When and How to Start Feeding Cannabis Plants with Fertilizers?

Cannabis needs 4 things to grow well. Light, carbon dioxide, water, and nutrients. Leaves are responsible for the first two, while the roots focus on acquiring water and nutrients. Find out everything you need to know about properly feeding cannabis plants

There are dozens of nutrients you can give to your plants, and each one causes a unique reaction from your crop. For instance, one nutrient could increase the pH of the soil while another will decrease it.

All the nutrients needed for cannabis plant development are naturally present in the environment. However, to help your plants develop even faster and produce a better end product, you’ll want to feed them with fertiliser—concentrated nutrients.



Cannabis plants require three nutrients in large quantities. These macronutrients are nitrogen (N), phosphorus (P), and potassium (K), and they form the cornerstone of cannabis plant health. As such, these three nutrients usually feature front and centre on fertiliser products in the form of an NPK ratio. The higher the number for each value, the higher the concentration of that particular nutrient.

However, cannabis needs more than just three nutrients to survive and thrive. It also counts on secondary nutrients like calcium, magnesium, and sulfur to play vital roles in plant growth:

Calcium is important for cell wall development, can help reduce soil salinity, and improves water penetration when used as a soil amendment.
Magnesium plays a key role in photosynthesis and carbohydrate metabolism, and also helps with the stabilisation of plant cell walls.
Sulfur is necessary for the formation of chlorophyll and the production of proteins, amino acids, enzymes and vitamins, and protects plants against disease.

Typically, cannabis fertilisers will vary in the four following areas:

  • Nutrient ratio: Different brands use different nutrient ratios they consider optimal.
  • Ingredients: Different fertiliser brands can achieve the same nutrient ratios using completely different ingredients, ranging from the most chemical (or “artificial”) to the most natural.
  • Soil or hydro: Soil nutrients are very different from hydro or soilless nutrient solutions. Make sure you only use fertilisers designed for your growing medium.
  • Supplements: Many fertiliser brands also make “supplements”. These products typically contain low NPK ratios and instead feature other nutrients designed to boost certain aspects of growth. Some supplements, for example, are essentially molasses.

When to Start Using Fertilizers with Cannabis

Depending on the phase your grow is in your plants are going to need certain nutrients in higher proportions; they need more Nitrogen in growth, and phosphorus and potassium for the flowering period. Cannabis plants absorb large quantities of these nutrients as well as others, so if they don’t get them through irrigation then they’ll probably end up showing deficiencies through stains on the leaves.

To start using nitrogen during the growth phase you’ll need to wait for your little plant to grow the roots out enough so that it becomes slightly stronger.

Once your plants have grown 3-4 sets of true leaves, you can give them their first feeding of nutrients. It usually takes about 3-4 weeks from planting your germinated seed for your seedling to have used up all the energy left in the seed.

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You should begin off with small dosages; if your product says 4ml/L for adult plants then you need to start off with 1ml/L, and only begin using it once the leaves on your plant have three points. Once those leaves appear you can start using your growth fertilizer in the irrigation water. Once the plant begins growing more then you should raise the dosage until you reach the maximum milliliters allowed, and always use it with every second watering.

Water your plant as needed, when the first top inch of soil is completely dry. For a plant this young, it might not need another watering until a week from now. Its root system is still small and won’t be able to suck up all that water just yet. The temperature of your grow area will also affect how often you need to water your plants.

  • Prepare your water. If possible, heat your water to about 22°C to increase absorption by the roots.
  • Add your nutrients according to the instructions on your fertiliser, and stir.
  • If necessary, adjust the pH of your feed using a nitric or phosphoric acid pH down supplement.
  • Once your PPM, pH, and temperature are right, feed your plants and measure your runoff using your PPM or EC meter to ensure your plants are taking up their nutrients properly.


To avoid over- or underfeeding your plants, it’s a good idea to always measure the PPM of your soil or grow medium to see if it still contains nutrients. If there are nutrients present in your medium at the time of your next feed, subtract your medium’s PPM from the PPM recommended by your feed chart to avoid overfeeding.

What Does PPM Mean?

PPM (parts per million) is a measurement that is used to identify the density of a nutrient solution. Using a PPM reader allows you to accurately measure the nutrients going into your garden, information that is vitally important when addressing nutrient lockout or deficiency.

Why Are pH Levels Important?

If the pH level is too high or too low, your plants cannot uptake the nutrients they need to thrive. Keeping a pH between 5.5 and 6.5 for hydroponic grows–or 6.0 and 6.8 for soil gardens–is absolutely necessary for your plants to reach their full potential.


Here are some quick tips to ensure feeding goes off without a hitch.

Embrace chelation

Most high-quality nutrients will contain chemical chelates. If you’re an organic grower, you can use natural chelates like fulvic and humic acid to help your plants better absorb mineral nutrients like iron or zinc. Chelates work by surrounding positively charged nutrients with a negative or neutral charge, allowing them to pass through the plant pore barrier.

Try foliar feeding

Foliar spraying—spraying cannabis leaves with a fine mist—can be a great way to tackle nutrient deficiencies or pests/disease. It is also particularly effective for short-term feeding with secondary nutrients like magnesium or calcium, or micronutrients zinc, iron, and manganese.

Always flush

Nutrients are super important, but you don’t want any of them leftover in your harvested bud. That’s why it’s important to flush your plants with pH-neutral water for at least one week before harvesting. Flushing forces your plants to consume any leftover nutrients they’ve stored, resulting in a clean, smooth smoke.


In general, we recommend you focus more on meeting your plants’ demands for macro and secondary nutrients before pumping them full of supplements. With the right genetics, just enough nutrients and water, and plenty of light, you’re well on your way to growing some exceptional weed.

  • Zenpype.com is an educational website dedicated to shedding the light on many sides of medical and recreational cannabis. Aside from informing people about cannabis, we also provide cannabis seeds and CBD products. Readers who show their support with purchasing, help us keep doing this. Thank you for your support and for helping us improve!

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