There are a lot fertilizer options to produce crops.  With the rapidly changing and generally rising cost of crop nutrients, a firm understanding of fertilizer mathematics is essential to determine where a few (or a lot!) of bucks can be saved by simply substituting one fertilizer source for another.  The process is relatively simple, but there is one underlying truth that needs to be understood:

Commercially marketed fertilizers are tested and have a grade analysis that is guaranteed.  State fertilizer laws ensure that labels on all fertilizers list the guaranteed available primary nutrients in order of N, P₂O₅, and K₂O as a percent by weight of fertilizer material. The fertilizer grade is determined on a weight basis regardless of whether the fertilizer product is liquid or dry. The content of each nutrient is determined in laboratory tests. The results are interpreted as the fraction of a particular nutrient from a fertilizer that would be available to plants. Therefore, two different materials that have the same analysis will have equal amounts of available nutrients in the fertilizer itself.

        What all that means is no one type of fertilizer is “more available” than another and fertilizers of equal analysis provide equal nutrients for plant growth. 

Cost per unit

        The best method to compare fertilizers containing the same nutrient is to determine the actual cost per pound of nutrient unit.  As an example, let’s compare dry fertilizer urea (46-0-0) with liquid urea ammonium nitrate (UAN - 28-0-0) as follows: 

Source   %N    $/ton    lbs N/ton             $/lb N             

Urea       46     $700     0.46x2000=920     700/920=$0.76

UAN       28     $600     0.28x2000=560     600/560=$1.07 

        In this simple example, the liquid UAN is costing $0.31 more per unit of N.  From a purely economic standpoint, the urea is a much cheaper source of nitrogen.  Unfortunately, life and farming are not always that simple.  Perhaps there are reasons why UAN has to be used (for example, the planter has a liquid rather dry applicator).  The important outcome is that you know how to determine your cost per unit, regardless of source. 

Multiple nutrient fertilizers

        Some fertilizers have multiple nutrients.  A good example of this is our primary phosphorus sources:  monoammonium phosphate (11-52-0) and diammonium phosphate (18-46-0).  Both products contain nitrogen and phosphorus so that our true cost of P₂O₅ can only be determined after we take out the value for the nitrogen.  The nitrogen value is usually based on the cost of a nitrogen source fertilizer.  In the following example, we’ll use $0.76 per lb N from our previous urea calculation and assume both MAP and DAP are priced at $1100 per ton. 

Source   %N       lbs N/ton             Value N/ton             

MAP       11         0.11x2000=220     220x0.76=~$167

DAP       18         0.18x2000=360     360x0.76=~$274

Source   %P    $/ton minus N value        

MAP       52      1100–167=$933  

DAP       46      1100-274=$826

Source   $/ton-N    lbs P/ton               $/lb P                 

MAP       $933        0.52x2000=1040   933/1040=$0.90

DAP       $826        0.46x2000=920      826/920=$0.90 

        In this example, the actual cost of P₂O₅ per pound is the same for both MAP and DAP when the value of N in each fertilizer is taken into account. 

Fluid fertilizers priced per gallon

        Sometimes the price of liquid fertilizers are quoted in $ per gallon.  In these situations, the best first step is to convert $/gal to $/ton.  Below are some common weights for several liquid fertilizers:

        Let’s assume 10-34-0 is being sold for $8.00 per gallon.  Remember, the analysis is based on weight so we need to determine both the amount of nutrients in a ton to figure the cost per unit for the P₂O₅ contained in 10-34-0 and make a comparison to other sources.  

Gal 10-34-0 per ton = 2000 / 11.6 = 172.41

$/ton = 172.41gal x $8.00/gal = ~$1379/ton 

        A case could be made for either urea (cheapest) or UAN (liquid N source) to be used as our base N value.  In this example, we’ll use urea ($0.76/lb N) so we can make a comparison of P₂O₅ value with the MAP and DAP example from above. 

Source   %N       lbs N/ton             Value N/ton             

10-34-0   10         0.10x2000=200     200x0.76=$152 

Source   %P     $/ton minus N value        

10-34-0   34       1379–152=$1227     

Source   $/ton-N    lbs P/ton               $/lb P                 

10-34-0   $1227      0.34x2000=680    1227/680=~$1.80 

        In this example, the $8.00 per gallon 10-34-0 equates to buying P₂O₅ at double the price of the MAP or DAP fertilizers ($1.80/lb. vs. $0.90/lb.). 

        The point of all this is not to lead you down one fertilizer path or another (the prices were made-up), but rather to emphasize the importance of pushing the pencil and comparing fertilizer inputs on a nutrient unit cost per pound basis.  Quick calculations like this also help to emphasize the value of manure and legume crediting.  Finally, don’t fall for the sales pitch that “our fertilizer is more available to plants than a competitors.”  It just ain’t so.