GROWING

What Nutrients Does Cannabis Need? NPK, pH and Organic Options

Nitrogen, phosphorus, potassium and the supporting cast: what weed plants eat, when to feed them and how to go organic.

BY CosechaLibre Editorial5 MIN READ

What Nutrients Does a Weed Plant Need?

Cannabis needs three groups of nutrients: the big three macronutrients, NPK (nitrogen, phosphorus and potassium), secondary nutrients like calcium, magnesium and sulfur, and small amounts of micronutrients such as zinc, iron and manganese. On top of that it pulls carbon and oxygen from the air and hydrogen from water.

The three main nutrients weed plants need to grow are known as NPK, short for nitrogen, phosphorus and potassium. NPK is the backbone of any cannabis fertilizer, and of any thriving weed plant. Cannabis plants also need carbon dioxide and oxygen, which they get through airflow, and hydrogen, which comes from water.

Weed plants also need secondary nutrients, which include sulfur, calcium and magnesium.

Finally, there's a bigger group of micronutrients that support healthy growth. These essential micronutrients include zinc, manganese, iron, boron, chloride, cobalt, molybdenum and silicon, among others.

How and When Should You Fertilize Cannabis?

Weed plants need different nutrient levels depending on their growth stage. In general, cannabis fertilizer goes in at least once a week, along with plenty of pH-balanced water.

As a rule of thumb, the ideal pH for cannabis grown in soil is between 6.0 and 6.8. Hydro growers should keep their water pH in the 5.5 to 6.5 range.

Cannabis plant getting fed in a grow

How to Feed Cannabis at Each Growth Stage

Seedling: little to no fertilizer. Wait until a few sets of leaves have come in before giving the first dose of a basic NPK fertilizer.

Veg: for week 1, use an NPK ratio of 2:1:2, meaning two parts nitrogen to one part phosphorus and two parts potassium. By week 7 you can bump the NPK ratio up to 10:5:7, followed by a 1:1:1 ratio in late veg.

Flower: at this stage, focus on raising phosphorus and potassium. Feeding helps a lot in early flower, but it's not as effective in the final weeks once the real buds have formed.

Pro tip: not all feeding charts are created equal. Start with less than the label recommends and work your way up to avoid overfeeding or nutrient burn.

What Is Fertilizer, Exactly?

Put simply, fertilizer is plant food made from natural or manufactured substances that growers apply to the soil and plants to get the most out of their growth. The nutrients in fertilizers can benefit lots of different plants, including potted houseplants.

Pineapple Larry OG cannabis plant in flower

Why Does Cannabis Need Fertilizer?

The nutrients in fertilizer help growers produce a healthier harvest, with lusher leaf growth and, eventually, flowers. If you're after big buds, the right fertilizer can help you get there. Without good feeding, your buds may never reach their full potential.

What's the Best Organic Fertilizer for Cannabis?

If you want to grow organic, you can use a natural cannabis fertilizer made with the following:

Beneficial bacteria and fungi
Humus (worm castings)
Fish meal
Blood meal
Bat guano

What Nutrients Does Cannabis Need in Flower?

During flowering, cannabis has specific nutrient needs to develop high-quality buds. Essential nutrients for flowering weed plants include the macronutrients nitrogen (N), phosphorus (P) and potassium (K), secondary nutrients like calcium (Ca) and magnesium (Mg), and micronutrients like zinc (Zn).

Nitrogen matters a lot in the early stages, but in flower you need to cut back, since too much nitrogen can hold back bud development. Phosphorus, on the other hand, plays a vital role in flower production and boosts bud formation. Potassium is also essential in bloom, since it supports resin and essential oil production, which improves bud quality and density.

Beyond these main nutrients, make sure your plants get enough of the secondary nutrients and micronutrients. Calcium, for example, is key for building strong cell structure, while magnesium plays a crucial role in producing chlorophyll, which drives photosynthesis. Micronutrients like zinc are needed in smaller amounts but still matter for healthy flowering plants.

Keep in mind that too many nutrients can also hurt flowering cannabis. Overfeeding can burn the leaves and drag down the plant's overall health. That's why it's smart to follow a cannabis nutrient and deficiency chart, so you're giving the right nutrients in the right amounts during flower.

Dense cannabis bud in late flower

Why and How to Feed Your Weed Plants

Feeding your weed plants is crucial to give them the nutrients they need for healthy growth and a strong bloom. The nutrients in your growing medium get used up as plants grow, so you need to replace them with fertilizer.

There are different kinds of fertilizer on the market, mainly organic and synthetic. Organic fertilizers come from natural sources and tend to release nutrients more slowly and steadily. Synthetic fertilizers, on the other hand, pack nutrients in a more concentrated form that roots absorb quickly.

When you feed your weed plants, follow the manufacturer's instructions and use the right dose. Start low and increase gradually based on what your plants need and how they respond. Overfeeding can do real damage, causing nutrient burn and imbalances.

Don't forget about the pH of your growing medium, either. pH controls how available nutrients are to your plants. For most cannabis strains, a slightly acidic pH between 6 and 7 is ideal. If pH runs too high or too low, nutrients can get locked out of the roots' reach, which leads to deficiencies.

How you feed also changes with the growth stage. In veg, plants need more nitrogen to fuel vigorous growth. In flower, they need more phosphorus and potassium to build buds.