[noreason]

Hi ICmaggers
forgive me if I write something bad

I want to share with you my theory about VPD and nutrients absorption.maybe it's not only a my theory,maybe someone already know it or know something more or more accurate about this.You're welcome to reply,because I want to understand how this kind of stuff work.

VPD (link to wiki)

This is a value take in consideration RH, air temp. and leaf\canopy temp.

To be short,a low VPD value coincide with high umidity

on the other hand an high VPD value coincide with low humidity in the air

VPD is a shortcut,because it takes in consideration two values together: RH and temperature.



This is a little scheme I found in the co2 thread here on IC...it gives you values about VPD.As you can see the ideal VPD is 0.85 and you can reach this value considering the second scheme





Knowing this value we can know how much the stomata are open.

You know the plant (thanks to rubisco) hook the C atom in the co2 molecule just through the stomata.
A low VPD value let the stomata to open wide,so the plant has a great co2 availability.
In the other hand there is low h2o absorption from the roots,because the leaves already have a lot of water inside and need not much h2o due to little transpiration.

Having an high VPD value leads the stomata reduce their opening.This means the plants has little co2 availability but there is a lot of water absorption from the roots due to more transpiration through the leaves.More uptake of water from the roots means also more nutrients absorption,from the substrate to the top of the plants to be used after.

At now I'm thinking about a cycle alternating low VPD period with high VPD period.
This could be helpful because in the high VPD period the plant can absorb a lot of nutrients and then with a low VPD the plants can use those nutes faster due to the more co2 availability.

I wonder if in the night hours when the lights are off,plants usually absorb nutrients or not...but I think yes.Transpiration works also in the night cycle,isn'it?
So,having an high vpd (low RH) during the ''night'' can help the plants to absorb more nutrients to be used then,in the next light phase?

This is only a theory,I have no evidence of anything...but using the logic this stuff make sense to me,so feel free to give me\us your two cents or maybe more

[!!!]

I don't think the plant absorbs nutrients at night. The only thing I know happens during the night cycle is that energy from photosynthesis is stored in the roots, and I'm only basing this on something I read on a random 420 forum awhile back.

[Rowdy420]

I think more folks need to take a look at VPD. It will exlpain alot about stomata activity during different times of relative humidity. In my opinion most people run RH to low and basically you are locking the stomata closed to fight against transparation.

Your theory is interesting with introducing both conditions, not sure if if the plant is using the nutes; rather storing and off gasing during night cycle. Would love to see more info on the subject fo shizzle.

[spurr]

Originally Posted by noreason

Hi ICmaggers
forgive me if I write something bad

I have written very extensively about "Air to Leaf Vapor Pressure Deficit" (i.e. VPD) in the past. And my work was plagiarized by Urban Gardener magazine/blog (assholes)!

I can post up much info about this topic, and why we should use VPD and not RH if you'd like me to. I have a simple guide for people that will allow them to reach ideal VPD (i.e. 0.8-1.0 kPa) and close to ideal VPD (1-1.25 kPa) while keeping ADT (Average Daily Temp) high (ex. > 70'F) and keeping RH below ~65%, which is important for growers who use carbon scrubbers (they don't work once RH is ~65%+).

If a grower doesn't use a carbon scrubber than they should shoot for VPD of ~0.8-1 kPa for veg and ~1 kPa for flowering. But if they have a carbon scrubber than they should shoot for ~1.25 Kpa all the time.

To find VPD one needs to know the canopy temp (under shade), and RH, and ideally average leaf temp.

Using VPD is ideal because that allows us to provide ideal stomatal conductance, which in turn allows for ideal rate of transpiration ("E") and carbon fixation; the former in turn allows for ideal uptake (and translocation) of Ca and Bo because they need to be 'sucked' up into roots with water, unlike other elements.

Let me know if you'd like me to help you, I can also upload a thread I posted at a different forum (TCC) with tons of info and good studies, etc. I would prefer to give you my thread because I covered most topics and concerns about using ideal VPD, ex. why to use it, mold issues, etc. This thread would just bring up topics I already covered in my thread, so I would prefer to just upload my thread for people to read which will answer most all questions and concerns.

For veg and flowering I use a VPD of ~1.20-1.25 kPa due to my use of carbon scrubbers (which means I can't allow the VPD to go too low, ex. ~1.0 kPa).

I want to share with you my theory about VPD and nutrients absorption.maybe it's not only a my theory,maybe someone already know it or know something more or more accurate about this.You're welcome to reply,because I want to understand how this kind of stuff work.

VPD (link to wiki)

This is a value take in consideration RH, air temp. and leaf\canopy temp.

You are bit mixed up. VPD is RH, canopy temp and leaf temp; but because leaf temp is hard to accurately measure and also be representative of all leafs, just using canopy temp and RH is fine for most people.

To be short,a low VPD value coincide with high umidity

Kind of, low VPD coincides with higher RH and lower temp.

on the other hand an high VPD value coincide with low humidity in the air

Kind of, high VPD coincidences with lower RH and higher temp.

VPD is a shortcut,because it takes in consideration two values together: RH and temperature.

It's more than a shortcut, it's the only way to quantify what the plant's 'feel' and the effects it has upon stomatal conductance.

This is a little scheme I found in the co2 thread here on IC...it gives you values about VPD.As you can see the ideal VPD is 0.85 and you can reach this value considering the second scheme

Ideal VPD is 0.8-1 kPa, with up to 1.25 kPa being fine. Less than 0.2 kPa means much increased danger of molds, etc. Lower than 0.8 kPa reduces stomatal conductance (in resposne to too great of rate of transpiration or VPD directly) and higher than 1.25 kPa also means reduced stomatal conductance (in response to rate of transpiration or VPD directly).

There are two modes of action a plant uses with VPD, either directly (VPD via. leaf chemicals) or indirectly (via. rate of transpiration); and that depends upon wither or not the plant has been in drought conditions before...

A low VPD value let the stomata to open wide,so the plant has a great co2 availability.

Yes, but VPD below ~0.8 kPa is bad, the stomatal conductance is too high, thus the plant will close down stoma in response, to reduce water loss due to rate of transpiration.

Having an high VPD value leads the stomata reduce their opening.This means the plants has little co2 availability but there is a lot of water absorption from the roots due to more transpiration through the leaves.

Reduced stomatal conductance (due to too high VPD; and to too low VPD) means both reduced Co2 fixation and reduced rate of transpiration.

More uptake of water from the roots means also more nutrients absorption,from the substrate to the top of the plants to be used after.

Pretty much, but only Ca and Bo depend upon rate of transpiration to enter roots; other elemts can freely enter root tissue. The elements/amino acids traveling xylem also depend upon rate of transpiration.

At now I'm thinking about a cycle alternating low VPD period with high VPD period. This could be helpful because in the high VPD period the plant can absorb a lot of nutrients and then with a low VPD the plants can use those nutes faster due to the more co2 availability.

Not a good idea. High VPD means reduced rate of transpiration (and thus reduced uptake of Ca and Bo and translocation of elements via. xylem), and it also means reduced Co2 fixation.

It's best to keep VPD the same, ex. 1-1.25 kPa. Over 1.25 kPa is not good, just like under 0.8 kPa is not good.

I wonder if in the night hours when the lights are off,plants usually absorb nutrients or not...but I think yes.Transpiration works also in the night cycle,isn'it?

Yes and yes. But at night plants respire via. leafs and roots, unlike the day.

So,having an high vpd (low RH) during the ''night'' can help the plants to absorb more nutrients to be used then,in the next light phase?

No. They will absorb more Ca and Bo during light hours because E is increased during light hours. It's better to keep VPD at the higher end, ex. ~1.25-1.5 kPa during night due to incrased chance of fungi attack when it's dark.

This is only a theory,I have no evidence of anything...but using the logic this stuff make sense to me,so feel free to give me\us your two cents or maybe more

Done and done. Yrs, spurr.

[noreason]

Thanks all for reply,this is an interesting topic to me so any imput is welcome

Let me know if you'd like me to help you, I can also upload a thread I posted at a different forum (TCC) with tons of info and good studies, etc. I would prefer to give you my thread because I covered most topics and concerns about using ideal VPD, ex. why to use it, mold issues, etc. This thread would just bring up topics I already covered in my thread, so I would prefer to just upload my thread for people to read which will answer most all questions and concerns.

Feel free to post whatever you want.More info I can get the better

If a grower doesn't use a carbon scrubber than they should shoot for VPD of ~0.8-1 kPa for veg and ~1 kPa for flowering. But if they have a carbon scrubber than they should shoot for ~1.25 Kpa all the time.

This is another interesting topic btw.I have a friend using carbon filters...it could help him if you can explain why they don't work under 1.25Kpa

Using VPD is ideal because that allows us to provide ideal stomatal conductance, which in turn allows for ideal rate of transpiration ("E") and carbon fixation; the former in turn allows for ideal uptake (and translocation) of Ca and Bo because they need to be 'sucked' up into roots with water, unlike other elements.

So,my theory about nutrient uptake is wrong except for Ca and Bo?Other macro and micro elements don't need water to climb to the top of the plant...right?

Kind of, low VPD coincides with higher RH and lower temp.

Kind of, high VPD coincidences with lower RH and higher temp.

It's more than a shortcut, it's the only way to quantify what the plant's 'feel' and the effects it has upon stomatal conductance.

right

Ideal VPD is 0.8-1 kPa, with up to 1.25 kPa being fine. Less than 0.2 kPa means much increased danger of molds, etc. Lower than 0.8 kPa reduces stomatal conductance (in resposne to too great of rate of transpiration or VPD directly) and higher than 1.25 kPa also means reduced stomatal conductance (in response to rate of transpiration or VPD directly).

Values under 0.8 and over 1.25 are not good,got it.

There are two modes of action a plant uses with VPD, either directly (VPD via. leaf chemicals) or indirectly (via. rate of transpiration); and that depends upon wither or not the plant has been in drought conditions before...

This is not clear to me (forgive me but I can't understand everything,my native language is not English)...maybe can you spent two mins to explain better please?

Yes, but VPD below ~0.8 kPa is bad, the stomatal conductance is too high, thus the plant will close down stoma in response, to reduce water loss due to rate of transpiration.

Reduced stomatal conductance (due to too high VPD; and to too low VPD) means both reduced Co2 fixation and reduced rate of transpiration.

right

Pretty much, but only Ca and Bo depend upon rate of transpiration to enter roots; other elemts can freely enter root tissue. The elements/amino acids traveling xylem also depend upon rate of transpiration.

So as said before,only Ca,Bo and other aminoacids depends on traspiration?Elements like N P K are not linked to rate of transpiration?

Not a good idea. High VPD means reduced rate of transpiration (and thus reduced uptake of Ca and Bo and translocation of elements via. xylem), and it also means reduced Co2 fixation.

It's best to keep VPD the same, ex. 1-1.25 kPa. Over 1.25 kPa is not good, just like under 0.8 kPa is not good.

Yeah,you're right.But try to consider fluctuation from 0.8 to 1 - 1,25 Kpa,avoiding lower or higher values than this.

Yes and yes. But at night plants respire via. leafs and roots, unlike the day.

So the plant respire via roots in the day...the night also in the leaves,right?

No. They will absorb more Ca and Bo during light hours because E is increased during light hours. It's better to keep VPD at the higher end, ex. ~1.25-1.5 kPa during night due to incrased chance of fungi attack when it's dark.

Because E has effects only on Ca and Bo it's useless keep a low VPD value at night,right?


Thanks a lot for your reply buddy

[spurr]

Originally Posted by noreason

Feel free to post whatever you want.More info I can get the better

Here is the VPD tread from TheCannaCabana, it's four full HTML pages. I compressed them into a zip file and encrypted it with the passphrase "ilovecanna" (without quotes). I uploaded them to a Russian file host, so it's 'safter' to download IP wise than a US file host:

• https://www.file-upload.net/download-...6/VPD.zip.html

I will post my easy to use guidelines later, I have to head off for dinner soon. The thread I uploaded should answer all your question, and much more. The studies I referenced you can read in full, I could upload them here, or you can use Google to search for them by title.

Originally Posted by noreason

This is another interesting topic btw.I have a friend using carbon filters...it could help him if you can explain why they don't work under 1.25Kpa

You could use lower than 1.25 kPa. But doing so, while keeping temps around 75'F to 80'F at the same time (for high rate of photosynthesis), means the RH would be at least 65%. And that would stop carbon scrubbers from working.

Originally Posted by noreason

So,my theory about nutrient uptake is wrong except for Ca and Bo?Other macro and micro elements don't need water to climb to the top of the plant...right?

So as said before,only Ca,Bo and other aminoacids depends on traspiration?Elements like N P K are not linked to rate of transpiration?

No, other elements also travel in xylem, thus transpiration effects the rate of translocation of all of them (in general). However, Ca and Bo rely upon rate of transpiration to be taken into root tissue, other elements can freely pass into root tissue. That is why low RH rooms often show Ca deficiency in plants, the rate of transpiration is too low to allow sufficient uptake of Ca into root tissue. Once all elements are in root tissue they (generally) use xylem to move to top of plant (translocate), etc.

Originally Posted by noreason

This is not clear to me (forgive me but I can't understand everything,my native language is not English)...maybe can you spent two mins to explain better please?

It's nothing you need to worry about, it's just very specific info about how and why a plant responds to VPD. Here is more info:

Leaf-to-Air Vapor Pressure Deficit (VPD) is basically a way to quantify the difference between water vapor in air outside the leaf and the inside the leaf. VPD controls stomatal conductance (Gs) directly or indirectly, and Gs affects many process in the plant. It has been found that Gs is controlled either directly by VPD, by the "feedforward" hypothesis. In which the leaf/plant senses VPD somehow and probably uses "abscisic acid" (ABA) to close stoma down (i.e, reduce Gs). Or Gs is controlled by the plant when it senses increasing rate of transpiration (E), which is controlled mostly by VPD (higher VPD means higher E). That is called the "feedback" hypothesis, where the plant changes its' Gs depending upon E. The plants use of either feedforward, or feedback hypothesis, seems to depend upon wither or not the plant has been in drought conditions (media water stress) in the past, ie., low media water status. If the plant has been under media water stressed it is thought the feedforward hypothesis controls Gs, otherwise the feedback hypothesis is thought to control Gs.

Originally Posted by noreason

Yeah,you're right.But try to consider fluctuation from 0.8 to 1 - 1,25 Kpa,avoiding lower or higher values than this.

Yea, flux in that range is fine, in fact, it's hard not to have flux. But for flowering keeping it from 1-1.25 kPa is a good idea if mold or other fungal pathogens are a concern.

So the plant respire via roots in the day...the night also in the leaves,right?

No, plants do very little respiration in day, if they do so they are in trouble, called "photorespiration" and it means greatly reduced rate of photosynthesis and carbon fixation. Normally this is caused form too much light or heat.

Because E has effects only on Ca and Bo it's useless keep a low VPD value at night,right?

No, it's good to keep VPD lowish at night to keep E highish which allows greater translocation of Ca, Bo, N, etc., into tissue via. xylem. The stomatal conductance is reduced in darkness, but it's still wise to keep the VPD from 1.25-1.5 kPa...

Thanks a lot for your reply buddy

No problem, glad to help. This is a confusing topic for most people, esp. someone who's first language isn't English. But you are getting it faster than most people I have tried to teach!

Here is the Urban Gardener Blog post on VPD where the author fully plagiarized my work, I wrote it under the nic "gojo" (from the thread I uploaded for you). I bitched at them for the plagiarism, but they didn't care...assholes!

"Plantworks: Part 1 – Humidity and Vapor Pressure Deficit"
https://urbangardenmagazine.com/2010/...essure-deficit

[spurr]

Edit:

I just re-uploaded the files.

[noreason]

Your posts are really interesting spurr.I'm understanding a lot of things,thanks.

You could use lower than 1.25 kPa. But doing so, while keeping temps around 75'F to 80'F at the same time (for high rate of photosynthesis), means the RH would be at least 65%. And that would stop carbon scrubbers from working.

just a little ot...why carbon scrubber would not work properly?I'm a noob on this,never used one.

No, other elements also travel in xylem, thus transpiration effects the rate of translocation of all of them (in general). However, Ca and Bo rely upon rate of transpiration to be taken into root tissue, other elements can freely pass into root tissue. That is why low RH rooms often show Ca deficiency in plants, the rate of transpiration is too low to allow sufficient uptake of Ca into root tissue. Once all elements are in root tissue they (generally) use xylem to move to top of plant (translocate), etc.

Are you saying the transpiration affect mainly Ca and Bo and in minor part other elements like NPK because these elements can pass trough the roots in any case?

Ca is an element plants use in a average amount...having low stomata conductance due both to too low VPD and too high VPD can bring problem in its upatake,am I right?

It's nothing you need to worry about, it's just very specific info about how and why a plant responds to VPD. Here is more info:

Leaf-to-Air Vapor Pressure Deficit (VPD) is basically a way to quantify the difference between water vapor in air outside the leaf and the inside the leaf. VPD controls stomatal conductance (Gs) directly or indirectly, and Gs affects many process in the plant. It has been found that Gs is controlled either directly by VPD, by the "feedforward" hypothesis. In which the leaf/plant senses VPD somehow and probably uses "abscisic acid" (ABA) to close stoma down (i.e, reduce Gs). Or Gs is controlled by the plant when it senses increasing rate of transpiration (E), which is controlled mostly by VPD (higher VPD means higher E). That is called the "feedback" hypothesis, where the plant changes its' Gs depending upon E. The plants use of either feedforward, or feedback hypothesis, seems to depend upon wither or not the plant has been in drought conditions (media water stress) in the past, ie., low media water status. If the plant has been under media water stressed it is thought the feedforward hypothesis controls Gs, otherwise the feedback hypothesis is thought to control Gs.

Thanks,I understood.Do you have any link to any sort of scentific tests?with value etc... (don't think I don't believe you)

No, plants do very little respiration in day, if they do so they are in trouble, called "photorespiration" and it means greatly reduced rate of photosynthesis and carbon fixation. Normally this is caused form too much light or heat.

So,plants use a little amount of oxygen during the day both trough the leaves and roots,right?
During night they suck a lot of oxigen and use it to produce ATP,is this correct?

No, it's good to keep VPD lowish at night to keep E highish which allows greater translocation of Ca, Bo, N, etc., into tissue via. xylem. The stomatal conductance is reduced in darkness, but it's still wise to keep the VPD from 1.25-1.5 kPa...

Do you mean ''to keep VPD high'' ?
Keep it high helps in mold prevention too.


Thanks again for your kindness,I'm sorry for that post on UGB :(

One more thing...reading your attachments I found this:

And for you, because you are so into nutrient and roots (pH, etc) I think VPD would matter because ideal VPD (or close to ideal) allows for ideal rate of transpiration (E) which allows for ideal nutrient uptake. Putting the nutrients in the water/media is the first step, then we should provide the plant with the ability to use those nutrients the best it can.

I'm definetely interested to know which elements are more or less involved in transpiration...if you have something more accurate to explain...it's welcome.

Have a nice day mate

Originally Posted by spurr

\Here is the Urban Gardener Blog post on VPD where the author fully plagiarized my work, I wrote it under the nic "gojo" (from the thread I uploaded for you). I bitched at them for the plagiarism, but they didn't care...assholes!

"Plantworks: Part 1 – Humidity and Vapor Pressure Deficit"
https://urbangardenmagazine.com/2010/...essure-deficit

on July 1st, 2010 I posted this:

https://www.icmag.com/ic/showpost.ph...53&postcount=4

Previously on June 28, I posted the same stuff to another forum (t h c f a r m e r ......)

And on July 12, 2010, the ubgangarden blog post was published....

It could be a coincidence.

BTW, the text files you attached are not readable.

[Scrogerman]

Great thread & i know only too well the damage VPD can cause in leaf tissue if not in the correct balance, RH to AirTemps, can mimick & cause certain deficiencys too ive observed.
Not sure weather to put a de-huey in my cab now actually, canopy temp is 74.4f lights on & drops to mid 60's lights off(63-65f), nice winter atmosphere, lower RH etc. Now my undercanopy RH is 45% lights on & 50-54% light off, but never any higher, i think this is perfect but im wondering about putting more lighting in, which is gonna increase temps & reduce lights-on RH, but this will also increase transpiration, so i'll prolly need a de-huey im thinking?. (use Liquid 'Si' too)
anyway great thread man!