For those of you who attended Steve Grattan's presentation at UCCE Monterey last week on managing salinity in vegetable production, you may recall he mentioned that strawberries in gypsiferous soils can tolerate a higher EC reading than the salt tolerance guidelines allow.  I wasn't quite clear about this, so I emailed about it and his answer concerning this is as follows:

"Crop salt tolerance is based on crops response to the electrical conductivity of the saturated soil paste (ECe). But in actuality, crops respond to the salinity in the soil water. These are different.

The field soil water content for many berries and vegetable crops is slightly above or below the field capacity. This means that about 1/2 of the pore volume in the soil is water and the other half is air. To make a saturated paste, distilled water (pure water without salts) is added to fill the extra pore space. Now if the salts in the soil water are largely sodium and chloride (very soluble), then the ECe would be about half the EC of the soil water in the field...which is what the crop is truly responding to. But if the soil contains a lot of gypsum (CaSO4), then by adding distilled water, more salts will become dissolved so that the ECe would be higher.

As an example, the literature indicates that strawberries can tolerate a maximum ECe of 1 dS/m, beyond which yields decline 33% for every 1 EC unit is increased beyond that. This was based on a chloride dominated water. Therefore, stawberries can tolerate about an EC of 2 in the field water. Gypsum has a maximum solubility of about 20 meq/l which is an EC of about 2 dS/m but this is just a generalization and can vary depending on soil chemistry. But for simplicity, if the salts were all gypsum (no sodium chloride) than if the soils had excess gypsum, the EC of the field water would be about 2 (no yield reduction). But to make a saturated paste, distilled water is added and more gypsum is dissolved so the resulting ECe is not 1 but remains at 2. Therefore an ECe of 2 dS/m is not growth limiting in this case.

The rule of thumb is that plants can typically tolerate a 1-2.5 dS/m higher ECe than the salt tolerance guidelines indicate in gypsiferous soils because of this relationship."