How to use nutrients and fertilizers to grow marijuana plants

A cannabis plant needs many nutrients, and pulls these from the soil. Left on its own, with good soil, plenty of light and water, and a temperate environment, a weed plant will grow fine, but nutrients will help the plant thrive and grow healthy and strong.

What are cannabis nutrients?

Growing high-quality weed requires more nutrients, or fertilizer, than most common crops. 

Outdoor cannabis growers typically add powdered nutrients to soil when transplanting a weed plant outside. This will give the plant all or most of the nutrients it needs for its entire life cycle, and if you want to add more nutrients to plants later, you can add them to the top of soil—called “top dressing.” 

Indoor growers typically use liquid nutrients and mix them in with water before watering plants. Using liquid nutrients is usually more time consuming, as you typically have to measure and mix them in water 1-2 times a week.

We recommend not using nutrients made for indoor growing for outdoor plants, as they are usually composed of synthetic mineral salts and can damage soil bacteria.

What nutrients does a cannabis plant need?

Your marijuana 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)

Other micronutrients that occur in very small amounts and that you don’t hear about as much include: boron, chlorine, copper, iron, manganese, molybdenum, and zinc.

Additionally, cannabis plants derive these non-mineral elements from air and water:

• Carbon
• Hydrogen
• Oxygen

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. 

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.

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.

How to use and mix cannabis nutrients

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, For example, a product that says “10-4-4” will contain 10% available nitrogen, 4% phosphorus, and 4% potassium by weight. 

A general rule of thumb is that a vegetative fertilizer should have high nitrogen, low phosphorus, and moderate potassium: for example, 9-4-5. As a plant transitions into flower, taper off the nitrogen and focus on phosphorus and potassium—seek a ratio around 3-8-7, for example.

Products are also generally divided into “grow” solutions, high in nitrogen needed for vegetative growth, and “bloom” solutions, high in phosphorus for flower development. You can stick to these general terms if you don’t want to get bogged down with numbers. 

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.

Liquid nutrients

Liquid nutrients are typically used for indoor growing, but can be used outdoors too. Liquid nutrients are used for weed plants in soil, hydroponics, and other grow media, and can be pushed through drip lines, misters, and hoses for easy and efficient delivery. 

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.

To use liquid nutrients, you’ll need a separate water tank, such as a dedicated garbage bin, to mix them into water. You’ll also need to know how much water is needed for all your plants. Depending on the amount of water you need, add the correct ratio of liquid nutrients according to the bottle’s directions.

When using liquid nutrients for cannabis plants, it’s important to have a watering schedule to write down and track:

• How much water you use
• How many and what kind of nutrients you use
• How frequently you water

You don’t want to use liquid nutrients every time you water—use them every other watering, or two waterings on, one off. It depends on the complexity of your soil and the health of your plants. Too many nutrients will damage your plants.

Giving weed plants the proper amount of nutrients requires careful monitoring. Many growers start at a solution dose lower than recommended and work their way up until plants respond optimally. Too little nutrients and the plants will have stunted growth, while too many can lead to nutrient burn and lockout.

Check your water pH levels

It’s important to get a pH meter and pH test kit 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.

The importance of timing and frequency when using nutrients and fertilizers

Weed plants need a consistent amount of nutrients as they get bigger. It’s a good idea to create a feeding schedule, which outlines when to water plants with nutrients, and the levels of nutrients needed at each watering.

Growers usually do not add nutrients at every watering because it can lead to nutrient lockout. It’s best to water with straight water as well.

When using fertilizers for outdoor growing, they are usually mixed into soil before a seed germinates, or when a small plant is transplanted into new soil. Plants can be top-dressed with additional fertilizer as needed.

Can you fertilize cannabis seedlings or clones?

We do not recommend adding fertilizers or additional nutrients to seeds, clones, or seedlings. A plant is very delicate in its initial stages and only water will suffice.

Best nutrients for a marijuana plant’s vegetative stage

As a cannabis plants grows, it will need different nutrients at different stages of its life. During the vegetative stage, cannabis plants need more nitrogen. These are usually nutrients with “Grow” in the name, to help the plant pack on stems, branches, and leaves.

In the N-P-K system of nutrient ratios, this is the first letter, “N.”

Best nutrients for a marijuana plant’s flowering stage

During flowering, a cannabis plant focuses its energies on producing buds, or flowers, instead of stems, branches, and leaves. When flowering, a weed plant needs more phosphorus and potassium, and less nitrogen. These are usually called “bloom” nutrients.

In the N-P-K system of nutrient ratios, phosphorus is “P” and potassium is “K.”

5 best nutrients for cannabis

There are many different cannabis nutrients out there and it may be overwhelming knowing where to start. Here’s a breakdown on some of our favorites.

Organic cannabis 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. 

Most of these fertilizers can be purchased cheaply at your local nursery and then mixed into soil before potting outdoors. Done correctly, you’ll only need to water your plants throughout the growing process, as all nutrients are in the soil. 

We recommend these organic fertilizers:

• Nitrogen: Worm castings, 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.

Benefits of organic fertilizers for cannabis plants

One of the best things about organic fertilizers is they improve the soil while also improving the quality of your plants. Other benefits:

• 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

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

The fertilization process can repeat itself year after year as the soil continually improves—next year, your soil will be even better than this year’s. 

Using organics is also great if you want to be more in-tune with your natural environment. Organic fertilizers are readily available from renewable sources and are an earth friendly option.

Disadvantages of organic nutrients for cannabis plants

There are some complications in working with organic fertilizers. The main issue is if your weed plants have a nutrient deficiency, it takes longer for a plant to absorb organic powder nutrients, which can increase the damage to plants. Liquid nutrients act much quicker. Other disadvantages:

• They take time to be absorbed by the plant
• Require microorganisms to break down nutrients, which may slow in colder temperatures
• Can introduce insects and pests

What’s the best soil for growing cannabis?

Cannabis plants love loamy soil that’s not too dense and not too loose. Some great brands include:

• PRO-MIX
• Fox Farms
• Happy Frog

Standard potting soil from any of these companies will work great for weed plants. Soil with mycorrhizae, a beneficial fungus, will help a plant’s roots tremendously.

How to make compost tea for marijuana plants

Compost is filled with beneficial microorganisms and nutrients, and you can take it one step further by steeping it in aerated water. This process, called “compost tea,” extracts the microorganisms and soluble nutrients into a water “tea” solution. 

The goal of compost tea is to introduce nutrients, fungal colonies, and beneficial bacteria to either the soil or foliage of a marijuana plant to aid growth and protect it from harmful disease, promoting bigger, stronger, and more resilient plants.

Compost tea should never be a 100% replacement for nutrients, but it can be a great complement to other nutrients. 

You can add compost tea to weed plants by:

• Spraying it on leaves
• Applying it to soil

When applied to soil, you’re adding to the soil food web by introducing a healthy population of microorganisms that are aerobic in nature. These organisms hold nutrients, aerate soil, aid water retention, increase nutrient absorption in the cannabis plant, help grow healthy roots, and help prevent diseases.

However, the benefits of compost tea are debated in the agricultural world. Many gardeners report quality results when using it, while others see no more benefit than applying straight compost. The uncertainty lies in whether or not growing and developing populations of microorganisms in the tea can actually benefit plants and prevent disease.

Compost tea recipe for marijuana plants

A healthy compost tea pulls soluble nutrients and microorganisms from compost, including bacteria, fungi, and protozoa.

Here are five key compost tea ingredients recommended by the Beneficial Living Center located in Arcata, California, to create a successful tea that will work best for your cannabis.

• Compost: A healthy compost should have large populations of microorganisms and nutrients, and sourcing it locally will ensure organisms are local pathogens. Compost that contains developed mycelium (fungal colonies) populations will help aid the development of fungal growth in the tea.
• Worm Castings: These byproducts expelled from a worm after digestion provide a high density of nutrients in a broken-down, refined form readily available for plants. Worm castings also introduce microorganisms.
• Fish Hydrolysate: This is produced by breaking down fish and crustaceans to create a nitrogen-dense product. Crustacean exoskeletons also have chitin, which works as an immune booster for plants. Fish hydrolysate also helps feed and increase fungi populations.
• Kelp: This serves as a source of food for fungi that grow while the tea is brewing. It’s also thought to provide a surface for fungal colonies to attach to and develop.
• Molasses: Serves as a source of food for bacteria that grow while the tea is brewing.

How to make compost tea in 5 steps

Build a compost tea brewer

Before building a compost tea brewer, you need to consider the size of your cannabis garden. Most homegrows use 5-gallon buckets. On the outside of the bucket, you’ll need to have an air pump connected to an aerator device at the bottom. The aerator and air pump will oxygenate water so microorganisms can breathe. 

You’ll also need a 400-micron mesh bag to place ingredients for the tea. While you can buy pre-built tea brewers, you can also easily make your own for cheap.

Build your schedule

Tea brewing takes time, so it’s important to figure out when you want to apply the tea. Most teas generally take 24-36 hours to brew. You don’t want to let your tea brew for too long because the microorganism populations will develop to a point where they won’t have enough oxygen or space to live, and will begin to die, which can damage your tea.

Only start a tea when you can apply it within 36 hours of brewing it. When using as a spray, apply in the evening or morning when the temperature is low and without direct sunlight. This period is also when the stomata—pores in the plant’s foliage—are open to receive nutrients.

Fill your compost tea bag

When creating a first batch of tea, keep the solution simple. If you use city water, allow it to sit and breathe so chlorine can break down. Once your tea is brewing, keep it out of direct sunlight and make sure the air pump is running and oxygen is being pushed through the water.

Finalize your compost tea

There are multiple products that can be added in the middle of your brewing process, toward the end, or right before application: Food for bacteria and fungi can be added halfway through the brewing process to increase the growth of microorganisms; products like SeaGreen and Actinovate can be added before the tea is applied to plants for additional benefits.

Applying compost tea on cannabis

The tea can be applied to roots or as a spray on leaves of your cannabis plants. Dilute the tea with water at a ratio around 1:20 when applying it to roots. A basic tea can’t harm or burn your plants, so you can apply a potent dose freely. As a foliar spray, compost tea is generally diluted with water at a 1:2 ratio.

Don’t put compost tea through drip irrigation lines because it will clog them up over time. It’s important to either gravity feed the tea or use a diaphragm pump—as opposed to a centrifugal pump—to avoid chopping up and disrupting the active microorganisms when watering.

Trevor Hennings contributed to this article.

Pat Goggins edited this article.

Ready to start growing your own marijuana?

Buy marijuana seeds on Leafly.