How to Troubleshoot Basil Plant Leaves Turning Yellow

Check soil moisture retention. Lack of sufficient moisture in clay or sandy soil may cause basil leaves to wilt and yellow. Poke holes in over dry soil around the base of the basil plant with a handheld garden fork tool, allow water to seep down to the roots. Water immediately and continue watering whenever the top half-inch of soil is dry to the touch.

Verify that the basil plant is receiving sufficient sunshine. Remove any weeds or plants that have grown up around the basil casting shade on it, if possible. If the basil plant is growing in a pot, move the container to a more sunny location.

Inspect drooping and yellowed leaves for pests and insects. Cut off the damaged leaves with sharp scissors and discard. Spray the plant thoroughly with insecticidal soap. Make sure the spray covers the underside of all the leaves where insects often attach themselves.

Feed a basil plant with yellowing leaves a cup of diluted 10-10-10 formulation fertilizer. Mix the fertilizer according to the manufacturer's label instructions and dilute with an additional equal part of water. Typically, fertilizer is not recommended as it may change the flavor of the leaves. Yellow leaves, however, may indicate an imbalance of nutrients in the soil that may be enhanced with a light application of fertilizer.