While animals have to search for their food to obtain energy, nature has equipped plants with the ability to photosynthesize the energy they require from light and convert it into chemical energy, making them self-sufficient in terms of food for survival. Cannabis plants will naturally thrive on their own given the right conditions, however with a little bit of outside help in the form of supplementary nutrition (also known as fertilizer), they can reach their full potential, making both the plant and the grower very happy. The right balance of nutrients will result in large healthy plants and a bountiful harvest.

Just like humans and other mammals, plants have specific nutritional requirements in order to survive. In addition to the elements they obtain from air and water (carbon, hydrogen and oxygen), they obtain other basic nutrients from the soil/medium they are in and the water that reaches their roots. They require elements called macronutrients, which are nutrients used by the plant in large amounts as building blocks for rapid growth, as well as micronutrients that are used in much smaller amounts, specifically for chlorophyll formation. Having the right balance of both micro and macronutrients is key to ensuring the proper development of plants. In addition to having the right nutrients to grow, plants will also require different nutrient ratios depending on the stage of the plant’s life cycle. This is where the grower’s ability to understand the plant’s needs will be crucial to ensure it is getting everything it needs.

the main nutrients for cannabis are potassium, phosphorus and nitrogen
The main nutrients for cannabis are nitrogen, phosphorus, potassium

The three main categories nutrients can be filed under are primary and secondary (aka macronutrients) as well as trace elements (micronutrients) nutrients. They can be further subcategorized into mobile and immobile nutrients. Anyone who has ever set foot in a grow shop or gardening center will recognize the nearly ubiquitous acronym of N-P-K (nitrogen, phosphorus, potassium), which constitute three of the five macronutrients cannabis plants require, the other two being magnesium (Mg) and zinc (Zn). Together they form a group referred to as mobile nutrients, meaning they can move around the plant, bringing elements to areas of deficiency as needed.

The remaining nutrients (micronutrients) – calcium (Ca), boron (B), chlorine (Cl), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mb), silicon (Si) and sulfur (S) are regarded as immobile nutrients, which means they do not move from the original location they were deposited, in other words in the older leaves. With respect to mobile nutrients, older leaves will be the first to show deficiencies as they will move to wherever the plant needs them to, usually newer foliage. The opposite is the case with immobile nutrients: symptoms of nutrient deficiency show up in newer leaves coming out of the top of the plant but the nutrients remain in the older leaves.

Different Stages, Different Nutrition

Cannabis will always require the same macronutrients throughout its life cycle, though their ratios will change depending on the stage of development the plant is at. This is because of the different roles each nutrient plays in the various biological processes taking place. Nitrogen is primarily responsible for ensuring green foliage during the vegetative stage, while phosphorus promotes strong root development and aids flowering while potassium is used to regulate CO2 uptake, thus enhancing photosynthesis and is also used during flower formation and seed production after fertilization. The table below provides  approximate N-P-K ratios for the various stages of the plant life cycle.

Life Cycle StageN-P-K RatioPPM (parts per million)
Cuttings + seedlings1-2-2200-300
Vegetative growth3-1-2600-800

Temperature and light intensity can also affect the need for nitrogen, so if your grow space tends to be hot, decreasing nitrogen by up to a fifth will help prevent stalks from weakening due to increased transpiration. If the same level of N is maintained, growers run the risk of concentrating this nutrient, which could lead to complications later on. The reverse is also true, under cold conditions plants should receive up to 20% more N as their water uptake will be reduced.

The Measuring Game

So you know what your plants will need at their various stages of growth, but how can you tell exactly how much of each nutrient? How can you tell what nutrients a plant has at all? To help explain this, we will go through a few more of those wonderful scientific acronyms, of which there is no shortage in cannabis cultivation.

pH – Potential (or power) of Hydrogen – This is nothing more than an expression of the acidity of a given solution, with pH 1 being very acidic, pH 7 being neutral and pH 14 being very basic/alkaline. The number is a representation of the concentrated hydrogen or hydroxyl ions of a given substance. For cannabis, this measurement is important because the majority of the nutrients your plants will use fall within a limited acidity range (5.5-6.5 for solid mediums, 5.6-6.4 for hydroponic solutions).

PPM – Parts Per Million – This is a unit to describe the concentration of elements out of a million parts in water or soil. A rough equivalent of one ppm would be 1 milligram of any element per liter of water (mg/l) or per kilogram of soil (mg/kg).

TDS – Total Dissolved Solids – A measure of the dissolved combined content of all substances in a solution or medium and is interchangeable with ppm.

EC – Electrical Conductivity – A measure of the amount of electrical current a material is able to carry. EC is used to measure the strength of the nutrient solution (salt concentration) and is the most consistent method of doing so.

As mentioned above, temperature can affect transpiration, which can affect the concentration of nutrients over time. The water in planting mediums will eventually evaporate and need to be replaced, which is the easy part. The trickier part is knowing how much and when to replenish the nutrients required for your plants’ health once there is a deficiency. This is where measurement tools come in. The two most common (and easiest) types are readily available at gardening centers or the web: meters and chemical test kits. Digital meters are precise and relatively inexpensive, though analog meters do the job as well and are generally much cheaper. The other type of measurement you can take to get a precise snapshot of your nutrient profile at any given time is a chemical soil test kit. They can require a bit of tinkering with chemicals and lab tools, but generally speaking they are easy to run and provide precise individual readings of nutrients, which meters cannot as they only give overall readings. Depending on the type of meter, the readout for your measurement can be in either EC or TDS/PPM, whereas some devices can read all three.

Ph Chart for Growing Marijuana

The optimum pH range for soil/solid mediums is 5.8-6.5 with 6.2 being the sweet spot.

For hydroponic setups the optimum pH range is 5.6-6.3 with 6.0 being the happy medium.

Regardless of whether you are using a solid medium or hydroponics, it is always a good idea to test the pH of your water for your nutrients. Depending on your water source, you could have very soft water (very little mineral content) or very hard (high amounts of Ca and other minerals). When mixed with soil, the overall pH level could vary widely as a result of the contents of the medium. Without testing it first, determining whether to raise or lower your pH is a gamble and can lead to problems down the road.

Feeding Time

Once you have decided on a grow method for your garden (solid medium/hydroponics), you will have to prepare your medium accordingly. If using soil or another solid medium, you can take a few steps to make sure you will provide your plants with a solid nutritional foundation so that supplemental fertilization can be reduced to a minimum. Having a well-balanced organic medium with proper drainage (so that roots are adequately oxygenated and have enough room to breathe) is a must. Grow shops will usually be able to recommend a proper medium and frequently will have pre-fertilized mixes so that you can hold off on adding your own. If making your own mix, make sure that you have a good N-P-K balance to start off with and that the mix has proper drainage. Adding microbes and mycorrhizae will also enable micronutrients to be better absorbed as well.

Cannabis plants being watered.
Feeding cannabis plants with water and nutrients

As far as fertilizer itself is concerned, a good rule of thumb is the old adage of “less is more”. Just like with humans and animals, overfeeding and malnutrition can lead to chemical imbalances that will affect overall long-term health, so it is important to maintain the right balance and not overdo it. Novice growers in particular tend to get very excited and overfeed their plants thinking there cannot be too much of a good thing for their garden, however this can lead to nutrient burn and lockout, which is why measuring pH and EC is key.

If using ready-made nutrient mixes/fertilizers, most will come with instructions on how much to mix with water to get the right concentrations, however it is never a bad idea to have a slightly more diluted concentration as you can always add more later if necessary. It is more difficult to remove excess nutrients than to deal with a slight deficiency. Hydroponic solutions come in all sorts of different ratios, so depending on the growth stage of your plant, choose the one that will suit the plant’s needs best, however make sure to check the pH of the nutrient solution once you mix it with water and BEFORE applying to plants as the mineral content of the water may require a pH adjustment. For solid mediums, simple watering with the right amount of fertilizer will work just fine, though foliar application by spraying can also be effective as leaves are also capable of nutrient absorption.

Top dressing is another method of feeding your plants by spreading a fresh layer of a rich soil mix directly on top of your grow medium. To do so, first water your plants to loosen up the soil, next remove an inch or two of the top layer just above the rhizosphere so as not to disturb the roots, then place your fertilizer (compost) on top.

Cannabis plant with fertilizer
Top dressing cannabis soil is another method of feeding your plants

Compost Teas and Homemade Mixes

The DIY approach for preparing nutrients for your plants can be quite simple and very effective in addition to being much cheaper than store-bought products. Many nutrient/fertilizer brands like to claim they are 100% organic or that their product is something plants will absolutely love ad infinitum, however in many cases this is just a marketing trick to boost sales.

In addition, if you are going to periodically flush your plants, you will also be flushing away whatever nutrients you’ve added, meaning you will be flushing away your money as well, so it is best to use supplemental nutrition sparingly and on an as-needed basis. That being said, homemade compost mixes and compost teas are a fantastic way of providing your plants with nutritional supplementation.

Compost tea provides several benefits to your plants, though its main functions are to inject microbes into your medium or onto plant foliage as well as to add soluble nutrients for the microbial life that is already present. In contrast to synthetic fertilizers and other chemical based additives such as pesticides, compost teas actually add beneficial microbes to your medium/foliage. In addition to the plethora of nutrients for your soil, compost tea can also add fungi, bacteria, eukaryotes and even nematodes to your grow medium, spiking the underground activity that plant roots will thrive in. If prepared properly, that is using high-quality compost, your tea will:

  • Provide protection to plant surfaces and prevent diseases
  • Improve nutrient retention and reduce the use of fertilizers
  • Provide more nutrients to the root system
  • Improve water retention reducing water use
  • Provide better soil structure required for the entire food web
  • Improve tolerance and resistance to stress and enable faster recovery

Compost tea is a strong concentrate and may spoil quickly so it is best used within 8 hours of brewing. Aeration is also vital to compost tea so use an air stone to keep the solution well oxygenated.

If you prefer a more solid fertilizer, homemade mixtures couldn’t be any simpler and you likely might already have some components lying around your house. These might include grass clippings, weeds, kitchen scraps, manure, tree leaves, coffee grounds/spent tea bags, banana peels and egg shells. There are multiple recipes available online or at your grow shop, so before throwing something away check to see whether you can throw it on the compost heap. Another reason why choosing to use organic fertilizers over chemical ones is that organic elements will allow microbes to thrive, meaning your plants will produce more of those desired terpenes that make harvesting and trimming (not to mention consuming) the wonderfully odoriferous experiences they are.

Overfeeding Cannabis - Too Much Of A Good Thing

As mentioned above, giving your plants more nutrients than they need will not necessarily make them healthy. In fact, it can wreak havoc on your entire crop if you aren’t careful. Excess nutrients are a common problem in many rookie gardens as many inexperienced growers believe more nutes means more health. However, cannabis plants can only convert light energy to chemical energy (photosynthesis), meaning they cannot convert chemical energy into stored energy like certain animals do when they turn excess carbohydrates into fat. Excess nutrients in cannabis (as well as other plants) can trigger a chemical reaction between the nutrient solution, the grow medium and the plant where various salts and other nutrients bond together or become incompatible, resulting in their inability to be used/absorbed by the plant, also known as nutrient lockout. A similar problem in cannabis gardens is nutrient burn, which is another type of stress plants face as a result of excess nutrients (high concentration), overwatering or using too many growth stimulants, which results in yellow or brown burnt leaf tips. Severe cases can be remedied by removing the dead plant matter and flushing your medium with pH-balanced water.

A pH up/down solution can be applied to adjust pH levels as needed. Store-bought versions are safe to use and are much less of a hassle than making your own using concentrated acids, however vinegar is a good and simple solution to bring pH down.

Deficiencies can appear at different stages (early, mid, late) and intensify over time, however if you can identify them in time, they can fixed.

Other issues that might arise if you have an excess/deficiency of certain nutrients:

NITROGEN deficiency in cannabis

Nitrogen - The most common deficiency, the lower leaves turn a pale green, then they yellow and die. If untreated, the deficiency will work its way up towards the top of the plant, leaving only new growth green.

PHOSPHORUS deficiency in cannabis

Phosphorus - Leaves are stunted and grow slowly. Older leaves turn dark green and weak. Bluish/purple hues may develop, and edges curl downward and turn brown.

POTASSIUM deficiency in cannabis

Potassium - Rare in hydroponics, plant tips and bottom leaf edges develop necrotic spots and turn tan or brown and die.

BORON deficiency in cannabis

Boron - Grow tips first turn brown or gray and die soon after. Brown necrotic dead spots similar to strawberry seeds may also appear. If the plant receives excess amounts of boron, yellowing of the leaf tips occurs, progressing inward. Leaves eventually fall off and the plant dies.

calcium deficiency in cannabis

Calcium - Large necrotic spots  of tan, dried tissue appears along lateral leaf edges. Stems and branches are weak and brittle.

copper deficiency in cannabis

Copper - Deficiencies are rare but can be deadly. Young leaves show necrosis and copper, blue, or grey coloring at tips and turn yellow between the veins.

iron deficiency in cannabis

Iron - New leaves turn bright yellow while the veins remain green, looks similar to Mg deficiency but only affects new growth.

magnesium deficiency in cannabis

Magnesium - Common indoors, veins stay green while the rest of the leaf becomes yellow (chlorosis).

manganese deficiency in cannabis

Manganese - Leaf tissue turns yellow and necrotic spots (tan/brown) appear in the center of the leaf.

Molybdenum deficiency in cannabis

Molybdenum - Very rare but can more likely appear in strains that turn blue/purple from cold temperatures. Middle leaves become yellow and new leaves become twisted, giving them a pale, awkward appearance.

Sulphur deficiency in cannabis

Sulphur - Deficiency starts with yellowing of young leaves, becoming brittle, smaller and mutated. Occasionally leaves may turn orange or red instead of yellow.

zinc deficiency in cannabis

Zinc - Blades become very twisted on new growth. Spotting, chlorosis and yellowing between older leaf veins.

How To Feed Autoflowers

When feeding cannabis plants, it's important to recognize that autoflowering varieties have distinct nutritional needs compared to their photoperiod counterparts. Due to their compact size and rapid growth cycle, autoflowers are usually more efficient in absorbing nutrients. This efficiency means that they require significantly lower volumes of nutrients, often ranging from 1/8 to 1/2 of what you would typically give when you grow photoperiod cannabis plants. The small size of autoflowers doesn’t warrant the same nutrient quantities needed by the larger, longer-growing photoperiod strains. They have also evolved to thrive in less-than-ideal conditions, making them excellent at making the most of what they get. Overfeeding them can lead to nutrient burn or stunted growth, so it's important to be mindful of these differences and use an autoflowering specific feeding schedule.