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In theory, water beads for plants take in water when immersed in it to gradually release it to the roots of the plants. This process happens through osmosis. When the relative humidity is high enough, water beads take in water. They release the water in contact with the roots, as the roots have a stronger “osmotic gradient.”

I understand there is no regulated standard for water beads, but I guess their chemical composition is fairly regular, e.g. when compared with plant roots, which may range from desertic to aquatic. The point for the equilibrium will be higher than for the roots, which take the water from the beads, and lower than 100%, as they absorb water when immersed in it.

A nice and unlikely possibility would be the osmosis equilibrium point of the water beads being well below 60%, e.g., 40%, and the roots for any plant (e.g., succulents) lower than that, e.g., 20%. Then plants on beads would remove humidity from the air when above 40%, making for a nice dehumidifier. Many articles on the Internet claim that plants help dehumidify houses, but they are more likely to humidify them, as they need watering.

Is (a range for) the relative humidity of the osmotic equilibrium point something known or easy to find out?

The answer won't be a number, it will be a range. We find confusion and misinformation on the Internet in this area.

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    $\begingroup$ As the beads swell the activity of the water within changes and there is for each value of the activity a corresponding equilibrium external water vapour pressure. There isn't therefore a single equilibrium point. Rather there is an equilibrium curve of % water content in the beads versus vapour pressure. $\endgroup$
    – Buck Thorn
    May 25, 2023 at 5:30
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    $\begingroup$ How would that curve look like? My guess: it's a power law, e.g. the first 10% drop in vapor pressure causes a 90% drop in water content. Is this something known or easy to know? $\endgroup$
    – Trylks
    May 25, 2023 at 8:51
  • $\begingroup$ Let me know if further clarification is needed. $\endgroup$
    – Trylks
    May 26, 2023 at 12:55
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    $\begingroup$ Your intuition about how the beads should behave relative to the activity in the soil looks reasonable, but the question at present is not clear. You are asking how to determine "the relative humidity of the osmotic equilibrium point". You might want to edit that. Generating a limited curve (a few points) of RH versus water content seems something a do-it-yourselfer can do with the right supplies. The curve is also known as a "moisture sorption isotherm". The change in activity typically accelerates with increasing water content so might follow a power-law depending on the system. $\endgroup$
    – Buck Thorn
    May 29, 2023 at 18:28

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