A word of caution for PPM. - Page 2 - The Planted Tank Forum
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post #16 of 18 (permalink) Old 08-04-2019, 11:36 PM
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If you normalize fertilizer rates W.R.T. plantable surface area, there is a large discrepancy between tank sizes.
Keep in mind PPM is milligrams per liter. So no mater the tanks size and shape the PPM number will not change. The issue hereis that for terrestrial plants any fertilizer you apply is going to stay near the surface. IN an aquarium the nutrients are dispersed throughout the water column.

In an aquarium with proper water circulation plants have access to all the nutrient. However for terrestrial plants most roots are restricted to the surface. Some plants can send roots very deep but most don't. So a terrestrial plant has a limited amount of area to collect nutrients from. So if you are fertilizing grass you need to take the root volume into account to dertmine how much fertilizer to apply. Also you need to account for nutrient loss through runoff and plant growth rates. The larger the area to the more you need to apply. But in an aquarium the substrate surface area really doesn't mater. What matters is how big the plants are and what their growth rate it. So the amount of fertilizer you apply to an aquarium is determined ty the growth rate and volume of the tank.

The basic problem hear is you are trying to adapt terrestrial fertilizer guidelines and math to aquarium. The terrestrial math doesn't apply to aquariums. For aquarium you need to dose to an initial level and then adjust the level as needed to to amount for the growth rate of the plant. And the best mathmatical way to do that is to dose to an initial PPM (milligrams per liter) value and adjust. For terrestrial plants you don't know the growth rate or nutrient loss or volume of the roots. so you dose per area (Kilograms per meter or pound per square foot). And then you adjust the dose based on growth rate.
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post #17 of 18 (permalink) Old 08-05-2019, 02:06 PM
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In the spirit of brain teasing, two questions:

1) Letís say you have a 60f and a 60p and 30ppm NO3 in each. Now you add enough plants to each tank so that the plant mass equals 20% of the volume of water in each tank and each group of plants consumes 30ppm / day of NO3 (even though the plant mass in the larger tank is more than the smaller tank). Which tank is going to run out of NO3 first?

2) Which is heavier: a ton of feathers or a ton of steel?

Maybe what we are all trying to say here is that if you have a 60f and a 60p and 30ppm NO3 in each and then add one plant that consumes 30ppm of NO3 / day, it is obvious that the smaller tank will run out of NO3 faster.
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post #18 of 18 (permalink) Old 08-05-2019, 07:26 PM Thread Starter
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Originally Posted by Deanna View Post
In the spirit of brain teasing, two questions:

1) Letís say you have a 60f and a 60p and 30ppm NO3 in each. Now you add enough plants to each tank so that the plant mass equals 20% of the volume of water in each tank and each group of plants consumes 30ppm / day of NO3 (even though the plant mass in the larger tank is more than the smaller tank). Which tank is going to run out of NO3 first?

2) Which is heavier: a ton of feathers or a ton of steel?

Maybe what we are all trying to say here is that if you have a 60f and a 60p and 30ppm NO3 in each and then add one plant that consumes 30ppm of NO3 / day, it is obvious that the smaller tank will run out of NO3 faster.
Thank you for clarifying. I'm not sure how to put it any more simply. It doesn't matter that the plants can "access" the water in 3d space. You are literally putting less grams of nutrient in the water. If all the plants are growing 1" per day they are using the same rate of fertilizer per surface area of tank regardless of how big your tank is. Therefore you would need to dose more ppm to keep up with the growth in a smaller tank.

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