Fertilizer Blend & Mix Your Own N-P-K

It works out how to mix straight fertilisers — urea, DAP and muriate of potash (MOP) — plus a filler to reach a target N-P-K grade. You enter the grade you want (say 12-32-16) and a batch size, and it returns the kilograms of each material to weigh out, the actual grade the mix delivers, and whether that grade is even achievable from these straights.

Three common straights: urea (46-0-0) for nitrogen, di-ammonium phosphate or DAP (18-46-0) for phosphate, and muriate of potash or MOP (0-0-60) for potash. An inert filler (such as sand, dolomite or dry soil) makes the batch up to the target weight when the nutrients alone are lighter than the bag.

DAP is set first because it is the only phosphate source, so the amount of DAP is fixed by the P₂O₅ you need — and that DAP also brings in some nitrogen. Urea then tops up whatever nitrogen is still short after the DAP's contribution. MOP supplies all the K₂O. Finally filler is added so the four materials together weigh exactly one batch.

Because DAP carries both phosphate and nitrogen (18-46-0), it must be fixed first by the phosphate requirement; only then can you see how much nitrogen it has already supplied. Urea fills the remaining nitrogen gap. Doing it the other way round would over-apply nitrogen, because the DAP's nitrogen would be ignored.

Each straight has a fixed analysis, so there is a ceiling to how concentrated a blend can be. If your target needs more nitrogen than urea-plus-DAP can supply at that phosphate level, or the nutrients alone already exceed the batch weight, the grade can't be made from these straights. Lower the grade, raise the batch size, or use a higher-analysis source.

Most blended grades are less concentrated than the straights, so the nutrient materials don't fill the whole bag. An inert filler makes up the difference in weight, keeps the spreader calibration honest, and helps the granules flow and spread evenly. If a grade needs no filler it's near the limit of what the straights can deliver.

Usually, yes. Buying straights and blending them yourself avoids the premium charged for factory complexes and lets you hit a grade matched to a soil test instead of accepting a stock formula. The savings are biggest at volume; weigh the materials accurately, because guesswork erodes both the agronomy and the cost benefit.

Weigh each material accurately on a clean, dry floor or in a small batch mixer, combine evenly, and break up any lumps. Use granules of similar size so the blend doesn't segregate when handled. Avoid mixing materials that react or cake together, and keep the area dry — urea and MOP both pick up moisture quickly.

Use it promptly. Home blends aren't conditioned like factory products, so they absorb moisture and can cake or segregate if stored. Mix close to application, keep it sealed and dry until you spread it, and apply within days rather than weeks for the most even, predictable result.

They're accurate for the standard analyses used (46-0-0, 18-46-0, 0-0-60). Real bags can vary slightly, and weighing error in the field matters more than rounding here. Treat the output as a precise recipe, weigh carefully, and check the actual grade the tool reports against what your crop and soil test call for.