Hey All, 
There’s been quite a bit of confusion lately about the Lucas formula and methods so I put this little tutorial together. I also needed an on-site link to a thread that just outlines how to use it, without all the theory behind it as well. The best part about these methods and formula is how simple they are and how well they work, even if you don’t quite understand the why behind it.
This write up covers nutrients from General Hydroponics and, though others have formulated similar formulas with other nutes, GH is all I have experience with. I know that, when the directions are followed closely, you’ll get excellent results as long as the rest of your grow is in good shape. (Ventilation/temps/etc.)
I highly recommend the Flora Nova Bloom for simplicity and super lush and healthy plants. I've used the Flora 3-part for years with great results but really prefer that extra kick the Nova gives.
This formula can be used in both veg and bloom.
If you are not constrained by growing space, you may want to use the FloraNova Grow during veg as it increases the internode length.
Note: This formula works 100% of the time with RO water. No guarantees for tap-water. If your res was not mixed according to these directions to start with, raising your pH to 6.0-6.1 during days 20-30 of 12/12 will prevent/remove the mag deficiency you would normally see. There is NO reason to use Cal/Mag.
Additional Note: DO NOT check pH or TDS/PPM/EC without first making sure the res is topped off.
Those numbers are useless to you and will only confuse you until you fully understand. (They’re still useless to me. LOL)
Lucas Formula
(Choose one of the following nutrient types)
Using GH Flora 3-part liquid nutes:
0ml of Grow, 8ml of Micro and 16ml of Bloom per gallon of RO.
0-8-16
Or
Using GH FloraNova Bloom One-part liquid nutrient:
8ml per gallon of RO
Lucas Method(s)
The method you use to maintain the nutrient levels in the reservoir, after you’ve mixed them, depends on the equipment you have and your level of understanding. There’s a simple change-out method and an add-back method. Both are very successful with the add-back having a higher yield but also being a bit more complicated and requiring an EC/TDS/PPM meter of some sort. I highly recommend anyone new to the community to use the simple change-out first and to only use pH drops or a pen for it. It will allow you to concentrate on the plant more.
Simple Change-out method
This simple method for using the Lucas formula only needs pH drops or a pH pen to be successful with. Let’s use a reservoir that holds 10 gallons of mixed nutrient solution as our example.
(Choose one of the following nutrient types)
Using GH 3-part:
Add 80ml of Micro and mix it in well.
Add 160ml of Bloom and mix it in well.
Adjust pH to 5.2
Or
Using GH FloraNova Bloom (FNB):
Add 80ml of FNB and mix well.
Adjust pH to 5.2
Mark the level of the nutrients in your reservoir and walk away for 24hrs.
Keep the level of the reservoir the same by adding RO water and keep track of how many gallons you’re adding.
Once you’ve added back the original number of gallons you mixed (in our case, 10 gallons) it’s time to dump the reservoir and mix a fresh batch of nutes.
That’s it! (wasn't that simple?
)
You’ll notice that I didn’t mention adjusting the pH again. That’s because you won’t have to. All you should do on your first grow is check the pH daily and mark it down. Keep track and see the pH swing as it goes up and down through the life of the res. Watch the plant too and write down any changes you see in growth. If your pH bottoms out below 4 you have root rot or some other issue, adjusting your pH won’t help.
Advanced Add-Back method:
If you have a EC/tds/ppm meter you can increase your yield by maintaining a constant level of nutrients throughout the grow. This is done by periodically adding back nutrients to your reservoir and works best when used in combination with a float valve to continually keep the reservoir topped off with RO.
Let’s take our same 10 gallon res and mix up a batch of nutes according to the simple change-out method. Once that’s done the fun begins.
Disclaimer: I just received my first tds pen a few days ago and have started my first add-back res. This means the following information is presented as I understand it from years of reading. I’m not perfect.
Test your fully mixed and pH'd res and you will get a ppm reading of around 950 (@.5 conversion) or 1400 (@.7 conversion). This is the nutrient level that you’re going to want to maintain throughout the grow. All you’re going to add are nutes occasionally to keep that ppm concentration constant.
Important: If you don’t have a float valve topping off your res, try to top off at least twice a day, beginning of lights on and just before lights off are good times to do it.
DWC/SWC/NFT Setups: Do NOT add nutrients directly to a res that has roots hanging into it! Please pull a gallon or so of water from the res, mix your nutes and then pour it slowly back in. Nutrients hitting your roots that have not been fully diluted yet will burn them and cause damage. Please don’t do this!
Note on 400w HPS and lower power lighting: You may find that your plants don’t have quite enough light to grow properly in a full strength solution. Start with 0-5-10. Increase to 0-6-12 if your pH steadily rises and goes above 6.1. Keep adjusting the strength until your pH swings between 5.2 and 6.1 without going past those points. Make sure you write everything down so that once you figure out what your plants like for your situation you can mix that strength to start with the next time.
Just remember that if your pH goes up out of range over time and doesn’t come back down, you’re not feeding enough. If your pH slowly goes down and down below 5.1, you’re feeding them too much. (This is of course assuming that everything else in the garden is good.)
This is only a basic guide to the Lucas formula and methods but it’s sufficient to grow excellent cannabis while you learn.
Final Note: This is not the ULTIMATE SOLUTION TO EVERTYTHING FORMULA! It is a general purpose formula that works in 100% of all gardens that are set up to properly grow cannabis. It is also just a starting point for when you learn enough to experiment on your own.
Each growing environment is different and has different strains/phenos so tweaking it to fit your specific plants will always increase yield when done properly. (Personally, I figure it will be another 4-5 years before I’ll reach that level of understanding. Thank you.) So, if you’re someone who’s never grown cannabis before, following the Lucas formula will give you awesome results while letting you focus on the plant, without getting bogged down in all the nitty gritty details about nutrients just yet.
Hope this helps and, if anyone would like to fill in the details on the add-back methods, I would sorely appreciate it. Thank you! 
There’s been quite a bit of confusion lately about the Lucas formula and methods so I put this little tutorial together. I also needed an on-site link to a thread that just outlines how to use it, without all the theory behind it as well. The best part about these methods and formula is how simple they are and how well they work, even if you don’t quite understand the why behind it.
This write up covers nutrients from General Hydroponics and, though others have formulated similar formulas with other nutes, GH is all I have experience with. I know that, when the directions are followed closely, you’ll get excellent results as long as the rest of your grow is in good shape. (Ventilation/temps/etc.)
I highly recommend the Flora Nova Bloom for simplicity and super lush and healthy plants. I've used the Flora 3-part for years with great results but really prefer that extra kick the Nova gives.
This formula can be used in both veg and bloom.
If you are not constrained by growing space, you may want to use the FloraNova Grow during veg as it increases the internode length.
Note: This formula works 100% of the time with RO water. No guarantees for tap-water. If your res was not mixed according to these directions to start with, raising your pH to 6.0-6.1 during days 20-30 of 12/12 will prevent/remove the mag deficiency you would normally see. There is NO reason to use Cal/Mag.
Additional Note: DO NOT check pH or TDS/PPM/EC without first making sure the res is topped off.
Those numbers are useless to you and will only confuse you until you fully understand. (They’re still useless to me. LOL) Lucas Formula
(Choose one of the following nutrient types)
Using GH Flora 3-part liquid nutes:
0ml of Grow, 8ml of Micro and 16ml of Bloom per gallon of RO.
0-8-16
Or
Using GH FloraNova Bloom One-part liquid nutrient:
8ml per gallon of RO
Lucas Method(s)
The method you use to maintain the nutrient levels in the reservoir, after you’ve mixed them, depends on the equipment you have and your level of understanding. There’s a simple change-out method and an add-back method. Both are very successful with the add-back having a higher yield but also being a bit more complicated and requiring an EC/TDS/PPM meter of some sort. I highly recommend anyone new to the community to use the simple change-out first and to only use pH drops or a pen for it. It will allow you to concentrate on the plant more.
Simple Change-out method
This simple method for using the Lucas formula only needs pH drops or a pH pen to be successful with. Let’s use a reservoir that holds 10 gallons of mixed nutrient solution as our example.
(Choose one of the following nutrient types)
Using GH 3-part:
Add 80ml of Micro and mix it in well.
Add 160ml of Bloom and mix it in well.
Adjust pH to 5.2
Or
Using GH FloraNova Bloom (FNB):
Add 80ml of FNB and mix well.
Adjust pH to 5.2
Mark the level of the nutrients in your reservoir and walk away for 24hrs.
Keep the level of the reservoir the same by adding RO water and keep track of how many gallons you’re adding.
Once you’ve added back the original number of gallons you mixed (in our case, 10 gallons) it’s time to dump the reservoir and mix a fresh batch of nutes.
That’s it! (wasn't that simple?
)You’ll notice that I didn’t mention adjusting the pH again. That’s because you won’t have to. All you should do on your first grow is check the pH daily and mark it down. Keep track and see the pH swing as it goes up and down through the life of the res. Watch the plant too and write down any changes you see in growth. If your pH bottoms out below 4 you have root rot or some other issue, adjusting your pH won’t help.
Advanced Add-Back method:
If you have a EC/tds/ppm meter you can increase your yield by maintaining a constant level of nutrients throughout the grow. This is done by periodically adding back nutrients to your reservoir and works best when used in combination with a float valve to continually keep the reservoir topped off with RO.
Let’s take our same 10 gallon res and mix up a batch of nutes according to the simple change-out method. Once that’s done the fun begins.
Disclaimer: I just received my first tds pen a few days ago and have started my first add-back res. This means the following information is presented as I understand it from years of reading. I’m not perfect.
Test your fully mixed and pH'd res and you will get a ppm reading of around 950 (@.5 conversion) or 1400 (@.7 conversion). This is the nutrient level that you’re going to want to maintain throughout the grow. All you’re going to add are nutes occasionally to keep that ppm concentration constant.
Important: If you don’t have a float valve topping off your res, try to top off at least twice a day, beginning of lights on and just before lights off are good times to do it.
DWC/SWC/NFT Setups: Do NOT add nutrients directly to a res that has roots hanging into it! Please pull a gallon or so of water from the res, mix your nutes and then pour it slowly back in. Nutrients hitting your roots that have not been fully diluted yet will burn them and cause damage. Please don’t do this!
Note on 400w HPS and lower power lighting: You may find that your plants don’t have quite enough light to grow properly in a full strength solution. Start with 0-5-10. Increase to 0-6-12 if your pH steadily rises and goes above 6.1. Keep adjusting the strength until your pH swings between 5.2 and 6.1 without going past those points. Make sure you write everything down so that once you figure out what your plants like for your situation you can mix that strength to start with the next time.
Just remember that if your pH goes up out of range over time and doesn’t come back down, you’re not feeding enough. If your pH slowly goes down and down below 5.1, you’re feeding them too much. (This is of course assuming that everything else in the garden is good.)
This is only a basic guide to the Lucas formula and methods but it’s sufficient to grow excellent cannabis while you learn.
Final Note: This is not the ULTIMATE SOLUTION TO EVERTYTHING FORMULA! It is a general purpose formula that works in 100% of all gardens that are set up to properly grow cannabis. It is also just a starting point for when you learn enough to experiment on your own.
Each growing environment is different and has different strains/phenos so tweaking it to fit your specific plants will always increase yield when done properly. (Personally, I figure it will be another 4-5 years before I’ll reach that level of understanding. Thank you.) So, if you’re someone who’s never grown cannabis before, following the Lucas formula will give you awesome results while letting you focus on the plant, without getting bogged down in all the nitty gritty details about nutrients just yet.
Hope this helps and, if anyone would like to fill in the details on the add-back methods, I would sorely appreciate it.
Thank you! 
