Cannabis plants require different nutrients at different stages of their life cycle, which can be split into three main stages: seedling, vegetative growth, and flowering.
Germination, or rooting clones, usually takes place for the first 1-2 weeks of the plant’s lifecycle. Depending on what medium (soil, coco, rockwool, etc) you’re using, your plants may already have all the nutrients they need for this stage. If not, because of the substrate being inert, a recommended PPM value of between 200-400 is recommended (0.2-0.4 EC), with a nutrient ratio along the lines of 2-1-2.
During this stage, plants will require a significant increase in nitrogen, as well as some phosphorus and potassium.
During the first 4 weeks of vegetative growth, PPMs can be gradually increased to max out at around 800, with a ratio of around 4-2-3. Note that many commercial grows typically do not surpass 4 weeks of vegetative growth. However, should you choose to do so, you can increase PPMs to 1000 for the next 2-3 weeks after that, and then adjust accordingly, depending on the plants’ needs.
As a general rule of thumb, during the vegetative stage, it’s good to keep phosphorus at around half that of nitrogen, and potassium at around two-thirds that of nitrogen, although this is also strain dependant, so being vigilant is key.
As cannabis plants transition into flowering, macronutrient requirements swing in the opposite direction, meaning they require a significant increase in phosphorus, to give them the appropriate resources in order to create large flowers. Some growers taper off their nitrogen and gradually increase their phosphorous over the course of a few feeds, but this isn’t necessarily optimal or required.
As a general rule of thumb, you should aim to reduce nitrogen to around half the amount of phosphorus, and potassium to around two-thirds.
A ratio of 5-10-7 or 4-10-7 is adequate for this stage, with PPMs potentially ranging up to 1500 and beyond, depending on flowering length and nutrient requirements (strain dependant).
Note that nitrogen can be brought down a further notch during the very last weeks of the life cycle, as the plant is now focusing all its efforts on flower production.