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Episode 18: Phosphorus and the Micronutrients

Show Notes

Today we’re talking about phosphorus and some plant micronutrients (not Iron, which we covered last episode). I will throw out the caveat that most of the nutrients that we talk about are not deficient in most aquaponic systems. The three big deficiencies that we see are potassium, magnesium, and iron. Occasionally, other deficiencies will show up, like phosphorus.

Why do I group phosphate with the other micronutrients?

Because it’s not that necessary in most aquaponic systems, because phosphorus is usually only deficiencies if you’re growing a lot of nutrient hogs. (Usually fruiting crops.) In fact, phosphorus is more often in excess, and is almost never a limiting factor to plant growth.

Phosphorus is available as phosphate – the oxidized for which is soluble and taken up through water. It’s especially important to setting fruit and fruit development.

The next nutrient is a micronutrient, Boron, which is important to cell wall integrity, which means that it’s important to plant growth. When you don’t have enough boron, you see problems with the meristems, immature plants, yellowing tips of leaves, and in extreme circumstances, die-back. It’s usually better to have more boron than less.

Once every two or three months, I add a little bit of borax soap to my system (not much or your fish won’t like it!). Just al little goes a long way here. If you’re having growth and development issues, boron could be part of the problem.

Copper is another micronutrient which is rarely deficient. Copper tends to find it’s way into the system through fish feed and other components. Copper is important to cell wall development and it’s part of a lot of catalysts. Copper is most available at low pH, so if a deficiency does happens, it’s usually at a high pH. Most deficiencies will show as tissue chlorosis, necrosis, and lack of flowers. It’s immobile in the plant, so young growth is most affected if a deficiency is present.

Fish do not like copper, so be really careful if you have to supplement it.

Chloride is common in feed as well as other supplements, and is important to osmotic regulation, especially in opening and closing the stomates. It’s also rarely deficiency as it is introduced so often. If you do see white spots along leaf edges, and if they expand, then it might be a chloride deficiency, but this is extremely rare. (If you do have one, call us up! I will be very impressed.)

Manganese deficiencies will show as very bleached out inter veinal chlorosis. This usually only happens in anaerobic conditions where there isn’t enough oxygen. Manganese is a key element to photosynthesis and carbon fixation, helping the plant to create chlorophyll and some carotenoids. Finally, it helps with nitrate uptake. Manganese binds with organic matter really well, which is a big deal in soil systems. In aquaponic systems it’s more important to know that it has a pretty antagonistic relationship with iron. An excess of iron can impact manganese uptake and vice versa – I’ve never seen it happen, but it could happen.

I’ve never had to supplement manganese in a commercial aquaponic system, but I know that there are products out there for it.

Molybdenum becomes unavailable at low pH – opposite of most of the other nutrients. Molybdenum presents itself as a nitrate deficiency, so if you see it but tests show plenty of nitrate, that’s probably what it is. Molybdenum is an important part of an enzyme nitrate reductase, which strips the nitrogen in nitrate of the oxygen connected to it, making the nitrogen available for the plant to use. Without molybdenum, the enzyme can’t be created, and the plant can’t use nitrate, even if it’s present. You don’t need a whole lot of molybdenum, however – sometimes the Molybdenum in the seed is enough to sustain the plant’s molybdenum levels for the rest of it’s life.

The last one is Zinc. Zinc and copper don’t play nice. A zinc deficiency stunts the plant growth, sometimes creates white bands along the midrib of the leaves, and the upper foliage gets yellowish while lower foliage turns more of a bronze color. Zinc is useful to cellular reactions and is also impacted by phosphate. (If there is too much phosphate, it can cause a zinc deficiency.) Because galvanized steel is pretty common in build, zinc is rarely deficient. Like copper, zinc is toxic to fish, so look out for that. (Don’t used galvanized steel!)

That’s it for the plant nutrients!

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