VPD vs. PPM

Clutchcargo_1 · Nov 7, 2020

I grow in my basement in the late fall and winter and the humidity is typically low 20-30% RH. Prior to my youtube knowledge of VPD, one strain on my first grow didn't go well because I maintained relatively high nutrient ppm (900 IIRC). I clipped a leaf off and brought it to my hydro store. She thought it was calcium deficiency and of course sold my a supplement. Hindsight 20/20, too much nutrients were being delivered.
So my question is ... why not simply lower the nutrient ppm to accomodate a high VPD? Is relatively high transpiration bad for cannabis?
What I've been experimenting with is to use the recommended PPM from GH but multiply by the forecasted RH for the week.
Example; GH ppm using flora series is 1000-1200 ppm, RH is 25%; so my nutrient mix would be 250-300 ppm. Does this sound reasonable or is there more to benefit from mechanically raising the RH in the tent?
TIA

vostok · Nov 7, 2020

the size of my ppm is related to the size of the plant not the vdp, which in my greenhouse is a direct comparison of water in the air

at 250-300ppm I expect the plant to be 2-4 inches tall and about 2-3 weeks old ...I grow in organics

good luck

Clutchcargo_1 · Nov 7, 2020

Let me see if I understand. As you're plants grow, the RH in your greenhouse goes up? That would make sense and supports my assertion. What is the RH in your greenhouse as the plants grow?

JoeBlow5823 · Nov 7, 2020

Plants should grow fine at 25% RH. RH doesnt really have much to do with nutrients......

Clutchcargo_1 · Nov 7, 2020

JoeBlow5823 said:
Plants should grow fine at 25% RH. RH doesnt really have much to do with nutrients......

That can't be true.
Water is the only thing transpiring from the plant leaving the nutrients in the plant.
At 90% RH, transpiration and VDP is much less. With that, nutrient ppm needs to be higher because the plant is not absorbing as much nutrients.
At 25% RH, transpiration is much higher so to get the same level of nutrients to the plant, the ppm would need to be lower simply because the plant is pulling in a lot more nutrients and water.

Nizza · Nov 7, 2020

when its dry out you need more water , when its humid out you need less water

JoeBlow5823 · Nov 7, 2020

I mean yeah but you dont translate 25% rh into using 25% of the nutes.... whether your at 25% or 50%, i doubt there is that much of a difference in how much water they go through. Might make a 5-10% difference.

Clutchcargo_1 · Nov 7, 2020

I agree. My formula might need some tweaking but I think you get my point in that the transpiration is a conveyor belt for nutes. The higher the humidity the slower the conveyor belt goes but the rate of nute consumption needs to be the same regardless of RH. The only lever that we have is to change the ppms to accommodate the changes in RH.

BIGBALLSJOE · Nov 7, 2020

yes EC is linked to vpd .. more the air is dry/hot more they will use water and less nutrient.
1.3mS i usually enough
if you're in the sweet spot of vpd, you will usually feed more, 1.6 1.7 mS

.Smoke · Nov 7, 2020

Clutchcargo_1 said:
I grow in my basement in the late fall and winter and the humidity is typically low 20-30% RH. Prior to my youtube knowledge of VPD, one strain on my first grow didn't go well because I maintained relatively high nutrient ppm (900 IIRC). I clipped a leaf off and brought it to my hydro store. She thought it was calcium deficiency and of course sold my a supplement. Hindsight 20/20, too much nutrients were being delivered.
So my question is ... why not simply lower the nutrient ppm to accomodate a high VPD? Is relatively high transpiration bad for cannabis?
What I've been experimenting with is to use the recommended PPM from GH but multiply by the forecasted RH for the week.
Example; GH ppm using flora series is 1000-1200 ppm, RH is 25%; so my nutrient mix would be 250-300 ppm. Does this sound reasonable or is there more to benefit from mechanically raising the RH in the tent?
TIA

GH trio/Calimagic sweet spot is .8-1.0EC when vpd is correct for hydro.

Try using a 3-3-1-6ml ratio for RO water. (Calmag/micro/grow/bloom)

You won't be disappointed, or have to worry about burns @ a lower RH %.

As if! · Nov 7, 2020

.Smoke said:
GH trio/Calimagic sweet spot is .8-1.0EC when vpd is correct for hydro.
View attachment 4736513
Try using a 3-3-1-6ml ratio for RO water. (Calmag/micro/grow/bloom)

You won't be disappointed, or have to worry about burns @ a lower RH %.

hey smoke. For 3-3-1-6ml, what stages of growth is that for? I have ro water and growing in coco.

also, that ratio doesnt hold near to true in the chart above?

.Smoke · Nov 7, 2020

As if! said:
hey smoke. For 3-3-1-6ml, what stages of growth is that for? I have ro water and growing in coco.

also, that ratio doesnt hold near to true in the chart above?

I use that all through flower.
The measurements aren't exact. 1/2tsp= roughly 2.5ml.
I've just took the chart as a base reference to start with and tweaked levels to what works for me.

TintEastwood · Nov 7, 2020

It's a symphony...

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BBQtoast · Nov 7, 2020

Clutchcargo_1 said:
I agree. My formula might need some tweaking but I think you get my point in that the transpiration is a conveyor belt for nutes. The higher the humidity the slower the conveyor belt goes but the rate of nute consumption needs to be the same regardless of RH. The only lever that we have is to change the ppms to accommodate the changes in RH.

The higher the humidity the wider stomata open and more densely grow making the same water use in lots of different humidity.

Your point is flawed, it's why one strain grown in two separate humidities look the same.

JoeBlow5823 · Nov 7, 2020

BBQtoast said:
The higher the humidity the wider stomata open and more densely grow making the same water use in lots of different humidity.

Your point is flawed, it's why one strain grown in two separate humidities look the same.

Agreed but the main point is that 25% vs 60% humidity will use pretty similar amounts of water as long as you have adequate air movement. So many variables. Temp. Light intensity. Air movement. The position of the moon in coordination to Orion's Belt. And lastly, humidity. The only time i'd ever worry about humidity is if its to high or if its uncomfortably low for my own taste. Weed grows just fine in the desert. Not worried about it.

BBQtoast · Nov 7, 2020

JoeBlow5823 said:
Agreed but the main point is that 25% vs 60% humidity will use pretty similar amounts of water as long as you have adequate air movement. So many variables. Temp. Light intensity. Air movement. The position of the moon in coordination to Orion's Belt. And lastly, humidity. The only time i'd ever worry about humidity is if its to high or if its uncomfortably low for my own taste. Weed grows just fine in the desert. Not worried about it.

Would assume in a well ventilated negative pressure tent humidity means very little if anything as well.

JoeBlow5823 · Nov 7, 2020

BBQtoast said:
Would assume in a well ventilated negative pressure tent humidity means very little if anything as well.

Exactly.

SpideyManDan · Nov 8, 2020

TintEastwood said:
It's a symphony...

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Wow very informative.

So RH affects transpiration which in turn would affect nutrient uptake. Now i dont know if the percentages OP is throwing around is accurate, but they were right about it afffecting nutrients and eventually photosynthesis.

Just like our bodies the plant is in a continuous battle for balance while in its environment.

TintEastwood · Nov 8, 2020

SpideyManDan said:
Wow very informative.

So RH affects transpiration which in turn would affect nutrient uptake. Now i dont know if the percentages OP is throwing around is accurate, but they were right about it afffecting nutrients and eventually photosynthesis.

Just like our bodies the plant is in a continuous battle for balance while in its environment.

Indeed.
The sitch is only compounded and accelerated when one supplements Co2.

blastinbob · Jul 17, 2021

This article explains it and has couple of scales https://www.cannabissciencetech.com/view/understanding-vpd-and-transpiration-rates-cannabis-cultivation-operations

VPD vs. PPM

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