Thanks Tai, I don’t know the answer to your question. I’ve reached the limit of my understanding here. Because of my background in building energy systems I have always used RH for energy calculations. My knowledge of plants is just based on what I read and a few years of growing. I also did a lot of work with desiccants and that is all about adsorption energy. When dealing with water on surfaces a few monolayers thick, adsorption energy is important in calculating mass transport. I just assumed that there was a water film over the entire leaf surface.
In the end, it probably doesn’t matter much, but it is fun to discuss with other folks on the board. My working alone can lead to very stupid mistakes so it is good to vent these things here.
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Hehe yea definitely man, it’s all out of passion for learning.
So leaf anatomy 101:
The cuticle is a waxy layer covering the leaf(and stems), it’s very resistant to water, so hardly any passes through, almost all water leaves and CO2 enters is though the stomatas which are mostly in the bottom part of the leaf. That way the plant can regulate it’s water loss, by closing and opening the guard cells.
As water can pass through the spongy mesophyll cells membranes, the air just under the stomatas is assumed to be at saturation and so water comes out(assuming air is not saturated) and CO2 comes in.
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Thanks Tai for the instruction. One question though, how do foliar sprays enter the leaf if the cuticle is waterproof?
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Nice to meet you, Ethan.
We retired old farts need to stick together.
I find it fascinating that we are bringing our experience with other species and horticultural practices to cannabis rather than the youngsters trying to recreate the wheel all over again.
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Well first it’s not 100% waterproof, but around 95% if I remember correctly, secondly there are some stomatas on the top of the leaves as well, so assuming they’re open some can enter that way, but for the most part it just doesn’t, foliar feed is really inefficient in terms of the amount of nutrients that actually get into the plant, most just stay on the leaf surface.
Recently a new study published in Nature, that used nano encapsulated nutrients in foliar feeding’, and the researchers managed to get to 30% nutrient use efficiency as opposed to 1% with regular foliar feeding.
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Tal, hope you don’t mind me picking your brain here, just anxious to learn. I read a book by Richard Spear and he devoted a chapter on water and nutrients for plants. He stated that water and nutrients are not taken up by the roots together. Water enters the plant via osmosis and nutrients much be “pumped” against the concentration gradient via ATP process. The bottom line was that the nutrient process of uptake would not allow large molecules, like sugars, to cross the semipermeable membrane of the roots. In essence he says that the only transport taking place from using magic plant enhancers other than the essential nutrients is a way of transferring money from your bank account to theirs.
Would appreciate your comments.
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Not at all Kirk, again that’s what were all here for 
What you wrote is partially true, some nutrients definitely enters the plant with the water flow with no extra energy needed by the plant while others need active transport.
What determines whether a nutrient enter by active or passive transport is it’s electrical charge and chemical concentration within the cell and in the nutrient solution, what’s called the electrochemical gradient.
All anions are actively transported into the cell as the cell’s membrane is negatively charged, so nitrate and phosphoric acid, for example, are always(saying “always” in biology is somewhat risky as there are always exceptions but you get the point…) actively transported into the cell.
While cations are mostly passively transported across the membrane, I say mostly because in terms of electrical charge there is no problem but sometimes the concentration in the soil’s solution is not enough and then the plant actively transport cations as well.
When talking about soil cultivation a lot of the time the thing that limits the plant is the concentration of a certain nutrient in the soil’s solution and the rate at which it moves from the solid soil into the liquid solution, and so calcium for example is very soluble and is usually at high concentrations in the soil’s solution(in calcareous soils) and as it’s a cation it mostly enters passively with the water flow, while iron on the other hand is also a cation but it’s concentrations are usually very low, so even though it can enter passively, the actual amount that enters with the water flow is not significant(even for a micronutrient) and the plant uses various mechanism of active transport to get iron inside the cell, either chelates or acidifying the rhizosphere(increases solubility).
The analogy you used with the money transfer is very precise, and to a large degree that’s the advantage of hydroponics over soil cultivation, we use various energy demanding processes to create concentrated nutrient solutions with chelates and by that we saved the plant a lot of energy and enable it to use all that energy for growth and biomass production. And regarding larger molecules, as a rule of thumb it’s true, they have a much harder time getting in, if at all, but some areas of young roots are not completely built and lack the casparian strip layer that blocks apoplastic movement(molecules then don’t go through a membrane) and then bigger molecules can enter the plant.
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Thanks Tal, best explanation I have seen. what part of the mj business are you in?
My sons and i are doing aeroponic. Designed the system myself mostly for cost reasons. Biggest problem we have run into is separating the low pressure circulation system from the 150psi system. One way valves work at first and then fail blowing up the chiller. Even solenoid valves have to be modified as they do not close fast enough.
We use the maxi series nutes from gh as we are dedicated to just npk and other essential nutes. My thoughts on nutrient compositions has been strongly influenced by Current Culture. Their nutrient line and feeding charts were supposedly developed by UC Davis on mj plants. Do you have any opinions on this?
Last question! Can nutes leave the plant when spraying the roots with only water? Trying to understand if flushing has any effect in aeroponics.
Again, thanks much for your insights.
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Thanks Kirk :), it’s a pleasure discussing with you.
I’m in the cultivation aspect, I work for a company that does greenhouse project management and agronomic counseling for cannabis and agricultural projects in general. I used to manage a medical cannabis farm and now doing my M.Sc in agricultural science. How about you?
Blowing up chillers does not sound fun… hope you manage to resolve that, I would offer help but to be honest, don’t have a clue about how to engineer an aeroponic system.
Unfortunately no, I’m from Israel and we have local nutrients producers here, so I really don’t know about gh or other brands.
Most definitely yes, pretty much any molecule that’s within the plant can and does “leak” out of the roots, sugars, organic acids, nutrients, hormones and many more has been found in root exudates\efflux. I don’t really know to tell you quantitatively how much nutrients can be lost due to flushing in an aeroponic system if you use only water, my instinctive thought is that it could drain the plant quite quickly but I’ll ask my teacher on plant nutrition and let you know.
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So I checked, she says that if you flush the roots with only water and no nutrients then in about 24 hours it should deplete all of the nutrient reservoirs in the roots.
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I noticed that only the root nutes were mentioned in your reply. The declared reason that people and some books recommend flushing with plain water before harvesting bud is that it is supposed to change the taste of the bud by drawing nutrients from the bud. I am very skeptical of this. How do you feel about it?
On nutes, our npk is 3-1-3 for all of veg and 1-3-3 for final week of bloom. we gradually change from 3-1-3 to 1-3-3 during the bloom cycle. Is this in general agreement with what you used? BTW, I have a PhD in mechanical engineering. Not one class on plants!
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Well the roots are actively washed so they are depleted directly, then maybe nutrients will start to flow from the top to try feed the roots and also start to deplete but also maybe they won’t, the plant is deep into senescence by then so maybe it won’t be so simple.
At either case from my experience, when growing with organic slow release fertilizers, flushing isn’t necessary, I would usually add slow release organic fertilizer in the beginning of flowering, just water after that and always had quality bud with no harshness or after taste. When using liquid nutes and coco-coir I feel stop adding nutes is enough, rather then actively flushing.
To be honest I doubt nutrients being drawn out of the buds, as all of the plant’s focus in that stage is putting nutrients into them and making them grow, I think it’s more about reducing nutrients reservoirs and incorporating them into plant tissue so you won’t smoke them in pure mineral form, but I can tell you that for a while we tried mineral fertilizer, and overfertilized as well I think, and the buds became really harsh and metallic tasting, now I’m not saying mineral fertilizer is bad, not at all, just that something definitely happens there and that you should pay attention to mineral concentration in your buds.
The ratios looks good, just make sure you also have enough calcium and micro’s, Cannabis is a big calcium feeder, we used 8-4-4 for veg and 4-4-4 for bloom, the organic slow release I mentioned, worked wonders.
Wow PhD in mechanical engineering, impressive! maybe you should go into Cannabis greenhouse engineering :).
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