How does fertilization impact lighting needs?


I finally overcame my stinginess and bought fertilizer - and two weeks in, there's already been a huge difference! But it got me thinking...

Now that there are more available nutrients in the water column, should I be changing my lighting schedule? Would there be a practical benefit to either increasing or decreasing lighting after making this change?

I can (and will) provide a few more details about my own lighting and plants, but before I do so, I'm just curious about the answer from a theoretical perspective. I know it would be easy for someone to look at my numbers and say "I've always done more" or "You should try doing less", but I want rationale more than I want opinions.

Best Answer - View -Mak-'s answer


It's more often viewed the other way around, lighting affects fertilization needs. Plants will grow based on how much light they get. Having more intense lighting drives them to grow faster, so they'll need more nutrients to sustain that. Having less intense lighting lowers their nutrient needs. This is also why slow growing plants like anubias may be described as "not needing fertilizers," their growth rate does not demand a large quantity of nutrients at a given time.

With more nutrients, perhaps you could increase the light intensity. But if you have unlimited light and nutrients, then the limiting factor is now CO2. This is what's happening in most low tech tanks. CO2 should be treated like another nutrient, because plants use several times more carbon than other macronutrients combined. And when carbon is limited you get similar effects as when the other nutrients are deficient: stunted or unhealthy growth, algae. So adding fertilizer is definitely an important step, but light can't be increased that much more without exacerbating the risks that accompany CO2 limitation.

In theory, light should be the limiting factor. Done right, excess CO2 and nutrients can work without leading to massive algae growth. On the other hand, excess light and lack of the two others is much more likely to lead to algae growth.
In reality, light is often not the limiting factor, especially in low tech tanks, which is why leggy growth forms, transplant melting, and algae are quite prevalent in low tech tanks. With experience and good maintenance practices, one can learn to reduce these problems.

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