All cannabis plants require essential nutrients to grow healthy and, depending on your specific setup, you’ll need to help your plants out by providing the necessities. You’ll need to know what you’re looking for before just grabbing the first mix that looks good. N-P-K, or nitrogen, phosphorus, and potassium, are all vital elements of plant nutrition, and essential for growth. Other nutrients are important for healthy growth in smaller amounts.  What and how much you add will depend on the stage of growth your plants are in as well as the grow medium used.

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Macronutrients for Growing Weed

Macronutrients are the elements necessary in large amounts for plants to grow properly. The most heavily required macronutrients for all plants are:

  • Nitrogen - Plays a critical role in photosynthesis as it facilitates the production of chlorophyll and amino acids.  A nitrogen deficiency is identifiable when the cannabis leaves begin to turn a pale yellow and eventually droop as the plant dies.
  • Phosphorus - Especially important in the process of photosynthesis, a cannabis plant’s metabolism, and the ability for the plant to absorb nutrients.  A phosphorous deficiency makes plants more vulnerable to pathogens and is identified by leaf discoloration from green to a purple-reddish hue.
  • Potassium - Helps plants grow earlier, fight disease better, and use water more efficiently.  Plants malnourished in potassium will show this deficiency with yellowing leaves.  However, a potassium deficiency distinguishes itself through the location of the yellowing; yellowing begins at the leaf’s edges.

This photo from Michigan shows neat rows of marijuana plants growing in black pots, connected by tubes to create a hydroponic growing system. The plants are lush and green. The plants are arranged symmetrically in the horizontal shot to create a central row with equal sides; additionally, green occupies the top half and black pots/white flooring compose the bottom. This photo could be used to depict either legal or medical marijuana cultivation. iStock / Rick_Thompson
In lesser quantities, cannabis plants also require secondary nutrients:

  • Calcium - Better enables plants to absorb other nutrients, maintain proper cell structure, and facilitate photosynthesis.
  • Magnesium - Plays a vital role in the creation of chlorophyll, making it essential in a plant’s ability to photosynthesize.
  • Sulfur - Aids in the development of strong roots, chlorophyll, and plant proteins during the cannabis plant’s vegetative stage.

If you’re purchasing nutrients, you should see an NPK ratio listed. These three are the most important for healthy growth, though the ratio will change as your plants develop. When shopping for nutrients, the ratio will always be listed in the order of nitrogen, phosphorus, and potassium.

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Nutrient Ratios

During the vegetative stage, a ratio of 3-1-2 is recommended, though there are always variations others have success with. The most important thing is that the nitrogen content is higher than phosphorus andÅÅ potassium. Higher nitrogen content is required for photosynthesis, a necessary process for the development of the plant during the vegetative stage.

When plants enter the flowering stage, you’ll need to lower the nitrogen content and raise the levels of phosphorus and potassium. While it varies by specific strain, we recommend a ratio of 1-4-5, though others seem to have success with slightly different ratios as long as the nitrogen content has been significantly reduced.

The secondary nutrients calcium, magnesium, and sulfur, all are required in lesser amounts for healthy growth. These are typically provided by the soil, so it’s important to adjust if you’re growing with hydroponics.

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Micronutrients for Growing Weed

Micronutrients are important for growth, but they’re not nearly as crucial as NPK and are required in much smaller amounts. Still, if your plants lack these elements they may become unhealthy. Necessary micronutrients include:

  • Chlorine
  • Molybdenum
  • Copper
  • Iron
  • Manganese
  • Zinc
  • BoronPlants typically aren’t deficient in these nutrients unless other factors have occurred, such as a pH imbalance.

Soil vs. Hydroponic Nutrients

Whether you’re growing in soil or hydroponically will determine the right kind of nutrients you’ll need to use. Nutrient solutions vary, and using the wrong kind can damage your plants.

Man's hand is sowing fertilizer. Important steps to take care of plants. iStock / sakchai_R

Soil Nutrients

While chemical fertilizers may be tempting, they diminish the soil quality over time and are not good for your soil in the long-run. We recommend using organic fertilizers in your grow because they’ll produce a better tasting and healthier plant.

You can either purchase pre-made organic fertilizer mixes or get your hands on the necessary components and add them yourself.

If you’re still new to growing cannabis, using an already-made mixture from the store may be your best option because you won’t have to worry about ratios and application methods and can instead just fertilize your plants as directed.

If you want to go the DIY route, some options are:

  • Compost for N-P-K, though nailing the right ratio can be difficult. Compost is a great way to prevent wasting food, as you can add leftovers to your compost bin and let it work its own magic over time. Eggshells, fruit and veggie scraps, and coffee grounds are all great additions. If you have chickens, chicken manure is also a great source of N-P-K, but you’ll need to properly compost it lest you burn or kill your precious crops.
  • Fish Meal has a high nitrogen content with lower levels of phosphorus and potassium. It’s a dependable source of nitrogen, though it may attract predators if you live in an area where animals might sniff it out.
  • Guano (bat feces) is good for nitrogen and phosphorous. It helps enrich the soil, improve drainage, and even acts as a natural fungicide. Pay attention to the source if you’re going to use guano, as some unethical practices of harvesting have posed a major threat to bats.

Hydroponic Nutrients

Without soil, the addition of nutrients is more crucial to grow healthy plants. They will still require the same necessary nutrients as if they were grown in soil, they’ll just need you to provide them in a consistent way. One benefit of growing hydroponically is the ability to control the type of nutrients and the portions given. Just make sure your nutrient mix does not contain organic matter, as this can harm your plants and clog the system.

Healthy soil typically provides the necessary micronutrients for cannabis plants without extra supplementation, but in hydroponic growing systems, micronutrients will need to be added. Make sure your nutrient mix has all the components for healthy plants included to prevent any deficiencies.

How Cannabis Absorbs And Transports Nutrients

How plants absorb NPK
Nutrients are absorbed in the roots through osmosis.  In plants, osmosis refers to the transfer of ions from water through the plant’s cell wall and into the cell.

This osmotic pressure—the movement of nutrient ions during osmosis—is a smart process.

Osmotic pressure moves according to the concentration of nutrients on each side, from the side with the highest concentration of ions to the one with the lowest. For example, when the cannabis plant uses phosphorous, the concentration of that ion is lower on the plant cell side than it is on the waterside.  Osmotic pressure then enables the plant to absorb phosphorous from the water.

If the concentration of phosphorus is the same inside of the plant cell as it is in the water, nothing happens.

There is no osmotic pressure. However, if there are more ions in the plant cell than there are in the water, the phosphorous will transfer to the water to create balance, causing a phosphorous deficiency. Once the nutrients are absorbed by the roots into the plant cells, they must move upward through the plant to get their work done. 

The pressure created by salts and oxygen present in the water as well as the leaves’ pores enable the nutrients to move through the cannabis plant. There are factors that can slow or stop the absorption and movement of nutrients.

If the humidity is too high, the leaves will struggle to create the right amount of pressure through evaporation, and the plant will absorb fewer nutrients.

If it is too hot, too much water may evaporate, and the plant will lose nutrients as a result of this excess evaporation.

This is why keeping cannabis at the right temperature is so important.  Reducing the plant’s sweating allows it to use the nutrients you’ve worked so hard to provide it. The materials you feed your plants with are converted into energy and the nutrients needed to create new plant cells and maintain existing ones.

The type of plant cells created depends on the stage of growth the plant is in; hence the different N-P-K ratios for each phase of cannabis growth.

Final Tips

The biggest factor in deciding the best nutrients for growing weed will depend on whether you’re growing in soil or hydroponically, so sort that out before picking up goodies for your plant.

The stage your plants are in is also crucial to pay attention to, as plants in the vegetative stage need high levels of certain nutrients while requiring smaller amounts in the flowering stage.

Your exact method will depend on many factors, such as growing medium, level of experience, and whether you want to be more in control of the process or just get a pre-made solution that will take care of it.

Whatever you decide, the bottom line is that you’ll probably need to do some experimenting before nailing the perfect nutrient solution, and that’s okay.