Why does ph drop when rez gets low ??

Here's what is happening in my res. Changed res on saturday morning, tds at 650, ph at 6.2. My res is appr 120 litres FYI. My tds as of today is 670 and my PH is at 6, yesterday it was 6.1. The run is almost done at 62 days of flower and I do find that as plant gets near the end ph does drop faster. Nutes may be a bit high as well but am starting flush next refill (typically 5 days) so not to concerned.

Detroit76 said:

My flood table runs just over 1000ppm in veg and almost 1500ppm in flower. Tap water can be almost 200ppm before you add anything. You need 500ppm for seedlings/clones. The water in a dwc system evaporates fast, leaving the nutrients behind and the ppm will increase due to the ratio of dissolved solids to the less water. Likewise your ph will lower because the salts make the water acidic.

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Explain to me, in simple terms, how a measurable amount of water evaporates from what is basically a sealed system. What makes it evaporate so much faster in your opinion?

superstoner1 said:

Explain to me, in simple terms, how a measurable amount of water evaporates from what is basically a sealed system. What makes it evaporate so much faster in your opinion?

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Does he mean that the plants are transpiring quickly and a lot therefore the res level is reducing fast.

I've had my PH dropping below 5.0 and the EC going lower. I looked at the canna aqua website and all I have to do is change nutes. Day 9 of 12:12

I found this so helpful. Check your manufacturers recommendations .
http://www.canna-uk.com/everything_about_ec_and_ph_using_aqua

PPM now is about 400 down from 470.... Gonna put 2 gallons of fresh nutes in tonight and shooting for a ppm of 600... Ph was also creeping down.

Meast21 said:

Can any expert's verify the quote above ??

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i don't feel comfortable calling myself an expert lol, but i don't think your problems are unique. the people i know in dwc, don't add nutes every time the rez/system gets low. they top off with ph'd water. i think the following may help you a bit.



EC goes up, pH goes down = Plants require less nutes
EC goes down, pH goes up = Plants require more Nutes
EC Stable & pH drifting up, = Equalibrium = Good thing!

Silky Shagsalot said:

i don't feel comfortable calling myself an expert lol, but i don't think your problems are unique. the people i know in dwc, don't add nutes every time the rez/system gets low. they top off with ph'd water. i think the following may help you a bit.



EC goes up, pH goes down = Plants require less nutes
EC goes down, pH goes up = Plants require more Nutes
EC Stable & pH drifting up, = Equalibrium = Good thing!

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My ppm was going down and ph was going down also due to underfeeding.... SUperstoner1 I don't get how my ppm was going down when my plants were underfeed drinking more water than nutes. Wouldn't there be a ratio of more nutes to water (3rd day after rez change) then original rez change ???

if the feeding lvls are low the plants will take relatively more salts than water and the ppm should drop afaik.

I could be wrong but I thought i had read somewhere that it is possible to under feed and still have readings (tds) rise. Possibly some place in the Lucas Formula guide???

Just an update! Checked things this morning and res was quite low, the girls are, I thought, just about done but r drinking loads! anyhoo I checked tds and it was right around the 650 mark so not much change except half the volume of water. PH had nose dived to 5.5 over 24 hours, I did top up with plain water that is 6.5 so it should come up a bit. Not to concerned as I am flushing Friday!

Keep it simple, make sure your ph is always within your preferred grow range, and make sure the ppm never gets over the max of what your feeding target is. Never let DWC bucket get below 40% level.
Do this and your babies will be fine, just like mine

Meast21 said:

My ppm was going down and ph was going down also due to underfeeding.... SUperstoner1 I don't get how my ppm was going down when my plants were underfeed drinking more water than nutes. Wouldn't there be a ratio of more nutes to water (3rd day after rez change) then original rez change ???

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the plants dont drink all nutes ( N.P.K & MICRO) evenly at the same time. they drink only what they need for that moment. what ever the plant doesnt drink will build up and make the PPM rise. but they always drink water. some plant eat more then others. start low and rise the the PPM's slowly. keep a eye on the plant see how she reacts. with that size rez its better to do a rez change once a week that why you always have fresh drank drank for the girl.

MEANGREEN69 said:

the plants dont drink all nutes ( N.P.K & MICRO) evenly at the same time. they drink only what they need for that moment. what ever the plant doesnt drink will build up and make the PPM rise. but they always drink water. some plant eat more then others. start low and rise the the PPM's slowly. keep a eye on the plant see how she reacts. with that size rez its better to do a rez change once a week that why you always have fresh drank drank for the girl.

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so in other words if your ppm's are rising , that means the nutes aren't being uptook as fast as the water is? good to know.
also this seems to have to do with proper NPK ratios too
ppm rising means ph dropping?
so then ppm dropping must mean ph rising, which is what you aim for (slightly ) ?

dont know how much NPK ratios affect it but some nutes are more acidic/basic then others. its better to to watch both PH and PPM i woundnt count on just one. something like a tri meter works very well. every time you go into the grow room take a look at the meter. after a few weeks you can learn how your plants eat. like i said some are light feeder some heavy. also diffrent nutes are more avilable at diffrent PH. so if your PH tends to drift up start your fresh rez at say 5.5 then let it drift up to 6.5 then ether change the rez or add what ever you think the plant needs.