Free Soil Amendment & Fertilizer NPK Calculator
Calculate exact pounds or kilograms of fertilizer required to hit recommended soil test Nitrogen (N), Phosphorus (P), and Potassium (K) targets.
🌱 Fertilizer Blend & Garden Area
📊 Required Fertilizer Application
Recommended Tools & Equipment
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Understanding N-P-K Fertilizer Numbers
The three bold numbers on any fertilizer bag represent the guaranteed percentage by weight of three primary macronutrients: Nitrogen (N) for leafy foliage growth, Phosphorus ($P_2O_5$) for root development and blooms, and Potassium ($K_2O$) for cold tolerance and cellular wall strength.
The Application Rate Formula
The "1 lb N per 1,000 sq ft" Rule
University agricultural extension services recommend applying no more than 1.0 pound of fast-release actual nitrogen per 1,000 sq ft in a single feeding. Over-applying nitrogen burns grass roots with high osmotic salt pressure, pollutes local watershed runoff, and causes rapid, spindly foliage susceptible to fungal mildew.
Frequently Asked Questions
Why doesn’t a 10-10-10 bag add up to 100%?
A 10-10-10 blend contains 10% pure N, 10% P2O5, and 10% K2O, totaling 30% active plant nutrients. The remaining 70% consists of carrier material (gypsum, sand, organic binder, micronutrients) necessary to allow uniform distribution through a broadcast spreader.
How many cups of fertilizer are in one pound?
Most standard dry granular inorganic fertilizers weigh roughly 0.5 to 0.6 lbs per cup, meaning 2 measuring cups roughly equals 1 pound. Lightweight organic meals (blood meal, feather meal) are fluffier, averaging 3 to 4 cups per pound.
Should I fertilize wet or dry grass?
Always apply granular fertilizer to completely dry lawn grass, and immediately follow with 1/4" of irrigation. If fertilizer granules stick to wet blades under direct sunlight, they dissolve against the leaf cuticle, causing brown chemical fertilizer burn.