It has been a few months since we started the University of Sheffield aquaponics system and the plants have been developing really well:
Although a closer look reveals something else! Last week one of our team members found out there was a kind of yellowing developing in some of our plants.
We set out to investigate what could be causing it and how we would sort it out.
First we examined the affected plants to look for obvious pests or other diseases and there were none. So it is probably some kind of toxicity or deficiency. As the system is new and toxicities are rare in aquaponics systems we supposed it is a deficiency. The complication is that there are more than a dozen possible nutrients so we need to follow a methodical approach to find out which one(s) are deficient.
The first thing to do to identify the missing nutrient is find out if the more affected leaves are new growth / upper leaves or the old growth / lower leaves. In our case the old growth was more affected.
This means the issue is possibly a mobile nutrient (instead of an immobile nutrient); a mobile nutrient has the ability to move from the old leaves to the new ones to support the plant development therefore the old leaves show the symptoms first. Great! This eliminates almost half of the possible nutrients.
Then the next step to follow is evaluate if:
A- The yellowing is on the entire leaf.
B- The yellowing is more pronounced but the leaf’s veins keep a green look.
Option A is our case.
The final step is defining if the plant has necrotic stop (dead tissue) or not.
As our leaves show no signs of dead spots the most likely nutrient missing is nitrogen / nitrate.
This is a very interesting result:
On one hand this is an expected outcome as we are feeding our system 3 grams of fish feed per metre square per day and the normal range for a media based aquaponics without solid removal is somewhere between 10 grams to 40 grams (the exact quantity will depend in plant growth/light intensity/plant type)
On the other hand the nitrate concentration is 20 ppm (parts per million) using a liquid test (API Fresh Water) and 50 ppm using a dip strip (6 in 1 Tetra test). This range is not necessarily low for aquaponics.
We will test our assumption by increasing the feeding quantities and checking the results in the following weeks.