How do SAR and ESP differ, and what are their effects on soil structure and infiltration?

• A. SAR uses irrigation water data to assess sodium risk in soils; ESP is the percentage of exchange sites with sodium; both can cause clay dispersion and reduced infiltration.
• B. SAR measures soil color; ESP measures soil texture.
• C. SAR indicates soil depth; ESP indicates soil moisture.
• D. SAR is the same as EC.

Answer: (A)

Sodium-related soil problems hinge on two related but distinct measurements: what the irrigation water could do to the soil versus what is already on the soil’s exchange sites. SAR looks at the sodium relative to calcium and magnesium in the irrigation water. It tells you how likely the water is to push calcium and magnesium off clay surfaces and replace them with sodium during irrigation, which can lead to dispersion if leaching isn’t sufficient. ESP, by contrast, is a soil measurement—the percentage of the soil’s exchangeable sites that are occupied by sodium. A high ESP means the soil itself is sodic and prone to dispersion, crusting, and very slow infiltration.

Both concepts describe sodium-driven changes in soil structure. When sodium dominates the soil–water system, clays tend to disperse, pores clog, and infiltration drops. A high SAR in irrigation water can raise soil ESP over time if salts aren’t leached away, whereas a soil with high ESP shows poor structure and infiltration even if the irrigation water isn’t extremely high in sodium. Management often involves improving leaching, adding calcium with gypsum to displace sodium, and ensuring good drainage to prevent infiltration decline.

That’s why this explanation fits best: it defines SAR as a measure based on irrigation water composition and ESP as a soil-based sodium saturation, both linked to clay dispersion and reduced infiltration.