Magnesium acts as the central molecule in chlorophyll and without it, plants aren’t able to generate the glucose from photosynthesis. No magnesium means no energy can be converted from sunlight.
Cannabis plants need different amounts of these nutrients throughout the different stages of growth: more nitrogen during vegetative growth, and more phosphorus and potassium during flower for bud production—also called “bloom” nutrients.
Because liquid nutrients are readily available to a cannabis plant’s roots, they are fast-acting, meaning they can damage plants if you feed them too much.
Your marijuana plants need the following primary nutrients, collectively known as macronutrients:
Yes, you can plant weed seeds in dirt and possibly grow a plant without any fertilizers or special nutrients. But chances are that you don't happen to live in a place with very rich, fertilized soil that carries just the right blend of nutrients needed by a marijuana plant to thrive.
While cannabis plants will grow in a lot of conditions, as a farmer, you are looking to optimize the size, weight, density, potency and overall quality of your buds. In order to do that, you will need to fertilize your cannabis plants in such a way that they can produce at their best.
Cannabis plants which don't get the right levels they need, especially in the flowering/budding phase, tend to grow small and airy buds. In extremely poor conditions, some cannabis plants may just sprout a few white hairs (pistils), and never actually form any real buds at all.
Because of that, I encourage growers to learn about the type of nutrients and special fertilizers needed by a cannabis plant, and at least start with good soil before you get started growing if you're determined not to use any nutrients.