The Q&A Archives: Underside of tomato leaves turns purple

Question: I have started several tomato plants and the under side of the leaves have turned purple. The stem is also slightly darker (purple) rather than bright green as I would expect. The plants seem OK otherwize except that they might be growing a little slowly. What is wrong?

Answer: Purple discoloration is most always a sign of phosphorous deficiency. In container soils, this happens when fertililer with phosphorous (the second number on a bag of plant food) isn't applied on a regular basis. In the open ground, phosphorous deficiency may occur if the soil pH is too low (acid) or too cold and wet. To remedy the first situation, apply light but regulatr aplications of a liquid fertilizer. To remedy the second, have a soil test performed to determine your pH. Young tomato plants do like soil temps of at least 60 degrees F.

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