This equation computes the amount of 15:2:8 fertilizer -- or an equivalent fertilizer with the same concentration requirements -- that you would need to cover a desired area.
Most commercial websites offering fertilizers specify the coverage rate using two terms:
Thus the commercial sites will specify, as an example, that a 15:2:8 fertilizer covers 7500 `ft^2` with 50 lbs of the fertilizer.
Green Roof at Walter Reed CC,
Flickr / Arlington County
CC BY 2.0
** Note that most commercial sites do not detail the concentration that is assumed for the coverage rate specified. In our example of a commercially available 15:2:8 fertilizer which was specified with a 50lb per 7500 `ft^2` coverage rate, the assumed concentration is 20 ppm.1 This concentration assumption is used in this equation to scale the ppm input.