Nutrients, products, and the gap between them
Every fertiliser recommendation you receive is written in pounds of nutrient per acre: 120 pounds of N, 60 of P2O5, 60 of K2O. Every fertiliser you can buy is a product with a percentage on the tag. The whole job of a blend calculation is dividing the first by the second, in the right order, with the right credits.
The order matters because the materials are not independent. Muriate of potash at 0-0-60 supplies nothing but potash, so it can be sized first with no consequences. Diammonium phosphate at 18-46-0 supplies phosphate and nitrogen, so once you have sized it for phosphate you have also committed to a quantity of nitrogen whether you wanted it or not. Only after that credit is taken can you work out how much urea is left to buy. Do it the other way round and you over-apply nitrogen by the DAP credit, which at 60 pounds of P2O5 is a little over 23 pounds of N per acre.
The blend that comes out has a grade of its own, and that grade is almost never a round number. The default figures here produce a 27.3-13.6-13.6 blend, which no manufacturer sells. That is the point of blending: you buy commodity materials in bulk and mix the ratio your soil test asked for, instead of buying a bagged compromise and accepting whichever nutrient is wrong.
The oxide convention, and why the tag is not elemental
Nitrogen is reported as elemental N. Phosphorus and potassium are not: they are reported as the oxides P2O5 and K2O. This is a nineteenth-century laboratory convention that has outlived its justification and is now locked into fertiliser law, tags and every recommendation you will read.
The conversions are fixed by molecular weights. Elemental phosphorus is 0.4364 of P2O5, so 60 pounds of P2O5 is 26.2 pounds of actual P. Elemental potassium is 0.8301 of K2O, so 60 pounds of K2O is 49.8 pounds of actual K. Neither compound exists in the bag — DAP is an ammonium phosphate, potash is potassium chloride — but the guarantee is stated in oxide equivalents and so is the recommendation, so as long as you keep both in the same units the arithmetic is consistent. Trouble only starts when a soil report from outside North America gives elemental P and K; convert before you enter anything here.
With the units settled, each material is sized by one division. Pounds of material equals pounds of nutrient divided by the tag fraction. Sixty pounds of K2O from a 0-0-60 is 60 ÷ 0.60 = 100 pounds of product. Sixty pounds of P2O5 from an 18-46-0 is 60 ÷ 0.46 = 130.4 pounds. That 130.4 pounds carries 130.4 × 0.18 = 23.5 pounds of N. Subtract it from the target, and the residue divided by 0.46 gives the urea.
The blend's own grade then follows from a mass balance: total nutrient in the batch divided by total batch weight. Because that denominator includes every pound of every material, adding filler lowers all three grade numbers proportionally while changing none of the nutrients delivered per acre.
Worked example: 120-60-60 on 100 acres
Your soil test calls for 120 lb N, 60 lb P2O5 and 60 lb K2O per acre across 100 acres. You have urea at 46-0-0 for $500 a ton, DAP at 18-46-0 for $750 a ton, and muriate of potash at 0-0-60 for $480 a ton.
- Potash first. 60 ÷ 0.60 = 100 lb of potash per acre.
- Phosphate second. 60 ÷ 0.46 = 130.43 lb of DAP per acre.
- Credit the DAP nitrogen. 130.43 × 0.18 = 23.48 lb N per acre already supplied.
- Nitrogen last. 120 − 23.48 = 96.52 lb still needed; 96.52 ÷ 0.46 = 209.83 lb of urea per acre.
- Total blend rate. 209.83 + 130.43 + 100 = 440.26 lb per acre.
- Blend grade. N: 120 ÷ 440.26 = 27.26%. P2O5: 60 ÷ 440.26 = 13.63%. K2O: the same 13.63%. The blend is a 27-14-14 for tag purposes.
- Batch size. 440.26 × 100 acres ÷ 2,000 = 22.01 tons — 10.49 tons of urea, 6.52 tons of DAP and 5.00 tons of potash.
- Cost. Urea at $500/ton is $0.25/lb, so 209.83 × 0.25 = $52.46. DAP at $750/ton is $0.375/lb, so 130.43 × 0.375 = $48.91. Potash at $480/ton is $0.24/lb, so 100 × 0.24 = $24.00. Total $125.37 per acre, or 125.37 ÷ 0.22013 tons = $569.52 per ton of blend, before the blending and spreading charge.
The cost-per-ton figure is what makes this worth doing. Quote a bagged 27-14-14 against $569.52 a ton plus your blend fee and you know immediately whether the convenience is worth its price.
Reading the blend you have designed
Look at the blend rate first. Somewhere around 200 to 500 pounds an acre is comfortable territory for a spinner spreader or an air boom. Much below 150 pounds and uniform distribution gets difficult, which is one legitimate reason to add filler. Much above 600 pounds and you are into a heavy single pass that may be better split, particularly for nitrogen, where a split application also reduces loss.
Then look at the grade. A high-analysis blend costs less per unit of nutrient to haul and spread, because you are moving less inert weight. A blend whose three numbers sum to under 20 is mostly not fertiliser, and unless you deliberately added filler that usually means a source analysis was entered wrong.
Then look at cost per unit of nutrient rather than cost per ton. Divide each material's price per ton by 20 times its analysis to get dollars per pound of nutrient: urea at $500 a ton and 46% N is 500 ÷ (20 × 46) = $0.543 per pound of N. Do the same for every nitrogen source available and buy the cheapest per pound of N, adjusting for the sulphur in ammonium sulphate or the handling penalties of anhydrous ammonia. This single comparison is where most of the money in a fertiliser programme is won or lost.
Finally, remember what the blend does not tell you. Dry bulk blends segregate in handling if the particle sizes of the components differ, so a well-mixed batch at the plant can arrive at the field with the potash at the bottom of the box. Ask your blender about size guide number and uniformity index matching, and spread from a full hopper rather than dribbling out the last of a load across a headland.
Guaranteed analyses of common blending materials
| Material | Grade | Other nutrients | Notes |
|---|---|---|---|
| Urea | 46-0-0 | — | Highest solid N analysis; subject to volatilisation if left on the surface |
| Ammonium nitrate | 34-0-0 | — | Half nitrate, immediately available; restricted in many places |
| Ammonium sulphate | 21-0-0 | 24% S | Acidifying; the usual sulphur carrier in a blend |
| UAN solution | 32-0-0 | — | Liquid, not blended dry; 28% and 30% grades also sold |
| Monoammonium phosphate (MAP) | 11-52-0 | — | Higher phosphate, lower N credit than DAP |
| Diammonium phosphate (DAP) | 18-46-0 | — | Carries 0.18 lb N per lb of product |
| Triple superphosphate | 0-46-0 | — | No nitrogen credit at all |
| Muriate of potash (KCl) | 0-0-60 | ~46% Cl | The standard potash; chloride matters for a few sensitive crops |
| Sulphate of potash | 0-0-50 | 18% S | Chloride-free, materially more expensive per unit K₂O |
| Potassium nitrate | 13-0-44 | — | Supplies both N and K; usually a fertigation material |
Grades are the standard guaranteed analyses registered under state fertiliser laws following the AAPFCO model bill. Individual products vary slightly; always use the analysis printed on the tag you were sold.
The tag is a legal document
Guaranteed analysis, the order N-P2O5-K2O, and the minimum percentages on it are set by state fertiliser law, which nearly every state has written from the Association of American Plant Food Control Officials model bill. The guarantee is a legal minimum, not a nominal figure, and state departments of agriculture sample and assay products against it. That is why you can size a blend on the tag number with confidence, and why an analysis you were told over the phone is not a substitute for the tag.
Mistakes that cost money in a blend
- Not crediting the nitrogen in DAP or MAP. At 60 lb of P₂O₅ from DAP that is 23.5 lb of N per acre applied twice, which is both wasted money and an agronomic risk.
- Confusing pounds of nutrient with pounds of product. 100 lb of urea is 46 lb of N, and 100 lb of N needs 217 lb of urea. Write the units on every number.
- Mixing elemental and oxide units. A report giving elemental P and K needs converting first: divide P by 0.4364 and K by 0.8301 to reach the oxide basis the tag uses.
- Comparing prices per ton instead of per unit of nutrient. A cheaper ton of a low-analysis material is often a more expensive pound of nitrogen once you have hauled and spread the difference.
- Ignoring particle size matching. Dry blends segregate when component particle sizes differ, so a uniform batch at the plant can spread unevenly in the field.
- Forgetting incompatible pairs. Urea and ammonium nitrate together take up moisture and go to slurry, and urea with superphosphate can release free water. Ask your blender before combining materials you have not blended before.
- Leaving urea on the surface. Urea hydrolyses to ammonia and can volatilise from a warm, moist surface. Incorporate, irrigate in, or use a urease inhibitor.
Where the blend fits in the fertility programme
A blend answers the question of how to deliver a rate, not what the rate should be. That comes from a soil test interpreted against a yield goal by your state's recommendation system or an agronomist. If pH is below the range for your crop, correct it before you spend on phosphorus at all: phosphate availability falls sharply in acid soil, and the agricultural lime requirement calculator sizes the correction in tons of your actual liming material.
If you have manure available, credit it before you size the blend, because manure often supplies all the potash and most of the phosphate a crop needs. The manure application rate calculator converts an analysis into a rate and reports the fertiliser value it replaces; subtract that from the target rates here and blend only the shortfall. Compost is a slower-release version of the same idea, and the compost C:N ratio calculator is where that batch starts.
Placement matters as much as rate for phosphorus in particular, so if you are banding a starter through the planter, work out how much lands per foot of row using the row feet per acre figure from the row spacing calculator. And if any part of the programme is going out through the sprayer as a foliar or a UAN application, size the loads with the acres per spray tank calculator.