Calculate precise mixing ratios for water-soluble fertilizers. Achieve target nitrogen (N) concentrations in parts per million (ppm).
Last updated: March 2026
Desired nitrogen concentration
First number in NPK ratio (e.g., 20-20-20)
Water-soluble fertilizers are concentrated nutrient formulations that dissolve completely in water, allowing for precise nutrient delivery through irrigation systems (fertigation), foliar sprays, or manual watering. Unlike slow-release granular fertilizers, water-soluble forms provide immediately available nutrients that plants can absorb quickly.
These fertilizers are labeled with NPK ratios (e.g., 20-20-20) representing the percentage by weight of nitrogen (N), phosphorus as P₂O₅ (P), and potassium as K₂O (K). The calculator focuses on nitrogen because it's the most commonly limiting nutrient and the basis for most fertilizer concentration recommendations given in parts per million (ppm).
Professional growers in hydroponics, greenhouses, and nurseries use water-soluble fertilizers because they allow precise control of nutrient concentrations, can be adjusted for different growth stages, and provide consistent feeding. The key is calculating the correct amount to dissolve in your water volume to achieve the target nutrient concentration.
Mixing 200 ppm Nitrogen Solution for Vegetative Growth
Dissolve 378.5 grams (13.4 oz) of 20-20-20 fertilizer in 100 gallons of water to achieve 200 ppm nitrogen concentration. This is suitable for vegetative growth in most crops.
Water-soluble fertilizers are identified by a three-number grade such as 20-20-20 or 15-5-30. These numbers represent the fertilizer's guaranteed analysis and indicate the percentage by weight of the three primary plant nutrients.
| Number | Represents |
|---|---|
| First | Nitrogen (N) |
| Second | Available phosphate (P₂O₅) |
| Third | Soluble potash (K₂O) |
Phosphorus and potassium are traditionally reported as their oxide forms (P₂O₅ and K₂O) rather than as elemental phosphorus (P) and potassium (K). Although modern fertilizers do not necessarily contain these exact compounds, this labeling convention remains the international standard for fertilizer analysis.
Example: A 20-20-20 fertilizer contains 20% nitrogen, 20% available phosphate (P₂O₅), and 20% soluble potash (K₂O) by weight.
Parts per million (ppm), electrical conductivity (EC), and fertilizer concentration are closely related but describe different aspects of a nutrient solution.
| Measurement | What It Describes |
|---|---|
| ppm | Estimated concentration of dissolved nutrients. |
| EC | Electrical conductivity produced by dissolved salts. |
| Fertilizer concentration | Amount of fertilizer dissolved into a given volume of water. |
Higher fertilizer concentrations generally increase both EC and ppm. However, many conductivity meters estimate ppm using different conversion scales, commonly 500, 640, or 700. As a result, the same EC reading may display different ppm values depending on the instrument.
Fertilizer calculations assume the irrigation water contains few dissolved nutrients. In practice, water quality can significantly influence the final nutrient solution.
| Grade | Typical Use |
|---|---|
| 20-20-20 | General-purpose feeding. |
| 15-5-30 | Flowering and fruiting crops. |
| 30-10-10 | Vegetative growth. |
| 10-52-10 | High-phosphorus starter fertilizer. |
| 13-2-13 | Hydroponics and greenhouse production. |
Seedlings: 50-100 ppm. Vegetative growth: 150-250 ppm. Flowering: 200-300 ppm. Mature plants: 300-400 ppm. These are general guidelines; adjust based on plant species, growing conditions, and visible nutrient deficiency/toxicity signs.
Nitrogen is the most commonly limiting macronutrient and most fertilizer recommendations are given in ppm N. The other nutrients (P, K) come along proportionally based on your fertilizer's NPK ratio. Some growers use separate fertilizers to adjust individual nutrients.
Tablespoons are approximate due to variable fertilizer density and packing. For small amounts or precision feeding (hydroponics), use a digital scale. For large volumes where +/- 10% doesn't matter, tablespoons work.
Hydroponic/soilless: every watering at lower concentrations (100-200 ppm). Soil: weekly to biweekly at higher concentrations (200-400 ppm). Container plants: every watering at 50-100 ppm. Adjust based on growth rate and runoff EC.
The calculator works with any NPK ratio — just enter the first number (N percentage). For 10-5-5, enter 10. For 24-8-16, enter 24. You'll need proportionally more of a low-N fertilizer to reach the same ppm.
Yes! Most fertilizers lower pH slightly. Check and adjust to your target range (5.5-6.5 for hydroponics, 6.0-7.0 for soil) AFTER dissolving fertilizer. pH affects nutrient availability.
Generally yes, but avoid mixing calcium-containing fertilizers with sulfate or phosphate fertilizers in concentrated form (precipitates form). Mix each separately into water, or use as A+B formulations designed for compatibility.
In opaque, sealed containers: 1-2 weeks. Exposed to light/air: 2-3 days (algae growth, precipitation). In irrigation systems: 1 week max. For best results, mix fresh weekly and store in cool, dark conditions.
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