chemistry question about flushing

asaph · Jun 21, 2011

heyo

i use RO water (also have tap of course), and I need to flush before harvest.

Now I have understood and made sense that the highest osmotic pressure (perhaps even too high) is achieved with RO water, as the gap between it and the medium/roots is the greatest, thus more particles are drawn to it through osmosis.

Then again, I have had a debate here somewhere where an educated person here said that RO water with no charged ions (cations?) in it will not "attract" other charged ions (with opposite charge I assume), and in order to "leach" salts out of the medium you need to have some (dunno how much) charged particles in it. (is h2o not charged? if so how?). This guy suggested adding some cal mag (or just mag, not sure) to the water to encourage leaching.

In other places I have read that it is best to use clearing products like clearex.

Now for me, all I have is RO water, tap water (that's 7.8 pH and 0.7 EC), molasses, enzymes, epsom salt, calcium additives for humans, organic nutrients (biobizz which I hardly use), and hydroponic nutrients (HESI coco). I have some plants in soil and some in coco.

No access to clearing products, calmag+, or other special products you may think of.

So I'm trying to figure out what would be the best way to flush my medium. With how much EC, and of what.

Vindicated · Jun 22, 2011

Non-salted water moves to salted water. Or if you prefer big words, a low solute solution will gravitate towards the higher solute solution. However, I think your way over thinking this. Stop your feeding 1-2 weeks before harvest and use clean water. Unless your doing lab experiments I highly doubt you or anyone will notice the difference between flushing with tap vs filtered water.

I grow organically and haven't noticed the difference between flushed and non-flushed bud, so I no longer flush. I consider it a waste of time and water. If you like heavy synthetics nutrients, take a look at Advanced Nutrients feeding schedule. They encourage growers to feed like crazy, yet recommend cutting back only one week before harvest and doing a quick flush. I know your not using AN, but I think over all its the same concept.

You don't have to flush, but if you choose to, do it by cutting nutes 1-2 weeks before and flush with water. The general rule of thumb to flushing is to use three times the amount of water that your container holds. Example... if you have three gallon containers, you should flush with nine gallons of water.

But again, I don't flush because I haven't met anyone who can tell the difference whether vaping or hitting through a bong.

asaph · Jun 22, 2011

Vindicated said:
Non-salted water moves to salted water. Or if you prefer big words, a low solute solution will gravitate towards the higher solute solution. However, I think your way over thinking this. Stop your feeding 1-2 weeks before harvest and use clean water. Unless your doing lab experiments I highly doubt you or anyone will notice the difference between flushing with tap vs filtered water.

I grow organically and haven't noticed the difference between flushed and non-flushed bud, so I no longer flush. I consider it a waste of time and water. If you like heavy synthetics nutrients, take a look at Advanced Nutrients feeding schedule. They encourage growers to feed like crazy, yet recommend cutting back only one week before harvest and doing a quick flush. I know your not using AN, but I think over all its the same concept.

You don't have to flush, but if you choose to, do it by cutting nutes 1-2 weeks before and flush with water. The general rule of thumb to flushing is to use three times the amount of water that your container holds. Example... if you have three gallon containers, you should flush with nine gallons of water.

But again, I don't flush because I haven't met anyone who can tell the difference whether vaping or hitting through a bong.

hey thanks man, you say some important stuff there, though i didn't feel my questions were answered

first off, about your first sentence, i would think it be the other way around, no? at least, the way I see it, salt in salted water moves towards non-salted water to make it salted. that's the way i understand osmosis.

and well, my question is more (and even more importantly) about flushing in general, especially flushing to solve problems, where it gets more critical to lower your nutrient concentration in the medium.

now about pre-harvest flushing, I know it's subject to much debate, and I'm actually not one that notices or differs smoke tastes (you know, it's all shit to me, really. I can't think of enjoying the taste of any smoke. it's smoke). But i don't grow organic, and I know that most people say that they CAN taste a bad difference in unflushed plants from synthetic nutes.

so yeah, i know the volume of water you should flush with, but I'm still wondering whether it's good to flush with pure h2o (RO water)

rollinronan · Jun 22, 2011

you seem to have quite a good understanding of it already

osmotic pressure discribes the pressure gradient over the semi-permiable membrane (roots in this context) due to osmosis, if the pressure is too high it can burst the membrane, this is why drinking too much water can kill a person, but not a plant because they have a cell wall that prevents this, it is noticeable when plants wilt as the water is not there to suppourt the cell wall so it goes limp, this is when the osmotic pressure is too low. when you add a small amount of salt or other impurities it will create a limit on how much water can be passed accross before the concentrations on both sides become equal.
in short, adding pure H2O will be absorbed better into the plant than saturated water

the best concentration for growth is .9% w/v of salts (thats for humans and its probably somthing similar for plants but im not sure, ill look it up and get back) as this is the concentration of blood so as the organisim uses up the water in its system already and transpires it through its leaves and the liquids in the plant cells become more concentrated..then drawing up extra water through the roots to equalise the concentration gradient

this can cause nutrient build up when the unabsorbed feed water evaporates and becomes so concentrated is falls out of solution

as for the ions, water has both as it is H2O this is [H+] + [OH-] therefore it has a cation and anion (positively and negatively charged molecules that are drawn together due to their charge) just like NaOH and HCl witch are alkaline and acid. it is a very strong bond between the molecules so the H in HCl is drawn to the OH- in water and so it dissolves in water easily. and as we know, plants can only absorb nutrients when they are dissolved in water.

and thats all i know off the top of my head about it, if you need more information i'd be happy to look into it for you

asaph · Jun 22, 2011

rollinronan said:
you seem to have quite a good understanding of it already

osmotic pressure discribes the pressure gradient over the semi-permiable membrane (roots in this context) due to osmosis, if the pressure is too high it can burst the membrane, this is why drinking too much water can kill a person, but not a plant because they have a cell wall that prevents this, it is noticeable when plants wilt as the water is not there to suppourt the cell wall so it goes limp, this is when the osmotic pressure is too low. when you add a small amount of salt or other impurities it will create a limit on how much water can be passed accross before the concentrations on both sides become equal.
in short, adding pure H2O will be absorbed better into the plant than saturated water

the best concentration for growth is .9% w/v of salts (thats for humans and its probably somthing similar for plants but im not sure, ill look it up and get back) as this is the concentration of blood so as the organisim uses up the water in its system already and transpires it through its leaves and the liquids in the plant cells become more concentrated..then drawing up extra water through the roots to equalise the concentration gradient

this can cause nutrient build up when the unabsorbed feed water evaporates and becomes so concentrated is falls out of solution

as for the ions, water has both as it is H2O this is [H+] + [OH-] therefore it has a cation and anion (positively and negatively charged molecules that are drawn together due to their charge) just like NaOH and HCl witch are alkaline and acid. it is a very strong bond between the molecules so the H in HCl is drawn to the OH- in water and so it dissolves in water easily. and as we know, plants can only absorb nutrients when they are dissolved in water.

and thats all i know off the top of my head about it, if you need more information i'd be happy to look into it for you

that's excellent info there man, +rep!

now i better understand what Vindicated said before too. So I had a different picture in my mind, where flushing the roots and medium with low concentration "draws" salts out of them and washes the salts away. But I see now that it's more like low concentration solution is drawn into the roots? and lowers salt concentration in them?

and yeah, what you said also explains why they say you should never give nutes to plants that are too dry. because the cell walls are limp? but then, i would expect adding pure water at this point would create a too high pressure?

rollinronan · Jun 22, 2011

asaph said:
that's excellent info there man, +rep!

now i better understand what Vindicated said before too. So I had a different picture in my mind, where flushing the roots and medium with low concentration "draws" salts out of them and washes the salts away. But I see now that it's more like low concentration solution is drawn into the roots? and lowers salt concentration in them?

and yeah, what you said also explains why they say you should never give nutes to plants that are too dry. because the cell walls are limp? but then, i would expect adding pure water at this point would create a too high pressure?

no problem, yes thats it!
well like i said plant cells wont burst like animal cells so i think adding RO water would be fine; because when the osmotic pressure gets high enough the water stops moving into the area of high pressure because the osmotic pressure prevents it, osmotic pressure is how your RO machine works by putting pressure on the higher concentration( unpure water) to move water into a lower concentration area. otherwise it would not work because the pressure would be equal without the it. its kinda like blowing up a balloon, where your lungs (RO machine) are putting pressure on the air (dirty water) to beat the elasticity of the balloon (clean water wanting to move back)
sorry if thats a little confusing...

chemistry question about flushing

asaph

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Vindicated

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asaph

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rollinronan

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asaph

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rollinronan

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