Determining water need with a meat thermometer

[fredfarmer]

I know. I know. What the $%*@! Is this? Others are thinking maybe junior member posts should be reviewed before posting. But please hear me out.

I have a number of young Tolumnia that have been “potted” in 2 inch clay pots sitting in oversized clay saucers with a very small amount of tree fern to hold the roots in place inside the pots. It works rather well. They have to be watered frequently (every 24 to 60 hours dependent on ambient temperature and relative humidity) and a fan is directed on them to maintain enough air flow to make the larger leaves move slightly in the breeze.

I noticed that after about an hour of watering and lasting well into the next day that the clay pots were cool to the touch which I attribute to evaporative cooling. What I didn’t expect was the extent or uniformity of the cooling – until I decided to measure it with one of those rapid reading meat thermometers. I just stuck the needle tip into the media right next to the clay sides of the pots. I also weighed the pots as is my habit with newly acquired orchids to monitor water retention levels. Some results with 9 tolumnias are below. Note that the watering liquid was at ambient temperature (80 deg. F) for at least 24 hours prio to watering.

Hrs post watering: 2 hrs , 24 hrs , 48 hrs

Ave. wgt.– wgt. of: 35 g. , 21 g. . 14 g.
dry empty clay pot

Amb. temp minus: 4.7 deg , 5.9 deg , -0.1 deg

thermometer readiing

As the average initial weight of the Tolumnias themselves were in the range of 9 to 12 grams, it appears that at 48 hours they were pretty dry and the temperature reading on the me at thermometer was basically the same as ambient temperature. As I can determine the thermometer reading without moving the Tolumnia filled pots, it’s a convenient way to determine when to water any given pot without getting out the scale and moving it back and forth into position under the LED lights.

Not sure this will work for everyone, but maybe some inside-under-lights growers will find it useful.

My next effort will be to figure out how to make a table inside an OB post.

[Roberta]

Interesting short cut to weighing! Probably wouldn't work for plastic pots, or even bark in clay. But for this particular case, looks like it works quite well, an indirect measure of how much moisture is there.

I suspect that for Tolumnias, if they go 60 hours before they need watering, they need to be in something that dries faster. Your setup seems to be a good alternative to mounting, but they'd want to be in that 24-48 hour range however they're managed (mount or pot), they do hate to stay wet.

[estación seca]

Your method sounds great.

I looked once but couldn't find a table function. The best I was able to do was copying and pasting parts of a spreadsheet.

[StephaneL]

That seems to be an interesting approach.

Have you tested it with different ambient relative humidity values? I suspect this will have an effect. For example if the ambient relative humidity is 100% there will not be any evaporation and no cooling.

[Roberta]

It wouldn't work in 100% humidity. But I don't think most orchids would be happy in that environment either. The technique wouldn't be suitable for all pot/medium conditions either, but where it does work, it's quite elegant.

as for constructing a table, maybe it could be done in html. Not easy. One could create a table in program of choice, convert to graphic (like jpeg) and display it as one would a photo. Not beautiful, but probably the simplest approach.

[estación seca]

There's still evaporation with 100% humidity. Just outside any zone of 100% humidity will be a zone of lower humidity. Water vapor will diffuse into this zone and permit more water to evaporate into the higher-humidity zone. Homes aren't closed tightly the way a jar could be.

[Ray]

There are a lot of variables here, not the least of which is the physical “structure” of the potting medium.

Evaporation from a pot occurs primarily from the top surface of the medium, with a secondary location being the medium/pot wall interface. If the pot is porous clay, that still holds true, but some of the moisture is wicked through the pot wall, as well. A third “source” of evaporation is the interior of the medium, but that is hugely variable, and is highly dependent upon its packing density - loose and airy, such as LECA or Fred’s tree fern, and it’s substantial; densely packed sphagnum and it’s insignificant.

Then, of course, there are factors such as the wicking and water-holding capacity of the media ingredients, air movement, temperature, humidity and light levels, all of which combine to mean that pretty much each pot will need its own “calibration curve” to understand that’s going on.

[fredfarmer]

Roberta – I’ve pretty much eliminated the “60 hour” problem –it didn’t happen often anyway. I did all of these measurements a few months ago – since then I discovered the culprit – algae. The moisture on the outer surface of the some of the clay pots apparently attracts the algae and when it builds up it delays the evaporation process. Not sure how that works – probably by clogging up the pores in the clay. A little spritz with dilute Physan 20 on the outer surface of the pots as soon as I see some green and the temperature reading differences were down to less than 2 degrees in about 36 hours and there are rarely any temperature reading differences at 48 hours.

You are correct that this doesn’t work with plastic pots or bark media. There are some temperature differences but they are much less and don’t seem to correlate will with weight losses.

P.S. Thanks for not laughing at me. Some of my weird stuff turns out to be useful so I finally got up the courage to post this study

[Ray]

Nobody on my end is laughing!

[fredfarmer]

Ray - and thank you also.