Saturated paste extractions are used by a number of laboratories for soil salinity evaluation since this is the method suggested for use by The USDA.
A soil is saturated with de-ionized water. Water is added to the sample in the condition it is received from the customer.
The paste is allowed to equilibrate over night, or for 12-18 hours.
It is then extracted using an air press.
It is then analyzed.
Some positive aspects of using a saturated paste are that it simulates a naturally occurring environmental condition where salts would be mobile and most probable to damage. Because of this, the measurements from a saturated paste extract can be used directly without any adjustment.
Some negative aspects of using a saturated paste deal with potential human error. The point of saturation for different soils is different due to organic matter, clay, sand, and silt present in the sample as well as the amount of certain salts in some instances. Because of this, human subjectivity introduces a large amount of error and little hope of accurate reproduce-ability. The time involved varies greatly for different soils to reach equilibrium in order that a representative amount of salt can be extracted. The timeline for extraction of saturated pastes is not well defined, and has a large amount of variability.