Beginning with the sandy clay in the upper hills of the Bay Area, I added 50% mushroom compost in raised beds to get amazing growth rates last summer. I watered with "softened" water, and this year switched to the usual local water (average CA mineral content - low to middling) and did some leaching to get out the sodium. This summer everything is very green, but with very low growth. Guess what, a soil test shows the Phosphate content was extremely low. Does the sodium "lock up" or grab the phosphate in some way? I have added superphosphate recently, no signs of improvement yet (1 week).
Possible Effect of Soft Water
Certified GKH Gardening Expert
Soils with a high pH and salt can tie up phosphorous by creating a reaction between phosphorous and calcium. How was the pH on the soil test? If it was higher than 7, also take steps to bring the pH down to help stop the reactions that are tying up the phosphorous.