DWC Nutrient PPM Dosage Theory

[Addie42069]

Hemp/CBD bagseed

5 gal DWC

Age: 58 days, week 2 flower

Light schedule: 18/6 (apparently she's an auto)

DLI: 25

VPD: 1.3


I first tested this 2 days ago based on my most recent ppm measurement (day prior) and was expecting a ppm reading of 1250, it read 1150. That's only an 8% difference so I figured fuck it that's close enough to look further into it.


3 gal = 588 ppm (fresh res)

2 days later

1.5 gal = 769 ppm

"Ate" = 386 ppm

Desired PPM = 535


To get the 1.5 gal in the res from 769 to 535 the 1.5 gal top off solution should be 301. To dose the 1.5 gal with 301 of nutes I cut the dosage down to 25% strength of what I mixed to get my starting res of 588.


After Mixing the top off solution the ppm was 332. Mixing 1.5 gal of 332 to 1.5 gal of 769 should be 550. It measured 566, close enough.


So in theory she will drink the same 386 plus, I'm guessing, a little extra as she gets older. If this holds true we can calculate, within a reasonable degree of accuracy, how much water vs nutrients each individual plant is taking up and adjust accordingly.


Even though I calculated that she ate 386 ppm, I didn't want to drop the ppm that low because she doesn't seem to mind it where she is.


Once I get a baseline of how much she's eating I plan to start adjusting variables to see what happens. My first move is to increase the DLI to see how it effects nutrient uptake. From what I've read, higher DLI should mean more nutrient uptake but I'd like to actually put a number to it. I'm going to increase DLI by 5 on a weekly basis on the same day as the res change. Once she starts to show signs of stress I will back off a little and leave the DLI alone.


Next I plan to adjust VPD to see how it effects the nutrient uptake, then I'll consider adding CO2. This obviously won't all be done with this same plant. The initial testing will be done one her but they next seed I plant is going to be a feminized CBD photo. Ideally I will keep her in veg long enough to test everything mentioned above. The goal is to put together a calculator where you can input your environment parameters and get an accurate reading of your plant's needs. No more guessing!


Any input is greatly appreciated. Is there something I'm not considering? Has anyone tried something like this before? I spent multiple evenings trying to get some information on this but I got nothing.



TLDR: The goal is to put together a calculator where you can input your environment parameters and get an accurate reading of your plant's nutrient needs. No more guessing!

 

[Bdubs]

Sadly the PPM doesn’t decrease unless you change the PPM of the top off water. PPM will only increase due to a lockout or imbalance.

Of the nearly 2 years of growing DWC my PPM never went down only up. I always fed with same PPM feed top off or very near.

I have seen a few guys claiming their PPM going down. It has to be because of inconsistency of top off.

Cannabis drinks like a straw, it will always take up what is available at the rate at which it drinks. So there will never be a reduction of ppm, only an increase from buildup of a nutrient in the root, lockout.

It’s a mistake to top off with water only and dilute. If your mix is consistent and PPM is on target, you should never see a decrease. It indicates you are doing something wrong.

Sorry, no easy way to put it.

 

[Bdubs]

When the reservoir level begins to drop, should I top it off with fresh water or nutrient solution?

If you drain, clean and remix the nutrients every 7 to 10 days, it’s okay to top off with fresh water daily. As plants consume nutrients and water, the nutrient strength in the hydroponic reservoir…

generalhydroponics.com

This is bad advice IMO. If you have a problem in the reservoir, you always swap entirely. No games. You’ll have loads of problems if you go playing with the water.

 

[Bdubs]

If you need to top off, always mix and top off with exactly the same as what you swapped into the bucket in the first place for that swap. Swap every week. Swap early if you have an aggressive ppm creep/spike. Swap early if your water temps are creeping or too warm. Swap early and reduce ppm to correct nutrient burn and ph increase/spikes. Swap to fresh water every 7 days no questions. Swap is your best friend, hydro isn’t for the lazy. You will thank me later to do it this way.

 

[Addie42069]

I agree topping of with plain water doesn't seem like a good idea, you need to replace what she ate.

Sorry I may not have explained things properly. My res ppm is rising. It started at 588 and ended up at 769 over 2 days and the water level dropped by 50%. To my understanding they don't drink equal parts water and nutrients unless you have it dialed in perfectly. So I did some math to see how many ppm she took up vs the amount of water she took up. I'm hoping to get it dialed in where ppm stays fairly stable as the water level drops.

 

[Addie42069]

If you need to top off, always mix and top off with exactly the same as what you swapped into the bucket in the first place for that swap. Swap every week. Swap early if you have an aggressive ppm creep/spike. Swap early if your water temps are creeping or too warm. Swap early and reduce ppm to correct nutrient burn and ph increase/spikes. Swap to fresh water every 7 days no questions. Swap is your best friend, hydro isn’t for the lazy. You will thank me later to do it this way.

So even if you res ppm is rising you mix your top off to the same ppm the res started at? Doesn't that increase your res ppm after top off?

 

[Bdubs]

So even if you res ppm is rising you mix your top off to the same ppm the res started at? Doesn't that increase your res ppm after top off?

If ppm rises, you swap entirely and reduce the PPM lower than what it was on that current swap.

 

[Bdubs]

Depending on how fast the creep is, take action. If it’s a spike over night, yes do it quickly. If it is a slow creep by like 50-80ppm….meh just try not to increase it the next swap mix. Creep is always a little too much and you should always get a small increase leading up to the 7th day of your swap intervals.

 

[Bdubs]

So if it is a little creep, don’t change anything that’s acceptable. Spikes are when you make a move. Reduce ppm by 50-100 depending on how much of a creep or how quickly it creeps. Once you reduce the ppm, keep that ppm into the next swap, she won’t take more if you give more, she is demanding less so consistency after adjustment.

 

[Bdubs]

But yeah even if it is a small creep, same mix give it to her. You’ll swap at your day 7 and that’s when you can make adjustment to lower it a hair.

 

[Addie42069]

Copy that, thank you very much! This is my first grow, how does she look in your opinion?

 

[Bdubs]

Looking real good! Now that you can master the feeding and reductions.
Keep an eye on the water temps and your Ph. Ph swings along with temps of the water even when Ph it is stable, so don’t trip if it moves a little - check against water temps from the day before in your analysis. Large spikes or drops can indicate root health issues. Water temps are a big culprit, because usually warmer water temps house more bad bacteria. Any amount of root rot can and will cause Ph swings just depending on how bad. If at the medium will cause acid rain effect and continual lowering of Ph. Keeping those three things tight and follow up any adjustments with consistency and you will grow fire .

 

[Bdubs]

Looks super healthy. What nutrients do you run?

 

[Addie42069]

Awesome! Great to hear! The water temps have stayed between 72-73.5 so far and the pH has stayed between 5.7 and 6.1. I'm running 1:20 diluted SAG just to be safe. So far the roots look great in my inexperienced opinion. There is some staining but no rot I don't believe. I'll take some pics shortly. I'm running GH light feed schedule. My mix is:

Armor Si
CaliMagic
Rapidstart
Micro
Gro
Bloom
Diamond Nectar
SAG

 

[Bdubs]

All sounds good. keep up the good work. Dont forget, if using Armor Si, add it first before you add Micro. If using Armor Si and CaliMagic, always mix them 2 first before Micro, and Armor Si always first. CaliMag first before Micro if not using Armor Si.

 

[AZreefer]

Awesome information here!

 

[Addie42069]

Yessir always Armor Si first. I did a ton of research on silica before I pulled the trigger on it. I just gave her a trim, Here are some pics and a few of the roots. The roots are about 4 feet long, maybe that's why she drank 7/8 of a gallon since top off 21 hours ago

 

[Bdubs]

That would def do it too lol

 

[DillyTrying2Learn]

Sadly the PPM doesn’t decrease unless you change the PPM of the top off water. PPM will only increase due to a lockout or imbalance.

Of the nearly 2 years of growing DWC my PPM never went down only up. I always fed with same PPM feed top off or very near.

I have seen a few guys claiming their PPM going down. It has to be because of inconsistency of top off.

Cannabis drinks like a straw, it will always take up what is available at the rate at which it drinks. So there will never be a reduction of ppm, only an increase from buildup of a nutrient in the root, lockout.

It’s a mistake to top off with water only and dilute. If your mix is consistent and PPM is on target, you should never see a decrease. It indicates you are doing something wrong.

Sorry, no easy way to put it.

Is this true? I had thought if you're feed is too light along with some other situations that your plant can eat faster than it is drinking and cause your PPM to go down.