I've been watching this thread with great interest since we produce TROPHY MAKER 12-61-0 water-soluble pond fertilizer. The observations have been enlightening (particularly the fire-ant remedy). However, I'm puzzled by the price per "unit" of P concept. What is the referenced "unit"?
In most fertilizer applications, costs are usually determined per lb. of N (nitrogen), P (phosphorus) or K (potash). With pre-blended fertilizers, it can be difficult to calculate the exact cost of each component (N,P or K) since each element represents a disproportionate amount of the blend's cost. Also, the source of each element impacts its cost-to-produce as well as its degree of plant-availability.
In the case of pond fertilizers, wouldn't it make reasonable sense to simply calculate the blended product's cost "Per lb. of P" (since it is the primary nutrient for phytoplankton development)?
If that is the case, then cost per lb of P might be a sensible means for comparing the relative costs of different blends (though the element's source and its availability for plant-uptake may differ significantly).
To compare costs per lb. of P, take the cost per lb. of blended product and divide it by the % of P that is contained within that lb of blended product.
ie. TROPHY MAKER 12-61-0 contains 61% phosphorus. "IF" TM sells retail for $1.20/lb., the cost per lb of P would equal $1.20 divided by .61 (or $1.97/lb. of P). Although this method ignores the value and/or benefit that the 12% of N contributes to the product's cost, it still give a cost-basis for the nutrient of primary concern (P). BTW: Trophy Maker's phosphorus is derived from monoammonium phosphate, which is very readily available for plant uptake and use. Other sources of phosphorus may require days before the P becomes available for a plant's use.
Other examples:
THRIVE 12-51-4 / If cost is $1.06 per lb. of blend, then the cost for P could be stated as $2.07 per lb. (Back-check: $2.07 x 51% = $1.06)
SOUTHERN EXCELLENCE 18-46-0 / If cost is $1.00 per lb. of blend, then the cost for P could be calculated as $2.17/lb. ($1 divided by .46)
What about liquids?: 11-37-0 contains 37% P by weight (dense liquid fertilizers weigh about 11 lbs/gal.). So, 1 gal of 11-37-0 contains about 4.07 lbs. of P. At a blended cost of $3.60/gal., the cost per lb. of P may be calculated at $3.60 divided by 4.07, equal to 88 cents per lb. of P (plus a LOT of labor and equipment to do the job right).
I'm open to correction if I'm miscalculating these fertilizer costs. But it seems to be a logical means of comparing the cost of different blends when one particular element is of primary concern. Any comments????
