At a recent meeting in Modesto covering drought and how it is being dealt with around the world, there was an interesting presentation by some Israeli researchers.  They looked at the use of recycled water from sewage treatment plants and the use of desalinated water from the Mediterranean.  The recycled water had much of the original mineral nutrients, but had been treated for microorganisms.  They wanted to know how much of the nutrients could be accounted in the fertilizer balance applied to apple, pear and nectarine orchards.  Their conclusion that after one year, there was a significant contribution and that leaf analysis had not changed, not yields after applying the effluent.  This was only for one year, but I could imagine that after many years there would be a significant impact.

They also looked at desal water, and found that the process removed most minerals except for boron.  They actually found that plants irrigated with this water ended up with calcium and magnesium deficiencies and more boron toxicity.  The reverse osmosis membranes used in this case were not very effective at removing boron.  In the case of many waters north of Los Angeles there are often high levels of boron in the water, and using RO water might accentuate the problem.  

RO water also is usually too pure and the lack of salt causes soil to deflocculate - lose structure.  Yes, you need some salt to have a healthy soil.

Read more about this trial at:

http://media.wix.com/ugd/7df67b_0bd44a0fe135459eb345ba31b96f232b.pdf

Bitter Pit in Apple caused by low Calcium