The number of emitters needed for each plant is determined by the following factors:
1. The Soil Type:
The subsurface area wetted by each emitter depends on the size of the soil particles. The descriptions of soils range from very coarse to very fine grade.
With fine soil water spreads out more underneath the surface. This spreading is the result of capillary movement. Capillarity is described as the attraction of water, because of its surface tension, toward a solid. In fine soils capillary attraction is much stronger because of the relatively short distance across the spaces between the small soil particles.
In coarse soils the larger, loose, single grains of sand, with bigger pore spaces between them, reduce capillarity. Water movement responds more to the downward force of gravity in sandy soil.
The illustration below shows what could be expected in subsurface wetted patterns if the soil grades were uniform with no stratification or layering
The table below defines the approximate ranges of the wetted areas per emitter in the given soil type:
The best way to determine the wetted area is to test it at the site. This is done quite often for cash crops. The test involves applying the water at a selected flow rate for one hour and then trenching across the wetted pattern for an actual measurement. I don't think the "actual test" method will be used quite so often on landscape drip projects.
If you know you have a sandy soil, but are unsure of its grade within the range of coarse soils, use the smallest wetted area listed. As you'll see in the formula for the number of emitters, using the smallest area increases your emitter number.
2. The Plant Area
The area of the plant, which was established for the gallon per plant per day formula, is also a factor in determining the number of emitters.
3. The Percent of Plant Area to be Irrigated
The other main factor is the area of the root zone to be wetted. Fifty percent is the absolute minimum. In most cases sixty percent is preferable (like for fruit producing trees or to provide more emitters for leaching salt from the soil).
NUMBER OF EMITTERS PER PLANT
Emitter# = Area per plant x % to be wetted (as a decimal)