Nutritional Lockout, One of the most frustrating difficult common problems growers encounter is Nutritional Lockout. Nutrient Lockout basically occurs when the optimal nutrient levels are not pH and nutritonally balanced so your plants cannot gain access to a specific nutrient or group of nutrients it needs, and consequently it developes a deficiency. Perhaps you are growing plants that ate all the nitrogen first and just snacked around the iron, magnesium and calcium, or vice-versa. It can also be caused by a chemical reaction in the medium/solution that causes a toxic substance to block the roots. There can be many causes but almost always there will be discoloration of the leaves, yellowing or rust spots, or curling up of leaf tips. It also becomes apparent when your plants were consuming a gallon or 1/2 gal of water a day, and then suddenly when you check the levels the next day they did not drink any water at all. This is a classic case of Nutritional Lockout.
- Ryan Riley, Growing Elite Marijuana