Download Otter for your meeting notes

GrowCast - Beyond VPD, the Soil Plant Air Continuum | Otter.ai

GrowCast - Beyond VPD, the Soil Plant Air Continuum

Jordan River57min

Jordan River

00:00

Greetings, growers worldwide. Jordan River here back with more grow cast, live long and prosper. Today we have Nick from rooted leaf back on the line. That's right. Nick was at cult cup. He was speaking up there, of course, rootedleaf.com code grow cast for my favorite nutrients around. And today's episode is a very interesting one. We're stepping away from soil, fertilizer, elements, and instead going a little bit deeper into some climate science, this is a very cool episode where Nick introduces a whole new concept. I know you're gonna love it, but before we jump into it, shout out to AC infinity. That's right. Acinity.com, code grow, cast one five, or go to Amazon and use grow. Cast one zero. Now you can get everything you need up at AC infinity.com they have whole Grow Tent kits, just one click ready to go. You need to know new two by two. You need a new propagation kit. You need new four by four for flowering. Just go and hit it up. They've got their awesome and affordable board style lights. They've got their heavy duty super, super, powerful bar style lights and everything else you needed to grow, tents, fans, filters, stakes, even everything that you need. It's up there. It's AC infinity.com. Use code, grow, cast one five on the website. And if you order from Amazon, use code, grow, cast one zero on Amazon, and that'll get you savings, lesser savings, on everything in your cart. That's AC infinity. So they've been a supporter for a long time. We love AC infinity. Go and check it out. Grow cast one five is the code on the website. And grow cast one zero on Amazon. That's acinity.com code. Grow cast one five. Thank you to AC infinity. All right, let's get into it with Nick thanks for listening and enjoy the show. Hello, podcast listeners, you are now listening to grow cast. I'm your host, Jordan River, and I want to thank you for tuning in again today. We are back. That's right. We've got a long time guest, favorite guest, but before we get into that, make sure that you share this show. That's right. Tell a friend. Tell a grower, or better yet, turn a smoker into a grower. That's the best way you can help us on our mission of overgrow. Check out growcast.com. The brand new website. There, you'll find a membership in the seeds and the classes and everything you're looking for it is up there. Special. Thank you to the members of growcast. You guys make this possible. As I said, we've got a fan favorite back on the line. You know him. You love him. Nick from rooted leaf is here to continue our series of deep dives as we move away from things like nutrients and soil fertilizer nutrients into more complex subjects. I'm very excited about today's episode, and Nick is here with us now. What's up? Nick, how are you doing? Man,

Unknown Speaker

02:33

Hey, Jordan, I'm doing pretty well. Man, how are

Jordan River

02:35

you excellent? Super Excellent. Not as good as I was a couple of weekends ago when I was at Colt cup with you and the whole rooted leaf team, thank you so much, dude, for supporting and sponsoring the cup, for coming out, for hanging out and smoking with everybody. Awesome education, awesome camaraderie. Just want to shout you out here on the show. It was cool you guys to do that. You know, rooted leaf support and community. We really appreciate you, man.

Speaker 1

02:59

We really appreciate you guys, too. It was a fantastic opportunity for us to go out there and to meet everybody. It was great to hang out with everybody that we had a chance to chat with. And you know, for the people that listened to the presentation, I'm glad you guys got a lot of useful information out of it. So, yeah, it was definitely an experience of a lifetime, and I'm looking forward to doing it again next year.

Jordan River

03:19

Well, listen, we're going to improve a lot. The event was big, definitely a few learning curves, but next year is gonna be 100 times better. You made it awesome this last year, my friend, this last weekend, I should say, and what did you think of the Grow cast family? Was it? Was it interesting seeing everybody together? Yeah.

Speaker 1

03:36

I mean, it's an incredible community. You know, have an opportunity to work with a lot of individuals and work with a lot of companies on a larger scale all across the US. But there's something special about the Grow cast community, and I love being a part of it. I love supporting it, and I'm very grateful that we've been able to meet as many cool people as we have and make as many good friends as we have, yourself included. So it definitely is something special. I would encourage everybody to check out. Grow cast. The Discord is awesome. Everybody's super nice, and it's just the best place that I found. I think it's a real gem.

Jordan River

04:08

Thank you, man, I listen. Your tolerance is very impressive. You know, like, I these cups always get me so fried by the end of the day, you just hang in there. Man, where does that? I don't see you consuming a ton like you. I know you work a lot, like, where does that tolerance come from? You always just have, like, a stable mind or what?

Speaker 1

04:27

Yeah, yeah, nutrition and diet are very important to me, and I find that those, like, if you treat cannabinoids, like dietary supplements, they can really fit into a larger, sort of, like, nutrient profile, if you will. You know, I kind of take a plant centric approach when we're talking about like NPK fertilizers and calcium and magnesium and all that stuff. But, you know, it's certainly true that you can view certain compounds that are produced by plants as being nutritional. I think cannabinoids, even though they're they can be quite powerful and very strong. Have very strong psychedelic effects. I also feel like they can, they're quite moldable in the sense that you can. To take that experience and shape it into something that you want it to be. So I found a way to make cannabinoids work for me, and they don't really seem to overwhelm me even considerably high doses. Let's

Jordan River

05:12

just say that's awesome, man. It was a blast real quick before we do get into today's subject, which is really, really interesting. You guys, you're gonna want to stick around for this one, but I did want to shout out a few of the competitors in the Colt cup really impressed with this year. Less submissions overall, away, higher quality. Shout out to both the competitors who made the brownies. I always dig into edibles first because they are fresh. Both of those brownies were incredible. Flower sample number 20, I want to shout you out on air. Flower sample number 20. You guys are going to all see pictures of this, by the way, they'll be up at the Grow cast, Instagram, at grow cast, so don't miss those posts. Flower sample number 48 and flower sample number 39 haven't tried them all, but those are a few I'm impressed with. So wanted to shout that out on air real quick before we get into this awesome episode. So Nick, we're kind of in the middle of a watering arc with the Grow cast members. It's so interesting because watering, it really plays into a lot of what you're saying today. But if you think about it, the act of hand watering, specifically, is the simplest act to perform in the garden. It's the easiest thing to explain, as opposed to topping or defoliating, it's the simplest act to perform in the garden. It's the hardest act to master. Meaning, yeah, it's harder to explain topping. But once you got it, you got it. Now you're, you know what I mean, you're all the way there, not watering. Takes years, decades, to really understand how to perfectly hand water, and it's probably the single most important thing when it comes to health of a plant. So really, it's an interesting subject, and we have been doing some episodes around it, and that really plays into today. But we tend to stop short when we think about watering, water goes into the soil, and then we don't think about much after that, to be honest, we kind of just stand around and sit on our thumbs and wait for the dry back. So today I want to talk to you about about soil moisture and about VPD and this continuum. Well,

Speaker 1

07:13

thank you. I appreciate it, and it's something that we've hinted at in the past when we've talked kind of briefly about topics related to watering, or the efficiency of watering. I may have brought up certain concepts, particularly the episode that we did on potassium. I think that I was stringing together, or at least weaving the foundation for this larger and more complex topic that we're going to get into today. And everybody loves acronyms. Everybody's familiar with BPD, so we decided to call this one the Spock, right?

Jordan River

07:41

That's fantastic. Yes, SP AC, soil, plant, air, continuum, or the Spock, live long and prosper. That's so funny that you just said that everybody cannabis loves acronyms, so here we go. So I want to talk about this continuum, and let's start from the hottest acronym of the past couple of years, VPD. Now I get to nickname Old Man VPD in my community, because I'll often say, you know, we tend to focus on this single metric quite a bit. It's an important one. But I like that, as you say, it's part of a larger kind of picture. Can I give you my understanding of VPD? And then you tell me if, if I'm correct, or if I, you know, should be corrected, and let you can elaborate on it. Oh, definitely. Yeah, that's a great place to start. So my understanding of VPD, or vapor pressure deficit, in layman's terms, is how thirsty the air is. And my understanding is that pressure deficit comes from, you know, nature abhors a vacuum, right? So if, if, if there's not a lot of moisture in the air, it's going to be quicker to pull moisture in to the air. So if the VPD means that the air is very, very thirsty, your plants are going to therefore dry out faster in that environment as the as the air kind of sucks the moisture out of it. That was my understanding of VPD, and that it is just a simple metric between temperature and humidity. Humidity telling you how much water is in the air, which is important, but then also temperature, which plays into as we all know, how fast things dry, right? So you kind of combine those to get this thirstiness metric is that, is that at least close to the correct

Speaker 1

09:22

definition? Yeah. I mean, that's definitely along the right lines, and the layman's explanation that definitely fits into the more complex scientific one. I guess you could think about VPD, like, think about when we're talking about VPD, really, what we're looking at here is a difference, or a delta. There's a difference between how much moisture the air can possibly hold, and then how much air it's currently holding. Those two variables will always be present at the same time, and those what go into defining VPD, right? So, when you have a when you're so the other thing to consider about VPD is the scales that we use to measure VPD. We've got. Variables, like you mentioned, we've got temperature and we've got humidity, that factor in to VPD as a measurement, but then we also have a scale that we use, right we as we approach zero on that scale, it indicates that the ability of the air to hold additional moisture is reduced, meaning it's going to be harder for plants to transpire, and there's going to be a little bit of a back pressure created because the air has to be dry in order for the plants to effectively transpire. Like you mentioned, they're trying to effectively the air being really dry creates a wick wicking effect on the plants, right? So as we approach zero on this VPD scale, really what we're doing is creating back pressure in the plants, because they need to move that water somewhere, and if the air can't hold it, they're going to have a really hard time moving it out into the air. It's just not going to be possible once we're at zero, because that means that the saturation vapor pressure is equivalent to the actual vapor pressure, and there's no ability for that air to hold additional moisture. So what ends up happening is the plants have to hold on to it.

Jordan River

11:01

So in a gross scenario, this means, like, theoretically, it's very, very wet and it's very, very cold, almost impossibly So, like you would never see this, to the point where the plant can't even transpire. It's basically being smothered or suffocated by the air around it.

Speaker 1

11:15

Correct? Yes, yes. And the important thing to understand here is that as we approach zero, we become wetter, meaning that the leaf will transpire less and less the closer we are to zero, makes sense, and the further away we move from zero, and we're moving up in the positive direction. By the way, we're going from zero to one and then from one to two, so on and so forth. So as we move away from zero in the positive direction, it means that the air is becoming more and more dry and that the vapor pressure deficit is increasing, meaning that there's a larger pull on the plants to transpire more water. So now that you understand the basics of this VPD system, we've got this, like, push pull thing going on, basically, right? We have, as we approach zero, we have the air being wetter, and the water is being kind of push back into the plants. There's a back pressure. But as the air gets drier and we move away from zero, we move up towards 12345, so on and so forth, the air becomes drier, and now all of a sudden we have a stronger waking effect on the plants, meaning they'll dry out a lot faster. And if we hit a particular VPD, and we don't have the soil variables dialed in, which we'll get into here in a second. It can create some kind of stress for the plants. They dry out, they become really brittle. The leaf edges may start to crinkle, and they actually crack. They become very brittle. So these are the types of things that we want to avoid. It's too dry. This

Jordan River

12:38

is an evaporative pressure. I've heard you describe it as before.

Speaker 1

12:43

Yeah, yeah. So we we want to think about positive pressure with VPD, because we're going away from zero in the positive direction, and it's evaporative, meaning that we don't have typically, generally speaking, we're not looking at how vapor moves in the air as though it's condensed liquid form, right? We're dealing with evaporation of liquid phase water into a gas phase, right? So it's a positive pressure, and it's evaporative pressure, and as we go away from zero, becomes the pressure increases. This is going to be an important thing for us to contemplate as we move into the soil dynamics, what happens in the soil, because it's actually kind of the opposite. You know, above ground, we've got this positive vapor driven evaporative system, but in the soil, it's actually quite the opposite. We have a negative pressure regime there. We have bound water, and we have water acting as a cohesive system. And there's something about pressures that will change too. But yeah, we'll get into that here in a

Jordan River

13:45

sec. Yeah. Well, let's talk about that. How should we be thinking about this? I think the

Speaker 1

13:49

easiest way for people to understand what we're talking about here is that VPD is just one variable in this larger soil, plant, air continuum. It starts in the soil, it works through the plant, it ends up in the air. This is the continuum that we're trying to build out. And the problem with focusing just on VPD is that you're only focusing on two variables that are measured in the air. You have temperature and you have humidity. But for those of you who have ever switched from HPS lights to LED lights, you may have noticed that the ambient humidity in the room increases as a result of swapping the lights out. So as it turns out, even VPD, there is a third hidden variable, which is light intensity. How intense is the light? And if you have more infrared energy, it actually directly modifies the ability of the air to hold additional moisture.

Jordan River

14:38

My God, but Nick, this is why I'm old man VPD, though you just proved you just proved me right without without knowing it, which is, people focus so much on that one piece, and now lighting has changed so much. I feel like that. I feel like the VPD number doesn't take in so much into account. And like you said, if we're focused on air and evaporative pressure and all this, of course. Intensity would play a role. And that chart, as great as it is, doesn't take that into account at all. That's so fascinating. Yeah,

Speaker 1

15:07

yeah. And one of the, one of the things that I want to try to stress to people is like, you have, you have something going on in the air that's separate from the plant biology, and you also have something going on in the plant biology. And with the plant biology, the HPS lights that emit infrared radiation, they create a there's an effect that happens with the VPD gradient that directly surrounds the leaf. I'm not talking about your VPD. You have a sensor hanging, you know, somewhere in the air. I'm talking about, what is the plant sensing, right? So, if you have infrared energy coming in, it can modify by temporary lower, lowering that local relative humidity, just in that super local, micro environment of that one leaf, right? So the moisture can move rapidly away from the plant into the environment at that with that kind of infrared energy being present. But when we swap to LEDs and we cut that infrared energy out, that means the local environment directly surrounding the leaf surface is now all of a sudden, significantly different, because there's no infrared energy to help assist modify the VPD gradient around that leaf in that micro environment. Jeez, man, it causes moisture to remain closer to the leaves. I mean, put it this way, if I have a glass of water and I set it out on a hot summer day, what's going to happen to the water? Sun is going to cook it right off. Well, what if I'm in Alaska and it's not let's just say it's 45 degrees and it's overcast. Maybe not Alaska. Let's say I'm in Washington State. Okay, 45 degrees, overcast, gray skies. What happens to that same glass of water evaporates much more slowly? Oh, interesting. Interesting. Okay, so imagine instead of a glass of water, we have a leaf or a leaf surface or some photosynthetic interface here, really, what we're trying to do is look at how the light quality, the various frequencies of light, including infrared, can affect the way that moisture moves through a system, whether that system is just a glass of water or that system is a vacuole and a chloroplast and a whole leaf cell is very complex, with the stomata opening and closing and all this wonderful stuff. The plant system is obviously more complex, but it still takes advantage of very naturally occurring phenomena, wow, which is evaporation is not driven by biology. So why would plants spend energy doing it?

Jordan River

17:15

You know, I just want to take a moment to appreciate how how many people walk up to me and say how much they love these episodes and these deep dives. Man, you really, you really make us think differently about plants. And I know even recently, we've had discussions off air where you you go deep on this stuff and you say, I'm really relating this stuff to quantum physics. And when you break it down to things that you've said to me before, like plants are a particle decelerator that catches energy, and they're just a system that moves water, and it's the way that you describe it makes me think very, very differently about this. So that was mind blowing, what you just said, and it makes a lot of sense. I would have never guessed that spectrum would would be significant. But of course, Nick in a natural system, nothing is insignificant. Everything belongs, right?

Speaker 1

18:04

Yeah, yeah. And plants are really trying to string everything together. I mean, even green light, which people have thought for the longest time to be useless. Green light, interestingly enough, has some very remarkable effects on plant physiology, and it actually can drive CO two fixation as well. We need to do a whole episode on quantum yield and specific frequencies and how much energy plants can utilize. And we'll kind of start unpacking this, because I mentioned this at the at the presentation that I did. You know over the past couple of years, you and I have done some awesome episodes where we look at individual micro and macronutrient elements and building off of that as a base, you know, really this year, and leading into 2026 we're going to continue to build off of those basic understandings. And we're going to start looking at some of these more complex topics, like the the Spock right, the soil, plant, air continuum, and how VPD is just a tiny, tiny little piece of the larger Spock right. And then, on the flip side too, we'll look specifically at maybe one of the episodes in the future. We'll look at what exactly 450 nanometers and 680 nanometers is, and what are all of the other pigments that can absorb light, and what is the light energy used for so I want to try to unpack medium to advanced level concepts for people, because you know, the episodes we did with the individual macronutrients. They really laid a solid foundation for these topics, and uncomfortable getting into the more advanced applications. Now that we've established that that base

Jordan River

19:33

come and join the greatest community in cannabis cultivation. It's grow cast membership, and it's waiting for you right now grow cast.com/membership there are so many perks that come with being a member. Not only do you get members only discounts on items you use every day from growers you know and trust and grow cast seed CO, our genetics Company, but you also get access to 24/7 garden advice. If you have any struggles in your garden, don't go to Google. Don't go to Facebook. You're going. Drive yourself crazy. Come to us, we will take care of you. It's insurance for your grow. And if you're already cruising along and you don't need any help, you still want to join, because the best part of grow cast membership is the community. Jump into our thriving worldwide community. Connect with gardeners in your area, attend meetups, hit up the trading bazaar. There is so much going on in the Grow cast member discord every single day. And I love connecting growers from all over the globe. Come and check us out. There's no community like it. We focus on uplifting each other as growers, instead of focusing on the differences and bickering. That's not what we do in Grow cast membership. Come and check out our vibe. You'll see what I mean right away. It's all waiting for you at grow cast.com/membership What are you waiting for? Go ahead and jump in now. Before we start our next Grand Final hunt, you're not going to want to miss it. I'll see you there. Grow cast.com/membership, okay, so let's get into numbers here. Let's back to this idea of the Spock. And we've, we've got a good grasp of VPD. Do you want to pick up there? Or do you want to talk about SWP, this idea of soil water potential, and how that plays into this continuum?

Speaker 1

21:06

So this concept is basically like, the soil can hold on to a certain amount of water, right? And depending on what you put in that soil, like, let's say you put more biochar in, well, now all of a sudden, it can hold more water. But if you put more rock wool in, and I'm not saying you should put rock pool in the soil, but just humor me here, like if someone was, you know, adding rock hold that that would decrease the overall capacity of the soil to hold or to retain onto moisture. So sure, it makes sense. There's this measurement or this concept that is going to be useful in describing how strongly the soil can hold on to water, and there's going to be some quantified metrics that we can use, like the field capacity. For example, there's something called the permanent wilting point as well, and the difference between the two of them basically defines what is the available water capacity for the soil. There's also something really, really interesting here that I want to unpack. I'm going to mention it right now, but we're going to unpack this. And this is that in terms of the units of measurements, when we measure VPD, you guys have maybe seen kPa. You got the lowercase k, you got the uppercase P, and then the lowercase A as well. That's kilopascals. It's a it's a way to measure pressure. Now, in the soil, you also have a way of measuring pressure. But as it turns out, instead of kilopascals, we have something called megapascals. And this is, this is, this is the aha moment. This is the thing that should start clicking for people, is that 1000 kilopascals is one megapascal. So look at that intense and extreme difference in pressures, just in terms of how we measure pressure in the soil. It's 1000 times greater the scale that we're using is 1000 times greater than the scale that we use to measure VPD. This is a massive difference in pressure regimes. You have a ton of pressure in the soil and not so much in the air.

Jordan River

22:50

That makes sense. So, so essentially, the soil is much more of an intense area where it's building up, almost building up like a rocket, right?

Speaker 1

22:58

Yeah, yeah. And remember earlier, I was talking about how VPD is measured in the positive direction, and we move evaporatively away from zero, meaning the closer we are to zero, the wetter the leaf surfaces. But we move away in the in the positive direction, getting drier and drier in the soil. It's actually kind of the opposite. As we approach to zero, we're actually approaching what's called the field capacity. So this is the measurement of how much water can the soil hold. If you saturate your soil, 100% of its maximum capacity. Technically, the water wants to jump out of the soil at that point, right? There's no pressure holding it into the soil. It wants to leach out of the soil, because there's so much, right? So now we're basically at zero, or as close to zero as we can possibly get and because this is not evaporative pressure, right? Vapor moves upward in the air. But what happens when we water the soil? It doesn't move up, it moves down. Because we have gravity acting on water in a cohesive manner, not in an evaporative manner, like we do in the air. So we have cohesive draining of water through soils. We have the loss of water. So when we saturate the field capacity, we're basically at zero. As the soil starts to get drier and drier, we're actually moving in the negative direction, away from zero. We're moving negative 0.1 negative 0.2 negative 0.3 and as we move away from zero in the negative direction, the soil becomes drier. We have negative pressure regime here. There's something called the permanent wilting point, which, think about it this way, like the permanent wilting point is the minimum water content possible to achieve in the soil before the plants start to experience wilt. So this is kind of like the lower bound of like, hey, there's a certain amount of moisture in the soil, but what's the minimum amount of moisture that the soil can hold? You know, a lot of you have experienced dry backs that have been too intense, and you may have noticed there's something weird that happens in the soil. It becomes a little bit hydrophobic, right? Why is that? Why is that? And what happens to the plants too, if the soil gets really, really dry, I wonder if. Pressure gradient can get reversed by accident. I wonder if the soil can get so dry that it starts to wick water out of the plants. You know what I mean? Because we're looking at VPD in the kilo Pascals being able to do this in the leaf surface. We have 1000 times greater pressure regime present, a negative pressure regime present in the soil. What's to say that if the soil becomes really, really dry, that it won't suck the moisture out of the plants, it's a ton of pressure. Man, let's,

Jordan River

25:28

let's dive deeper on on this subject. How does this translate to us as home growers? So what are the specific kind of variables that we might run into where this would play a role? Well,

Speaker 1

25:41

I'm a big fan of keeping plants on the drier side of the VPD curve. Honestly, I think the vapor pressure deficit should be higher, keeping plants closer to 40% humidity in flowering when the temperatures are around 78 degrees is way outside of the VPD curve. If you just look at it online, I forget what the exact number is, but it's, it's so far outside of the curve that most people wouldn't ever you know, you wouldn't look at that curve and think, right? I should end up here at 40% but what I'll say is this, now that we understand how air can move moisture from the soil through the plants, right? We have the we have the moisture starting off in the soil, we've established that there's a particular pressure gradient that's created in the soil. The plants themselves are kind of in the middle, right? They're what they're trying to do is move water from an area of high concentration to an area of low concentration. This is just naturally how things are going to work in nature. Even without the plants being present, they're just diffusion. But it just so happens that if we can keep our air dry and if we can keep our soil wet, this is the best way to get the water pump to start pumping water, we need to have the soil be wet and the air be dry, so that the plants can lift the water out of the soil, which is a much higher pressure gradient, obviously, because we water several pounds of water into the soil, but we would never apply five gallons of foliar spray on a plant, you know what I mean? So and even if we could, the air can't actually hold that much moisture, right? Because the soil itself can hold on to moisture. I

Jordan River

27:06

don't think we give enough credit to what you just said. Imagine if you spill that all over your floor instead how much water that is, yeah, and it just goes right into the pot. And all that weight and all that, like you said, tremendous amount of pressure that's applied in there. That's really fascinating,

Speaker 1

27:22

yeah, and it just so happens that, because the air has a positive evaporative pressure regime, the plants are like, oh my goodness, this is great. I have wet soil, and I need to lift it up into the air where it naturally wants to get wicked out. I think with transpiration in particular, there are certain things that occur in nature, right in the air. We have moisture lifting up off of the leaf surface, driven by evaporation. This is a positive pressure in the soil that doesn't happen. We have water collecting. We have water condensing. We have water coming together and forming giant pools and puddles and hanging out in the soil below ground, where there's by the way, there's no thermal radiation as well, like the infrared energy band that we were talking about earlier, and how HPS lights can help plants burn some of that additional moisture off that doesn't occur in the soils because the soils are underground, there's no infrared light energy that wait Well, I mean, there's some, but it doesn't work its way far enough to have the same evaporative pressure. Now, having said that, if you have really bad soil chemistry, if you have really poor soil chemistry, and you live in a really hot and dry area, then it is true that the soil itself may also experience evaporative pressures. And you may have some some farmers may have noticed this. If they apply, like urea fertilizers, for example, and it's really, really dry and hot outside, that will evaporate out of the soil too. So there, there's, there's something to be said about that. But generally speaking, our pressure regime here is that we're moving from the soil into the air, and the plants are kind of in the middle of that. So by optimizing your environment, you can really get the plants to go through wet, dry cycles very efficiently. This will decrease disease pressures. This will increase your yields and quality, and it will remove limiting factors for growth too. I think it's important if you live in a humid environment and you can't bring the humidity to a spot in there where it's really, really dry, I think it's a good idea to use a type of growing medium that does not have such a high field capacity. In other words, if you're living in a really humid environment, maybe you would benefit from using cocoa or rock wool as opposed to peat. However, if you live in an environment that's a hot, dry desert, like if you're in Bend Oregon, or if you're anywhere in New Mexico or Arizona, Southern California, you would probably do much better having a wetter soil, because the air is going to be drier anyways, and this pressure gradient that's created is to the benefit of the plants. If they can rely on the soil having a higher field capacity, if they can store more moisture, the plants will pump that moisture out into the drier air.

Jordan River

29:53

That's such a good point, because what ends up happening is it gets away from you, and if you're ready for it, that's one thing. So for instance. You say, Coco, this thing is perfect for high frequency fertigation to begin with. But if you bring it down into a hot, dry environment, you better be ready with some high frequency you know what I mean? It's almost like you have to keep up with that. Yeah.

Speaker 1

30:13

And what ends up happening is there's these little micro pressures that are created, you know? Like, what we've done basically, is we've established, like, okay, when we're measuring pressure in the soil, it's megapascals. When we're measuring pressure in the air, it's kilopascals. Big pressure difference, right there too. Well, what ends up happening in the soil too, is that there's a certain point at which the pressure differential dovetails and you end up breaking the plant, basically. And what I mean by that is, if the air is really dry and the soil is also really dry, the plant is going to get stressed. On both ends of the equation, right? The air is going to desiccate it. It's going to cause it to lose too much moisture. And the plants will automatically, as a direct result of that, close their stomata, which decreases photosynthesis. And then on the flip side, too, if the roots are really, really dry and there's a little bit of a vacuum pressure that's generated against them. They'll fully shut that down the stomata, because they can't afford to have the transpiration pump working when the pump has no water to pull on from the soil, right? The soil is actually doing the reverse. So the reverse of the pressure gradient is what you want to avoid. That will break the plants, and that will also break the soil too. If you get the soil so dry that it starts wicking moisture back out of the plants, and you have dry air. That is a recipe for disaster. I don't understand why people go through the painstaking process of needing to water the plants 10 to 15 times a day, or doing this funny crop steering stuff like it, just putting the plant in the wrong kind of stress. Or they're trying to form a continuum. Here is what plants are trying to do. They want to form a steady, predictable, reliable continuum. They want to have wet soil in dry air, because that's how it works in nature. If I put a glass of water out again in front of a, you know, in front of the sun on a hot summer day, that water is going to evaporate and plants are trying to do exactly the same thing. I

Jordan River

31:56

love it, man. I mean, I think that's a really, really interesting way to look at it. And if it's almost impossible to achieve as as one person, you need automation for certain styles of growing. And when you put it like that, I can imagine it that is that that wilting effect looks like the result of the phenomenon you just described. That makes perfect sense in my head. If you could kind of like, you know, X ray vision into the soil and watch that thing happen in real time. It's interesting. You said a lower humidity and flower because we talk about that a lot, and usually in the context of, oh, let's avoid mold and mildew, like you said, lower disease pressure and stuff like that. But also what it does to transpiration in general, I think that people get away with a higher humidity earlier on in the plant's life cycle, obviously, if it's coming from like a cutting, and then in veg, people run a little bit more humid. They run a little bit more warm. That's cool, that's fine. But then once you get into flower, man, I like to dry it out a little bit. I would agree with that. And I like to turn down those temperatures late, late in flower. And it's interesting looking at cannabis as an annual, which it is. I know, technically, you can keep them alive for long times, but cannabis acts as an annual in so many ways. And the drinking habits of the cannabis plant before harvest is really interesting. Nick when they're first drinking up for bulk, and then they kind of stop bulking, and they start senescing a little bit, and you notice that the earth box ain't, ain't drying as quick as it used to. That's that nothing else has changed, just the biology of the plant, the genetics of the plant signaling these things.

Speaker 1

33:34

Yeah, yeah, definitely. Um, like you mentioned, when the plants are sort of like getting into flowering, they'll start ramping up their water intake, and then right around week three to five, they really start packing on a lot of water. And you know, you can get that bulking phase happening for, you know, five to seven weeks in total, depending on the genetics, of course, but, but really, what we're looking at here is something going on within the plants that drives them to take up a large amount of water, larger the volume of water. And the reason for that specifically, we've gone through this on some of our episodes in the past, but terpenes and cannabinoids are very highly energy intensive molecules to produce. They are not cheap. They are not inexpensive. These are luxury goods. These are like the high end, ultra premium metabolites that are found in nature. This is why you can't shade grow your way to above 35% total cannabinoids and above 5% terpenes on dry flower. You can't do it with the plant that's off in the corner, getting minimal sunlight, only getting watered once a week, growing maybe three to four inches, you know, per week, that's not going to be the plant that performs well. And the reason, again, specifically, is this all comes down to thermodynamics. Ultimately, this is just a transfer of energy. There's massive amount of energy being transferred from the sun through the plants and being sunk into the terminal forms of these metabolites. What a cannabinoid is literally represents the power of the sun. What a terpene is also literally represents the power of. The sun, right? So when we have more power coming in to the plants, when our light intensity is higher, the plants have a drive to produce more of these compounds, right? But as we've looked at so far, the ability of the air to hold moisture is one of the critical defining factors in this sort of like Spark air fuel equation to make terpenes and cannabinoids, right? We would use Spark air and fuel to make torque and horsepower in a car engine. But in a plant, the spark is the light intensity. The air is obviously air quality is important too, but it's the CO two, and then the fuel is the water right? Plants do take up water molecules. They split those molecules of water apart, and they use that, the resulting protons and the electrons to do the work that's required to make these cannabinoids. So everything that we want to focus on in flowering should revolve around getting plants to produce more terpenes and to produce more cannabinoids. One of the ways to do that most one of the most straightforward ways to do that, is to focus on getting your plants to drink more water. They need to drink more water if you want them to produce more terpenes and more cannabinoids. What's the best way to get them to get them to drink more water, put more light intensity on them and make the air drier? Right? You can increase the light intensity, that'll increase the transpiration rates in the plants, but even if you just drop the humidity in the room, it'll force the plants to transpire the water faster, because there's a bigger pressure deficit between the bound water that's present in the soil versus the evaporative water that's present in the area. All that tension, evaporative pressure, if you lower the humidity, it will make the plants automatically drink faster, and you can shine a higher light intensity on them as well.

Jordan River

36:34

Really good point. You know, another thing to consider is outside of that, that kind of bubble of the soil is the larger bubble of the tent, and you can alter those factors even more with things like circulation turning up or down your exhaust as the air in the tent will collect humidity. You understand what I'm saying. So for instance, if you live in a place that's too dry, you might actually want to have a little less intense ventilation to allow the humidity to build up within your tent, thereby making your air not too thirsty. In layman's terms, yes,

Speaker 1

37:11

that is true, and it's also, it should also be mentioned that there's a, like, there's a, there's there's a, there's an interaction between what's going on in the soil in terms of, okay, how do we measure the ability of the soil to retain water? We're looking at compounds with a certain affinity to hold on to water. Humic substances, for example, biochar, things like that. So in reality, when we're talking about the water that the plants have to access in the soil, it's bound water. It's in a cohesive system, right? It's surface interactions. There's adhesion of the water. There's cohesion of water molecules. There's a little bit of tension present in the soil as well. So effectively, what I'm getting at is, when you bring the soil to field, capacity, that water just just wants to jump out, right? Gravity will drain it. If the roots are there, the roots will automatically get passive water flowing by them, because the moisture just isn't going to be possible to hold in the soil. But eventually, over time, that soil starts to dry up, and it starts to get drier and drier, and now, all of a sudden, it's still holding on to water. It's bound. But because of these surface interactions that occur now there's pressure, the plants actually have to find a way to tap in to this negative pressure, bound water, cohesive system that we're talking about here. They got to tap in and pull it out. Does that make sense? So it's bound in soil particles. The water is right. So this is a negative potential, but the leaves have a positive potential. The air is pulling water away through evaporation, fundamentally different.

Jordan River

38:41

And I'm thinking of all the little things that make a difference in that. Like you said, obviously you have the the plant itself, but what about how well or how little, or how much you defoliate the plant? What about whether or not you have a mulch layer covering the top of your soil? What about it Nick if you're in a fabric pot versus a terra cotta pot versus a plastic pot. These are all things that adjust and effect that continuum. And if you if you have too many compounding factors pushing you in one way or the other, that's where people run into problems. Man, it's not that plastic pots are like, let's just say, solid pots, that there's anything inherently wrong with them. It's just, if you happen to use them in an environment that's also wet and also cold, naturally, and you mulch, and, you know what I mean, these factors compound and steer you off the road eventually, yeah,

Speaker 1

39:31

yeah. And, you know, I'll throw this out there too, if you live in an environment that's a little bit too dry, maybe one of the things that you can do is just get a slightly bigger pot and water it a little bit more, and the natural as the air, as the moisture, evaporates, either through the plant or even directly out of the soil. If it's really hot and really dry, the moisture that's present in that larger medium is going to end up saturating the air, and so you'll end up with this equilibrium of how the soil dries out more naturally, because it'll release moisture to. The air, as opposed to just going through the from the soil, through the plant into the air, via this continuum that we're talking about that that's more along the lines of like this biologically driven continuum, the Spock soil, plant air continuum. There's also a non biological equilibrium effect that can be achieved if you have slightly more soil, you water that larger volume of soil with a larger volume of water, and then the air that's naturally drier is going to pick up the moisture that's in the soil, right? Of course, I'm assuming that you're not just watering in a dark tent with no lights and with 40 degrees Fahrenheit, right? Because then, in that case, the moisture is going to sit there. But if you're actually growing a plant in that tent, that's an example of where this type of approach would actually work out really well. Having the soil hold on to more moisture may actually kind of smooth things out for the plant, the soil water interactions themselves. Like we didn't really get too far into it, but like the type of medium, you know, peat versus cocoa. You mentioned that PEAT holds onto water a lot better, and part of it is just the moisture retention capacity. But there's also a couple of other variables, like capillary action. We mentioned, surface tension, adsorption, cohesion, the aggregate size of the particles, how, how much water, you know, can go into each soil particle? Like, are you dealing with very fine soil that will compact down into a fine dust? Are you dealing with something that has rocky aggregates that's like really bulky and and maybe the water can only get between the cracks of but it can actually fully penetrate and evenly saturate the full so there's so many physical variables present here, the air water interactions that we're talking about with VPD. Those are totally different man, those are vapor diffusion gradients, thermodynamics, like I mentioned earlier, relative humidity differences, the influence that light energy and light intensity and really light quality has, these are entropy driven evaporative processes. That's what causes this phase change from liquid water flowing through the plant to turn into water vapor at the leaf surface and then get lifted off. That doesn't happen with soil. Your VPD

Jordan River

41:59

could be on point, but if you don't consider what's going on in the soil, you have less than half the picture at that point. Perfect example is your temperature and humidity and VPD could all be perfect, even perfect for whatever particular cultivar you have. But if you have that pot of your sitting on a cold concrete ground, the soil temperature is what's going to throw off the Spock. It has nothing to do with the air, right? But it's equally as important, if not, they play into each other so much it's inseparable, is what I'm trying to say. And you really hit it on the head with this one man, that it all starts in the soil, yeah?

Speaker 1

42:36

And really, the continuum starts in the soil. Because even if we're talking about what happens in nature, you know, most plants drink water out of the ground, and yes, it rains and they get wet on the foliar surfaces, but in reality, the roots are the the parts of the bits and pieces of the plant that have become optimized to lift water via hydraulic conductance, up, up and away from the plant, right where the evaporative positive pressure regime can kick in in the air. So yeah. I mean, plants have evolved these very, very specific tools of working with nature like this. And I will say too, as the moisture and the leaf evaporates out and forms clouds, what happens to the clouds is they condense and they rain back, right? They rain back down, and the plants get fed again. So really, it's about the transfer of water that originates in the soil, and you can't measure air quality in terms of VPD, you know, temperature and humidity, without actually looking at what goes on in the soil. And what are some of the properties of the soil. Does it really hold on to a large amount of water, and is it going to be very difficult for the plants to access, or does it hold on to a very low amount of water, like rockwool? Is it going to be easy for the plants to access, because Rockwell has no CEC. It just says, Here you go. But it's very quick to give everything up, which is why you have to water it 65 times a day. There's trade offs, you know

Jordan River

43:50

exactly, and that's why it's not a satisfying answer. But I like to tell people, you know, Nick, you give me your thoughts. But when people come to me and they say, you know, I've been trying to hit my target, VPD, and my plants look sad, and my answer is usually an unsatisfying one, which is, first of all, I'll work with them one on one, on their temperature and humidity and things like that. And basically just say, you need to learn how to read the plants over time. And you can learn how to read the plants over time. When you see that your plants are drooping, they're not praying like they should be like a nice, 45 degree angle, lift, turgid, happy, nice and sturdy. If they're not like that, there's something off. Now, let me be clear. Here at growcast, we have a very, very strict tenant, which is, if you're happy with your garden, that's all that matters. So that's the first thing I'll say. But a lot of people want a perfect garden. A lot of people want plants that pray from beginning to end they have even healthy green leaves with no deficiencies showing from beginning to end. That's kind of the goal of this, right? So for those of you who do concern yourself with that, you have to learn how to read the plant, and there is no singular number that is going to give. You the magic silver bullet to make a happy plant. It's a combination of a lot of different numbers and, more importantly, kind of forgetting the numbers and looking at the plant and saying, Oh, I know this. I know what an over lit plant looks like when it's driving too hard and it's running low on micronutrients, or, Oh, I know this. This is an over watered plant. That's why it's drooping. It's not thirsty. It's actually the opposite. It's over watered. So So I guess Nick my question to you is, like, this is a bigger picture. And what do you recommend to people who are who are trying to dial things in, but seeing the results and saying, I know my plant isn't quite optimal,

Speaker 1

45:37

I would say learn how to play with these variables. You know, at the beginning of this episode we had mentioned and we had talked about how learning how to water the plants is one of the most difficult things that you can do as a grower, and it really takes the longest amount of time. I mean, it's the simplest thing, because it's the first thing you have to do in order to grow a plant, but it's the very last thing that you actually end up figuring out and dialing in. So I think the reason that it takes people a long time to figure out how to water is because they don't understand this soil, plant, air continuum. They don't understand Spock, right? They just need to understand Spock a little bit better, because then everything will make sense. I feel like that's the quantum wormhole for people. Once you understand Spock, you will understand VPD, and you will understand how to learn the optimum way of watering your plants. Oh, let me put it to you this way, if you understand Spock, and let's just say hypothetically, you over water your plants. If you understand Spock, the first thing you're going to do is crank that humidity down and turn the light intensity up just a little bit. Or hypothetically, on the reverse end of the equation, let's say you underwater your plants, and you're concerned that they're going to get too dry too fast. What would you go in and do? You would go in and raise the humidity, allow the air to get wetter, create a little back pressure on the plants? Not a big deal, as long as it doesn't actually slow down metabolism. Plants are very plastic. They have ranges that are optimum for them. They don't have any one distinct number or any precise thing that is accurate, that will always work out best for them. It's always a combination of variables, and the optimum is really just a range. It's a sweet spot. So I love that. Being said, your plants will hang out in an optimum spot if you can learn how to work with them, if you underwater them, you can maybe turn the humidity up a little bit and let the air get wetter, and you don't have to worry so much about light intensity either. That way your plants burn through that moisture a little bit slower, but you're not kicking them out of the equilibrium. The equilibrium is maintained between the pressure differences that are created in the soil being negative, much higher pressure versus in the air being positive and much lower pressure. Those two exist in conjunction in an equilibrium with each other, and that equilibrium has a massive, massive range, if you understand it properly. So I hope people will take that to heart and kind of start, start playing around a little bit and understanding that it's really just the soil, plant, air continuum thing that matters way more than VPD. And as we looked at already VPD is, you know, just VPD by itself is missing a third and very, very important variable, which is the light intensity and light quality. VPD is useless without measuring light intensity and light quality. So why don't we see that third variable there as a cross section of temperature and humidity? The other thing that I want to point out real quick, because I had this come up a couple of times in my head so far, and I don't want to miss out on this thought relative humidity. I don't think people should focus on a percentage of relative humidity as being the important thing. I think when you're measuring humidity, the important thing to do is figure out how much water you're removing. What is the volume of water you're removing? Because if I set let's say I have two rooms that are identical in terms of their percentage humidity. Let's say I'm in week six of flower, 78 degrees. I want those plants to be at 40% relative humidity. So if I just look at the percentage humidity in both rooms, I'll see it set to 40% and that might give me the indication that both rooms are performing exactly the same. I mean, why wouldn't they be right? The humidity is set to the same percentage, but the actual variable that's important to measure is how much water is being pulled out to maintain that 40% humidity. Let's say, in one of those rooms I have a 320 pint dehumidifier, but in the other room I have 5000 pints of dehumidification, right? The 5000 pints of dehumidification, staying at 40% humidity is gonna let me water those plants 10 times as much, right? And if I'm watering plants 10 times as much, and they're going through wet dry cycles either 10 times as fast or 10 times more efficiently. I'm getting 10 times more productivity out of a room that's also set to 40% relative humidity, just like the other one is. It's not about the percentage humidity, it's about how much moisture did you pull out? It seems to

Jordan River

49:37

me like it really combines some of the most important metrics in your grow, which include M bars in your soil, which is the pressure of your soil, but also how that relates to your temperature, your humidity, that combined VPD number and that creates the spot continuum. I love it. Man, it's making sense to me. Nick,

Speaker 1

49:55

yeah, and I think if people understand the basic concepts here, it'll eventually reveal. Itself, because this, this is the way that nature works, is oftentimes more simple than it is complex. And these are the very easy things to understand, like, there's a huge pressure difference here. You know, things move from areas of high concentration to areas of low concentration naturally. So it makes sense why, if the air is really dry compared to the soil, why moisture wouldn't naturally move? And it also makes sense why we see plants doing this. So plants are really just trying to take advantage of naturally occurring phenomena. They're trying to leverage things that occur in nature without a need for them to spend energy on doing it. I mean, if they can set up an environment where the soil stays really wet and the air stays really dry, that's the optimum environment for them to pump water. I mean, there's no pressure. The pressure flows one way, the humidity being really low keeps disease pressures down, because that back pressure effect is not happening. So makes it much more difficult under such a high positive pressure regime. It makes it extremely difficult for molds and mildews and things like that to actually effectively take a hold, because the water pump is pumping under more pressure, if the air is drier, right? Because the pressure deficit is greater, and what's going to fill the pressure deficit is additional moisture being delivered through the plant. So you have an increase of water pressure inside of the plants, and the microbes are trying to get in, and the plants like, Sorry, dude, I have, you know, 100, 100 gallons of water flowing through here, so I won't be able to let you in. But if there's back pressure, if the soil is really dry and the air is really wet, those molds and mills just come flooding in through the stomata. They will just tear through a plant immediately, because there's no pressure differential there anymore. We're breaking the environment. We're breaking the plant and forcing diseases into the plant at that point.

Jordan River

51:39

Wow, man, that is incredible. That that is a that is a mic drop moment. Do you have any final thoughts on this SP, AC, soil, plant, air continuum that you've so generously introduced to us?

Speaker 1

51:52

Yeah, the thing about the humidity that I was just talking about, you know, try to measure humidity in terms of how many pints you you moved out of the room, rather than just what is the percent, you know, meter saying. The reason that this is really important is because if you're in a sealed room, you have no other way of getting moisture out of that room. If you roll in a 50 gallon trash can and you're about to water your garden, and that's a perfectly sealed room, where's that water going to go and how's it going to get there? Right? You water the soil, the plants move that moisture through them and out into the air, right? This is the soil plant air continuum thing. But where does it go once it's in the air in a sealed environment, you have to have your air conditioners and you have to have your dehumidifiers take that moisture out. So this is why it's important to measure the volume of moisture that is being removed in order to maintain that set point at 40% this is a huge, huge, huge for people that are in truly sealed flowering rooms,

Jordan River

52:48

it's really interesting how you talk about this stuff. Man, I appreciate this today. Nick, I learned a lot in this episode, not just like rethinking the way that I think about VPD and dryness in the air, but how it all works is one and really what you said about how this is something that naturally happens in physics and in nature, and the plants just kind of mimic it and leverage it, as you said, really, really mind blowing. Very, very good episode, man. Thank you for spending the time with us today and taking the time to do this on growcast. Yeah,

Speaker 1

53:18

you're welcome, but glad to you know have the opportunity to do it here. It's always a pleasure to bounce these ideas off you and to get feedback from people that listen to this episode. So, you know, for those of you that are watching this, you know, if it's two, two to three weeks or whatever, after we've recorded this, feel free to, you know, hit me up on Instagram or talk to me on Discord. I love to unpack these things for people in the community as well. Yeah, absolutely.

Jordan River

53:39

What do you got going on? Any teasers or upcoming events or upcoming products?

Speaker 1

53:46

Yeah, you know, in May, we're gonna launch a micronutrient product. And we wanted to do it for the 420 sale that we have currently going on right now, but unfortunately, we couldn't get everything ready in time. So it'll probably be sometime the next couple of weeks, but definitely in May, we're going to launch the micronutrient product, so keep your eyes open for that. And yeah, rootedleaf.com

Jordan River

54:05

code grow cast, always for the biggest savings. Code grow cast@rootedleaf.com at the rooted leaf on Instagram to give them a follow and hit them up in the discord, of course, hanging out in the growcast members discord always appreciate that. Nick, any final words, this is a great episode. Man,

Speaker 1

54:24

yeah, I feel great about this episode too. I hope people gathered some useful information, and I hope that they can better understand how to water their plants and how to treat watering their plants as part of a

Jordan River

54:34

continuum. Man, it is a endlessly complex subject, and you did a really good job breaking it down today. So thank you again, and listeners, make sure Stay tuned for more Nick coming back on the show and check out the playlist on our YouTube channel, youtube.com/grow, cast, we have all of Nick's episode conveniently in a playlist for you there titled nutrient deep dives with Nick. We'll have to change that to grow science deep dives as we go deeper and. Deeper. Thank you, Nick. I appreciate you, man, and we'll see you next time soon. Okay,

Speaker 1

55:04

sounds good. Thank you, Jordan. Thanks for everyone for listening. You got it.

Jordan River

55:08

Thank you listeners. We appreciate you, and go to growcast.com for all the things you need until then. This is Jordan River and Nick from rootedleaf signing off, saying, Be safe out there. Everybody. Stay tuned with grow cast so you never have to grow alone. Bye, bye. That's our show. Thank you so much for tuning in. Thank you to Nick from rooted leaf for this awesome episode and special. Thank you to the members of growcast.com/membership you make this whole show possible. We got a whole bunch of stuff going on in there. You're not going to want to miss, folks. So check it out before we go. Of course, one more quick shout out to rootedleaf.com use code growcast for the best carbon based nutrients you can get your hands on. No need to pH, just follow their medium feed chart. Water to a good run off and watch the explosive growth happen. The quality is the biggest difference that I noticed with rooted leaf, and I know you will too. Code, grow cast for 20% off@rootedleaf.com and that's all, folks. I am cranking out some awesome member content this upcoming May, and got some big things rolling out. You're gonna want to stay tuned for hard at work behind the scenes. I appreciate you tuning in to every episode, so be safe out there, everybody, and we'll see you next time Bye, bye. You

00:0000:00