The Q&A Archives: Tomato Plants have Purple Leaves

Question: The lower half of my tomato plants turn purple. They are potted in good potting soil with a generous amount of bone meal added. My greenhouse is warm 105-112 degrees. They are on drippers which cycle on every day for 4 minutes with an adjustable flow I would guess to be about 5 gph. The ones that are heat tolerent do well for a while even setting fruit although growth seems somewhat stunted.

Answer: It sounds like a phosphorous deficiency. Phosphorous deficiencies show up as slow/decreased plant growth with
leaves that are purple or have purple veining. The nutrient
may be lacking because there is not enough available in the soil or, it is in a form that is unavailable to the plants. Although you have added bone meal, a source of phosphorous, unlike nitrogen, phosphorous moves very slowly through the soil, so it may not be where the roots can take it up. Also, bone meal is very slow to decompose so that can be another factor. If you want to stick with organic fertilizers, you can try adding a little soil sulfur, which helps speed the decomposition of bone meal. It's also a good idea to mix the phosphorous in at the bottom of the planting hole, where it's more readily available to the roots.

Also, make sure your drip irrigation is watering all the way down to the bottom of the pot, with water draining out, not just wetting the top layer of soil. Tomato pollen isn't very viable much over 90 degrees, so if you are having any fruit set at all, that's pretty unusual in such high temperatures. I hope this info helps!

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