Yellow patches in lawn: 6 causes

Yellow patches in a lawn are a diagnostic signal, not a single problem. The color tells you something is wrong; the pattern, season, and associated symptoms tell you what. Treating the wrong cause -- applying fertilizer when the problem is grubs, or treating for disease when the cause is soil pH --.

Yellow patches in a lawn are a diagnostic signal, not a single problem. The color tells you something is wrong; the pattern, season, and associated symptoms tell you what. Treating the wrong cause — applying fertilizer when the problem is grubs, or treating for disease when the cause is soil pH — wastes time and money and lets the real problem continue.

1. Iron deficiency / high pH

What it looks like: Interveinal chlorosis (leaf veins stay green, tissue between veins turns yellow). Affects entire areas, not random patches. More severe in high-pH, sandy soils or where excessive lime has been applied.

Why it happens: Per Penn State Extension, iron becomes unavailable to plants when soil pH rises above 6.5—7.0. Centipede grass and bahia grass are particularly sensitive; Kentucky bluegrass shows iron chlorosis at pH above 7.0 in sandy soils.

Diagnosis: Soil test. If pH is above 6.5 and interveinal chlorosis is widespread, iron deficiency is likely.

Treatment: Short-term — foliar chelated iron spray (ferrous sulfate, 2 oz per gallon). Long-term — lower soil pH with elemental sulfur based on soil test recommendation. Do not apply more lime.