Study: Sources of humidity in a micro growbox

[Beak]

I got in a bit of a debate with a fellow grower regarding how much the topsoil evaporation contributes to the overall rise in humidity in a small grow cabinet, so I decided to put it to the test.

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HYPOTHESIS: in a microgrow cabinet, topsoil evaporation contributes significantly to relative humidity

ABSTRACT: in a microgrow cabinet, there are two key sources of increased humidity

1. Humidity exhaled by the plant as it transpires
2. Humidity released by wet topsoil exposed to air

This study aims to determine whether soil evaporation contributes significantly to the overall rise in humidity in a micro growbox.

A recently watered plant was used for this experiment. The plant is in a plastic container so that the surface of the soil is the only part of the soil exposed to air and thus can undergo evaporation. Incoming air to the box (room humidity) is 40% controlled by a dehumidifier.

(plants and box used for this study)

First, the soil was covered by two layers of plastic wrap, fitted tightly around the trunk of the plants, wrapped around the sides of the container, and then clipped in place. The box was then closed and allowed to rest for two hours. Next, the plastic was removed, and then the box allowed to come to stasis again for approximately two hours.

If soil contributes significantly to overall humidity in the grow box, then we should expect to see a significant change in RH when the plastic is removed.

RESULTS:
With plastic covering the soil, the relative humidity inside the box increased from 40% (incoming air) to 55%. With the plastic removed, the relative humidity increased to 56%.

CONCLUSION: soil evaporation is not a significant contributor to the relative humidity inside a micro grow box. The rise in humidity inside a grow box is due almost exclusively to plant transpiration.

The other thing that’s really interesting to me is how much the plant cools the box by breathing. Incoming air is 28°, and then there are three led light panels making additional heat. Meanwhile air temp near the top of the box is 26°. As soon as the soil dries out and the plant stops transpiring the temp leaps up to 31°. The plant is dropping the temperature inside the box by about 5°C.

[Foreigner]

I can tell you anecdotally that my RH rises after a water but not significantly. I didn’t wrap it in Plastic though.

[Beak]

Once the soil dries enough the plant slows transpiration. OFC when the topsoil is wet it’ll contribute somewhat to rH but I think mostly it’s plant transpiration picking back up.
@Foreigner

[Foreigner]

Yeah I wouldn’t call it significant and it should be noted my current plants are pretty big to be crammed into such a small space.

[Beak]

That’s basically where I’m at. I got too much plant in too little box. It’s a battle to keep humidity down and VPD in a good zone.

[SonsOfAvery]

This is actually quite interesting, I do usually notice a rise in humidity in my small tent whenever I water. I had assumed that a lot of that was down to the evaporation. I’ll be honest I’ve never taken any exact readings and tracked them.
But interesting to see that the increas is negligible.

[Beak]

Remember this is bro science so definitely take it with a grain of salt @SonsOfAvery

[Gpaw]

Excellent experiment!

@Beak

The plant is dropping the temperature inside the box by about 5°C.

That squares with my results as well.
(IR thermometer)

Cheers
G

[cannabissequoia]

@Beak

Remember this is bro science so definitely take it with a grain of salt @SonsOfAvery

I think the only additional control to elevate it’s scienceyness is to repeat it 10000 times

total volume & surface area could be a variable… but since you’ve laid it all out I think we’re fairly convinced.

[the_bot]

what was the humidity level when the pot was dry btw?

[Loggershands]

Great write mate that was an interesting read.

@Beak

The plant is dropping the temperature inside the box by about 5°C.

This was definitely the most interesting conclusion. I’m not sure how it would translate in old growth forest transpiring directly into the atmosphere but it’s wild to think about the implications for temperature outside as well

[Mr.Sparkle]

Plant Evaporation being #1, also as some mentioned their RH increases after watering the plant just is taking up more during that period.

Now fabric pots that sit in trays… those do increase the RH alot no, but more than a standard pot

[Beak]

When the pot is dry and the lights are on the humidity goes < 40% and the temp goes > 30°C but I never leave them like that for long enough to find out how low it’ll go. These plants are under potted so when the soil gets dry it only takes a few hours for it to get dangerously dry.

[Beak]

Exactly. In fact I had the opposite humidity problem a while back in my veg box (humidity dropping too low). You might remember that. You and I chatted about humidifiers. One solution I tried was putting a big pot of water in the cabinet. It did nothing to help. The only thing that helped was putting wet washcloths right at the air intakes so that the incoming air blew over them. That raised humidity up from 40% to 50%.

Passive evaporation isn’t as easy as people think. A big plant is basically a giant evaporator, pulling the moisture from the soil and breathing it out into the air.

Anyone can test this. Put a few gallons of wet soil into your grow cabinet. Come back a week later, it’ll still be very heavy. Meanwhile my plants are blowing a liter of water into the air every day. A big healthy plant can probably go through a gallon or two per day.

[Beak]

By the way I’m going to follow up this test with another test. After I harvest these, I plan to water the soil in the pot and put it back in the box. My hypothesis is that humidity will stay very low, near 40%.

[YOLO.T]

Well done !
It’s something I already thought about, but your experiment is confirming that fact.
I have these strawberries and the one in the middle had an underwater stress with sun burning and did not make it as well as the other ones, and now, when I come to water them, I see the pot being heavy in weight and still moist whereas the others drank all the water and are lighter, so I have the same conclusion that yours.
Thanks for posting this, bro

[webeblzr]

Years ago, when growing in boxes, I found sinking my pot through the floor of the box to very helpful, with a few different things.
Sucking out at the top/back of my box, and adding vents, lower down, around the base of the plant, to allow air to in flow, and go upward. I never used fans to blow air around, as I found that useless.
I made hollow walls and doors, then used soffit vents, with poly batting filters, to keep dust out.
I could swap out totes and run hydro set up, with my reservoir sitting of the floor, underneath the box.
Some rock salt will lower humidity also.
Love your set up!!
I spent years in small boxes, and that was back in the HID days, so LOT’s of heat needed shuttled about also.
Hope it all works out for you!
webe

[Andrexl]

@webeblzr This guy knows his stuff!I would listen carefully to his words.
I actually use Coco coir and found out that when running Coco with HFF the RH that the plant puts out Is very very much more than when growing in soil,obviously of the conditions are right and plant can transpire good enough