When spacing emitters over a plant's area, keep in mind the size of the emitter's subsurface wetted pattern and how to use it for control of salt build up. Where sodium and/or chloride are in the irrigation water or soil in significant concentrations, salt leaching is required to provide for plant health.
A reduced water intake occurs where salt build-up accumulates in the plant's root zone. Properly spaced, emitters can prevent this build up and provide for leaching the salt away from the zone.
We once worked out an emitter number problem for a tree, spaced the emitters and showed the diagram below.
Salt builds up where evaporation and drying occurs at the edges of the wetted area. In fact, salt problems are easily identified by white rings which will appear around the emitters after operating the system for a period of time. The above spacing would have produced salt built up in the areas shown in the next diagram.
A large concentration of salt formed in the center of the root ball in the tree trunk area. It's important to overlap these subsurface wetted patterns in a manner similar to the way sprinkler throws are overlapped above the surface.
One final note on salt leaching. After a careful balance has been struck and salt concentrations are pushed away from the root area, a light rain can change the whole picture. Incoming surface moisture puts the salts back into solution and spreads them out again.
In a salt prone area, turn on the system during light rains or until about two inches of rainfall occurs. When the system is on, water is continually moving outward through the root zone restricting salt transfer back from the zone perimeter.
In many cases the wetted area of a single emitter exceeds the plant root zone area. This simplifies spacing and provides for salt leaching automatically.