Aside from the most common killers of hydroponic crops (Read it Here), nutrient deficiencies can be harder than finding your way through labyrinth to correctly diagnose. Luckily hydroponic nutrients are usually nutritionally complete formula so the issue is more simple to approach, Aquaponics on the other hand is a more nuanced agriculture system with more nutrient pitfalls.
Here's a great resource from MSU to visually identify your Mobile and Immobile Nutrient Deficiencies. Know the difference between immobile and mobile and how they affect new and old growth!
The first thing you should verify once you identify a nutritional deficiency (be sure to completely rule out pests and disease!) is your pH. It is critical to have your pH at a level around 5.5-6 in hydroponics and 6.5-7 in aquaponics. This is crucial for your crops to be able to absorb the essential nutrients that keep them healthy and producing.
HOW TO SOLVE YOUR HYDROPONIC OR AQUAPONIC pH PROBLEMS:
While the solution for you aquaponic pH problem is more complex (check out our entire blog post deticated to aquaponic pH problems), it's a little more simple in hydroponics as you do not have the biological factor of the fish. For the most part, you'll use muriatic acid to drop your pH if it's above 7, and to fine tune it to 5.5-6 use phosphoric acid. Remember to do everything VERY slowly as you don't want to overshoot your pH. To raise your pH I would use Potassium Silicate or Potassium Bicarbonate.
If your issue does not resolve itself once you correct the pH, or if your pH was perfect to begin with, this is where things can get interesting. Unfortunately I mean this in a convoluted, much more complex way that many would rather call "frustrating". This is due to the fact that plant nutrient solutions must exist in the proper ratios of nutrients. Because plants take in nutrients through ion (negative or positively charged) channels, these ions must exist in the right amounts so they don't over-compete with each other and "lock" each other out. Paraphrasing to give an example, since Calcium and Potassium are both positive ions they will compete with the same positive ion channel so too much calcium can give you the visual deficiency effect of not having enough potassium! This of course is simplifying biology a bit, but the point is that your ratios of each nutrient must be correct, otherwise identifying your deficiency is nearly impossible.
Now that you have furthered your understanding of why deficiencies are hard to detect, let's talk about how to approach the solution. As I've mentioned earlier, your first priority is to check you pH. Afterwards, ensure your nutrient schedule is not due for a hydroponic water change, if applicable. These are the equivalent to "have you tried turning it off and on again" in the soil-less agriculture world. The next step is to then identify the affected leaves; if the older leaves are affected that will tell you that you're dealing with an immobile nutrient such as
If you find that the younger leaves are affected then, as you can guess, mobile nutrients are your point of concern.
Your issue is most likely either Nitrogen, Phosphorus, or Potassium since those are used the most. If those are not the culprit, the second most likely group is Calcium, Magnesium or Sulfur. If those miss the diagnosis then only the micronutrients are left. Look at deficiency charts and flow diagrams specific to your crop and pay attention to how younger and older leaves are affected and you will be able to correctly diagnose your specific problem.
If you have any questions, or need help with your deficiencies drop your comments below and we'll be sure to take a look!
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Cheers and Good Growing!