Fertilizing Vegetables: What N-P-K Means, and When to Feed

By Growing Home Greens Team · July 31, 2026

The three numbers on a fertilizer label (N-P-K) stand for nitrogen, phosphorus, and potassium, in that order, and each does a different job. Nitrogen drives leafy green growth, phosphorus supports root and flower development, and potassium supports overall plant health and fruit quality. A fertilizer’s three numbers show the percentage of each by weight, so a 10-10-10 is balanced across all three, while something like 5-10-10 is deliberately lower in nitrogen relative to the other two.

Match the fertilizer to what the plant is doing

A leafy crop like lettuce, grown entirely for its leaves, benefits from a nitrogen-forward feed throughout its short life. A fruiting crop like tomato benefits from balanced or lower-nitrogen feeding once it starts flowering, too much nitrogen at that stage pushes the plant into producing more leaves at the expense of fruit, a common reason for a huge, healthy-looking tomato plant that sets very little fruit.

Heavy feeders need more than light feeders

Some crops, corn among them, pull far more nutrients from the soil over a season than others and benefit from a supplemental feeding partway through the season, beyond what goes in at planting. Root vegetables and most herbs are lighter feeders and often do fine on what’s already in a reasonably amended bed without additional fertilizer at all. Over-fertilizing a light feeder doesn’t help it and can burn roots or push root crops toward excess leaf growth instead of the root itself.

Compost handles a lot of this on its own

Well-finished compost worked into a bed provides a slow, balanced nutrient release across a season, which is why a well-composted bed often needs less supplemental fertilizer than the same bed built from unamended soil. Synthetic fertilizers act faster but don’t build soil structure the way compost does over time, the two aren’t really competing approaches, and many gardens use both, compost as the base and a targeted feed for specific heavy feeders.

Signs to watch for

Pale, yellowing lower leaves on an otherwise healthy plant often point to a nitrogen shortage. Purple-tinged leaves, especially in cool weather, can indicate a phosphorus shortage. Brown leaf edges (called leaf scorch) can indicate a potassium shortage. These are useful diagnostic signals, but they overlap with other problems like underwatering or a pH imbalance that locks nutrients up regardless of how much is in the soil. See Soil Testing and Amending if a plant’s symptoms don’t clear up after a reasonable feeding.

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Fertilizing Vegetables: What N-P-K Means, and When to Feed: The three numbers on a fertilizer bag correspond to different jobs, and matching them to what your plant is doing matters more than feeding more often.

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