain salts

Why is gypsum (CaSO₄) commonly used to remediate sodic soils?

The Ca2+ ions in gypsum can replace the Na+ ions in sodic soils, promoting flocculation

Why is lime (CaCO₃) generally not effective in sodic soil remediation?

not very insoluble at high pH conditions, and sodic soils generally have high pH, so adding lime is not an effective way to provide Ca2+ ions in sodic soils

What conditions must be present for sodic soil remediation to succeed?

Adequate drainage, clean leaching water with an effective sink, and continual treatment

Name three factors that would indicate whether a soil is more likely to develop salinity problems in the future.

A hot/arid climate, coastal proximity, and a shallow water table