Lighting LED and CalMag discussion.

[Power.P]

Ok i have been told another theory as to why we are seeing Cal/mag def under Led... The theory goes like this, because LED's don't give out the same radiant heat as HPS the plants don't transpire as they normally would, this in turn changes how the plant uptakes nutrients. i have been adviced that a way around this, is to increase the temps and the RH in the grow room. this is what i am doing at the moment..running temps between 82-86F and an RH of 70-80%, so far no problems. Will keep you all updated on how this theory works out.

Isn't this risky in flower due to possible mold?

 

[dildano]

Ok i have been told another theory as to why we are seeing Cal/mag def under Led... The theory goes like this, because LED's don't give out the same radiant heat as HPS the plants don't transpire as they normally would, this in turn changes how the plant uptakes nutrients. i have been adviced that a way around this, is to increase the temps and the RH in the grow room. this is what i am doing at the moment..running temps between 82-86F and an RH of 70-80%, so far no problems. Will keep you all updated on how this theory works out.

Now I'm really confused, my leaf sides curl up a bit which I read indicates heat stress, it is hot inside the house because it's summertime I struggle to keep temp below 30c (86F) in my tent, now I read that heat is good. My seedlings are only 5 days old and I fear the heat stress will make them males or herm or some other problems? Well not so much I can do about the heat, so at least I hope you are right about the cal/mag problem.

 

[robwars]

Isn't this risky in flower due to possible mold?

Yes you're right bro. Hazy is experimenting at his own risk :Holy Moly:

Now I'm really confused, my leaf sides curl up a bit which I read indicates heat stress, it is hot inside the house because it's summertime I struggle to keep temp below 30c (86F) in my tent, now I read that heat is good. My seedlings are only 5 days old and I fear the heat stress will make them males or herm or some other problems? Well not so much I can do about the heat, so at least I hope you are right about the cal/mag problem.

I say again... Hazy is experimenting. This is a discussion not a how to thread. The calmag issues are legit. :smokebuds:

 

[evol]

If the cal mag issue was due to temps I think we would have seen the same problem with cfl and t5 grows... I think it has more to do with the increased red spectrum of LEDs. Just my

 

[dildano]

@robwars yea I know I was just saying I hope he is right because there is not much I can do to lower my heat too much anyways :/

 

[CLMonkey]

Yupp evol i have to agree, i used 1800 watts of CFL's and was seeing temps at almost 90f @ 40%RH and i didnt have as much problem till i added the LED. once i added that my temps went even higher, so i now have a window shaker keeping the room cool @ 75f and my issues are just getting worse every day. i am now starting to lose my bud sugars to yellowing and spots ;( i believe i am losing my battle.
I am a newb, but i just dont think that the cooler temps are working, at least not for me.
:karma Cloud:

 

[evol]

Also wouldn't running a higher rh decrease transpiration? I think higher temps and lower rh would be better if the temp theory is correct..
A hydrated leaf would have a RH near 100%, just as the atmosphere on a rainy day would have. Any reduction in water in the atmosphere creates a gradient for water to move from the leaf to the atmosphere. The lower the RH, the less moist the atmosphere and thus, the greater the driving force for transpiration. When RH is high, the atmosphere contains more moisture, reducing the driving force for transpiration.

 

[CLMonkey]

your right evol in nature the lower RH would "wick" more fluids, but at certain temps the leaves close up as self preservation. so i guess temps matter in that aspect. ? am i stoned? lol
I was up all night worrying about my girls so i am a bit cloudy

PS
I do want to add for anyone reading through this thread with cal/mag/LED issues, that my case is a little different than most because my 2 girls are root bound..and in all fairness is also crippling my uptake.
I didnt want to look like a poo slinger
:karma Cloud:

 

[cres]

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 of 24 torr (~4.65 pound-force per sq "[24*{19.337*10^-3}]=4.65 psi pound-force). >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.

a 3'x3' plant can transpire a gal. during high vpd (low rh), or a cup during low vpd (high rh). vpd/rh measurements must take into account air temps.
generally, keeping rh as high as possible (~55-75%) keeps the plant in a comfort zone to its genetic desires. lowering rh provides a method of tailoring nute formulas, pushing parameters, etc - as they will be taking up much more water. keep in mind that the water uptake during low rh is mostly just that - water. keeping nute ppm/ec in balance w/ ph is good idea.

prevention of mold/diseases/condensation is the reason for decreasing rh, but it can be higher w/ air exchange/movement w/ tempered evaporation (air movement over/around plants). also, growth slows during high rh, but in fruiting, slow growth is not an issue, fruit production is. lower rh also increases translocation of calcium (why gardeners use calcium additives during fruiting - it is sent to every part of plant during increased water uptake via roots).

not having problems because the plants' comfort zone is pretty broad w/ comparable temps+air movement - just like there's a 'domain+range', so to speak, in nute levels, etc.

note: 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.

rh does not need to be >50%, it needs to be tailored to specific environment, w/ specific temps/air circ variables accounted for. can manipulate growth/water uptake from that comfort zone point (~45-65%, again, depending on gardens other inputs).

[FONT=Arial, Helvetica, sans-serif]ow rh formula is basically water add-backs, as old lucas formula/threads discussed. was not discussed so much because of rh, but because plants generally utilize more water than nutes during any res/watering/feeding session/change. ppm of nutes will be higher w/ less water in res, if recirculating, but if hand-watering, feed every 2-3 waterings, etc.

plants want 100% humidity in root-zone+100% humidity in atmosphere. they are constantly functioning to balance this ratio out. 100% rh is inapplicable, so gardeners try to get as close to this as possible, w/out incurring disease/rot/pests, etc.

less nute application is best standard formula to begin w/ due to non-linear absorption. different nutes assimilate @ different ph's, etc.

higher rh promotes growth, as plant is not using energy to simply transpire in attempt to balance in/out vpd. as fruiting commences, nutes+light regime+lower rh encourages fruiting. plant has broad comfort range and will adapt to changes within range.

higher rh w/ good air flow is doable. plant will draw up less root-zone water/transpire less. gardeners seek to 'push' the plant during flowering/fruiting by reducing rh for greater nute absoption/water uptake.

yield is inter-related to all garden variables: cultivar, rh(vpd), diff, root-zone temps+mass, light regime(s), etc. hard to standardize yield assesments w/ thousands of cultivars/various garden environmentals.

plants will uptake+transpire more h20(water) during low rh. nute assimilation/absoption is non-linear/non-correlative to water absorption. the plant will not use as much nutes as water during low rh, as plants only have a need for/can only process a limited amount of nutrients. they need water 100% of the time.

pre-mix organic mixes w/ water only throughout the cycle are documented here. possible to use less nutes, w/ more water add-backs, in hydro, w/ success.

if the garden is producing good vegetables, keep the comfort zone you have established for your own particular environment, regardless of what books/threads, etc say. best indicator of good garden is gardener's own pleasure w/ fruits/vegetables.
[/FONT]

 

[cres]

Vapor Pressure Deficit - The Hidden Force on your Plants

Once you understand what vapor pressure deficit is, all those environmental factors you're trying to juggle in your mind suddenly click into place and you start to think and feel like a plant. Take a few minutes to understand why VPD management is key to creating the perfect indoor growing environment! Your plants will thank you for it!
Humidity is HUGE when it comes to growing plants. An important milestone in becoming a competent and responsive grower is developing an understanding of what humidity is, how plants respond to it, and how you can manage and manipulate it
Firstly, let's make sure we're all on the same page. When we speak of the "humidity" of or in the air we are basically referring to the amount of water in the air. "In the air?" What do we mean? Well, water can only truly stay in the air when it is in gas form - aka "water vapor". We're not talking about tiny droplets of water in the air here (e.g. fog or mist.)

Unsurprisingly, temperature plays a crucial role when it comes to humidity. The warmer the air, the more water vapor it can potentially hold. As the amount of water air can hold constantly changes with temperature it can be difficult to get a handle on what we need to measure. Fortunately an answer comes in the form of the concept of "Relative Humidity" (RH) - this is a measurement in terms of percentage, of the water vapor in the air compared to the total water vapor potential that the air could hold at a given temperature. So, when we say there's a relative humidity of 50% - we mean "At this specific temperature, the air is carrying half the potential water vapor possible."
The Effect of Relative Humidity on Your Plants

RH can be easily measured using digital or analogue meters called "hygrometers." They are available for around $15 at your local indoor gardening store. But what do the readings mean for your plants?
Turns out-they mean a great deal! While many novice growers focus solely on keeping temperature in range, many take their eye off the ball as far as RH is concerned-perhaps because they don't fully understand what it is or how to manipulate it to their advantage.
Have you ever been to Florida in July? You'll know that it's not just the heat that's oppressive, it's the humidity! You feel constantly wet with sweat - the whole place feels like a sauna you can't escape from! (Sorry Floridians!)
RH has an ever more direct effect on plants. Plants need to "sweat" too - or rather, they need to transpire (release water vapor through their stomata) in order to grow.
The amount of water plants lose through transpiration is regulated, to a point, by opening and closing their stomata. However, as a general rule, the drier the air, the more plants will transpire.

Under Pressure

All gasses in the air exert a certain "pressure." The more water vapor in the air the greater the vapor pressure. What does this mean? Well, in high RH conditions (think of Florida again) there is a greater vapor pressure being exerted on plants than in low RH conditions. From a plant's perspective, high vapor pressure can be thought of as an unseen force in the air pushing on the plants from all directions. This pressure is exerted onto the leaves by the high concentration of water vapor in the air making it harder for the plant to 'push back' by losing water into the air by transpiration. This is why with high RH plants transpire less. Conversely, in environments with low RH, only a small amount of pressure is exerted on the plants' leaves, making it easy for them to lose water into the air.
What is Vapor Pressure Deficit (VPD)?

Okay, so now that you have RH firmly implanted into your conceptual map, we move on to Vapor Pressure Deficit or VPD. As implied by the word "deficit" we're talking about the difference between two things. In this case, it's the difference between the theoretical pressure exerted by water vapor held in saturated air (100% RH at a given temperature) and the pressure exerted by the water vapor that is actually held in the air being measured at the same given temperature.


The VPD is currently regarded of how plants really 'feel' and react to the humidity in the growing environment. From a plant's perspective the VPD is the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If we look at it with an RH hat on; the water in the leaf and the water and air mixture leaving the stomata is (more often than not) completely saturated -100% RH. If the air outside the leaf is less than 100% RH there is potential for water vapor to enter the air because gasses and liquids like to move from areas of high concentration (in this example the leaf) into areas of lower concentration (the air). So, in terms of growing plants, the VPD can be thought of as the shortage of vapor pressure in the air compared to within the leaf itself.
Another way of thinking about VPD is the atmospheric demand for water or the 'drying power' of the air. VPD is usually measured in pressure units, most commonly millibars or kilopascals, and is essentially a combination of temperature and relative humidity in a single value. VPD values run in the opposite way to RH vales, so when RH is high VPD is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant.
As mentioned above, VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control. The only problem with VPD is it's difficult to determine accurately because you need to know the leaf temperature. This is quite a complex issue as leaf temperature can vary from leaf to leaf depending on many factors such as if a leaf is in direct light, partial shade or full shade. The most practical approach that most environmental control companies use to assess VPD is to take measurements of air temperature within the crop canopy. For humidity control purposes it's not necessary to measure the actual leaf VPD to within strict guidelines, what we want is to gain insight into is how the current temperature and humidity surrounding the crop is affecting the plants. A well positioned sensor measuring the air temperature and humidity close to, or just below, the crop canopy is adequate for providing a good indication of actual leaf conditions.
Managing Humidity

Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant's water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth.
So what should you aim to keep your humidity at? Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but it's not the whole story as it fails to take into account the air temperature.

Photo credit: Aquaculture Hydroponics, UK.

By looking at this example we can see that at 70% RH the temperate should be between 72-79°F (22-26°C) to maintain healthy VPDs. If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.) The problem with running a high relative humidity when growing indoors it that fungal diseases can become an issue and carbon filters become less effective. It is commonly stated that above 60% RH the absorption efficiency drops and above 85% most carbon filters will stop working altogether. For this reason it is good practice to run your RH between 60-70% with the upper temperature limit depending on your crop's ideal VPD range, in the example it would be 64-79°F (18-26°C.)
The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress.
Please understand that by presenting this information we do not want you to go to your indoor gardens and run your growing environment to within strict VPD values. What's important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can.
If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:
Measure the leaf temperature and look up the vapor pressure at 100% RH on the table below.

Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on the above table. Subtract the air vapor pressure from the leaf vapor pressure:
Example:
Leaf Temperature = 24°C (100% RH) Leaf VP: 29.8
Air Temperature = 25°C @ 60% RH Air VP: 19.0
VPD= 10.8
Humidity's Effect on Plants

Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.

When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.
For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavorable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant - the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.

Most growers are well aware that with high humidity comes and increased risk of fungal diseases. Water droplets can form on leaves when water vapor condenses out of the air as temperature drops, providing the perfect breeding ground for diseases like botrytis and powdery mildew. If humidity remains high it further promotes the growth of fungal diseases. The water droplet exuded through guttation also creates the perfect environment for fungal spores to germinate inviting disease to take hold.

Quick reference chart:
Low VPD / High RH
High VPD / Low RH
Mineral deficiencies
Wilting
Guttation
Leaf roll
Disease
Stunted plants
Soft growth
Leathery/crispy leaves
Words: Gareth Hopcroft and Everest Fernandez