Some notes from a fellow Cacti grower on Atmospheric pressure & how it relates to nutrient strength.
A note on altitude and how it affects the required nutrient strength for best plant performance. The transpiration rate of a plant is a measure of how much water flows through the plant over a given period of time. Since nutrients are generally taken-up with the movement of the water through the plant, this is an important concept when considering altitude effects. Transpiration rates, measured at the plant canopy, are directly dependent on the atmospheric vapor pressure. Atmospheric vapor pressure is a function of humidity and is dependent on temperature and altitude. For a given air temperature and relative humidity, as altitude increases, atmospheric vapor pressure decreases. As a result, under the same growing conditions, at altitude less water moves through the plant than at a lower altitude. When less water is moving, less nutrients are available for the plant. We compensate by increasing the strength (EC) of the feed.
If we go up 1 mile from sea level, we see a directly proportional reduction in vapor pressure. That reduction for 1 mile up is about 20%, so to say if someone is using an EC at sea level, and they go to altitude, given same humidity and temp and sun exposure, they should likely need approx 20% increase in EC. So say you like 3.0 down there, you might like 3.6 up here. Add some extra sun, and maybe it could be bumped up a bit more to accommodate the increase in photosynthesis rate. Humidity and temp will also affect vapor pressure.
Those in Colorado & other high altitude areas, what EC are you pushing for your plants? I am wondering if anyone else has noticed this.
Interesting concept makes since though , not a hydroponic grower my self but seems that this is all related to VPD and the charts associated with it when one gets VPD dialed in in Indoor one can experience faster than normal growth with their plants haven’t seen it first hand but the sources Iv read here and there indicate so . Does seem like there should be altitude adjustments to VPD charts just like they do in home canning and pressure cooker canning buy adjusting length and pressure at higher altitudes . Great thread interesting to see what falls out in this one!
Could be tested with pressure chambers. & cacti are sooooo different from cannabis… I would think “burning” would be an issue unless temperatures were lowered.
Makes me think of older carbureted cars; I’m young enough to have never dealt with driving from sea-level to the mountains & back in anything that old. Always fascinated me to read about the compensation done automatically by fuel-injection.
Just a quick example, a friend in Colorado pushes 4+ for some of her plants. I live pretty much right at sea level and an EC of 4+ would for sure MELT my plants. I’ve found even 2+ can be problematic.
Interesting idea, I have definitely noticed in my lifetime how altitude affects yeast requirements when it comes to baking, so it wouldn’t surprise me if it also affects fertilizer rates…
Sorry folks, but I think there is a fundamental problem with the basic premise/assumption here. Its possible I have a misunderstanding about what is being proposed here, but it sure sounds like stoner science to me.
Vapor pressure has zero relation to altitude or humidity. It is solely and only dependent on the temperature of the liquid - nothing else.
If there is any connection between altitude and necessary feed strength (and there may well be one), it has nothing to do with vapor pressure.
Its also my understanding that transpiration rate is dependent on relative humidity as well as temperature, and wind, or other mechanical effects. Relative humidity is also a direct function of temperature - not altitude. Altitude has no effect here except in a very indirect way. Temperatures tend to drop as you go up in altitude, which lowers the relative humidity - BUT - its the changing temperatures and not the change in altitude that is the cause.
Also, if you are growing indoors, you have total control over temps, so I dont see how altitude applies anywhere except maybe in an outdoor grow where temps would be a good bit lower than typical - but who grows outside in the cold?
Good question, but once again, VPD is a function of humidity. It is defined as the difference between how much water the air can hold - at that exact temperature - in relation to how much water is actually in it. In other words, its a measure of how much LOWER the relative humidity is vrs what it could or should be if the system was in equilibrium. By definition, if there is any VPD, then the system is not in equilibrium.
But remember - humidity is a function of temperature only. Altitude does not enter into it in any direct way.
I should say this again - I do not know if altitude does or does not play a part in how hi an EC you could or should run when growing. It may well be true that growing at hi altitudes allows you to run higher EC or even that you must run higher EC for healthy plants. I have no clue one way or the other.
Off the top of my head, I cant think of any reason altitude would make higher EC possible or needed. My first thought would be that the opposite is true due to lower oxygen levels at altitude. Less O2 should mean lower plant functions in general, and therefore less nute use from slower growing.
But thats just an off the cuff guess
I tend to distrust theories that are based on incorrect science. It is very very dificult to come up with the correct answer when you have the wrong facts to start with.
On the other hand, people do sometimes get the right answer even if they dont understand the science behind it. Sometimes.
Humidity and temperature effect Vapor pressure though? I am interested what you think the connection between Altitude and EC is if you do not think it is Vapor pressure? How can someone in colorado feed at 4+ when that would melt my plants at sea level? Just curious as to what you think it is.
Once again, interested to know what you think the connection is then.
Transpiration is dependent on vapor pressure, and at altitude the atm has a lower vapor pressure, therefore for the same given conditions a plant at altitude will transpire less. Yet at alt the sun is more intense. So photosynthetic rates are higher than lower down. Now if we reduce the TR then how do we maintain the proper nute dosing? Less water is moving, so to get an equivalent dose, the EC must be higher. Now this is for nominal conditions. So if we look at the formula for determining the vapor pressure we see it can be related directly to the alt. That means if we go up 1 mile from sea level, we see a directly proportional reduction in vapor pressure. That reduction for 1 mile up is about 20%, so to say if someone is using an EC at sea level, and they go to alt, given same humidity and temp and sun exposure, they should likely need approx 20% increase in EC (guessed at this) so say you like 3.0 down there, you might like 3.6 up here. Add some extra sun, and maybe it could be bumped up a bit more to accommodate the increase in photosynthesis rate.
First - you were typing at the same time I was. Look up at the post above yours.
No. Vapor pressure is a function of temperature only. Humidity is the result you end up with when you have water vapor - at a certain temperature - in the air.
This is the basic error - the atmosphere has no “vapor pressure”.
This is a definition of vapor pressure:
Vapor pressure is the pressure caused by the evaporation of liquids. Three common factors that influence vapor press are surface area, intermolecular forces and temperature. The vapor pressure of a molecule differs at different temperatures.
Note that there is no mention of atmosphere or altitude. You measure vapor pressure by putting the liquid in a vacuum chamber, pump out ALL the atmosphere, and wait to see what the final pressure is - at that exact temperature. No atmosphere involved at all.
If transpiration rate is based on humidity - then it cant be effected by altitude.
But again, there may be some other factor involved. I doubt it, for the reasons I listed in the other post above yours, but its possible.
One more point on transporation. Transporation rates are dependent on two things primarily.
Number one is relative humidity. Remember, humidity is determined by temperature only.
Altitude has no effect at all on humidity, vapor pressure or transporation rate.
The other thing that changes transporation rate is mechanical action - wind blowing across a leaf will strip away water molecules which increases the evaporation rate.
It sounds like you are talking about growing outside then? If you are outdoors, AND if the temps were lower, then I could see tansporation rates dropping, but how well will your plants grow at low temperatures?
It still seems to me that altitude (lower temps, lower O2) will lower the metabolism of the plant. I dont see how that connects to higher EC levels. It might be true, but I am doubtful.
Sorry, but that website is incorrect - in several ways.
Its true that humidity CAN be, and often times is lower at altitude, but the ONLY reason it would be lower is that temps at hi altitudes are generally lower.
The air pressure has nothing to do with it at all.
Google the definition of humidity, relative humidity, vapor pressure, and VPD. You will find exactly zero reference to altitude or air pressure.
VPD relies on two things. Temperature and relative humidity difference between two things.
If the relative humidity is different at a different altitude, then the VPD will also differ. If on the otherhand RH and temperatures are the same, then there should be no change in the VPD. Air pressure is not considered.
See these equations:
Vapor pressure is calculated by RH * saturation pressure. Vapor pressure deficit is the saturation pressure of the plant matter subtracted from the saturation pressure of the surrounding atmosphere. Plant matter is considered 100% saturated. Saturation pressure relies on temperature.
The variable that would affect VPD is the RH and temperature.
This is not saying the effect on transpiration (and subsequent uptake) is not occurring at altitude. What it does say is that perhaps we are looking at the wrong thing. Maybe the measurement of RH is problematic. Maybe the stomata close down for some reason. Maybe the leaf temperature differs (who measures that?). idk. Interesting though. Dew point, on the otherhand, …
This is interesting as I’m finally able to get into the correct ranges of VPD at various stages of growth reason I’m bumping this thread is that on various websites and such charts are noted for values at “sea level” just curious is all and living in higher elevations myself then most probably but I have a hard time believing that altitude doesn’t have some sort of effect on VPD The “thinner air” we have here has to effect pressure inside the leaf correct?
Interesting read thanks @Northern_Loki but here’s my analogy on it ! If one ever buys a sack of chips at 5000’ and takes them into the mountains at even 8000’ the damn things are about to explode! Having a hard time wrapping my head around it. Lol
& cacti are sooooo different from cannabis… 
I would think “burning” would be an issue unless temperatures were lowered.
