Reservoir Nutrient uptake, Newbie tip/Rule of thumb?

When I made my own aeroponic system out of a rubbermaid tub a pump and sprinklers I had no problem getting the plants to survive and grow. One thing I never had a problem with even with my very first time experimenting with hydroponics was fluctuating PPM

So I was wondering, Is it a general rule of thumb if you dump 5 gallons of water into you reservoir and have an EC of 1200 PPM and a week goes by with no topping off and nothing but letting the system run and the water level has reduced the PH slightly shifted but the PPM are still between 1000 and 1300. Does that mean The plants are taking up nutrients and water at an equal level?


I have read of problems you can have with hydro systems. Like PH gets shifted too much and the plants drink the water but the nutrients are locked out so the PPM in the water skyrockets.

Simplified and short My question is. If the water level drops in the reservoir and the PPM stays somewhat the same as when you filled the res after a week does that mean everything is running optimal?

exactly. the sweet spot. but all strains are different so that number can change.

So really the only time the PPM rapidly fluctuates is when there is a PH issue or lockout and the plant starts uptaking water and not the nutrients? and if the PH is right but the PPM is too high then you will just burn your plant right?

To me it sounds like in all cases it is unlikely for the PPM in the water to fall at all.... What scenario would have to take place in order for the PPM to fall after a week of a running system?

It seems unlikely that the plant can just take up the salts without taking water with it so I can't see how an EC can fall but I can easily see how an EC can skyrocket.

Or is it even possible for an EC to be lowered without adding additional liquid to the reservoir.

My guess is the plant will take up what it needs, tho I don't use your type of system and am hydro my ppm does rise after a few feedings and I just adjust the ppm back to where I started, can't really say I've noticed any problems due to this ocurring unless I forget to check and the ppm being too high burns my plant. btw mine is a feed and drain in hydroton.

Hypothetical scenario on ppm's:
1 gallon jug empty.
Add 1/2 gallon of 0 ppm RO water.
Add 500 ppm's of nutes.
1 gallon jug filled half way with 500 ppm nuted RO water.
Fill rest of jug with 1/2 gallon of 0 ppm RO water.
Take a measure and you get 250 ppm.
With all things being equal in this hypothetical scenario,the ppm's was made less concentrated with the volume increased with 0 ppm water.

Evaporation,nute uptake by plant changes the ppm's.Nute uptake is (all things being equal) the main factor in PH shift.

With the right balance of starting nute dose,when to add back plain water,and when to add back nutes?Determines how long (besides other factors of an unhealthy res) one can go between res change or reset.

the key is to have a res that is large enough to last between res changes without having ton"add back" anything.

i usually measure it when res is 50% low (my system is 40g so re-check when water is at 20g) takes my system 1 week usually to drink up 50% of the res, if i start at 1500ppm and in a week with 50% water gone take another reading and ppms are right around 1500 still then im hitting the perfect feeding spot (plants eat equal to what they drink) if after a week i checked and the ppms where say 500 less i would be underfeeding, if they rose 500 then im technically overfeeding. ph should drift up when plants are feeding correctly i set mine to 6 and let it get up to 6.5 withing the week allowing for more nutrients to be taken in at different ph's (i like ph of 6 when using hydroton prably 5.8 for a aero system). gradual ph drifts are good drastic ph fluctuations are bad, if system and everything is dialed in correctly you should have a nice ph drift as they eat and drink.

I feel like some of these answers should put be in a new thread and get stickied..just my opinion ( I already bookmarked )

drgreentm said:

i usually measure it when res is 50% low (my system is 40g so re-check when water is at 20g) takes my system 1 week usually to drink up 50% of the res, if i start at 1500ppm and in a week with 50% water gone take another reading and ppms are right around 1500 still then im hitting the perfect feeding spot (plants eat equal to what they drink) if after a week i checked and the ppms where say 500 less i would be underfeeding, if they rose 500 then im technically overfeeding. ph should drift up when plants are feeding correctly i set mine to 6 and let it get up to 6.5 withing the week allowing for more nutrients to be taken in at different ph's (i like ph of 6 when using hydroton prably 5.8 for a aero system). gradual ph drifts are good drastic ph fluctuations are bad, if system and everything is dialed in correctly you should have a nice ph drift as they eat and drink.

Click to expand...

I understand most of your post except the part about PPM dropping 500....I still haven't really been given an answer or shown how the PPM can actually drop. It is my understanding that the only way to drop PPM in water is to add more water.

When a plant feeds the roots uptake the water and all the chemical reactions and separation of water and salts happens inside the plant not at the site of the roots, and the plant doesn't spit back out water after it takes the salts out of the water. That being said how can the PPM actually drop?

Or I can be totally wrong and the salts get separated at the root site but I doubt that.

I just want to get the scientific answer to that question. To me it doesn't seem right that PPM can drop in water without more water physically getting added to the reservoir.

the ppm can easily drop when the plants are taking more nutes from the water.

Yup I was under the impression the plant takes up the water before the salts and other elements were separated. I learned that they are all separated at the root site all at the same time water is taken up.

Now I understand how it can drop

When a plant takes up magnesium to support photosynthesis for growth.The Mag,or parts per million drops,and less of Mag in res.
Now that some Mag was used by the plant there is less PH down buffering from the physical property of Magnesium in a H2O solution.
With less PH down buffering the PH rises.This is just one factor that alters the res,and why PH goes up in vegetative growth stage.

In flower PK is used to produce bud,and less PK in res or lower PPM's as plant takes for the bud.P or K has the physical property of being a PH up.With less PH up buffering the flower stage res PH will drop along with ppm's


All things being equal yadda yadda blah blah good stuff.