Best Hydroponics Nutrients | Lucas Formula

Easiest nutrient schedule and grow guide for vegetation and flowering when using hydroponics, specifically deep water culture
Estimated 6 min to read
June 15, 2021


The Lucas Formula is an extremely simple nutrient recipe using General Hydroponics Micro and Bloom designed for growing cannabis plants in hydroponics. See the chart below for the ratios, the example feeding schedule, and try out the add-back calculator at the end of this article for a quick way to calculate how much nutrients you need to add back into a nutrient reservoir.


The Lucas Formula is a method for feeding cannabis plants that are grown using hydroponics, many times Deep Water Culture (DWC). This is a formula many new hydroponic growers begin with because of its simplicity. With that said, if you are a new grower to cannabis in general, hydroponics will likely be difficult and result in smaller initial yields because of the many obscure “gotchas” involved in hydroponics. Instead, I would highly suggest beginning with a soil-less medium like coco-coir to keep a lot of the benefits of hydroponics, like higher yield and faster grow times, but with substantially less maintenance and potential problems. Here is my guide to get started growing weed with a soil-less medium like coco-coir.

Some of the earliest documentation of this method was here on rollitup forums in 2007 - but it existed before then. It has been well tested over the years and is considered a staple method for many hydroponic growers, and as such, is a safe way and easy way to feed your cannabis plants growing in a hydroponic solution. This formula is so popular because it provides a great starting point for most beginners in cannabis cultivation using hydroponics, since using full-strength manufacturer recommendations for nutrients is almost a sure-fire way to over feed your cannabis plant, resulting in burnt tips, stunted growth and potentially nutrient lockout. Lucas Formula was designed to provide all the nutrients your cannabis plant needs, while minimizing the risk of over feeding your plant. The Lucas Formula is based off of the General Hydroponics 3 part nutrition system.

Importantly though The Lucas Formula only uses the Micro and Bloom nutrients. This is because the GH grow solution provides too much nitrogen for cannabis plants, which typically results in nitrogen toxicity. The Micro solution provides enough nitrogen for the plant to thrive, while also providing other important nutrients like calcium and magnesium.

Lucas Formula Ratios


Grow PeriodMicroBloom
Vegetation5 ml/Gallon10 ml/Gallon
Bloom8 ml/Gallon16 ml/Gallon

Concentration after adding nutrients

Grow PeriodApprox ECApprox PPM @ 0.5
Vegetation1184 µS592
Bloom1894 µS947

Important Tips

  • Don’t start at full strength. When using the Lucas Formula (or any formula for that matter) you should always slowly ramp up your nutrient solution over time. Start at 1/4 strength and increase the PPM every time you need to refill the res. Before refilling make sure to check the PPM, and ensure it is still dropping. If the PPM is increasing, your plant is drinking more water than nutrients and you should decrease the concentration.
  • Each plant is different, you may never need full strength nutrients. Make sure you are carefully monitoring your plant, watching PPM and pH, to ensure you aren’t over or underfeeding.
  • Ensure your pH is between 5.5 and 6.2 - if its out this range, your plant may not eat or drink!

Example feeding schedule

Growth stage GH Micro (ml/Gallon) GH Bloom (ml/Gallon)
Week 0 (Germination) 0 0
Week 1 (Seedling) 2 4
Week 2 (Vegetation) 3 6
Week 3 (Vegetation) 5 10
Week 4 (Vegetation) 5 10
Week 5 (Transition) 5 10
Week 6 (Early Bloom) 6 12
Week 7 (Early Bloom) 7 14
Week 8 (Mid Bloom) 8 16
Week 9 (Mid Bloom) 8 16
Week 10 (Late Bloom) 8 16
Week 11 (Late Bloom) 8 16
Week 12 (Harvest) 8 16

Water type

This method is designed to be used with regular tap water, not reserve osmosis water (RO). Tap water is typically a decent source of Calcium and Magnesium which is needed for your cannabis plant. With that said, you still may need to supplement with cal/mag for a handful of reasons, such as if your water is very soft, or your cannabis strain has higher requirements. If you start to see calcium/magnesium deficiency feel free to add a calcium/magnesium supplement, like this one.

Common Deficiencies

Unfortunately, when growing with hydroponics, deficiencies frequently occur for new growers. This is a result of many different factors including grow space temperature, tap water irregularities (pH and nutrient), strain preferences, environmental bacterial growth and more. See this article for more information about deficiencies.

Addback Calculator

Lucas Formula add-back Calculator
Current Concentration (PPM)
Target Concentration (PPM)
Reservoir Capacity (Gallons)
0 mL Micro
0 mL Bloom
How to use the add-back calculator: When you need to refill your res, fill up a second container with the amount of water to top off your main res back to the maximum volume. Measure the current PPM for your res. Enter in the maximum volume of your res, the current PPM and goal PPM of your res into the calculator. Add the ml Micro and Bloom into the second container with water and fill your res with this solution - you should be at your target PPM!


Suppose you have a 5 gallon reservoir, and the plant drank 1 gallon, so the water level is now at 4 gallons. The PPM for the 4 gallons of solution measures to be 510, but your target is 592.

  • Add 1 gallon of water to a bucket
  • Enter target PPM of 592, current PPM of 510 and res gallons of 5 into the calculator. The calculator computes 5.5 mL of Micro and 11 mL bloom is needed.
  • Add the bloom and micro the calculator suggests to the 1 gallon of water. Mix this solution
  • Add the solution back to the main reservoir! Now the solution will be back to the target gallons (5) and PPM (592)!

This calculator uses this formula:

((target - current) / target) * 8 ml per gallon * res gallons = Flora Micro (ml)