Feeding your Food: Plant Nutrient Deficiency and Toxicity in Aquaponics Systems

by Tim Day

The major benefit of aquaponics is being able to harvest the most delicious and fresh ingredients just in time for your next meal. But if you want the tastiest and healthiest ingredients possible, you will want to insure that your plants are being well fed first! In order to do that you need to avoid nutrient deficiency in aquaponics systems.

In Aquaponics, it is much simpler to optimize your plants’ diet than in soil gardening and considerably less maintenance to do so compared to using hydroponic methods. The natural processes that are the cornerstone of aquaponics will do most of the work for you, and the vast majority of the nutrient that your plants need will enter the system through your fish feed. If you also occasionally throw your fish a handful of some natural food like insects or worms, then that’s even better. That being said, if your plants are lacking in something, or receiving too much of something else, they will tell you about it if you know how to listen.

Why even bother?

nutrient deficiency in aquaponicsHungry (nutrient deficient) or fat (nutrient toxic) plants are vulnerable to attack by insects, because they are not in their healthiest defensive state. Could you fight off a swarm of locusts if you were drunk and hadn’t eaten? It’s sort of the same thing. Further, your plants won’t produce as well, won’t be as healthy, and won’t taste as good. Like humans, plants require a varied diet. If something is missing or the plant is overindulging then the plant will suffer. If you ate nothing but French fries you would get sick. If you fed your fish nothing but French fries your plants would probably get sick too (I haven’t tried this). Stunted growth, off-colored leaves, a lack of production and even dead bits occur in plants with an unhealthy diet.

How can I tell if my plants are deficient in something?

The word ‘nutrient’ refers to the elements plants ‘eat’. Some of these include nitrogen, potassium, phosphorus, magnesium, iron and calcium.. To help figure out if your plant is deficient in something, we can split these nutrients into two groups: ‘mobile’ and ‘immobile’.

Mobile Nutrients: Yep, these nutrients move around inside the plant. In fact, these nutrients move from older leaves to newer leaves to assist with growth. If the plant is deficient in a mobile nutrient, the symptoms will show up in the older leaves first, because the nutrient will move up to where the new leaves are growing and won’t be replaced in the old leaves.

Immobile Nutrients: These nutrients can’t move once they have been used in plant growth. This means that when a plant isn’t getting its fill, the symptoms will show up in the newer leaves first, as the old leaves still have their immobile nutrients fixed there, whilst there isn’t sufficient new supply for the new leaves.

Here’s a quick reference list:

Mobile Nutrients

Immobile Nutrients

Nitrogen Iron
Phosphorus Calcium
Magnesium Copper
Potassium Zinc
Chloride Sulfur
Molybdenum Manganese
Boron

 

Ok, so now you can tell if your plant has a mobile or immobile nutrient problem, and you have a list of potential offenders. Yes it’s a long list, but don’t worry – most of those nutrients should be well supplied by your fish food. To help you diagnose nutritional deficiencies, I strongly recommend that you check the fantastic free resource from Montana University called ‘Plant Nutrient Functions and Deficiency and Toxicity Symptoms’ by MaCauley. Jones and Jacobsen. This post includes a great flow chart to step you through the diagnostic process.

Now, to help you describe your problem, it’s time for some big science words:

Chlorosis – General yellowing of the plant tissue

Interveinal Chlorosis – Leaf veins remain green, but the leaf is yellowing in-between the veins

Mottling – Irregular spotting

Stunting – Growth is stunted, or reduced from normal

Necrosis – When plant tissue dies

So now you have your immobile/mobile list and you know the basic language needed to describe plant deficiency symptoms. You can either use the previously mentioned flow charts or chance it on using Google to try and pinpoint the problem. Another great resource is the Aquaponic Gardening Community. If you do not find a post that immediately answers your question, you will quickly discover that the site is frequented with some amazingly intelligent members who will likely be able to share some solid advice.

Here’s a quick quiz for you! Your plant has yellowing in-between green veins on the newer leaves to the top of the plant. Is it a mobile or immobile deficiency and what would you call the symptom?

Two common Issues in Aquaponics

nutrient deficiency in aquaponics - chlorosis

Nutrient Deficiency in Aquaponics – Chlorosis

Wow! Correct, you agricultural scientist! That would be an immobile nutrient inducing the symptom of interveinal chlorosis in your plant. This is a common one for aquapons… it’s probably iron. Iron is one of the few nutrients that isn’t adequately supplied to your system through fish food, and if your plants are showing these systems you probably need to supplement your system with iron chelate.

One important note on iron – it is possible that you have plenty of iron in your system but that your plants aren’t taking it up due to high pH levels. If your pH is much above 7.0 then your plants will struggle to consume it efficiently. This is why you should aim for a pH of 6.8 in your system. It is a nice compromise between the fish, the bacteria, the plants and the iron uptake of those plants that keeps everything moving along nicely.

Another issue not unheard of in aquaponics is nitrogen toxicity, that is, your plants are doing the plant equivalent of sitting on the couch and eating lots of fried chicken. The telltale sign of this is when the leaves turn a deep green to black color – sometimes disrupting fruiting. You need to stop feeding those fatty plants so much! You either have too many fish producing more nitrogen then your plants can handle, or not enough plants or grow bed space to remove enough nitrogen from the system. The best course of action is probably to add more plants and stop feeding your fish for a couple of days. Might be a good idea to fire up the BBQ and have some fish tacos as well.

It can be tricky to pick

Insects, disease, pH, water quality, transplanting stress – there are many other things that can lead to plants showing signs that look like nutrient issues, so make sure to check out your whole system if your plants are struggling. A high or low pH can actually affect the ability of some plants to uptake certain nutrients, so the issue might not be a lack of a nutrient in the system, but just that your plants aren’t hungry because your pH is off. So check that one. In addition to that, plants can also be deficient or toxic in more than one nutrient at a time, which can make figuring it out more challenging. Insects and diseases can also easily be mistaken for nutrient problems, but insects and disease are also often occurring as a result of a deficiency anyway. Tricky.

So for the best food, make sure to feed your food in the best way! It might seem a little complicated but be happy in the fact that aquaponics is one of the simpler ways to grow plants with a good nutrient balance, and treat any issues like a detective game! Nature is amazing and will reward you with tasty food if you give her a bit of attention every now and then. Luckily for me, as a lazy twenty-something male, the amount of attention required is minimal!

References:

Plant Nutrient Functions and Deficiency and Toxicity Symptoms, Nutrient Management Module 9, A. McCauley, C. Jones & J. Jacobsen, Montana State University, May 2009. <http://landresources.montana.edu/NM/Modules/Module9.pdf>

Crop Nutrient Deficiencies and Toxicities, Integrated Pest Management, G. Stevens, P. Motavalli, P. Scharf, M. Nathan, D. Dunn, University of Missouri-Columbia, 2002, <http://ipm.missouri.edu/ipm_pubs/ipm1016.pdf>

Nutrient Deficiencies and Toxicities, D. Nunns, Land and Air Water Resources University of California, blog post <http://www.hydrofarm.com/resources/articles/nutrient_deficiencies.php>

7 thoughts on “Feeding your Food: Plant Nutrient Deficiency and Toxicity in Aquaponics Systems

    • Ph is good, about 6.8, ammonia is almost 0, plants growing well, but only thing is, I’m not getting any flowers whatsoever. what gives? any help would be appreciated.

  1. Pingback: Feeding your Food: Plant Nutrient Deficiency and Toxicity in Aquaponics Systems | Green Energy Approach

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