Nitrogen, phosphorus and potassium—commonly referred to as NPK—are the three essential macronutrients. They’re called macronutrients because plants require them in large quantities, which is why they’re often regarded as the most important for plants.
Each plays vital roles in plant development. Nitrogen is a fundamental part of both amino acids, the building blocks of protein, and nucleic acids (DNA and RNA), while phosphorus is essential for photosynthesis, and potassium regulates water use.
Because growers must supply these nutrients in hydroponics, it’s important to understand the different fertilizer sources for each, as well as the ideal amounts and ratios to provide.
Nitrogen fertilizers
In water-soluble fertilizers, nitrogen may be in the form of nitrate, ammonium or urea. Each form behaves differently in terms of plant uptake, assimilation, substrate interactions and pH:[1]
- Nitrate nitrogen: Highly mobile and freely moving through the substrate, nitrate[2] does not bind to substrate particles and has potential basicity, meaning its uptake tends to raise the substrate pH.[3]
- Ammonium nitrogen: Often listed as “ammoniacal nitrogen” on fertilizer labels, ammonium readily attaches to organic matter, reducing leaching. Unlike nitrates, it has potential acidity, lowering the substrate pH during uptake.[4]
- Urea: Although moving like nitrate in the substrate, urea is not in a plant-available form. It must first convert to ammonia, then ammonium. Without this conversion, it may be lost through leaching or volatilization.[5] This process also affects the pH. First, urea converts to ammonia and releases hydroxide (OH–) ions, temporarily raising pH. If ammonium undergoes nitrification—microbial conversion to nitrate (NO₃⁻)[6]—it releases hydroxide ions, gradually lowering pH. One study found that pH of the hydroponic nutrient solution either slightly increased or remained stable when urea was used.[7]
Nitrogen fertilizers are categorized by the form(s) of nitrogen they supply:[8]
- Nitrate-containing fertilizers (NO₃⁻-N): Sodium nitrate (NaNO3) contains 16% nitrogen and calcium nitrate (Ca(NO3)2) 15.5%.
- Ammonium-containing fertilizers (NH₄⁺-N): Ammonium sulfate ((NH4)2SO4) contains 20% nitrogen, ammonium chloride (NH4Cl) 24–26% and anhydrous ammonia 82%.
- Ammonium- and nitrate-containing fertilizers: Ammonium nitrate (NH4NO3) contains 33–34% nitrogen and calcium ammonium nitrate contains 20% nitrogen.
- Amide fertilizer: Urea (CO(NH₂)₂), an amide compound, contains 46% nitrogen.
Nitrogen fertilizers are categorized by the form(s) of nitrogen they supply:[8]
- Nitrate-containing fertilizers (NO₃⁻-N): Sodium nitrate (NaNO3) contains 16% nitrogen and calcium nitrate (Ca(NO3)2) 15.5%.
- Ammonium-containing fertilizers (NH₄⁺-N): Ammonium sulfate ((NH4)2SO4) contains 20% nitrogen, ammonium chloride (NH4Cl) 24–26% and anhydrous ammonia 82%.
- Ammonium- and nitrate-containing fertilizers: Ammonium nitrate (NH4NO3) contains 33–34% nitrogen and calcium ammonium nitrate contains 20% nitrogen.
- Amide fertilizer: Urea (CO(NH₂)₂), an amide compound, contains 46% nitrogen.
Choosing a nitrogen fertilizer
Phosphorus fertilizers come in a wide range of physical and chemical forms, including insoluble and water-soluble types. Rock phosphate is the raw material used in most phosphorous fertilizers (expressed as P2O5), including:[9]
- Single superphosphate (OSP): Produced by treating ground rock phosphate with sulfuric acid, it contains both 18–25% water-soluble phosphorus (P2O5), 27% sulfur trioxide (SO3), a small amount of unreacted rock phosphate and gypsum (CaSO4).
- Triple superphosphate: Also produced by treating rock phosphate with sulfuric acid, it is thereafter treated with phosphoric acid, resulting in 46% water-soluble phosphorus and a small amount of elemental sulfur.
- Ammonium phosphate: Available as mono ammonium phosphate (MAP) and diammonium phosphate (DAP), these are produced by reacting rock phosphate with phosphoric acid and ammonia. They contain 46–55% phosphorus.
- Nitrophosphates: Produced by treating rock phosphate with nitric acid, these supply roughly 20% phosphorus and 20% nitrogen.
Phosphorus fertilizers
Phosphorus fertilizers come in a wide range of physical and chemical forms, including insoluble and water-soluble types. Rock phosphate is the raw material used in most phosphorous fertilizers (expressed as P2O5), including:[10]
Phosphorus fertilizers
Phosphorus fertilizers come in a wide range of physical and chemical forms, including insoluble and water-soluble types. Rock phosphate is the raw material used in most phosphorous fertilizers (expressed as P2O5), including:[10]
- Single superphosphate (OSP): Produced by treating ground rock phosphate with sulfuric acid, it contains both 18–25% water-soluble phosphorus (P2O5), 27% sulfur trioxide (SO3), a small amount of unreacted rock phosphate and gypsum (CaSO4).
- Triple superphosphate: Also produced by treating rock phosphate with sulfuric acid, it is thereafter treated with phosphoric acid, resulting in 46% water-soluble phosphorus and a small amount of elemental sulfur.
- Ammonium phosphate: Available as mono ammonium phosphate (MAP) and diammonium phosphate (DAP), these are produced by reacting rock phosphate with phosphoric acid and ammonia. They contain 46–55% phosphorus.
- Nitrophosphates: Produced by treating rock phosphate with nitric acid, these supply roughly 20% phosphorus and 20% nitrogen.
- Single superphosphate (OSP): Produced by treating ground rock phosphate with sulfuric acid, it contains both 18–25% water-soluble phosphorus (P2O5), 27% sulfur trioxide (SO3), a small amount of unreacted rock phosphate and gypsum (CaSO4).
- Triple superphosphate: Also produced by treating rock phosphate with sulfuric acid, it is thereafter treated with phosphoric acid, resulting in 46% water-soluble phosphorus and a small amount of elemental sulfur.
- Ammonium phosphate: Available as mono ammonium phosphate (MAP) and diammonium phosphate (DAP), these are produced by reacting rock phosphate with phosphoric acid and ammonia. They contain 46–55% phosphorus.
- Nitrophosphates: Produced by treating rock phosphate with nitric acid, these supply roughly 20% phosphorus and 20% nitrogen.
- Single superphosphate (OSP): Produced by treating ground rock phosphate with sulfuric acid, it contains both 18–25% water-soluble phosphorus (P2O5), 27% sulfur trioxide (SO3), a small amount of unreacted rock phosphate and gypsum (CaSO4).
- Triple superphosphate: Also produced by treating rock phosphate with sulfuric acid, it is thereafter treated with phosphoric acid, resulting in 46% water-soluble phosphorus and a small amount of elemental sulfur.
- Ammonium phosphate: Available as mono ammonium phosphate (MAP) and diammonium phosphate (DAP), these are produced by reacting rock phosphate with phosphoric acid and ammonia. They contain 46–55% phosphorus.
- Nitrophosphates: Produced by treating rock phosphate with nitric acid, these supply roughly 20% phosphorus and 20% nitrogen.
Choosing a phosphorus fertilizer
Hydroponic growers should always use water-soluble phosphorus fertilizers, as insoluble forms are designed for soil cultivation where weak soil acids help dissolve them.[11] MAP and mono potassium phosphate are available in liquid form, making them ideal for hydroponics by reducing the risk of emitter clogging. Most commercial pre-mixed fertilizers use mono potassium phosphate as their phosphorus source.[12]
Use caution when using fertilizers containing superphosphate. In highly acidic substrates, they may react with iron or aluminum and become insoluble.[13] Since cannabis prefers a slightly acidic pH of 5.8–6.3, these fertilizers may precipitate and limit phosphorus availability.
Potassium fertilizers
A limited number of fertilizer materials can supply potassium, with the most common being potassium chloride (KCl). Also known as muriate of potash, KCl accounts for more than 90% of potassium fertilizer sold in the US and Canada and contains 60–62% potassium (expressed as K₂O). Other sources include:
- Potassium sulfate (K2SO4): Contains 50% potassium and 18% sulfur. With a chloride content below 2.5%, it’s commonly used for chloride-sensitive crops like fruit trees.[14]
- Potassium magnesium sulfate (K2SO42MgSO4): Contains approximately 22% potassium, 11% magnesium and 22% sulfur.[15]
- Potassium nitrate (KNO3): Contains 44% potassium and 13% nitrogen.[16]
- Potassium thiosulfate (K2S2O3): Contains approximately 17% potassium.[17]
Chloride and sulfate forms of potassium dissolve readily in water, ionizing into K⁺, Cl⁻ and SO₄²⁻ ions for plant uptake.
Choosing a potassium fertilizer
Potassium chloride is an affordable source that can be used as part of a potassium program, provided it’s supplied in appropriate amounts that won’t harm plants. Potassium sulfate and potassium nitrate offer the added benefit of sulfur or nitrogen, respectively. However, the nitrogen content of potassium nitrate may limit the use of other nitrate-containing fertilizers, such as calcium nitrate.[18]
Optimal NPK ratios and amounts for cannabis
Fertilizers are labeled with three numbers, which indicate the percentage, by weight, of NPK. For example, a fertilizer with an NPK value of 2–1–6 means it contains 2% nitrogen, 1% phosphorus and 6% potassium.
Grow experts recommend an NPK ratio of 3:1:1 during the vegetative phase of cannabis growth. Once flowering begins, nitrogen should be reduced while phosphorus and potassium increase. A 1:3:2 ratio is ideal during early flowering, while 0:3:3 suits late flowering.
Several studies have examined the optimal nutrient amounts for cannabis:
Nitrogen study
One study found 160 mg/L of nitrogen to be ideal throughout both vegetative and flowering phases. Inflorescence biomass increased up to that level but not beyond, while lower rates reduced overall plant development.[19]
Phosphorus studies
Although some growers believe high phosphorus levels during flowering promote bud growth, research does not support this. One study found no difference between plants given 100 mg/L of phosphorus and those given 30 mg/L during the vegetative stage. Another identified 60 mg/L as the optimal rate.[20]
Not only that, but high phosphorus levels can cause nutrient antagonism, in which the phosphorus locks out nutrients like iron, zinc, magnesium, calcium and copper.
Potassium studies
One study showed potassium requirements vary by genotype. Cannabis was supplied potassium at rates from 60 to 340 mg/L, which had no effect on inflorescence yield. However, 240 mg/L reduced fresh shoot and root biomass in one genotype while increasing it in another.[21]
Another study found that yield increased linearly in aquaponic cannabis with potassium rates from 15 to 150 mg/L, though the researchers noted that the 75 mg/L nitrogen rate they used was lower than typical hydroponic levels and may have limited growth and yield.[22]
Conclusion
As the primary macronutrients in plant growth, NPK is essential for healthy cannabis development. Choosing the right fertilizer forms ensures nutrients are available for plant uptake, while adjusting amounts and ratios throughout the grow cycle supports strong yields and consistent quality.
Emerald Harvest Team
[1] Gimondo, Jaden, and Erik Runkle. 2023. “Understanding the Forms of Nitrogen in Water-Soluble Fertilizers for Greenhouse Growers.” Michigan State University, June 12. https://www.canr.msu.edu/news/understanding-the-forms-of-nitrogen-in-water-soluble-fertilizers-for-greenhouse-growers.
[2] Often referred to in the plural: nitrates.
[3] Gimondo, Jaden, and Erik Runkle. 2023. “Understanding the Forms of Nitrogen in Water-Soluble Fertilizers for Greenhouse Growers.” Michigan State University, June 12. https://www.canr.msu.edu/news/understanding-the-forms-of-nitrogen-in-water-soluble-fertilizers-for-greenhouse-growers.
[4] Ibid.
[5] Ibid.
[6] Mengel, David B. n.d. “Types and Uses of Nitrogen Fertilizers for Crop Production.” Purdue University Cooperative Extension Service Agronomy Guide. Accessed March 12, 2025. https://www.extension.purdue.edu/extmedia/ay/ay-204.html.
[7] Ikeda, Hideo, and Xuewen Tan. 1998. “Urea as an Organic Nitrogen Source for Hydroponically Grown Tomatoes in Comparison with Inorganic Nitrogen Sources.” Soil Science and Plant Nutrition 44 (4): 609-615. https://doi.org/10.1080/00380768.1998.10414484.
[8] Nadarajan, Stalin, and Surya Sukumaran. 2021. “Chemistry and Toxicology Behind Chemical Fertilizers.” In Controlled Release Fertilizers for Sustainable Agriculture. Academic Press. https://doi.org/10.1016/B978-0-12-819555-0.00012-1.
[9] Gimondo, Jaden, and Erik Runkle. 2023. “Understanding the Forms of Nitrogen in Water-Soluble Fertilizers for Greenhouse Growers.” Michigan State University, June 12. https://www.canr.msu.edu/news/understanding-the-forms-of-nitrogen-in-water-soluble-fertilizers-for-greenhouse-growers.
[10] Finch, H.J.S., A.M. Samuel, and G.P.F. Lane. 2014. “Fertilisers and manure.” In Lockhart & Wiseman’s Crop Husbandry Including Grassland (Ninth Edition). Woodhead Publishing.
[11] Nadarajan, Stalin, and Surya Sukumaran. 2021. “Chemistry and Toxicology Behind Chemical Fertilizers.” In Controlled Release Fertilizers for Sustainable Agriculture. Academic Press. https://doi.org/10.1016/B978-0-12-819555-0.00012-1.
[12] Hochmuth, George J., and Robert C. Hochmuth. 2018. “Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida.” University of Florida IFAS Extension, October 4. https://doi.org/10.32473/edis-cv216-1990.
[13] Nadarajan, Stalin, and Surya Sukumaran. 2021. “Chemistry and Toxicology Behind Chemical Fertilizers.” In Controlled Release Fertilizers for Sustainable Agriculture. Academic Press. https://doi.org/10.1016/B978-0-12-819555-0.00012-1.
[14] Ibid.
[15] Ibid.
[16] Ibid.
[17] University of Minnesota Extension. 2018. “Potassium for Crop Production.” Accessed March 9, 2025. https://extension.umn.edu/phosphorus-and-potassium/potassium-crop-production.
[18] Hochmuth, George J., and Robert C. Hochmuth. 2018. “Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida.” University of Florida IFAS Extension, October 4. https://doi.org/10.32473/ediscv216-1990.
[19] Saloner, Avia, and Nirit Bernstein. 2021. “Nitrogen Supply Affects Cannabinoid and Terpenoid Profile in Medical Cannabis (Cannabis sativa L.).” Industrial Crops and Products 167: 113516. https://doi.org/10.1016/j.indcrop.2021.113516.
[20] Bevan, Lewys, Max Jones, and Youbin Zheng. 2021. “Optimisation of Nitrogen, Phosphorus and Potassium for Soilless Production of Cannabis Sativa in the Flowering Stage Using Response Surface Analysis.” Frontiers in Plant Science 12: 764103. https://doi.org/10.3389/fpls.2021.764103.
[21] Ibid.
[22] Ibid.