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pupillacharites

Media water retention test

PupillaCharites
10 years ago

Any have any ideas on how to compare the water retention of one growth medium to another? Let me try to start a discussion:

- If you weigh an amount of test medium, and also an amount of some 'old faithful' medium dry, and then soak & drain quickly, you can get two things -

* % water capacity
* total water capacity

You can also track it over time by letting it dry.

Neither of those is accurate from the plants point of view IMO. What I think we are after is similar to the field measurements of available root water pressure like the Irrometers (FWIW (not much! I think that's what the company that makes them calls them) makes.

Does anyone have any creative (and poor man) suggestion to get something close to this sort of measurement, quick and dirty, so we can better experiment with what's happenin' to the part of the roots in our nettie pots?

Comments (6)

  • grizzman
    10 years ago

    What does "available root water pressure" mean?

  • PupillaCharites
    Original Author
    10 years ago

    It was an attempt to describe "medium water tension" to compare different media, without sounding too geeky. "Available" is when the medium water tension is weaker than the plants ability to extract/drink it into their roots which depends also on type of plant.

    From Wikipedia:

    "When water is in contact with solid particles (e.g., clay or sand particles within soil), adhesive intermolecular forces between the water and the solid can be large and important. The forces between the water molecules and the solid particles in combination with attraction among water molecules promote surface tension and the formation of menisci within the solid matrix. Force is then required to break these menisci. The magnitude of matrix potential depends on the distances between solid particles��"the width of the menisci (see also capillary action)--and the chemical composition of the solid matrix. In many cases, matrix potential can be quite large and comparable to the other components of water potential discussed above.

  • grizzman
    10 years ago

    Oh, okay. I am familiar with the concept. I'd have probably gone with something like available soil moisture or free moisture in the medium. The pressure part is what really threw me off.
    I believe you'd have to have two samples for your test. one without plants and the other with active growing plants. you could then compare the decrease in water content over time of the two to see how much water is consumed vs the baseline of when both samples were dry.
    At least that is my initial thought on the matter.

  • PupillaCharites
    Original Author
    10 years ago

    Right, "free moisture" is the same way I began to think about this until I realized that although 'available' or 'free' water in the medium is the concept I was after, the moment you try to nail down what you mean by 'free moisture', you really must define what you mean by 'available'.

    It turns out that it is all about picking the pressure a root needs to basically pump (or suck) water out of the medium. This is so easy to confuse (at least for me). But it makes a lot more sense on the second go round when you realize the media work best frequently when they are not saturated and draining water (that is where the word 'free moisture' is probably better applied), but rather sucking it up themselves (at that same pressure just like the roots have to overcome from the irrigation source.)

    I like your first idea, but at the same time I'm looking for a quick method that will not depend on the plants since it is really just a pressure or suction measurement characteristic of the specific medium that should be doable in some way without waiting.

    Of course putting the plants in is the final goal, but with evaporation and transpiration and plant size it would get mixed up with the plant when what I'm hoping is just for a medium specification which could be very useful, especially when providing the preferred root pressure environment for the plants, rather than depending on chance to keep it there.

    I've had an idea to make a fake root pressure test to answer myself: Put in an appropriate piece of dry paper in the control and in the test, recently drained media. Leave it in for say 1 minute, then remove and weigh water gain. Making the reproducable contact would be a little challenging, but I think not hard to work out.

    It really is a great thing to think about doing IMO.

    http://cdi.uvm.edu/collections/item/mapleimage036a

  • grizzman
    10 years ago

    If you know the pressure, you could weigh dry media. Saturate and reweigh the media. Then place is under the appropriate pressure for "x" amount of time then weigh the medium again to see how much was removed.

  • PupillaCharites
    Original Author
    10 years ago

    Maybe something along those lines, at this point it is still not very clear to me the difference between a real test and what would be a fun thing that won't prove anything (paper inserts, expose to pressure, or other ideas).

    Not sure how to execute your constant-pressure suggestion though probably with a pressure gauge, chamber and place to separate media from lost water. Not sure if it is the same measurement though, as there will be no differential across the surfaces to simulate the capillary root suction, which is the pressure the plant feels. Does cereal suck up less milk in a pressurized jet cabin - or something like that?

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