NPK ratios – Why is Weekly Feed Phosphate so low?

Some people ask “Why does the Mittleider formula have 13-8-13 when so many other available mixes have a much higher phosphate content, such as 18-46-0?” 

Plants use about twice the amounts of both nitrogen and potassium as they do phosphorus during their life span.  Therefore, we feed the plants just what they need.  However, 50% of the phosphorus plants need is required in the first 20-25% of their life cycle.  So if only one or two feedings are being made, is it common to apply more phosphate immediately.  It is not volatile, so it doesn’t leave, but it does become “fixed” or unavailable fairly quickly on contact with the soil, so much more is used than actually gets to the plant.  This is wasteful, and contributes to excess phosphates in our streams and rivers. 

In addition, because tractor farmers are unable to get into their fields to feed many times, and because of the labor and equipment costs of doing so, they have adopted the practice of substantially over-feeding P and K, by applying 18-46-0 and 0-0-60, with the ultimate result that their plants get enough to produce a crop.  And knowing nitrogen’s unique properties, they will usually apply that 3 times per crop.  The end result is that a Mittleider-fed garden will produce substantially more than a typical field-grown crop, with much more efficient and conservative use of fertilizers.  The practice described above may have contributed to the anxiety many “organic” growers have about the over-use of commercial fertilizers by farmers.