Fertilizer prices are quoted per ton of product, but the number that matters is cost per pound of actual nutrient. A ton of 46-0-0 urea at $600 contains 920 pounds of actual nitrogen at $0.65 per pound of N. A ton of 28-0-0 UAN solution at $400 contains 560 pounds of actual nitrogen at $0.71 per pound of N. The cheaper product per ton is actually more expensive per pound of nutrient.
This guide covers NPK analysis interpretation, cost per pound of nutrient calculations, custom blend optimization to hit a soil test recommendation at minimum cost, and real-world considerations of application uniformity and handling logistics.
Understanding NPK Analysis and Nutrient Content
The three numbers on a fertilizer label represent the guaranteed minimum percentage by weight of nitrogen (N), available phosphate (P₂O₅), and soluble potash (K₂O). A bag labeled 10-20-10 contains at least 10% nitrogen, 20% phosphate, and 10% potash by weight.
Soil test recommendations are given in pounds of actual nutrient per acre. If your soil test says apply 150 lbs N, 60 lbs P₂O₅, and 80 lbs K₂O per acre, calculate how much of each product delivers those amounts. For 150 lbs N from urea (46-0-0): 150 / 0.46 = 326 lbs urea per acre.
The interaction between multi-nutrient products is where blending gets interesting. DAP, MAP, and potash are the three most common granular sources. Blending them in the right ratio to hit your soil test at minimum cost is a linear optimization problem.
Cost per lb N = Product price per ton ÷ (2,000 × N percentage)
Example: Urea at $600/ton: $600 ÷ (2,000 × 0.46) = $0.65 per lb actual N
DAP at $750/ton: $750 ÷ (2,000 × 0.18) = $2.08 per lb N (but also delivers P)
Fertilizer Blend & Cost Calculator
Compare fertilizer products by cost per pound of actual nutrient delivered. Build custom blends, see excess nutrient waste, and generate printable blend tickets for dealer negotiation.
Optimizing a Custom Blend to Hit Soil Test Targets
Start with the nutrient that has the fewest source options — usually phosphorus. Choose your P source (DAP or MAP), calculate how much you need to hit the P target, then account for the N and K those products contribute as a side benefit.
A typical recommendation of 160-60-80 per acre might be filled with: 130 lbs DAP for P, 133 lbs potash for K, and 297 lbs urea for remaining N. Total cost per acre around $170 compared to pre-mix approaches that require supplemental applications.
Pre-mix blends like 19-19-19 rarely match soil test recommendations, meaning you either under-apply some nutrients or over-apply others. Custom blending delivers exact ratios at lower total cost.
Application Rate and Spreader Calibration
Spinner spreaders have an effective swath width that depends on product density, particle size, and spinner speed. Different particle sizes in a blend fly different distances off the spinner, causing nutrient segregation in the spread pattern.
Calibrate by collecting material in pans placed at regular intervals across the swath width. A properly calibrated spinner spreader shows a triangular pattern with 50% overlap between adjacent passes.
Variable-rate application using GPS-guided equipment typically saves 10% to 20% on total fertilizer cost while improving yield in nutrient-deficient zones.