Cannabis plants need the following primary nutrients, collectively known as macronutrients:

  • Nitrogen (N)
  • Phosphorus (P)
  • Potassium (K)

These micronutrients are needed as well, but in much smaller quantities:

  • Calcium (Ca)
  • Magnesium (Ma)
  • Sulfur (S)

Cannabis plants also need carbon dioxide and oxygen, which they obtain through airflow, and hydrogen, which comes from water.

Finally, there is a more expansive group of trace, or “micro”, nutrients that boost marijuana plant growth. These essential micronutrients include zinc, manganese, iron, boron, chloride, cobalt, copper, molybdenum, and silicon, among others. These are needed in much lower quantities, but are still essential for healthy plant growth.

In general, macronutrients are absolutely required, whereas micronutrients help ensure a thriving plant.

Macronutrients

Nitrogen

Nitrogen is mainly responsible for a cannabis plant’s development during the vegetative stage of its life. It’s an essential part of chlorophyll and without it, a plant can’t turn sunlight into energy and it won’t be able to grow.

Nitrogen is also part of amino acids that act as building blocks for proteins in a plant. Without the necessary proteins, your cannabis plants will be weak and frail. Nitrogen is also a part of ATP, which allows plant cells to control the use of energy.

Nitrogen is also necessary to create nucleic acid, an essential ingredient in DNA or RNA, and without it, cells won’t be able to grow and multiply.

Phosphorus

Phosphorus is important for producing large, healthy buds. The key role of this element is to help make nutrients available for the plant to uptake. These nutrients are used to build the structure of a plant as it grows from its roots to its flowers.

Without adequate phosphorus, marijuana plants will show signs of undeveloped roots and might not even flower. Early signs of phosphorus deficiency shows up as a purple hue in the veins of leaves.

Potassium

Potassium has a number of jobs that largely help regulate the systems that keep a plant healthy and growing. It plays a large role in osmoregulation, the passive regulation of water and salt concentrations in the plant. Potassium accomplishes this by controlling the opening and closing of the stomata—the pores in the leaves—which is how a plant exchanges CO2, H2O, and oxygen.

Potassium also triggers the production of ATP, which works to store energy produced in photosynthesis by creating glucose. This glucose is then used as energy for the plant as it grows. Without sufficient potassium, you will see weak plants starved for energy that appear burnt because they are unable to successfully regulate the exchange of CO2, H2O, and oxygen.

Micronutrients

Calcium

Calcium is responsible for keeping the structure of cell walls in a plant together. Without calcium, new growth won’t develop properly and the plant won’t function as it should. New growth will be stunted, leaves will curl, and rusty spots will show up on the plant.

Magnesium

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.

Once magnesium has helped create glucose, it helps metabolize glucose to make it available for the plant to grow. Without sufficient magnesium, you will find yellowing leaves, with discoloration reaching the veins as well.

Fertilizing Cannabis Plants

Nutrient solution bottles and fertilizer bags will indicate how much of the three main nutrients are in the product, in the form of N-P-K: Nitrogen, phosphorus, and potassium

Cannabis plants need different levels of nutrients depending on their growth stage.

A general rule is that a vegetative fertilizer should have high nitrogen, low phosphorus, and moderate potassium. As a plant transitions into flower, taper off the nitrogen and focus on phosphorus and potassium.

  1. Seedling: Minimal or no fertilizer. You may want to wait until your plants have sprouted a few leaves before administering the first dose of primary NPK fertilizer.
  2. Vegetative: For Week 1, use an NPK ratio of 2:1:2 — that’s two parts nitrogen to one part phosphorus to two parts potassium. By Week 7, consider increasing the NPK ratio to 10:5:7, followed by a 1:1:1 ratio in the late vegetative phase.
  3. Flowering: At this juncture, stop feeding nitrogen to the plants and focus on elevating the phosphorus and potassium levels. It is useful to fertilize plants during the early flowering stage but not as effective in the latter weeks after true buds have formed. 

In the final week or so before harvest, be sure to give your plants only water to clear any nutrient buildup in the buds—this is called flushing.

It’s important to get a pH meter to check the pH level of your water when mixing nutrients. Cannabis prefers a pH between 6 and 7 in soil, and between 5.5 and 6.5 in hydroponic media. Letting the pH get out of this range can lead to nutrient lockout, meaning your plants are unable to absorb the nutrients they need, so be sure to test your water regularly and make sure the nutrient mix you give plants falls within the desired range.

Organic Fertilizers

Organic fertilizers are nutrients that come from organic sources such as animal and vegetable waste. They also include sediments like glacial rock dust and gypsum that contain beneficial minerals for the soil and plant. They are common for outdoor growing and usually come in powder form.

Organic fertilizers and nutrients can be more forgiving than liquid nutrients. They usually contain less immediately soluble nutrients and more elements that are beneficial to soil organisms.

We recommend these organic fertilizers:

  • Nitrogen: Worm casings, blood meal, fish meal, bat guano
  • Phosphorus: Bone meal, rock dust
  • Potassium: Wood ash, kelp meal
  • Calcium and magnesium: Dolomite lime

Commercial soil blends also exist that already contain the proper mix of these nutrients.

Integrating these ingredients into regular soil can help you create a super soil with a diverse wealth of nutrients. While you can also purchase commercial fertilizers in liquid or powder form, natural materials often provide the most powerful nutrients.

Some growers also find that growing organically increases the flavor profile of finished cannabis as well as increases yields.

Organic fertilizers improve the soil while also improving the quality of your plants. Other benefits include:

  • The slow release of nutrients protects plants from too many nutrients
  • Over time, organic fertilizers will improve the quality and diversity of life in soil
  • Improved airflow and water retention in soil
  • Renewable and sustainable
  • Organics stay in the soil with a lower chance of nutrient run-off