Plant Problem Diagnostic
How to Spot Nutrient Deficiencies in Plants
The short answer
Where and how a plant yellows tells you which nutrient is short. Nitrogen deficiency shows as uniform yellowing of the older, lower leaves first (the plant moves nitrogen up to new growth). Phosphorus deficiency shows as purple or reddish coloring on older leaves with stunted growth. Potassium deficiency shows as yellowing and brown, scorched edges on older leaves while newer leaves stay green.
Because both nitrogen and potassium hit older leaves first, the tell-apart is the pattern: nitrogen is even yellowing, potassium is browning at the margins. When in doubt, a soil test confirms it — and rules out look-alikes like overwatering or pH lock-out.
By nutrient
- Nitrogen (N): uniform yellowing of older/lower leaves first; overall pale, slow growth.
- Phosphorus (P): purplish/reddish or bronze tint on older leaves; stunted.
- Potassium (K): yellowing with brown, dead, scorched leaf margins on older leaves; newer leaves stay green.
- Iron (common in high-pH soil and blueberries): yellowing between the veins of NEW leaves, veins staying green.
Before you feed
Deficiency-like symptoms are often not a true shortage — overwatering, root damage, cold soil, or a pH that locks nutrients up can all mimic them. A soil test is the reliable way to know what (if anything) to add, and how much. Adjusting pH is frequently more effective than adding more fertilizer.
⚠ Don't guess-and-dump fertilizer. Confirm with a soil test, and check pH first — nutrients present in the soil can be unavailable if the pH is wrong.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- West Virginia University Extension — Nutrient Deficiencies in Plants — Nitrogen deficiency causes uniform yellowing of older leaves; phosphorus causes purpling of older leaves; potassium causes marginal yellowing/scorch of older leaves.
- University of Georgia Extension — Nutritional Deficiencies in Vegetables — Deficiency symptoms differ by nutrient and leaf age; a soil test is recommended to confirm before correcting.