How can I tell nutrient deficiency from overfeeding?

Mobile nutrients like nitrogen, phosphorus, potassium and magnesium move to new growth, so a lack shows on the older, lower leaves first. Calcium, iron and sulphur don't move, so those problems show on the newest leaves. Overfeeding looks different: dark green, shiny, claw-shaped leaves and brown tips.

Before you change the feed, check pH and run-off EC, because lockout from wrong pH looks like a deficiency. Change one thing at a time. A pH/EC meter will save you a lot of guessing.