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- Author: Steve Grattan
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."
The key to understanding this concept is knowing that for things which are more difficult to dissolve, the addition of more water will put more of it into solution. Gypsum is one of those substances which are a little bit more difficult to dissolve.
At field capacity the 1/2 of pore space filled with water has the highly soluble sodium and chloride already dissolved, but yet some of the gypsum is not because it needs even more water to fully dissolve. Add the distilled water from "saturated paste" test as described in Dr. Grattan's letter, and the higher volume of water allows more of the gypsum to dissolve - which in turn raises the EC read because more gypsum is in now in solution than was before in just the soil water.
Nevertheless, if we were dealing with just gypsum and no sodium or chloride, the higher EC isn't really negative for plant health because gypsum is not saline and therefore not harmful to plants.
In conclusion it pays to be careful about interpreting a "gypsiferous soil" EC print, since one is also going to be looking at more gypsum, which is not harming the plant like sodium and chloride, in solution and in turn raising the EC number.
Does that make sense? Thanks for bringing this to my attention.