ESP / SAR
How many cation seats sodium took — not crust thickness.
Also called: exchangeable sodium percentage, sodium adsorption ratio
- ESP is sodium’s share of the seats.
- No CEC, no ESP.
- Salt is EC. Alkali is ESP.
- Rates live in the saline guide.
How it appears on the sheet
Some sheets print ESP%. Others expect you to divide Na by CEC. SAR is more common on irrigation-water tests.
A sketch. Do not fertilise from the picture alone.Common units
ESP is usually %. SAR is dimensionless. Read with exchangeable Na and CEC.
Soil:water ratio · ESP and SAR are not the same number. No CEC, no ESP.
How to read the number
Salt is EC. Alkali is pH, Na and ESP. High ESP: drain and calcium first, not urea.
Rates live in the saline-alkali guide.
Thresholds only, not a prescription. Rates live in the field guide. This page does not hand you a bag count.
| Cell | Low | Mid | High |
|---|---|---|---|
| ESP% (sketch) | Few Na seats | Structure starts to fail | Sodic — drain before gypsum |
Regional classes differ. This table is a comparison, not a prescription.
What it looks like in the field
Poor structure, slurry when wet, clods when dry — that matches sodicity better than a white film.
Don’t do this
- Don’t treat ESP as EC.
- Don’t invent ESP without CEC.
- Don’t gypsum every crust.