How much fertilizer per square foot
Recommendations come in pounds of nutrient per 1,000 sq ft; bags come in pounds of product. The bridge is one division — and this is the table that does it for the bag you actually bought.
The conversion, in one line
Extension recommendations are written in pounds of nutrient per 1,000 square feet. Bags are sold in pounds of product. The bridge is one division:
pounds of product = (pounds of nutrient wanted × 100) ÷ the percentage on the bag.
So 1 lb of nitrogen from a 10-10-10 is 1 × 100 ÷ 10 = 10 lb of product. From urea (46-0-0) it is 100 ÷ 46 = 2.2 lb. That is the entire arithmetic, and it is the step most bag directions skip.
Product per pound of nitrogen
| Product | %N | %P2O5 | %K2O | lb of product per 1 lb N |
|---|---|---|---|---|
| Balanced 10-10-10 | 10 | 10 | 10 | 10.0 |
| Balanced 12-12-12 | 12 | 12 | 12 | 8.3 |
| Balanced 8-8-8 | 8 | 8 | 8 | 12.5 |
| Garden 5-10-5 | 5 | 10 | 5 | 20.0 |
| Garden 5-10-10 | 5 | 10 | 10 | 20.0 |
| Complete 16-16-16 | 16 | 16 | 16 | 6.2 |
| Urea 46-0-0 | 46 | 0 | 0 | 2.2 |
| Ammonium sulfate 21-0-0 | 21 | 0 | 0 | 4.8 |
| Calcium nitrate 15.5-0-0 | 15.5 | 0 | 0 | 6.5 |
| Triple superphosphate 0-46-0 | 0 | 46 | 0 | — |
| Superphosphate 0-20-0 | 0 | 20 | 0 | — |
| Muriate of potash 0-0-60 | 0 | 0 | 60 | — |
| Sulfate of potash 0-0-50 | 0 | 0 | 50 | — |
The last column is just 100 ÷ %N — the whole conversion in one step.
Per square foot
Divide the per-1,000 figure by 1,000. At the standard 1 lb N per 1,000 sq ft, a 10-10-10 goes down at 10 lb per 1,000 sq ft — 0.01 lb per square foot, or about 0.16 oz. For a 4×8 raised bed (32 sq ft) that is 0.32 lb of product: a large handful, not a scoop. Small areas are exactly where per-acre and per-1,000 figures get misread by an order of magnitude.
The nitrogen cap
Do not exceed about 1 lb of nitrogen per 1,000 sq ft in a single application. More than that burns foliage and the surplus leaches past the root zone — it feeds the storm drain rather than the bed. A season total higher than that is fine; it just has to be split.
Why one bag rarely hits all three numbers
A report asking for 1 lb N, 0.5 lb P₂O₅ and 1 lb K₂O per 1,000 sq ft cannot be satisfied by any single balanced bag: hitting the nitrogen with a 10-10-10 over-delivers phosphate by half. That is not a rounding error — phosphorus accumulates, and many home soils are already high in it. The honest move is to hit the limiting nutrient with the blend and top up the others separately, and to know which one you are over-applying.
Your own lab's recommendation wins. Loam converts what the report already says into what your bed and your bag need — it does not set rates.
FAQ
How much fertilizer do I need per square foot?
Take the recommendation in pounds per 1,000 sq ft and divide by 1,000. At the common 1 lb of nitrogen per 1,000 sq ft, a 10-10-10 works out to about 0.16 oz of product per square foot — roughly 0.32 lb for a 4×8 raised bed.
How much 10-10-10 per 1,000 square feet?
10 lb of product delivers 1 lb of nitrogen, because the bag is 10% nitrogen (100 ÷ 10 = 10). That same 10 lb also delivers 1 lb of phosphate and 1 lb of potash, which is usually more phosphate than a home soil needs.
What does 10-10-10 actually mean?
Percentages by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 10-10-10 holds 5 lb of each. The second and third numbers are oxide forms, not elemental phosphorus and potassium — which is why lab reports state recommendations the same way.
How much fertilizer is too much?
About 1 lb of nitrogen per 1,000 sq ft in one application is the widely published ceiling. Beyond it you get leaf burn and leaching rather than growth. Split larger season totals into several applications.
Getting the numbers this needs
The conversion is trivial; knowing what to convert is not — that comes off a soil test.
- A mail-in lab test or a home NPK + pH kit is what turns "feed the garden" into an actual pound-per-1,000 figure.
- A balanced 10-10-10 is the default when a report asks for all three; single-nutrient products are how you avoid over-applying the other two.
- A cheap kitchen scale beats a measuring cup — these rates are weights, and bulk density varies enough to matter on a small bed.
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Sources
Rates and the single-application nitrogen cap are typical published extension figures for home lawns and gardens; your own lab's printed recommendation wins.