Why Mix Your Own Fertilizer Solution?
Whether you are a greenhouse grower or a hobbyist, there are numerous benefits to learning how to make fertilizer solutions, stock solutions, or hydroponic nutrients. By using the water soluble fertilizer calculator, you can:
- Save money. Mixing nutrient solution is often less expensive than pre-made nutrient solutions.
- Transport and store fertilizer efficiently. Dry fertilizers take up less space than liquid fertilizers, weigh less, and can be stored safely for extended periods before being made into a liquid.
- Gain complete control over nutrient composition. By mixing a custom solution, you control every macro- and micronutrient your plant receives.
- Ensure consistent, healthy plants. Correctly calculating your nutrient formula helps ensure a consistent growing environment, leading to better results.
- Become a more knowledgeable horticulturist. Understanding what you are feeding your plants leads to better decision-making and more satisfying results.
Water Soluble Fertilizer — A Guide to the Label
There are two important components of a water-soluble fertilizer label:
- The N-P-K numbers (e.g., 20-8-20); and
- The "guaranteed analysis" section on the back of the bag.
The N-P-K numbers represent the three primary nutrients as a percent of the fertilizer's weight: nitrogen (N), phosphate (P₂O₅), and potash (K₂O). For example, in a 20-8-20 fertilizer:
- 20% of the bag's weight is nitrogen (N);
- 8% is phosphate (P₂O₅); and
- 20% is potash (K₂O).
Note that the weights of elemental phosphorus (P) and potassium (K) are less than the weights of the compounds phosphate (P₂O₅) and potash (K₂O), because the compounds include the weight of oxygen. The calculator handles these conversions automatically using the standard factors:
- P (elemental) = P₂O₅ weight ÷ 2.29133
- K (elemental) = K₂O weight ÷ 1.20460
How to Use the Water Soluble Fertilizer Calculator
- Enter the N-P-K numbers from the fertilizer label (nitrogen, phosphate P₂O₅, and potash K₂O percentages).
- Select the target element for which you have a concentration goal: N, P (elemental), K (elemental), P₂O₅, or K₂O.
- Enter the desired concentration in ppm (mg/L) or mmol/L (millimolar).
- Set the final volume of the solution you want to prepare (liters or US gallons).
- Optionally, enter your bag weight and price to get a cost estimate in your currency.
- Click Calculate to see the fertilizer recipe and the resulting concentrations of all nutrients.
Calculating a Nutrient Solution — Step-by-Step Example
Goal: 200 ppm nitrogen in 100 L of water using a 20-8-20 fertilizer.
- Rewrite ppm as mg/L. Since 1 ppm = 1 mg/L: target = 200 mg N per liter.
- Find the fertilizer concentration. Divide the nitrogen concentration by the N percent weight (as a decimal):
fertilizer concentration = 200 mg/L ÷ 0.20 = 1000 mg/L = 1 g/L - Calculate the weight of fertilizer. Multiply the concentration by the volume:
fertilizer = 1 g/L × 100 L = 100 g - Find concentrations of other elements using the same fertilizer concentration (1 g/L):
- P₂O₅: 1 g/L × 1000 × 0.08 = 80 ppm → P = 80 / 2.29133 ≈ 34.9 ppm
- K₂O: 1 g/L × 1000 × 0.20 = 200 ppm → K = 200 / 1.20460 ≈ 166.0 ppm
Converting Concentrations from mmol/L to ppm
Millimolar (mmol/L or mM) is a molar concentration unit. To convert to ppm (mg/L), multiply by the element's molar mass:
- N (14.007 g/mol): 1 mmol/L × 14.007 = 14.007 ppm
- P (30.974 g/mol): 1 mmol/L × 30.974 = 30.974 ppm
- K (39.098 g/mol): 1 mmol/L × 39.098 = 39.098 ppm
The calculator accepts both ppm and mmol/L as input for the target concentration and always displays both in the results table.
Estimating Container Volume — Simple Method
If you don't know your tank's exact volume, you can estimate it:
- Rectangular tank: Volume (L) = Length (m) × Width (m) × Height (m) × 1000
- Cylindrical tank: Volume (L) = π × radius² (m) × height (m) × 1000
- Round/irregular: Fill with a known number of 1-gallon jugs and multiply by 3.785 to get liters.
What Water Source Should You Use?
Your source water quality affects the final nutrient solution:
- Reverse osmosis (RO) or distilled water is ideal — it starts with near-zero background minerals, giving you full control.
- Municipal tap water often contains calcium, magnesium, and chlorine. Use a water test to check its electrical conductivity (EC) and pH.
- Well water may contain significant calcium, magnesium, iron, or sodium that can interfere with nutrient uptake.
- Adjust pH to 5.5–6.5 for most hydroponic crops after mixing nutrients.
5 Helpful Tips on Mixing Water Soluble Fertilizer
- Weigh, don't guess. Use a gram-accurate scale for small batches. A tablespoon can vary significantly in weight depending on how packed the fertilizer is.
- Mix concentrates separately. If using multiple fertilizers, dissolve each one in a small volume of water before combining to prevent chemical reactions.
- Stir thoroughly. Allow several minutes of agitation or recirculation to ensure complete dissolution.
- Measure pH and EC after mixing. The final solution pH and electrical conductivity should match the requirements of your crop.
- Start with clean containers. Residual minerals or biological material can alter your recipe and promote disease.
What If I Want to Use Tablespoons?
While weight measurements (grams, ounces) are more accurate, tablespoons can be used for small batches. The conversion depends on the fertilizer's bulk density, which varies by product. As a rough estimate, 1 tablespoon of a water-soluble fertilizer weighs approximately 6–10 grams. For precise results, weigh a tablespoon of your specific product once and use that value going forward.
US and Metric Units
This calculator supports both US Imperial (oz, lbs, US gallons) and Metric (g, kg, liters) systems. The mixing ratio is always shown in both g/L and oz/gal for easy reference regardless of which system you select.