Diagnosing Phosphorus Imbalances in Hydroponics

Back in 1995 I produced my first educational video on “Introduction to Hydroponics”. I barely knew what hydroponics was at the time! But after doing my homework and interviewing NASA scientists, agriculture teachers, commercial growers and other leaders in the hydroponics industry, I was off to a good start.
I’ll never forget my first visit to a hydroponic tomato greenhouse in Northern Michigan. I brought my trusty video camera with me, took the grand tour, and interviewed the main grower about the benefits of hydroponics. I was impressed! But after I packed up my gear and was getting ready to head out the door, the grower stopped to ask me a question. He said, “Maybe you can help us. We’ve been having a problem with a nutrient deficiency, and it’s severe enough that we lose the first two or three sets of fruit early in the spring. A leaf analysis shows a zinc deficiency, but the nutrient solution analysis shows that we have plenty of zinc. What do you think the problem is?”
Remember, I was still “green” and asking all the questions. So I said, “Hmmm… I’ll do a little research and see what I can find out.” So I went back to the books, and what do you know, there it was! Phosphorus toxicity often shows up as a zinc deficiency. Taking a closer look at phosphorus, I also found out that phosphorus is temperature dependent. If the nutrient temperature is too cold, plants won’t take up enough phosphorus. Then I got it! The greenhouse in Northern Michigan wasn’t heating their nutrient solution, so under cold temperatures the plants weren’t taking up much phosphorus, and the extra phosphorus was accumulating in the tank. Then, sometime later in March, the greenhouse would warm up four or five days in a row, the nutrient temperature would rise, and phosphorus would be taken up by the plants to toxic levels. Hence, the zinc deficiency. In fact, as phosphorus toxicity worsens, the plants could also show signs of iron deficiency, copper deficiency and even magnesium deficiency. So I got to share my findings and return the favor to the growers!
More current research shows that adding silicon to the nutrient solution can also help treat or prevent phosphorus/zinc imbalances. So to prevent phosphorus problems; start with balanced nutrient formula, keep the nutrient temperature between 68-75 degrees F. and add a generous dose of silica to the nutrient solution. Silicon is the only nutrient that can’t accumulate in the plant to toxic levels so don’t be afraid of it! Just make sure that you add silica to the water, not to concentrated fertilizers.
Phosphorus deficiencies are much easier to diagnose than phosphorus toxicities. Shoot growth is restricted, and the undersides of the leaves, including the stems, become purple. Check the nutrient temperature first, of course. But you can also correct the problem with a phosphorus additive such as mono-ammonium phosphate (MAP). Make sure you use only the purest, water-soluble phosphorus available. If the guaranteed analysis is 9-61-0, you are on the right track. High-purity MAP dissolves into a clear solution that can be used as a foliar spray or added to irrigation water. Trial and error can be costly. So never underestimate the value of a good, fundamental education in hydroponics and plant nutrition!
Copyright© 2013
Harley Smith

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