Step 6: Diagnosis
Phosphorous-induced iron deficiency
The symptoms on the plants were classic iron deficiency chlorosis. This seemed strange because the plants were getting fertilized every day with a fertilizer that included chelated iron. However, not knowing the exact amounts of fertilizer being applied really sticks out. Greenhouse growers are going to fertilize their plants almost to the point of toxicity in order to get their plants finished and ready to ship as fast as possible. There is probably already a high amount of fertilizer in the pots when they come in, and the plants have already absorbed quite a bit of it. Problems can occur when there is too much of a certain nutrient. In this case, I would conclude that this is a phosphorous-induced iron deficiency. There is too much available phosphorous, and plants tend to absorb P instead of some other micronutrients like iron and zinc. The fertigation technique being used should probably be re-visited, and proper amounts of fertilizer should be applied at a known rate, not just guessed.