How much fertilizer? (general)

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Equation / Last modified by mike on 2015/07/29 08:15
MichaelBartmess.How much fertilizer? (general)

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.


  • ppm - the desired concentration of fertilizer you want to achieve
  • cov - the area you wish to cover at the specified concentration
  • wt - the product specification of weight of fertilizer that will cover a standard area
  • stdArea - the product specification of are covered by the product weight (wt)


Most commercial websites offering fertilizers specify the coverage rate using two terms:

  • a standard container weight for the fertilizer, which is the input wt
  • a standard coverage area, which is the input stdArea

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 Community Center (3926468274_1185491b6b_m).jpg  
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.


  1. ^ Dr. Clayton Rugh, Manager & Technical Director at Xero Flor America LLC