101 Questions answered
Why High ESP Results Might Mislead: The Importance of Pre-Wash Testing
The exchangeable sodium per cent (ESP) is a measure of soil sodicity and often used to gauge soil dispersion and/or to calculate gypsum rates. As dispersion is affected by more than the ESP, the number is best used as a guide only. See Question 22. ‘How do I tell if I have a dispersive soil?’ and Question 27. ‘What’s the difference between sodic soil and dispersive soil?’
A soil with an ESP > 6% in the topsoil and > 15% in the subsoil is considered sodic and could be dispersive.
Sometimes, lab results don’t match up with what you know about your soil, such as when the ESP is very high e.g. 25% but you know the soil isn’t highly dispersive.
This can happen when a soil is both saline and sodic. The salinity interferes with the test result making the ESP appear higher than it really is. The test measures both soluble and exchangeable sodium, but it is only the exchangeable sodium (attached to the soil) that impacts soil dispersion, not the soluble sodium (in the water).
To get a true measure of ESP, you need to ask the laboratory to do a pre-wash, which washes out the soluble sodium before testing. Ask for a pre-wash if the salinity (EC1:5) is 0.3 dS/m or more. If you don’t know the EC, you can request a pre-wash and have the samples tested again, or if you know you already have saline soil, ask for a pre-wash to begin with.
One clue the ESP reading is off is the cation exchange capacity (CEC). If there’s a high CEC when you would expect it to be low, such as in sandy soil with low organic matter, the ESP is also probably inflated.
