Do I Need Both Compost and Fertilizer?

What compost does (and doesn't do)

Compost's main job is the soil, not the plant. University of Georgia experts note its real value is loosening soil, improving drainage, and increasing the soil's nutrient-holding capacity, and that its organic matter literally provides food for beneficial microorganisms and earthworms. That's soil-building work fertilizer can't do.

What compost is not is a concentrated plant food. Georgia notes the average compost contains only about 1.5% nitrogen, phosphorus, and potassium (and manures often less — cow manure around 0.5%). Those nutrients are real but dilute, which is exactly why compost is a conditioner first and a fertilizer a distant second.

Why hungry crops still need fertilizer

Two things keep compost from feeding a demanding crop on its own: concentration and timing. On concentration, Georgia's arithmetic is stark — you'd need about 70 pounds of compost to add the same nutrients as 10 pounds of 10-10-10 fertilizer. Spreading that much on a bed isn't always practical.

On timing, Georgia notes the nitrogen release from compost and manure isn't immediately available and can take a few months to be converted by soil bacteria. So it's necessary to supplement the vegetable garden with additional nitrogen during the season — most vegetables need two or three nitrogen applications to maximize yield. That's the gap fertilizer fills: available nutrients, when the plant is actually growing.

Using them together without overdoing it

The sensible pattern is compost as the foundation and fertilizer as the top-up. Georgia is direct that compost is not a substitute for fertilizer if you're growing crops with high nutrient demand — so build the bed with compost each year for structure and biology, then add fertilizer to hit the crop's nutrient needs. Our guide to starting composting covers making your own, and how to tell compost is finished tells you when it's ready to use.

Both together doesn't mean more is better. NC State Extension warns against the urge to overfertilize, which can produce lush growth but decrease flowering and fruiting and increase pest problems. Let a soil test set the fertilizer rate rather than guessing, and if you're adding manure, do it safely — see using manure safely in the vegetable garden.

Sources

Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.

• University of Georgia Extension (CAES Field Report) — Compost Enriches Soil But Doesn't Replace Fertilizer — The average compost contains about 1.5% nitrogen, phosphorus and potassium (cow manure only about 0.5%); you would need about 70 pounds of compost to add the same amount of nutrients as 10 pounds of 10-10-10 fertilizer; compost is not a substitute for fertilizer if you're trying to grow crops that have a high nutrient demand, and its value is loosening soil, improving drainage, increasing nutrient-holding capacity, and providing food for beneficial microorganisms and earthworms; the nitrogen release from compost and manure is not immediately available and can take a few months, so it is necessary to supplement with additional nitrogen during the growing season, with most vegetables needing two or three nitrogen applications to maximize yield.
• NC State Extension — Home Vegetable Gardening: A Quick Reference Guide — Avoid the urge to overfertilize, which can produce lush plant growth but decrease flowering and fruit development and increase pest problems.

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