Excellent article posted on the Salinas Valley blog by colleagues Richard Smith and Tim Hartz on zinc nutrition of crops and soils in the Salinas Valley.

Key takeaways:

1- Historically zinc deficiency was common in California, but now because of widespread use of zinc fertilizers, zinc deficiency is pretty rare.  I concur, and as a matter of fact have yet to find a single plant sample which was deficient for zinc.

2- Bioavailability of zinc is limited by increasing soil pH, high clay content, high phosphorous and low soil temperature.

3- Tissue zinc sufficiency is between 15- 30 ppm (anecdotal note- blackberries tend to be in the range of 40 ppm)

4- Most common soil zinc test is DTPA extraction, which gives a good estimate of what is plant available.  Generally, soil DTPA extracts from 0.5 ppm - 1.5 ppm means crop plants in that soil would probably respond to the addition of a zinc fertilizer, while a test above 1.5 ppm means there likely will be no plant response to zinc addition.

You really should read the whole article, it's quite good and definitely worth the while:

http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=13163