Every year I find I have purple coloring on many of my plants. It's always in the spring when things are still a little cool, and the plants are continuing to struggle to grow big. After selling organic fertilizer for many years, I know that this means my plants probably aren't taking up enough phosphorus.
Phosphorus is the second most necessary plant nutrient needed, right behind nitrogen. Phosphorus is a major component in proteins, enzymes, and other molecules of plants and animals including part of the energy molecule ATP (no cringing please), DNA, and a structural part of cells (phospholipid bilayer for you geeks like me).
If I see purple I know my plants are struggling, and that is something any gardener wants to avoid.
Here's what some of my broccoli looks like:
|Notice the purple on the underside of the leaf|
So what do you do? Is your soil deficient? Maybe not, but only a soil test will prove it. I usually just blame the weather. That's because phosphorus doesn't move within the soil very much like nitrogen does. It tends to stay pretty much right where it is. It is possible the roots have used up what is immediately surrounding them, and aren't accessing the remainder of the available nutrient because the roots aren't growing fast enough. This is usually the case for me in the spring when I see purple. It's cold so the plants aren't growing very fast, but they ARE using up all the phosphorus that is in the soil surrounding the roots. The answer for me is to apply a little high phosphorus bat guano in a water solution. This means the phosphorus will be easily available to the plants because I replace what they have used up in the soil. This little kickstart is sometimes all they need to get back on track and start growing again even when mother nature provides a few more weeks of cool, gray weather.