Fertilizer Blend Calculator (Urea, DAP/MAP, Potash & AMS per Acre)
Build a dry fertilizer blend to hit target pounds of N, P₂O₅, K₂O and sulfur per acre from common products — urea 46-0-0, DAP 18-46-0 or MAP 11-52-0, potash 0-0-60, ammonium sulfate 21-0-0-24S — with pounds of each per acre and per field, the tons to order, the blend's analysis, cost per acre, and the nitrogen credit the phosphate source supplies.
How it works
AMS lb = S ÷ 0.24 · phosphate product lb = P₂O₅ ÷ analysis (DAP 0.46, MAP 0.52) · potash lb = K₂O ÷ 0.60 · N make-up lb = (N − N from DAP/MAP − N from AMS) ÷ analysis (urea 0.46) · cost/ac = Σ lb × $/ton ÷ 2000 · blend analysis = nutrient lb ÷ total lb × 100
Fertilizer is sold as products with fixed analyses, so hitting a soil-test recommendation for several nutrients means solving them in order: the products that carry only one nutrient are set last, and those that carry two — DAP or MAP with their nitrogen, ammonium sulfate with its sulfur — are set first so their by-product nitrogen can be credited before the urea is calculated. The pounds per acre of each product then scale to tons for the field and to a spreader rate, and dividing each product's price by its nutrient content shows what each pound of N, P or K really costs. Urea's tendency to evaporate off the surface is the practical caveat that follows the maths onto the field.
Worked example
A 150-60-80-15S target for corn: 62 lb of AMS supplies the sulfur and 13 lb of N; 130 lb of DAP supplies the 60 lb of P₂O₅ and another 23 lb of N; 133 lb of potash gives the K₂O; so only 113 lb of N is left for urea — 247 lb. The 573 lb/ac blend analyses about 26-10-14-3S and costs around $154 an acre — roughly $18,400 and 34 tons for 120 acres, with urea nitrogen at 57¢ a pound.
Frequently asked questions
Should I count the nitrogen in DAP?
Yes — it is real ammonium nitrogen and reduces the urea needed. It is applied at P timing, though, so if the P goes on in autumn some of that N may be lost before a spring crop uses it; many agronomists credit only part of it in that case.
Why not just buy a bagged 10-10-10?
Balanced blends rarely match a soil test — you overbuy the nutrients you do not need. Straight products blended to the recommendation are cheaper per pound of nutrient and let you put N, P and K where the soil test says.
Related calculators
Agronomic rules of thumb and standard grain-trade formulas from extension services. Local elevators, labs and equipment dealers publish their own schedules and specifications — use those where they differ, and confirm lime and fertilizer rates with a current soil test.