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Thread: Still don't understand the ins and outs of RDWC

  1. #1

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    Default Still don't understand the ins and outs of RDWC

    Take this this scenario as an example. I am 4 weeks into flower, 300 litre RDWC 8 plant system with a 200 litre header feed tank nearly empty. My feed chart says at week 4 to add 10ml of PK per 10 litres and top up with base feed to 1.2 ec. The only way I could get that solution into my system at 4 weeks would be to drain and refill the system. If I put the PK solution into the header tank to drip feed it would never really be at full strength PK as they would only be drinking around 20 or so litres each day and the 20 litres coming from the header tank would be diluted with the 300 litres in the system.

    In coco my first feed at 4 weeks would contain the full 10ml per 10 litres of PK and into the 5th week it could be reduced exactly to 5ml per 10 litres. It is the slow dilution from the header tank into the system. I don't understand how can i time the feed changes accurately from week to week.

    Another thing is, in RDWC the nutrients are fed at a much lower ec so do I also half the quantities recommended for boosters and pk?
    Last edited by toblue; 22-02-19 at 11:14 AM.
    TO GROW A PLANT YOU NEED TO THINK LIKE A PLANT


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  3. #2

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    Quote Originally Posted by toblue View Post
    Take this this scenario as an example. I am 4 weeks into flower, 300 litre RDWC 8 plant system with a 200 litre header feed tank nearly empty. My feed chart says at week 4 to add 10ml of PK per 10 litres and top up with base feed to 1.2 ec. The only way I could get that solution into my system at 4 weeks would be to drain and refill the system. If I put the PK solution into the header tank to drip feed it would never really be at full strength PK as they would only be drinking around 20 or so litres each day and the 20 litres coming from the header tank would be diluted with the 300 litres in the system.

    In coco my first feed at 4 weeks would contain the full 10ml per 10 litres of PK and into the 5th week it could be reduced exactly to 5ml per 10 litres. It is the slow dilution from the header tank into the system. I don't understand how can i time the feed changes accurately from week to week.

    Another thing is, in RDWC the nutrients are fed at a much lower ec so do I also half the quantities recommended for boosters and pk?
    Mate this is always something I’ve questioned myself about when adding nutrients and in the right ratios

    What I have been doing and I think it could be wrong if someone could correct me

    But say I’m adding grow and root stim. Manufacturers guidelines say 7ml/l grow 2.5ml/l root stim. And I have a 100litre tank say and I’m looking to set the ec to 0.5.

    I add them at 1/8 or 1/16 strength of them both and see what the ec is at. If I’m under at say 0.3/0.4 I try and add both root stim and grow and the same ratios I used at the start to get the 0.5ec

    Again hope that makes sense what I am trying to say lol

    But I have always questioned is this the right way to add them? Like should I be adding 1/2 strength root stim check the ec and then top up with grow until I reach 0.5?

    Honestly mate I’m gonna keep an eye on this thread to see if someone can shed some light on the correct way to add nutrients and in what ratios



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    Im no pro mate but I think you gita have rez changes every week or two and then that gives you the opportunity to add different nutrients.

    Feed should always be lower as plants are eating more.

    I would do a full rez change and start again.
    Although i do remember you saying you like to keep going with your rez.

    Hope you get sorted mate as i like these setups and will change over to these in the future.


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    Quote Originally Posted by tom222 View Post
    Im no pro mate but I think you gita have rez changes every week or two and then that gives you the opportunity to add different nutrients.

    Feed should always be lower as plants are eating more.

    I would do a full rez change and start again.
    Although i do remember you saying you like to keep going with your rez.

    Hope you get sorted mate as i like these setups and will change over to these in the future.


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    I change the reservoir and system water every week mate. I’m just not too sure how to add the nutrients and in what amounts


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    Mmm, its tricky one Junkie mate.

    The only way I can see how to do it properly is to drain at 2 weeks (not the last bits in the bottom just from the epicenter) and add full strength bloom and base to whatever ec you chose with a changeout, then just add bloom and base to your header tank descending with bloom to where you want to be at 3-4 week. Then drain again at week 4 add full strength PK at the desired strength and whatever else you use to correct ec, then fill your header tank to where you want to be with pk etc from weeks 5-7/8/9 depending on strain. Then if you use a finishing feed like Dragon Force drain again 10 days before they are ready and fill system and header with full strength finishing feed. Then drain when you want to flush add water to dilute the bottom bit then drain again, then fill system with water for the flush.

    I think you could probably get away with adding bloom from end of week one to be at full strength by week two, I don't think timing with bloom is as critical as it is with PK, saving you at least one changeout.

    Seems like a lot of work, getting all the solution to the right temperature etc. I drained my 300l system yesterday due to over feeding and it was a pain, luckily I have a 200l header tank. I just used the hot tap to get the temperature more or less.

    On a different note I bought an electric puddle pump to drain my system which is the dogs bollocks. I also added a T joint on my line from the header tank to the ballcock in the epicenter attached a short piece of hose with a shut off valve on the end so I can fill from the header quite quickly instead of waiting for it to go through the ballcock.

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    Quote Originally Posted by tom222 View Post
    Im no pro mate but I think you gita have rez changes every week or two and then that gives you the opportunity to add different nutrients.

    Feed should always be lower as plants are eating more.

    I would do a full rez change and start again.
    Although i do remember you saying you like to keep going with your rez.

    Hope you get sorted mate as i like these setups and will change over to these in the future.


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    The argument regarding whether or not to do changeouts is a hot topic on the American forums and the opinion seems to be divided. I opted for zero change out for easines but on reflection I may change at 4 weeks for the addition of PK then again for finishing booster, if I didn't use the finisher I wouldn't bother just drain for the flush. I have a different system for veg so the solution is new when I flip.

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    In nutrient science there a well known thing called "Law of the minimum" have a quick google about it as it will explain it better than i can. And i think you might find the answer your looking for,

    People are often over feeding and dont actuslly realise they are doing it. Often enough most people think by having a nutient solution at the max strength as the plant will allow will then give them max yield possible.

    Also note if your using an additive such as sillica you also adding potassium to your tank. You can change much less ofthen than you think. Providing its clean as microbes can build up quick in hydro. Also have a look through one of jessies old diarys mate. Im sure he done a few with little to no changes

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    Quote Originally Posted by BritishBotanics View Post
    In nutrient science there a well known thing called "Law of the minimum" have a quick google about it as it will explain it better than i can. And i think you might find the answer your looking for,

    People are often over feeding and dont actuslly realise they are doing it. Often enough most people think by having a nutient solution at the max strength as the plant will allow will then give them max yield possible.

    Also note if your using an additive such as sillica you also adding potassium to your tank. You can change much less ofthen than you think. Providing its clean as microbes can build up quick in hydro. Also have a look through one of jessies old diarys mate. Im sure he done a few with little to no changes

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    Nice post BB I will have a look at law of minimum. Do you mean Jessie Pinkman?


    I have been having a think and done just done a little experiment.

    Drilled a 1mm hole in a bucket and filled it with 5 litres of water it took half an hour to run through so my idea is to switch off the header feed tank and let my system run maybe 20 litres low then add PK to 20 litres of water until I have hit my desired amount of PK (haven't done the maths yet} or untill my 20 litres of pk/water hit my desired ec, which ever comes first. I will then drip it into the epicenter through the 1mm hole. Just a few short bits of timber suspended over the epicenter to support the bucket and two hours later all done....Well that is the plan. I may even use this method with a five litre bucket if I need to raise/lower ph or increase ec quickly.

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