Plant Nutrients in Aquaponic Systems

Posted by Nate Storey on September 4, 2013

Plants are funny things. We’re surrounded by them, whether in the food that we eat, making up the lawn that we walk across, or the cactus on our desk at work. If you’re a commercial hydroponic or aquaponic producer, plants the most economically important part of your system; but most of us don’t really know how they work or what they need.

Misunderstanding plant nutrients is a costly mistake

Unfortunately, this misunderstanding of the needs of plants is also common among aquaponic growers.

This leads to mis-diagnoses of disease, deficiency, and pest symptoms, and frustrates many aquaponic growers to the point of quitting (hang in there!).

This post is meant to begin a series talking about:

  1. Nutrients,
  2. what they do in the plant,
  3. how you supply them,
  4. and how you know when they’re deficient.

This is a big subject, so there will be many posts on this subject over time.

Why plant nutrients matter [in aquaponics]

To begin, plant nutrients are elements that are necessary for plant growth and reproduction; without certain elements, plants cannot grow, cannot flower, fend off pests, photosynthesize sugars, or engage in any of the important things plants must do to stay alive and produce offspring.

The interesting thing is that oftentimes, even if all of the other nutrients are there but one is missing, the plant simply cannot grow.

So it is important to understand going into this subject that nutrients are limiting, and there are few puzzles as frustrating for a grower as suffering from a single “most limiting nutrient.”

What plant nutrients matter, anyway?

There are sixteen plant nutrients; three of them are non-mineral, and thirteen are mineral. The three non-mineral nutrients are pretty obvious and are not really supplemented by growers except in the form of water (and in some cases) carbon dioxide.

These non-mineral nutrients are:

  • Oxygen, O
  • Hydrogen, H
  • Carbon, C

Carbon

Like everything on earth, plants are carbon-based life and a majority of the plant structure is composed of carbon chains (with hydrogen attached) and water.

Water

Water is relatively cheap in nature, so plants use water to support themselves. Instead of making very thick, tough cells that can stand on their own, many plants build rigid cell walls and then pump their cells full of water like a balloon to build structure.

Plants are great at maximizing growth with the least expensive resources. Not only is water (hydrogen and oxygen) a useful building tool on it’s own, but plants have the amazing ability to break water apart, scrapping the oxygen, and using the hydrogen for all sorts of things, including hydrocarbon chains. Basically, water is used as a molecule, but is also broken by the plant to use for spare parts.

Mineral nutrients are fussy

When we think of plant nutrients, we usually think of mineral nutrients- these are what are commonly supplemented by growers in the form of a fertilizer, manure, compost, or other organic nutrient addition. Many of them are also available or unavailable in the soil, depending on where you live and what kind of soil you have.

By and large, mineral nutrients can be a bit fussy, and just because they’re present in your system doesn’t mean they’re available.

Lots of different things influence whether mineral nutrients are available, but the most important thing is pH. Some minerals are only available at higher pH values, but more are only available at low pH values. This can make the lives of aquaponic growers pretty difficult at times.

When a nutrient is present in your system, but not available, usually that mineral is what we called “precipitated out of solution.”

Basically this means that instead of being dissolved in the water in the system, the nutrient becomes a solid and usually settles on the bottom of the tank, or attaches to a piece of gravel, or other surface in your system.

This means that while the nutrient is technically there, it’s not available for the plant to take up.

The best example of this is iron. Iron is often present in aquaponic systems, however, because of system pH and dissolved oxygen, it typically isn’t very available to the plants. Instead, it often exists as a solid precipitate in the system.

Mineral plant nutrients are divided into three groups- primary plant nutrients, secondary plant nutrients, and micro-nutrients.

 

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The primary plant nutrients are:

Nitrogen, N
Phosphorus, P
Potassium, K

These are the most consumed nutrients, and are the three concentration numbers on fertilizer labels: N - P - K

The secondary plant nutrients are:

Calcium, Ca
Magnesium, Mg
Sulfur, S

These are most commonly supplemented in aquaponic systems that use pH-raising nutrient supplements to moderate pH- primarily calcium and magnesium in the form of agricultural lime, a hydroxide composed of Ca and Mg.

It isn’t necessary to supplement sulfur in most systems, but it is commonly supplemented in neutral or high pH system if growers use Epsom salts (magnesium sulfate) or potassium sulfate.

The micronutrients are:

  • Copper, Cu
  • Zinc, Zn
  • Boron, B
  • Molybdenum, Mo
  • Iron, Fe
  • Manganese, Mn
  • Chloride, Cl

Of these micronutrients, iron is most common in aquaponic systems.

Copper is commonly included at high enough levels in the feed.

Zinc is common in fish feed as well as the galvanized steel components that inevitably make their way into our systems (despite our best efforts to exclude them!).

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Boron is required at low enough levels that the levels existing in most system are sufficient (Although I have been known to drop a pinch of borax soap in my system every 9 or 10 months).

Molybdenum is also required at low levels, to the extent that all of the molybdenum a plant needs for it’s entire life can often be found in the seed that it germinated from. (It’s only after growing some plants for several generations in a molybdenum-free environment that the deficiency occurs!)

Manganese is seldom deficient, and chloride commonly enters the system in the fish feed, and in the form of salts.

See more on individual nutrients!

All of these are necessary for plants to grow and be healthy, and so, over the course of the next year, we’ll dig in and talk about each of these important plant nutrients in detail- including how to manage them in your system, adding them to the system, recognizing deficiencies.

Check out our Youtube playlist and our other blog posts on nutrients in aquaponics.

Topics: Aquaponics

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