Lucas Formula, sometimes known as the Lucas Ratio, is less complicated than it initially seems. It is nothing more than a basic recipe of nutrients to give your plants during both the vegetative, and the flowering state.
The original recipe uses 2 parts of the General Hydroponics 3-part FLORA series (Gro, Micro, Bloom) nutrient system.
Lucas Formula using General Hydroponics 3-Part System
General Hydroponics makes a 3-part system named the Flora series. It consists of Flora Gro, Flora Micro, and Flora Bloom. These are 3 bottles you buy, that you would normally use together throughout the entire grow. You may use it as per the instructions on the bottle, with excellent results, but…
Lucas figured out that Flora Micro contains enough nitrogen, and everything else that Flora Gro contains, that you use it with Flora Bloom alone, saving you from having to buy Flora Gro.
How do you use it? Simple. Add 8ml of Flora Micro (dark red stuff, there are Hard and Soft water versions ) to 1 gallon of water*, mix, and then add 16ml of Flora Bloom (pink stuff) into the water, and mix. Done – unless you’re growing in coco.
Low light ratio, and flowering vs veg.
Lucas Formula should be used in the same ratio throughout the grow. There is a misconception that one should use 5ml of Micro and 10ml of Bloom, when in veg, and switch to 8ml of Micro, and 16ml of Bloom, when flowering.
This was never stated by Lucas himself. He originally stated that the 0-5-10 ratio was for low light situations (fluorescent lighting like PL/L and CFLs, or HID lamps less than 400W), whereas 0-8-16 is for medium to high light (400W+)
Lucas Formula for seedlings/cuttings
For young plants (< 4 weeks old) or cuttings, which may not be able to handle a full strength feeding formula, may require that you dilute the nutrient solution before feeding. This is OK, just make sure you keep the correct ratio when mixing. For example, 50% strength would be 4ml Micro and 8ml Bloom.
If you’re growing in coco, you may need to adjust the Lucas formula slightly to compensate for a property of coco which may result in a calcium or magnesium deficiency. How? You may be able to get by simply by adding 1-2ml of CalMag, or 1 gram of Epsom salt (any pharmacy carries this), per gallon of water, before feeding the plants.
There is a modified formula specifically for Coco that consists of using a different ratio: 6ml of Flora Micro, and 9ml of Flora Bloom, per gallon of water, along with 1 gram of Epsom salt per water.
The original formula uses reverse osmosis, or RO, water, and is intended to be used without having to measure pH and ppm. Reverse Osmosis water makes this possible because it should, in theory, be the same for everyone. It is filtered water, which you can get either by buying a reverse osmosis filter system, or just buying bottled water. One may also use dH2O (distilled water) with good results.
Lucas Formula with tap water
You may use Lucas Formula with tap water successfully, but this may require pH adjusting on your part. Keep the pH close to 5.8.
Can I use additives with Lucas Formula?
You may. It is recommended that you do not introduce additives until your setup is dialed in. After you’ve ironed out any deficiencies and fully understand how your plant and strain likes to be fed, feel free to begin using additives – not that the additives will help. For increased quality and yield, focus on good genetics, and ample light and temperature control, rather than trying to compensate for those with additives.