What do the three numbers on fertilizer mean?

Percent by weight of nitrogen (N), phosphate (P₂O₅), and potash (K₂O). A 10-10-10 bag is 10% of each — a 50-lb bag holds 5 lb of each nutrient.

How much 10-10-10 for 1,000 square feet?

At our vegetable nitrogen target of 2 lb N per 1,000 sq ft, you need 20 lb of 10-10-10 product. That load also delivers 2 lb P₂O₅ and 2 lb K₂O.

Is more fertilizer better?

No. Excess nitrogen grows leaves at the expense of fruit, burns roots, and can pollute runoff. Size to a target, preferably after a soil test.

Do I still need compost if I fertilize?

They do different jobs. Compost feeds structure and biology; fertilizer delivers measured nutrients. Many gardens use both — compost for soil, fertilizer for known gaps.

N-P-K Fertilizer Basics for Home Gardens

The three numbers on a fertilizer bag are not a personality test. They are percent nitrogen (N), phosphate (P₂O₅), and potash (K₂O) by weight. Our home-garden math starts from a seasonal nitrogen target of 2 lb N per 1,000 sq ft, then backs into pounds of product. The locked worked example: 1,000 sq ft, 2 lb N, 10-10-10 → 20 lb of product. Run any analysis through the fertilizer calculator. Memorize that one example and you can sanity-check every bag in the cart before the cashier asks how your tomatoes are doing this season at home.

Reading the label

Bag label	N fraction	P fraction	K fraction
10-10-10	0.10	0.10	0.10
5-10-10	0.05	0.10	0.10
12-0-0 (example blood-meal-style analysis)	0.12	0	0

A 50-lb bag of 10-10-10 contains 5 lb of each nutrient (50 × 0.10). That does not mean you should dump fifty pounds on a small bed.

Organic products use the same three-number language when labeled. “Natural” does not cancel arithmetic.

The product formula

lb product = (target lb nutrient per 1,000 sq ft ÷ nutrient fraction) × (area ÷ 1,000)

For nitrogen with 10-10-10 on 1,000 sq ft:

(2 ÷ 0.10) × (1,000 ÷ 1,000) = 20 lb product

Same target on 500 sq ft → 10 lb. On 2,000 sq ft → 40 lb.

That 20 lb of 10-10-10 also supplies 2 lb P₂O₅ and 2 lb K₂O. If your soil test already shows high phosphorus, a balanced 10-10-10 may be the wrong tool even when nitrogen is short — choose a product that matches the nutrient you actually need.

Why we lead with nitrogen

Vegetables are often nitrogen-hungry through the season, and N is the number gardeners under- or overshoot most dramatically. Our default target 2 lb N / 1,000 sq ft is a maintenance-style baseline for mixed vegetable ground — not a license to ignore a soil test that says otherwise.

Phosphorus and potassium deficiencies are real, but stacking P every year “just in case” builds up in soil and waterways. Let the test drive P and K; let the calculator size the bag once you pick which nutrient is limiting.

Worked examples you can redo by hand

Example A (locked): 1,000 sq ft, 2 lb N, 10-10-10 → 20 lb product → 2 / 2 / 2 lb of N / P₂O₅ / K₂O.

Example B: 1,000 sq ft, 2 lb N, 5-10-10 → (2 ÷ 0.05) = 40 lb product. Note you also apply 4 lb P₂O₅ and 4 lb K₂O — more P and K than Example A.

Example C: 200 sq ft bed, 2 lb N / 1,000 target, 10-10-10 → 20 × 0.2 = 4 lb product.

Write the area on the bag with a marker after you weigh. Future you will thank present you.

Organic sources and burn risk

High-N organics still follow the same fraction math — read the bag; a 12-0-0 label is only an example analysis, not a site-wide constant. Keep concentrates off stems and water them in. “Organic” burns if you pile it against seedlings.

Compost is a mild nutrient source and a soil builder — size piles with the compost calculator, but do not pretend a light top-dress replaces a documented 2 lb N seasonal target when crops are pale and a test confirms nitrogen need.

pH before you chase N-P-K

If pH sits outside roughly 6.0–7.0, fix that path first with the soil amendment calculator. Nutrients you apply on locked soil are expensive mulch.

Split applications and side-dressing

Many gardeners split nitrogen: some pre-plant, some as a side-dress when heavy feeders take off (corn at knee-high is the classic image). The calculator sizes the seasonal total; you decide how to stage it. Never exceed label rates on specialty products, and never assume two full seasonal doses are better than one measured dose.

Mistakes that waste money

Handful-per-plant folklore with no area math
Repeating 10-10-10 yearly without a test while P climbs
Sizing the bag from the middle number when nitrogen was the goal
Ignoring that a low-N bag means more total product for the same N target (see Example B)
Fertilizing drought-stressed wilted plants instead of watering first

Liquid feeds and “bloom boosters”

Water-soluble products still have N-P-K percentages. Convert the label to fractions and use the same target logic, or follow the product’s own square-foot directions if they are clearer — but do not stack a full seasonal granular dose and a full seasonal liquid dose without doing the arithmetic. Bloom boosters heavy in phosphorus are rarely the missing piece in soils that already test high in P; they are marketing with a hose-end sprayer.

Foliar feeding can green leaves quickly; it does not replace a soil-level plan for a 1,000 sq ft garden’s 2 lb N seasonal budget.

Storing and spreading

Keep bags dry. Spread with a scoop and a bathroom scale once, or calibrate a drop spreader on a tarp. Broadcast before rain or irrigate in. Sweep granules off leaves. For beds, weigh the 4 lb for a 200 sq ft example into a bucket and walk it in two passes at half rate for even coverage.

Log what you applied. Next year’s soil test is more useful when you know last year’s 20 lb of 10-10-10 actually happened. If you switch to a 5-10-10 midseason, recompute — do not assume “a similar scoop” delivers the same nitrogen.

Bottom line

N-P-K are percents. Target 2 lb N per 1,000 sq ft for a mixed vegetable baseline unless your test says otherwise. With 10-10-10, that is 20 lb of product per 1,000 sq ft — the example our fertilizer tool is built around. Scale by area, watch the P and K you hitchhike along the way, and confirm every bag in the fertilizer calculator.