VPD and nutrients absorption

[noreason]

& the pics, funny how VPD breaks down the clorophyl in the leaves, mine seemed to dissolve-(the green colour), leaves were showing like a Mg def but the leaf coulour turned 'Golden', very similar to your pics. nice example btw.

Is possible you are talking about photorespiration?the upper part of the plant,with a lot of light and low RH,can be affected by photorespiration quite easily.Also fans that blow air on the leaves increase photorespiration.
I say this because you wrote ''golden'' and my mind think to this...

 

[Scrogerman]

no definately VPD damage bro/or rather VPD that caused/led to the damage.

wasnt this one:
Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP (a sugar) has oxygen added to it by the enzyme (rubisco), instead of carbon dioxide during normal photosynthesis. This process reduces efficiency of photosynthesis in C3 plants.

or this:
Rubisco favors carbon dioxide as a ligand to oxygen,[1] however, photorespiration occurs when there is a high concentration of oxygen relative to carbon dioxide. The first reaction produces phosphoglycerate and phosphoglycolate (PPG), PGA re-enters the Calvin cycle and is simply converted back to RuBP.
PPG, however, is more difficult to recycle and has to move from the chloroplast to the peroxisomes, and then to the mitochondria, undergoing many reactions on the way, before the atoms can return into the Calvin cycle.

Now this is startying to get over my head, was anyway, i like to keep things simple.!
as long as enviromentals are in correct balance you dont get VPD damage anyway. now thats simple. fascinating subject i will endevour to learn more about, bio-chemistry & plant/botanical science is a weak point of mine.

 

[noreason]

SM,with photorespiration I mean that process that affect C3 plants like cannabis.
Rubisco usually fix a C atom taken from CO2 molecule in the air.

When Co2 levels are low in the leaf,eg when stomata are close due to high VPD (low RH),rubisco has the trend to fix oxygen instead of carbon.

The resulting molecule is useless for the plant,but for re-gain a part of the work,the plant convert this molecule into Co2 + H2O.
This process contrary to respiration (the plant gain 2 ATP with respiration) has no benefit for the plant,no gain and usually you can see it in indoor grows at the top of the plants beacause there is lower RH and high temperature,so the leaves often close the stomata and start this process.

This is why evolution ''create'' C4 plants,because C4 plants don't use rubisco to fix C from the air,but another enzyme that also at high temperature and low RH can't fix oxygen instead of carbon,so photorespiration it's not a trouble

I hope I helped someway

 

[Scrogerman]

I sort of get what you mean but its a little over my head tbh with you NR, all i know is i have the basic idea of VPD & how to avoid the damaging symptoms it can cause/bring about. I will endevour to learn on this fascinating subject, thanks for your time & your great thread & long may it continue my friend!
best of luck!

 

[*mistress*]

It's an interesting point here...plants can absorb minerals regardless from water uptake...but does water uptake influences mineral uptake or not at all?I mean,when water enter the roots does it carry on also minerals?The answer should be not,am I right?

water mainly carries the heavy metal calcium, which is immobile in the plant - except that it moves up w/ water..... this is why calcium deficiency appears @ the tops of plants 1st, where calcium is in its least abundance...... & why lack of water supply may induce calcium deficiency.....

so here comes the VPD instead of RH.RH is relative to temperature,and tell you only how many water (in %) is in the air at a certain temperature,but it doesn't tell you what plants ''feel''...thing that VPD does.Is this thought right?

vapor pressure deficit is the actual pressure of the external water vapor, pressing down on the plant..... the plant regulates how much water it pushes out based on this factor, along w/ air temps, root temps, water supply @ roots, etc..... vpd is an actual pressure of force upon the plant......

Every info on the topic is welcome.I just want to understand how all these stuffs work to give my plants a great setup.

keep air circulating 24/7.... both laminar & turbulent flows.... it carries away the moisture..... plants transpire mostly during the day, but seem to trickle @ night, seemingly.... most of the water coming out of the garden will be drained during lites on, though.... keep rh in balance w/ the environment that the garden is in.... what is not desired: condensation.

Mistress if you have some time can you link me something that explain phloem counter-flow please?I know what phloem is but I don't know how it works and what is the counter-flow.

#832
__________________

 

[spurr]

Mistress wrote:

vapor pressure deficit is the actual pressure of the external water vapor, pressing down on the plant

No it's not! According to J.Prenger and P.Ling (n.d.), "Vapor pressure deficit (VPD) is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated." <<< That is the correct definition.

Saturated = 100% RH


@ all:

I will try to respond to posts in this thread by Sunday, there is some wrong info being posted in this thread about what VPD does and does not do. The article I cited in this post is a good resource, as are the papers I posted in my thread that I uploaded on page 1. Please read them if you want to better understand VPD until I get a chance to post further.


"Greenhouse Condensation Control: Understanding and Using Vapor Pressure Deficit (VPD)"
Jessica J. Prenger, Research Engineer and Peter P. Ling, Assistant Professor
Ohio State University Extension Fact Sheet AEX-804-01

• (full text HTML) http://ohioline.osu.edu/aex-fact/0804.html
• (full text PDF) http://ohioline.osu.edu/aex-fact/pdf/0804.pdf

 

[spurr]

Mistress wrote:

keep air circulating 24/7.... both laminar & turbulent flows

Do you even know what laminar flow is!?! Growers are not using laminar flow in grow rooms, but they do use turbulent air flow. Laminar flow is a "nonturbulent streamline flow in parallel layers (laminae)" (link to definition). It takes special equipment, like a "laminar flow hood" in a room, to make laminar flow. Laminar flow is not a long distance phenomenon.

 

[*mistress*]

this is from the link in post # 15 of this thread:

rh...vpd...etc...
relative humidity is an expression of the actual water vapor pressure, expressed as a percentage of the maximum water vapor pressure possible under certain air+atmospheric pressure conditions.

@ room temp (~60*f), 100% humidity exerts a vapor pressure of24 torr (~4.65 pound-force per sq. in.) 1 torr = 1 mmhg/19.3*10^-3 psi...

24*{19.337*10^-3}]=4.65 psi pound-force.

less than (>) 24 torr of vapor pressure exerted on leaves, and leaves sense a vapor pressure deficit.

leaves stomata opening/closing influenced by difference between internal/external vapor pressure. opening/closing of stomata regulates gas exchange+transpiration, which in turn regulates growth/fruiting.

vapor pressure deficit is a lack of water pressure upon plant. this would be a low rh. it is indirect measure of water loss from plant. as plant attemps to balance internal/external vapor pressures, they draw up more water from roots and transpire it into the atmosphere. hence the de-humidifiers used in gardens.

air movement over plant+high temps+low rh reduce plants available water for sugar production. the roots uptake much more water during low rh. but all inter-related to ambient temp(avg. surrounding air temp), actual leaf temp, and root temp.

in general...
lower rh(high vpd)=increased transpiration, translocation, water uptake, greater calcium absorption/transport.

higher rh(low vpd)=slower transpiration, translocation, water uptake, slower evaporation, increased growth.

"@ room temp (~60*f), 100% humidity exerts a vapor pressure of 24 torr (~4.65 pound-force per sq. in.) 1 torr = 1 mmhg/19.3*10^-3 psi...

*24*{19.337*10^-3}]=4.65 psi pound-force.*

less than (>) 24 torr of vapor pressure exerted on leaves, and leaves sense a vapor pressure deficit."

the plant feels a "deficit" when the pressure is decreased......... that is because water exerts a pressure, just like atmosphereic pressure exerts a force of ~ 14-16lbs per square inch..........
the water in the air exerts a pressure, or force, upon the leaves - which the plant must overcome to expel water from w/in.........

w/ a greater external pressure, it becomes more physically difficult for the plant to push up & release water into the atmosphere, thus, vapor pressure deficit is the measure of the lack of pressure upon the plant - from outside......

the amount of water the air can hold has more to do w/ temps & air flow than anything else....

Do you even know what laminar flow is!?! Growers are not using laminar flow in grow rooms, but they do use turbulent air flow. Laminar flow is a "nonturbulent streamline flow in parallel layers (laminae)" (link to definition). It takes special equipment, like a "laminar flow hood" in a room, to make laminar flow. Laminar flow is not a long distance phenomenon.

haha.........tee.....hee............

see the oldest thread in this section...... @ #3

laminar simple means vertical air flow, or up & down..............
mainly up, in gardens. but the method can be used in other garden types.....

 

[spurr]

Mistress wrote:

most importantly, view the actual mathematical process by which force is calculated......

that *math* is not in books

You have got to be kidding me, of course the math is in books and available online. Where do you think I got it from? That said, your definition of VPD is wrong, like I showed you with proper citation of academic level reference.

You didn't invent VPD, but you sure seem to misunderstand it really well

Mistress wrote:

laminar simple means vertical air flow, or up & down..............
mainly up in vertical gardens, w/ bare bulbs......

What!?! Laminar flow is not only vertical flow. It can be, but it doesn't have to be. Laminar flow (in terms of air) is normally horizontal. There is no laminar flow in a garden, that is plain fact, unless the grower is using something that creates laminar flow. A fan does not make laminar flow, nor does natural vertical convention of air flow.

I ask you once again to post legit academic references in this sub-forum, not like the non-legit reference you posted for traicontanol.

In all honesty mistress, IMO, you should not be posting in this sub-forum until you learn how to reference legit academic sources for your claims and stop posting wrong info. I don't want to get in trouble with mods again, so I will leave it here, however, if this was my sub-forum you would not be allowed to post.

I won't posting to you any longer, I think people can realize for themselves the quality (or lack thereof) of your posts.

 

[*mistress*]

...............

 

[Scrogerman]

Oh noooo, the teachers are at it now, i think the both of you are really clever people, its a shame you dont seem to see eye-to-eye on things sometimes.
I followed Hazy's(& yours, you gave soo much info) Mollasses vs CBA Thread & you guys actually did trails with triacontinol if im not mistaken. from what i read(not that i know anything) the firm producing tria(cant remember their name) gave their own doses & trail information(albeit non MJ related). if i remember rightly results/ of tria, they had the same results using small ppm's like 10ppms as they did at 100ppms, those numbers may be wrong but i remember reading that & thinking you dont need to waste this stuff. can you correct the dosage info quickly for me please M*.

As for VPD, the deficit is what? we know what VP is right!
thats why i wanted the charts simplified to include safe RH to temp ranges as i listed earlier.
Do you know the chart i speak of M*/Spurr. ie- Temp-70f-80f & RH 50%-60% & so on. We have recommended RH & tempretures for growing MJ so we can avoid things like VPD, we have to make our plants comfortable so they can perform optimally.
If my RH ~~& temps are out of comfortable range i see damage in the form of yellow elipticle blotches(irregular), i have always known this damage as caused by VPD, it always happens when the numbers are wrong, so i know what it is. set the environment balance back to where it should be (75f & 60% RH etc) & the symptoms dissappear. Am i seeing damage caused due to VPD? or is it something else. Thats not the only symptom of damage ive seen & attributed it to VPD.

I hope you 2 can get along, there is too much bs in the forums atm.

best of Luck guys!

btw
is there such a thing as positive & negative VPD?(+-)

 

[noreason]

People can have different opinions,one can be wrong,one can be right but everyone should be peaceful with the next.Don't have a row please

I think spurr and *m* are saying pretty the same thing on what is VPD.Terms changes but the meaning seems to be the same to me.

"Greenhouse Condensation Control: Understanding and Using Vapor Pressure Deficit (VPD)"
Jessica J. Prenger, Research Engineer and Peter P. Ling, Assistant Professor
Ohio State University Extension Fact Sheet AEX-804-01

• (full text HTML) http://ohioline.osu.edu/aex-fact/0804.html
• (full text PDF) http://ohioline.osu.edu/aex-fact/pdf/0804.pdf

This is a very interesting read.


Regarding the laminar flow description(unknown to me before I read it here and did some read on the web) it seems to be not only a simple air movement up\down or viceversa.I understood fluid speed is a crucial factor to create a laminar flow.The fluid need to move slow to not become turbolent and I think normal fans can't create a laminar flow itself.However my question is:what benefits growers can have using this kind of flow?

I don't know what triacontanol is,but this thread is VPD related.Please don't get OT,thanks

is there such a thing as positive & negative VPD?(+-)

VPD is a value.It starts from 0, with 100% RH at all temperatures, and goes up as the RH decrease.Negative deficit is not possible.
Try to read the link posted by spurr,you should understand well what is VPD and why gardners use it

• (full text HTML) http://ohioline.osu.edu/aex-fact/0804.html
• (full text PDF) http://ohioline.osu.edu/aex-fact/pdf/0804.pdf

 

[Scrogerman]

OK thanks buddy, i thought i understood what it was & the damage it can cause, but obviously i must be getting it wrong somewhere, im not the sharpest knife in the kitchen by any means.
thanks NR respect!

btw, search for triacontanol & check out the Mollasses Vs CannaBoostA Thread, theres a lot of info from Mistress posted there. Its a growth Hormone i think(if i remember without looking), certainly remember that it has the same effect, its in alot of on the shelves products including CannaBoostAccelerator, think its found in Kelps & things like that.
If your interested in making your own growth boosters as appossed to paying people like Canna silly money, then its well worth checking out bro!

here you go bro: http://www.super-grow.biz/Triacontanol.jsp

Im gonna wade through those links spurr put up, Thanks guys!

 

[Scrogerman]

(from Spurr's very usefull link:below!)

Figure 1 shows how VPD relates to the customary thinking about humidity. Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition. Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition.
Figure 1. Vapor Pressure Deficit (VPD) enhances or inhibits the crop’s ability to transpire.

Therefore, vapor pressure deficit is a useful way to express the vapor flow in the system, both for condensation and transpiration. Higher VPD increases the transpirational demand, influencing how much moisture from plant tissues is transferred into the greenhouse air. Consequently, VPD is being used to predict crop water needs in some commercial irrigation systems. In contrast, very low VPD indicates closer proximity to the dew point, meaning harmful condensation can begin to develop. Using the canopy temperature to determine VPD gives the best indication of condensation risk, showing particularly how close the canopy is to the dew point.

VPD calculation is an improvement over relative humidity (RH) measurement alone because VPD takes into account the effect of temperature on the water holding capacity of the air, which roughly doubles with every 20°F increase in temperature. Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation. For example, a typical 100' long x 30' wide x 10' high greenhouse with 80% relative humidity has about 14 lb of water in the air at 50°F, while 70°F air holds about 28 lb of water at the same RH. This is reflected in the VPD values of 0.036 psi (0.25 kPa) and 0.072 psi (0.50 kPa), for the lower and higher temperature conditions, respectively (see Figure 2). Thus, VPD can be used to identify healthy air moisture conditions for plant production, while taking into account different temperature levels.


Do you know what, after reading that id say im not so blunt afterall & my understanding was right, a few points i wasnt realising but i certainly had the jist & basic concept of it right.

Cheers Guys!

 

[Scrogerman]

atm, now this terrible cold winter weather is here my atmosphere has changed again & RH is rather low, temps are optimal 74f-78f lights on & 60f-65f lights off(i dont like the 60f lights off temp so have turned up the central heating ), still my RH is 25%-40% tops now, this is measured under the canopy & in the middle of it(where its most humid?), the temp probe is right at top of canopy, so am i assumming correctly that the actual leaf temp is very close to this 74f?
I have no intention of putting in a de-huey or a huey for that, & some may say ideal, but the 25%RH is a bit low & this is High VPD & could start to cause damage, but im not 100%, things were OK untill this cold snap of weather. The lower the RH the more the plants will transpire! but with these numbers in the last 2 weeks of 12/12 im thinking everything is OK?
Any advice guys?

 

[Scrogerman]

Whoo-Raaaa! This is like the chart i was refering to bro's & sisters!, except it doesnt list the optimal range like the exact one i had in mind but rather the limits/threshold!

Table 1. Relative humidity thresholds for disease prevention, corresponding to 0.030 psi (0.20 kPa) VPD. ​

Temperature Relative Humidity Threshold
°F °C
50.0 10 83.0%
60.8 16 89.0%
68.0 20 91.5%
86.0 30 95.5%

Its in the link that Spurr kindly put-up, Nice one man

Wow, ive just disscovered the equasion calc's! exellent, it gets better all the time, ill get back to you tomoz with the exact VPD for my cab at this present time! happy camper!

its not very easy to get my head around but im gonna study it untill i crack it, im gonna have to get busy with a speadsheet too, this is very usefull information indeed! Thanks Spurr!
if this could be simplified haha! Hellllpp!(cant seem to crack it but i have all the numbers-'scratches head', maths was never a strong point of mine) bet its easy too lol!

 

[Scrogerman]

cracked it! i think

cracked it! i think

So the VPD for my cab is as follows:

approx lol: VPD = 0.42(VP sat) - 0.35(VP air) = 0.070psi
so my VPD is 0.070psi. Ok now ive got that bit, just!
The old cogs are really turning to push this lot out, its been a long time!


kpa = is 2.75, does that make any sense, my brain hurts lool!

 

[spurr]

Don't use the math mistress posted, you should use kPa or mbar (1 mbar = 0.1 kPa). I wouldn't trust her math if it was the last math on earth. Here is a uber easy way to find VPD via. an online calculator. This calculator uses the standard, and proven, equation to find VPD, unlike what mistress tried to cobble together:

To find VPD you need to find the temp at canopy (under shade) and RH at canopy, and ideally leaf temp (but using leaf temp is hard to do correctly).
​

http://www.autogrow.com/vpd_calc.php
​

0.8-1 kPa is ideal for everything but flowering if mold is a concern, or if you use carbon scrubber (and thus > 65% RH is not good).

I think using 1-1.25 kPa is a good goal for cananbis growers because reaching 0.8-1 kPa isn't easy. In terms of RH, molds and other fungi are most worry at >75-80% RH, and for VPD, < 0.2 kPa is danger area for molds and other fungi, etc.

 

[Scrogerman]

Thanks Brotha Spurr! Top man!

yeah it says my VPD is 1.63Kpa & with a slightly cooler leaf temp(-2c) its surly cooler than my air temp, so i just knocked a degree or two off & it says 1.3Kpa? Mmm completely differnt from the other charts figures ah? whys that, anyway im just glad youve made it simple for me, my RH & Temps are pretty clever at this time of year 74-78f & 42%RH (Lights-On, usually, but is dropping lower the last day or 2) i think me addressing the ambient inlet airtemp has helped there. Thanks for your help fella!

just coming up to Flush in a weeks time or so!

 

[spurr]

yeah it says my VPD is 1.63Kpa with a slightly cooler leaf temp(-2c) its surly cooler than my air temp so i just knocked a degree or two off & it says 1.3Kpa?

Nice, that isn't too high to be of major worry (1.63 kPa), I have seen gardens with VPD over 2 kPa, that is when it's really worrisome. And you made a good assumption with leaf temp, it is what I do too. Leaf temp is generally a few degrees (F) cooler than air temp (IIRC when air temp is less than ~85'F or so), thus leaf temp would also be a couple of degrees (C) cooler than air temp.

Mmm completely differnt from the other charts figures ah? whys that

What chart are you referring to? Here is another great resource for VPD with great tables to find VPD:

"Understanding Humidity Control in Greenhouses"
Floraculture FactSheet
http://www.agf.gov.bc.ca/ornamentals/floriculture/humidity.pdf



Here are some other good resources:

"Stomatal Conductance and Yields of Irrigated Crops"
http://4e.plantphys.net/article.php?ch=7&id=252


"Mist and Fog Equipment for Propagation"
http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/jb_MistandFogEquip.htm