Don Smith has it right. Lots of inert material.

You probably asked for a blend that had 50 lbs of actual P and 80 lbs of actual K. The 10.6 lb comes as a bonus because phosphate has some N in it.

To calculate the blend, you need to remember that 11-52-0 means that 11% of phosphate is N, 52% is phosphorus and that 0-0-60-0 is 60% K.

So to get 50lbs of actual P, you need 50 / 0.52 = 96 lb/ac of 11-52-0 which also gives you 10.6 lb of N (96 lb/ac x 11%)

To get 80 lbs of actual K, you need 80 / 0.60 = 133 lb/ac of 0-0-60.

Therefore your blend has 96 lb/ac of 11-52-0 plus 133 lb/ac of 0-0-60 which totals 229 lb/ac.


To calculate the % analysis the way the retailer showed it as 4.6-21.8-34.9 :


N - 10.6/229 = 4.6% of the total blend weight is Nitrogen

P - 50/229 = 21.8% of the total blend weight is Phosphorus

K - 80/229 = 34.9% of the total blend weight is Potassium   



As a general rule of thumb, you calculate the phosphate, potash, and sulfur needs first and do the N last because both phosphate and sulfur both have Nitrogen components in them that contribute to the N requirements.


Hope that simplifies the fertilizer mystery.