When feeding cannabis crops in hydroponic systems, growers must first prepare a stock tank solution—either using a complete fertilizer line from a reputable manufacturer or by mixing and matching fertilizers themselves. With a complete fertilizer line, creating a stock solution is straightforward: simply follow the manufacturer’s instructions. But for those who prefer to mix their own fertilizers, the process is more complex. Growers must understand the nutrient concentrations in each fertilizer, how much they want to feed and the dilution ratio.
In this blog post, we explain how to prepare a stock solution to ensure the nutrients are provided in the correct amounts and remain available for plant uptake.
Decoding NPK
Providing the right amount of nutrients to crops starts with understanding the nutrients each fertilizer provides. All fertilizers are labeled with three numbers that represent, in order, elemental nitrogen, phosphate (expressed as P2O5) and potash (expressed as K2O). These numbers are the guaranteed minimum quantities of each nutrient, based on their percentage by weight. This is often referred to as their NPK values, although technically it is N‒P2O5‒K2O:[1]
- N = percentage of elemental nitrogen
- P = percentage of phosphate
- K = percentage of potash
This clarification is important because it affects how much of each nutrient is actually available to the plant.
For example, say a 100-lb bag of fertilizer has a guaranteed analysis of 3–5–8. It would be easy to assume this means 3 lb of nitrogen, 5 lb of phosphorus and 8 lb of potassium. But because the latter two numbers represent phosphate and potash—not elemental phosphorus and potassium—we have to account for the atomic weights to calculate the true amounts available. The following menu explains the calculations:
Decoding NPK
Providing the right amount of nutrients to crops starts with understanding the nutrients each fertilizer provides. All fertilizers are labeled with three numbers that represent, in order, elemental nitrogen, phosphate (expressed as P2O5) and potash (expressed as K2O). These numbers are the guaranteed minimum quantities of each nutrient, based on their percentage by weight. This is often referred to as their NPK values, although technically it is N‒P2O5‒K2O:[1]
- N = percentage of elemental nitrogen
- P = percentage of phosphate
- K = percentage of potash
This clarification is important because it affects how much of each nutrient is actually available to the plant.
Decoding NPK
Providing the right amount of nutrients to crops starts with understanding the nutrients each fertilizer provides. All fertilizers are labeled with three numbers that represent, in order, elemental nitrogen, phosphate (expressed as P2O5) and potash (expressed as K2O). These numbers are the guaranteed minimum quantities of each nutrient, based on their percentage by weight. This is often referred to as their NPK values, although technically it is N‒P2O5‒K2O:[1]
- N = percentage of elemental nitrogen
- P = percentage of phosphate
- K = percentage of potash
This clarification is important because it affects how much of each nutrient is actually available to the plant.
For example, say a 100-lb bag of fertilizer has a guaranteed analysis of 3–5–8. It would be easy to assume this means 3 lb of nitrogen, 5 lb of phosphorus and 8 lb of potassium. But because the latter two numbers represent phosphate and potash—not elemental phosphorus and potassium—we have to account for the atomic weights to calculate the true amounts available.
The following menu explains the calculations:
Nitrogen is expressed as N.
Calculation is direct: 3% of N in 3–5–8 fertilizer bag of 100 lb = 3 lb
Phosphorus is expressed as P2O5.
Atomic weight of P = 31 and O = 16
P2 = 2 × 31 = 62
O5 = 5 × 16 = 80
Total weight of P2O5 = 62 + 80 = 142
Proportion of P in P2O5 = 62 ÷ 142 = 0.437[2]
Amount of P in 100 lb bag of 3–5–8 fertilizer = 5 × 0.437 = 2.18 lb
Potassium is expressed as K2O.
Atomic weight of K = 39 and O = 16
K2 = 2 × 39 = 78
O1 = 1 × 16 = 16
Total weight for K2O = 78 + 16 = 94
Proportion of K in K2O = 78 ÷ 94 = 0.83[3]
Amount of K in 100 lb bag of 3‒5‒8 fertilizer = 8 × 0.830 = 6.64 lb
Therefore, that 100-lb bag of 3–5–8 fertilizer contains:
- 3 lb of nitrogen
- 18 lb of phosphorus
- 64 lb of potassium
In summary:
- First number: N (elemental nitrogen) percentage
- Second number: P2O5 (phosphate) percentage—multiply by 0.437 to get elemental phosphorus
- Third number: K2O (potash) percentage—multiply by 0.83 to get elemental potassium
Determining fertilizer rates
Now that the true amount of NPK has been determined, growers can calculate the correct amount of fertilizer needed to prepare stock solutions.
Growers typically deliver nutrients to plants using one of two methods: batching and inline-injection fertigation. Since most commercial hydroponic facilities use inline-injection, this section focuses on preparing stock solutions for those systems, which requires an understanding of injector ratios and dilution.
Injector ratios and dilution
Fertilizer injectors operate across a range of dilution ratios, typically from 1:50 to 1:500. A 1:100 ratio means the stock solution is 100 times more concentrated than the final solution delivered to plants. At this setting, the injector draws 1 part of the stock and mixes it with 99 parts of water. In contrast, a 1:50 ratio requires only 49 parts of water per 1 part of stock solution to reach the target concentration.[4]
Calculating nutrient content
Fertilizer labels often include instructions to help determine application rates, but these recommendations may not align with a grower’s specific goals or their injector’s dilution ratio. When product instructions are insufficient, growers can calculate the required fertilizer amount for a stock solution using this formula:[5]
Amount of fertilizer per gallon or liter of stock solution = (desired concentration in ppm × dilution factor) ÷ (percentage of the element in the fertilizer × conversion constant)
Calculating nutrient content
Fertilizer labels often include instructions to help determine application rates, but these recommendations may not align with a grower’s specific goals or their injector’s dilution ratio. When product instructions are insufficient, growers can calculate the required fertilizer amount for a stock solution using this formula:[5]
Amount of fertilizer per gallon or liter of stock solution = (desired concentration in ppm × dilution factor) ÷ (percentage of the element in the fertilizer × conversion constant)
Use the table below to convert between units, if needed:
Use the table below to convert between units, if needed:
The dilution factor is the larger number in the injector ratio (e.g., 100 in a 1:100 ratio), and the conversion constant depends on the desired output units. For converting ppm to ounces per gallon, use 75 as the constant.[6]
The dilution factor is the larger number in the injector ratio (e.g., 100 in a 1:100 ratio), and the conversion constant depends on the desired output units. For converting ppm to ounces per gallon, use 75 as the constant.[6]
For example, a grower wants to apply 150 ppm of nitrogen as a constant feed using a 1:200 injector and a 17–5–24 fertilizer.[7] The variables are:
- Desired concentration: 150 ppm
- Dilution factor: 200
- Element percentage: 17
- Conversion constant: 75
Plugging into the formula: (150 × 200 = 30,000) ÷ (17 × 75 = 1,275) = 23.53
The grower would need to mix approximately 23.5 ounces of 17‒5‒24 fertilizer per gallon of stock solution to deliver 150 ppm of nitrogen.
How to prepare the stock solution
At minimum, growers should use two stock tanks to prevent nutrient interactions that can cause fertilizer salts to precipitate. One tank should contain calcium-based fertilizers, and the other should be reserved for phosphates and sulfates. Mixing calcium with phosphate or sulfate can result in the formation of insoluble compounds like calcium phosphate or calcium sulfate.[8]
A third stock tank may be used to hold a dilute acid solution to help correct the pH. This is especially helpful when working with alkaline water or nitrate-heavy feeds, both of which can increase the pH over time.[9] Since nutrient availability depends heavily on pH, it’s critical to monitor and adjust it as needed.
The water source also plays a major role in nutrient uptake. We recommend using reverse osmosis (RO) water, as it has been purified through multiple filtration steps. If using another water source, always test the water quality to identify any contaminants that could interfere with fertilizer performance.
Using dry fertilizers
Dry fertilizers must fully dissolve in the stock tank to be effective. The dissolution rate depends on several factors, including the amount of fertilizer added, the water temperature and the level of agitation. To speed up dissolution, use hot water and stir the solution thoroughly. Continuous agitation isn’t required after mixing, but briefly stirring before dilution helps ensure uniformity. Water quality can also influence the long-term consistency of the nutrient solution.
Using dry fertilizers
Dry fertilizers must fully dissolve in the stock tank to be effective. The dissolution rate depends on several factors, including the amount of fertilizer added, the water temperature and the level of agitation. To speed up dissolution, use hot water and stir the solution thoroughly. Continuous agitation isn’t required after mixing, but briefly stirring before dilution helps ensure uniformity. Water quality can also influence the long-term consistency of the nutrient solution.
In batch systems, dry fertilizers are dissolved directly into a ready-to-use solution, typically in a larger holding tank. In systems with fertilizer injectors, the fertilizer is first blended into a concentrated stock solution and stored in a smaller tank for automated dosing.
Always store dry fertilizers in a cool, dry place away from sunlight, and keep them sealed in their original packaging to prevent moisture absorption. When properly stored, dry fertilizers can last indefinitely.
Using dry fertilizers
Dry fertilizers must fully dissolve in the stock tank to be effective. The dissolution rate depends on several factors, including the amount of fertilizer added, the water temperature and the level of agitation. To speed up dissolution, use hot water and stir the solution thoroughly.
Continuous agitation isn’t required after mixing, but briefly stirring before dilution helps ensure uniformity. Water quality can also influence the long-term consistency of the nutrient solution.
In batch systems, dry fertilizers are dissolved directly into a ready-to-use solution, typically in a larger holding tank. In systems with fertilizer injectors, the fertilizer is first blended into a concentrated stock solution and stored in a smaller tank for automated dosing.
Always store dry fertilizers in a cool, dry place away from sunlight, and keep them sealed in their original packaging to prevent moisture absorption. When properly stored, dry fertilizers can last indefinitely.
Using liquid fertilizers
Liquid fertilizers are pre-dissolved and pre-mixed into the correct ratios, so growers only need to ensure accurate dosing when adding them to the stock tank.
Measure each liquid fertilizer separately according to the Feeding Chart instructions, then add them individually to their corresponding tanks, often labeled Part A and Part B, and mix thoroughly. Regularly check the pH and EC to maintain the proper nutrient concentration.
Using liquid fertilizers
Liquid fertilizers are pre-dissolved and pre-mixed into the correct ratios, so growers only need to ensure accurate dosing when adding them to the stock tank.
Measure each liquid fertilizer separately according to the Feeding Chart instructions, then add them individually to their corresponding tanks, often labeled Part A and Part B, and mix thoroughly. Regularly check the pH and EC to maintain the proper nutrient concentration.
Using liquid fertilizers
Liquid fertilizers are pre-dissolved and pre-mixed into the correct ratios, so growers only need to ensure accurate dosing when adding them to the stock tank.
Measure each liquid fertilizer separately according to the Feeding Chart instructions, then add them individually to their corresponding tanks, often labeled Part A and Part B, and mix thoroughly. Regularly check the pH and EC to maintain the proper nutrient concentration.
When using Emerald Harvest’s product line, the process is straightforward. Do not premix concentrated nutrients. First fill the reservoir with water, then add the nutrients, beginning with Part A followed by Part B. Maintain the water temperature between 60‒72°F (16‒22°C).
Store liquid fertilizers in opaque containers to protect them from degradation caused by exposure to light, particularly for chelated nutrients.
Troubleshooting common issues
Issue
Cause
Solution
Precipitates in stock tank
Mixing incompatible nutrients
Use separate A/B tanks
Clogged irrigation lines
Calcium or phosphate buildup
Flush the system
pH drift (too high or too low)
Imbalanced nutrient uptake
Correct pH using appropriate acids or bases
Inconsistent EC readings
Uneven mixing or injector malfunction
Check injector calibration and thoroughly mix solution
Troubleshooting common issues
Cause: Mixing incompatible nutrients
Solution: Use separate A/B tanks
Cause: Calcium or phosphate buildup
Solution: Flush the system
Cause: Imbalanced nutrient uptake
Cause: Uneven mixing or injector malfunction
Solution: Check injector calibration and thoroughly mix solution
Conclusion
Preparing a stock solution correctly is critical for hydroponic success. Understanding NPK values, injector ratios, nutrient calculations and proper stock solution preparation ensures your cannabis crop consistently receives the nutrients it needs to thrive.
Emerald Harvest Team
[1] Arnold, J.G., J.R. Kiniry, R. Srinivasan, J.R. Williams, E.B. Haney, and S.L. Neitsch. 2012. “Soil & Water Assessment Tool Input/Output Documentation.” Texas Water Resources Institute. https://swat.tamu.edu/media/69296/swat-io-documentation-2012.pdf.
[2] Ibid.
[3] Landschoot, Peter. 2022. “How Much Phosphorus and Potassium Are Really in Your Fertilizer?” PennState Extension, December 19. https://extension.psu.edu/how-much-phosphorus-and-potassium-are-really-in-your-fertilizer.
[4] Boyle, Thomas. 2006. “Grower 101: Calculations Part III: Fertilizers.” Greenhouse Product News, June. https://gpnmag.com/article/grower-101-calculations-part-iii-fertilizers/.
[5] Ibid.
[6] Boyle, Thomas. 2006. “Grower 101: Calculations Part III: Fertilizers.” Greenhouse Product News, June. https://gpnmag.com/article/grower-101-calculations-part-iii-fertilizers/.
[7] Ibid.
[8] Sanchez, Elsa, Francesco Di Gioia, Thomas Ford, Robert Berghage, and Nick Flax. 2024. “Hydroponics Systems: Nutrient Solution Programs and Recipes.” PennState Extension, updated March 18. https://extension.psu.edu/hydroponics-systems-nutrient-solution-programs-and-recipes.
[9] Mattson, Neil. 2018. “Fertilizer Calculation Basics for Hydroponics.” e-GRO Edible Alert 3 (5). https://greenhousehort.ca.uky.edu/sites/greenhousehort.ca.uky.edu/files/2022-03/Fertilizer%20Calculation%20Basics%20for%20Hydroponics.pdf.
