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Very simple question but hour long search with no answer. Concentration of fertilizer is relation ship of amount of fertilizer (either mass or molar) with volume, NOT with area, yet all fertilizers tell you how much fertilizer to apply per area. I understand, this comes because it is difficult to obtain volume of your soil in your garden. On the other hand, for container planting, you are in the opposite situation, you need to know how much fertilizer per volume of soil.

All in all, there has to be a starting number - amount of fertilizer per volume which then the manufacturers use to calculate the area on the package. They have to assume some standard depth to achieve the desired effect.

So the question. How do i determine the amount of fertilizer per volume from the information per area?

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  • The factory made fertilisers I have bought, always have two sets of instructions when using on container plants, the majority of which are houseplants in my case: 1 : How much to dilute the solid or liquid fertiliser with water to make the necessary solution 2 : how often to water that solution onto my container growing plants. They then add some extra info for summer crops like tomatoes. When it comes to things like poultry poop and blood & bone meal it is very much up to you. Ultimately it all comes down to the size of the container and the substrate's ability to prevent leaching.
    – Nikki
    May 28, 2023 at 16:37
  • You need fertilizer per area, really! We do not care about fertilizing the soil, but to have construction material for the plant which growth on surface. For volume, we may get the number of "reserve" (but than it is a complex topic: some materials must be processed by microorganisms, so "long term reserve" which cannot be (all) used quickly. Useful for N). Or then it depends on soil: water may move them away from your patch. Calculation are done by area: by taking all vegetation and measuring what they have inside (and roots). per surface area. May 30, 2023 at 12:33
  • @GiacomoCatenazzi I do not understand the second sentence. Maybe you are talking about the penetration of the surface present fertiliser into deeper layers of the soil. In that case, if you put 1 or 2 cm layer of fertilizer on the surface, you will have some penetration to the soil and then twice as much (actually more but that could be neglected) and you are back to volume. Also, take 0-0-60 fertilizer and recalculate the recommended fertilisation 0.5lb per 100sq ft to a 12" inch pot. You get, you need to use about 7 grams (that is a about a tea spoon). Great but there are two problems:
    – atapaka
    Jun 1, 2023 at 21:01
  • 1) teaspoon is about 10 grains of potash, it is impossible to distribute that so that you get some uniform soil permeation of potassium. 2) when you deal with pots, you usually have a volume of soil that you want to fit into pots but you do not apriori necesarilly know how many pots of what size you can fit. And finally you can have you can have a very low pot with the same diameter as a high one, in on you will have way less soil than in the other but roots take up nutrients based on concentration in the soil and even though you have the same area, you will have very different concentrations.
    – atapaka
    Jun 1, 2023 at 21:04
  • @GiacomoCatenazzi Yes, concentration will give you availability, nothing more, but surface area does not even give you this. Area measurement (as you explain at the end) makes no sense. It is only experimentally derived quantity - what probably works to give the plants what they might need. It is great if you have in large field and totally useless if you have pots.
    – atapaka
    Jun 1, 2023 at 21:06

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