I'm going to talk about “purple haze.” No, not Jimi Hendrix, not pot, or Colorado, and not anything else semi-legal or semi-anything. I'm talking about your little tomato seedlings and their beautiful purple under-leaves and veins, and very green upper leaves. Wait! Jim, are you saying that my tomato babies have varicose veins? And now that you've mentioned it, yesss... my tomato seedlings are a disturbingly unnatural shade of purple! What gives, Jim?
This phenomenon has occurred with almost every tomato variety, and some other non-tomato plants, that I have grown in my passive wick Hydrosock Plain 2 Grow System. When I was new at this, the sight greatly perturbed me. As a soil grower, I knew that purple under leaves and veins on plants meant either a phosphorus deficiency or a disease. One time, I had twenty tomato seedlings under way and all twenty were spindly and purpley. I was really upset because I thought they were doomed.
Remember I said in a past blog that one of the drawbacks in hydroculture is that plants can go from green to brown in a seeming heartbeat? That's still true, but I'm happy to tell you that might not be the case with your hydroponic tomato family that you are lovingly nurturing.
Yes, it could be a disease, but probably not. If you did not start your seedlings near other diseased plants, or you started them indoors under hygienic and insect-free conditions, then you can probably rule out a disease like beet curly top virus (BCTV). But is it a phosphorus deficiency? Probably. Argh! Phosphorus is a macro-nutrient! It's the “P” in “NPK,” right? Yes, it is. My babies are going to “keel over” and “kick the bucket!” Oh nooo!
Nah. They are going to do just fine. Maybe. Here's a brief explanation of what's going on in your tomatoes and then a few tips on what to do to keep them happy and fed.
Nitrogen (N), phosphorus (P), and potassium (K) – the “NPK” of macro-nutrients, is the “elixer of life” for ANY plant. These three elements must be available to a plant for it to survive. P is absolutely necessary to make the nucleic acids (proteins) and ATP (metabolic cycle) that creates the plant cells. Without getting deeply into plant physiology (and I am not a college-trained expert), phosphorus, or P, is very hard to get into a plant.
A plant can deplete the soil around it of P very quickly. This is especially true when the plant is immature, with a small root system, low rhizobia count (bacteria that can convert nitrogen and phosphorus), excessively wet soil, and the soil and ambient temperatures are below normal. Since we are talking about tomatoes right now, I'll say that most tomato varieties do not like a root system temperature below 60 degrees Fahrenheit, or 15 degrees Celcius, and really prefer a temperature dozens of degrees higher. Also, most phosphorus in soil is in an unavailable form to the plant.
Let's play detectives. I've given you almost all the data you need to figure out what's going on, except that I was talking about soil and not hydroculture / hydroponics. Now it gets interesting!
Hydroponic roots are in constant contact with water. Most hydro root systems run on the cold side, especially mine. My average nutrient fluid temperature is 68 degrees Fahrenheit. There is significantly less microbial action in the roots in a hydro system, particularly if you are using a chemical nutrient mix and not an “organic” or natural one. Hydro root systems are generally small in diameter: My eight-foot tall tomatoes on You Tube were growing in a one-gallon milk jug and using around a half-gallon of nutrient solution. Okay, Jim, you're telling me that my plants might be doomed before I start!
No. Nada. Nope. Nah. Nyet. Non. Lay. Mhai' . If you use a commercial chemical nutrient system like General Hydroponics and mix it according to directions, you might still get the purpling. Or you might get it with home-brew nutrients. I am currently experimenting with an interesting chemistry of grocery-store nutrients that has had identical results to the GH chemistry when it comes to P deficiency. The experimental two week-old Jet Star hybrid tomato I am growing right now is very cold, very wet, and very purple. It has been in both nutrient solutions. Do I think it will flop over and “push up daisies?” (I love American slang!) Well, I can't say that I'm not concerned. But I have YET TO KILL a single tomato seedling or any other plant I have grown because of P deficiency! Okay Jim, you're getting into magic fairy dust!
No magic. Not a lot of mystery either. Again, God's genius and simple science explain how this tomato is going to survive it's adolescence and grow into an adult ( I'm SO ready for my teen to do so).
- Your root system is very immature and small. It will grow larger, and more importantly, due to the unique properties of hydroponics, become very voluminous due to a whole bunch of tiny hairy roots. That huge increase in surface area is one of several secrets to how your tomato (and any other plant) will survive.
- Most of us start our seeds in a fairly wet substrate or medium. I use peat pellets, which in the beginning, are pretty darn dripping wet. Lately, I have been “squeezing out” excess water from the peat pellet before I put it in my system. As the plant grows and the system settles down, the roots become more selective of moisture uptake. Most importantly, the roots begin to differentiate into “air roots” (see my blog on air roots) and fully submersed or wet roots.
- Probably the most critical part of this initial deficiency is getting and keeping the pH of the solution at the optimal level for the tomato. The pH can critically affect the uptake, mineralization of, and “locking out” of P. Here is my blog on pH. The range for tomatoes is 5.5 to 7.5 . My personal experience is that most tomatoes find a range of 5.7 to 6.9 to be ideal.
- Ambient temperature is important but root temperature is critical. If your tomato roots are sitting at 55 to 69 degrees, then the tomato will experience a whole range of maladies including the P deficiency. Use a heat mat or something to get the nutrient solution and roots up to around 75 to 85 degrees. I've had tomatoes survive with solution temperatures as low as 45 and as high as 95, but that isn't a great way to grow tomatoes, or any other vegetative plant.
- The sap (fluid) transport vessels, known as xylem and phloem, are still small and immature. As they grow longer and bigger in diameter, they can handle the larger ionic compounds such as phosphate (the “salt” of phosphorus.)
- Lastly, oxygenate, aerate, hyperventilate! Bubble or move your nutrient solution constantly. I've hand-aerated, using a turkey baster. I've used aquarium pumps. It doesn't matter how you do it, but make sure there is as much oxygen in the nutrient solution, and around the roots, as you can get. If there is no oxygen saturation, then the nitrogen-phosphorus cycle will break down, microbial action will slow down, and algae and fungi will begin to grow in the solution chamber, robbing the plant of ALL macro and micro nutrients.
I can't guarantee anything. As I've said before, some of what I do is standard practice and is well-known, while other things that I do are much more experimental.