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Blending two fertilizers to hit a target ratio
Every fertilizer bag carries three numbers, the NPK ratio, which gives the percentage of nitrogen, phosphorus, and potassium inside. A 10-10-10 bag holds 10 percent of each, while a 20-0-0 bag holds 20 percent nitrogen and none of the other two. Gardeners often own two half-used bags and wonder what they get by mixing them, or they want a custom ratio for a crop that neither bag matches alone. Blending by guesswork gives an unknown feed, but blending by weighted average gives an exact one.
This calculator does that weighted average for you. Enter the NPK numbers and the weight of each fertilizer, and it gives you the blended NPK ratio and the total weight of the mix. The result is only as honest as the numbers you enter, so read the guaranteed analysis printed on each bag rather than a rounded number from a shop listing.
How the blend math works
A blend’s nitrogen percentage is the total nitrogen from both bags divided by the total weight. If you mix 10 pounds of 10-10-10 with 10 pounds of 20-0-0, the nitrogen is 1 pound from the first bag plus 2 pounds from the second, which is 3 pounds in 20 pounds of mix, or 15 percent. The same weighting gives 5 percent phosphorus and 5 percent potassium, so the blend is 15-5-5.
Unequal weights shift the result toward the heavier bag. Mix 30 pounds of a 0-0-0 filler-free plain material with 10 pounds of 20-20-20 and the blend lands near 5-5-5, because the larger share dominates. That weighting is the whole trick: the calculator simply does for three nutrients at once what you would do by hand for one, and it never lets a small bag outweigh a big one.
A worked example
Suppose you have 10 pounds of 10-10-10 and 10 pounds of 20-0-0 left over from last season. Enter both, and the calculator returns 15-5-5 and 20 pounds total. That blend suits leafy growth, since it is rich in nitrogen and modest in the other two, but it would be a poor choice for flowering plants that need more phosphorus and potassium.
Now try a different goal: a balanced feed from a high-nitrogen bag and a high-potash bag. Mixing 10 pounds of 20-0-0 with 10 pounds of 0-0-30 gives 10-0-15, which feeds leaves and roots but gives no phosphorus at all. If your soil already tests high in phosphorus, that gap may not matter, but if it tests low, the blend will starve flowering. The calculator shows the gap plainly so you can fix it before spreading anything.
Tips for sensible blending
Blend only dry, free-flowing granules of similar size, and mix them thoroughly in a bucket before spreading, because uneven mixing gives stripes of strong and weak feed across the lawn or bed. Keep the blend dry, since damp granules clump and react, and label the bucket with the blended ratio and the date so next month you know what you made.
Always start from a soil test when you can. Blending is most useful when a test shows one nutrient high and another low, because then a custom ratio corrects the imbalance instead of adding more of what the soil already has. Without a test, a balanced commercial feed is safer than a home blend built on guesses about what the soil needs.
Common mistakes
The most common mistake is averaging the ratios without weighting by amount, which says a pound of one bag counts as much as a sack of another. The second mistake is forgetting that the blended percentage is lower than the strongest bag: mixing a 20 percent feed half-and-half with a 0 percent material gives a 10 percent feed, so you must spread twice as much to deliver the same nutrient, and many people forget that adjustment.
People also blend materials that should never meet, such as urea with superphosphate in damp storage, or lime with ammonium fertilizers, which can lose nitrogen as ammonia gas. If you are unsure whether two products are compatible, keep them separate and apply them on different days.
Safety and limits
NPK blend results are general guidance estimates only. Always follow the product label for fertilizer rates, never exceed label rates, and local climate and soil conditions govern how plants respond. A blended ratio is not a recommendation: the right ratio depends on your soil test, your crop, and your local rules, and over-applying nitrogen burns roots and pollutes waterways. Wear gloves and a dust mask when handling dry fertilizer, and store blends away from children, pets, and damp.
Frequently asked questions
Can I blend liquid fertilizers the same way? Yes, if both labels allow mixing and you weight by volume, but liquids of different densities make the math less tidy. Check compatibility first, since some liquids form solids when mixed.
Does blending change the total nutrient applied? No. Blending only changes the ratio in the bag. The nutrient you apply is still the weight spread times the percentage, so a weaker blend needs a heavier spreading rate to deliver the same nutrient.
Use the calculator above with the exact numbers from your bags, check the blend against your soil test, and spread at label rates. A blend you understand beats two half bags you guess about, and your plants will show the difference within weeks.
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