Solved: Why Are My Tomato Plant Leaves Turning Black?

     Are your tomato plant leaves turning black, but you're not growing a black, blue, or purple colored tomato? I'm pretty sure I know why! Would you believe a simple deficiency could cause such a dramatic change in plant color? (I also have the YouTube video with footage linked at the end, so that you can see the progress!)

My tomato plant turned black out of nowhere- no other plants were affected. This deficiency was easy to resolve!

Why Are My Tomato Plant Leaves Turning Black?

    When a tomato plant turns black, especially when the tomato leaves have black edges or the entirety of the leaves are black, it usually points to a phosphorus deficiency. Don't reach for the fertilizer yet, though. You could have very rich soil and still encounter symptoms of a phosphorous deficiency. Confusing, right?

The leaves on this young tomato plant turned dark very quickly, pointing to phosphorous uptake issues. This is more likely to happen during the wet season.

Uptake vs Deficiency: True Phosphorous Deficiency, or Not?

    Much like blossom end rot from calcium deficiency, symptoms of a phosphorus deficiency don't necessarily mean that there is a lack of phosphorus in the soil. Tomato plant phosphorus deficiency is usually a result of the tomato plant being unable to uptake phosphorus from the soil, rather than a lack of availability phosphorus. Even though there's plenty of phosphorous there, the plants can't utilize it. Think of it like a roadblock on a busy highway- something is keeping the phosphorous from traveling into the plant.

This neighboring plant was unaffected by phosphorous deficiency. It's green & healthy in this photo!

Identifying the Culprit: Sudden Torrential Rainfall & Waterlogging

    If you have a black tomato plant that seems to change color in the blink of an eye, I'd take a moment to look back at recent weather patterns. Saturated soil can prevent tomato plants from taking up the phosphorus that they require from the soil. The tomato plant with black leaves that you see in this video was transplanted into a recessed row, and we wound up having several inches of rain shortly after; I'm talking 6 to 8 inches over the course of a weekend.

    While the plant showed clear signs of phosphorus deficiency, I knew that was impossible. The plant looked fine at transplant, and I had placed a busted egg beneath it which is actually rich in phosphorus. After the massive amount of rainfall, this one single tomato plant turned black. None of my other varieties had black leaf edges, telling me that this particular variety seems susceptible to phosphorus deficiency due to saturated soil. Kind of like how some tomato varieties are prone to blossom end rot. There wasn't a gradual change in color either, which would be more characteristic of dwindling available phosphorus.

The trench that I planted my tomatoes in in an effort to collect rainwater backfired. It led to saturated soil and phosphorous deficiency for this lone finnicky plant. This trench was otherwise helpful in hot, dry weather to fight drought conditions.

Addressing Phosphorous Deficiency: Will the Tomato Plant Stay Black?

    Now for the most important question: "How did I fix it?" I didn't. The plant simply needed to dry out. We don't get that much rain at one time generally, and if we do, it's only once a year or so. If you're watering your garden daily and you see this, you should withhold water for a while. Test soil moisture an inch below the surface of the soil before watering. It can be easy to over water a garden; for me, since I don't hand water, I was focused on moisture retention and capturing rainfall, as we have had some rough drought conditions in the last few years. That clearly backfired during this wet spring.

5 days after the phosphorous deficiency started; the tomato plant is beginning to grow young, green leaves, as the plant begins taking up phosphorus from the soil again.

9 days after the phosphorous deficiency started; the tomato plant is beginning to grow young, green leaves, as the plant begins taking up phosphorus from the soil again.

True Phosphorous Deficiency: What If Water Isn't the Cause?

    If watering isn't the problem, then I definitely recommend reaching for the fertilizer! Bone meal is an excellent source of phosphorus, or you could opt for a fertilizer that is specially blended for roots and blooms. Specialty tomato fertilizers are great, as well! There are a lot of products you can use to help your tomato plants thrive.     Dealing with a tomato plant nutrient deficiency can be a little complex at times, because you have to consider their environment, the weather, soil composition, and other factors. Hopefully, this blog entry & video cleared up tomato phosphorus deficiencies and how to differentiate between waterlogging and true deficiency!

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