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water retension experiment

Posted by sgull 6 (My Page) on
Fri, Jan 24, 14 at 16:20

I'm trying an experiment with my old potting soil to try to judge its water retention. The container (pictured) has a metal screen bottom. The container is 16 inches high and I put 8 inches or so of the old potting soil in it, then watered it with one gallon of water using a watering can. The container is sitting in a big tub where I can later observe/collect how much of the water infiltrates down through the soil in a particular amount of time. So far, I've noticed, after measuring, that in one hour just about half the gallon of water poured onto the this soil in this container as described has infiltrated through.

Follow-Up Postings:

RE: water retension experiment

sgull, so, after one hour the old soil has allowed half a gallon of water to drain through "8 inches or so of the old potting soil".
Is this good? Bad? Normal?
One way to tell is to treat some new potting soil the same way and use the observations on the new as the 'normal'.
Then you will be able to conclude that the old is better/worse/ as good as - the new.
Your post takes me back to long ago and far away when we did these measurements with lysimeters. The task could change elation into frustration in a heartbeat!

RE: water retension experiment

Is this good? Bad? Normal?

Yeah, what's the goal of the experiment? What do you hope to learn from this? Not quite clear to me from your description.

In real life water retention will be higher because in settled pots smaller particles sink to the bottom and raise the perched water table.

Also, PWT will be higher still if you use a narrower containers in real life - water will be clinging to the walls to a higher or lower degree depending on the container material. This bucket looks pretty wide to me.

If the goal is to saturate the soil, you may need to use more water than a gallon, use a watering can with a wide and gentle spray and to water twice (once a little, let it settle and diffuse, then water again to determine max water holding capacity).

Finally, the experiment may show wrong results because the soil likely contains some water already before you start, and your retention weight will be underestimated. If the mix is totally dry, it is also likely hydrophobic, a lot or a little depending on the components, and then your water retention weight will also be underestimated because not all of it will be absorbed like it would be in real containers.

So, be careful how you design the experiment. "Garbage in, garbage out", you know.

RE: water retension experiment

I think there is a more accurate water absorption method, as done with the engineering materials in construction , like wood, brick, etc

Take a sample of some dry mix. Lets take 1 lb .
Soak is in water (as long as needed)
Let it drain( just the water, not the particles).
Weigh it again.

By comparing the dry and soaked/wet weight you can determine its water retention capacity.

PS: you can do it with just a couple of oz or even less.


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