Calculate mixing ratios for liquid fertilizer concentrate. Covers PPM targets, dilution rates, stock solutions, and injector ratios.
Water-soluble fertilizers are the backbone of greenhouse, hydroponic, and professional container production. Unlike granular fertilizers that release slowly, water-soluble formulations deliver precise nutrient concentrations in every irrigation — but only if you mix them correctly. The difference between a thriving crop and burned roots or nutrient deficiency is often a miscalculation in the mixing room.
The key concept is parts per million (PPM) — the target concentration of a specific nutrient (usually nitrogen) in the final irrigation water. Professional growers target 100-400 PPM nitrogen depending on the crop, growth stage, and season. Achieving a target PPM requires knowing your fertilizer's analysis (N-P-K), the injection ratio of your dosing system, and the volume of stock solution you're preparing.
This calculator handles the full workflow: from target PPM to ounces of fertilizer per gallon of stock solution, accounting for injector ratios from 1:10 to 1:200. It also covers direct mixing for hand watering, multiple fertilizer blending, and common commercial formulations like 20-20-20, 15-5-15, and 20-10-20. Whether you're a greenhouse grower managing 50,000 square feet of production or a hobbyist mixing a watering can, this tool takes the guesswork out of fertilizer preparation.
Accurate fertilizer mixing prevents crop damage from over- or under-feeding. This calculator eliminates manual math errors that can burn roots at high concentrations or starve crops at low ones — critical for commercial greenhouse operations and precision container gardening. This water-soluble fertilizer calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
Fertilizer oz/gal (stock) = (Target PPM × Injector ratio) / (N% × 75.2). Direct mix: oz/gal = Target PPM / (N% × 75.2). Resulting P₂O₅ PPM = N PPM × (P% / N%). Resulting K₂O PPM = N PPM × (K% / N%). The constant 75.2 converts the relationship between ounces per gallon and PPM for percentage-based fertilizer analysis.
Result: 13.3 oz fertilizer per 5 gallons stock
At 1:100 injection: 200 PPM N × 100 ÷ (20 × 75.2) = 13.3 oz per gallon of concentrate. For 5-gallon stock tank: 66.5 oz (4.15 lbs). Delivers 200 PPM N, 100 PPM P₂O₅, 200 PPM K₂O.
**20-20-20 (General Purpose)**: The most widely used greenhouse formula. Balanced ratio suits most crops during vegetative growth. At 200 PPM N it delivers 200-200-200, providing good overall nutrition. Contains ammoniacal and urea nitrogen — best for crops that tolerate acidic root zones.
**15-5-15 (Cal-Mag)**: Designed for constant liquid feed programs. The 3:1:3 ratio is ideal for most floriculture crops. Contains calcium and magnesium, eliminating the need for a separate cal-mag supplement. Preferred for dark-leaf crops sensitive to boron.
**20-10-20 (Peat-Lite)**: Formulated for peat-based media with the 2:1:2 ratio. The nitrate-dominant nitrogen source keeps root zone pH from dropping in acidic substrates. Excellent for petunias, calibrachoa, and other pH-sensitive crops.
**Venturi-style (Dosatron, Dosmatic)**: Water-powered proportional injectors. Most accurate and reliable. Typical ratios: 1:10 to 1:200, adjustable. Calibrate by timing how long it takes to draw a known volume of concentrate. **Positive displacement (Anderson, Smith)**: Electric pump-driven. Very precise for large operations. Common in multi-zone systems. **Siphon mixers (Hozon)**: Low-cost, gravity/venturi, fixed ratio (~1:16 or 1:128). Fine for hobbyists but imprecise for commercial production. Verify output PPM with an EC meter regardless of injector type.
Your fertilizer nitrogen form directly affects root zone pH over time. **Ammoniacal nitrogen (NH₄⁺)**: Acidifies the substrate as plants release H⁺ during uptake. Use with substrates running high pH (>6.2). **Nitrate nitrogen (NO₃⁻)**: Neutral to slightly alkalizing effect. Use with low-pH substrates (<5.8). **Urea nitrogen**: Converts to ammoniacal in the substrate, then acidifies. Most liquid formulations list the percentage of each form — match them to your substrate pH trend for stable root zone chemistry.
A 1:100 injector draws 1 gallon of concentrated stock solution for every 100 gallons of water that passes through it. Common ratios: 1:100 (most greenhouses), 1:128 (Hozon siphon), 1:15 (Dosatron low range), 1:200 (high dilution systems). Higher ratios mean more concentrated stock solutions.
General guidelines: seedlings 50-75 PPM, vegetative growth 150-250 PPM, flowering/fruiting 200-300 PPM, maintenance 100-150 PPM. Specific crops vary: poinsettias 250-300, petunias 200-250, tomatoes 150-200, lettuce 100-150.
Only if they're compatible. NEVER mix calcium-containing fertilizers (calcium nitrate) with sulfate or phosphate fertilizers in concentrated stock — they'll precipitate. Use two-tank injection systems for incompatible fertilizers. Most complete fertilizers (20-20-20, etc.) are single-tank compatible.
Electrical Conductivity (EC) measures total dissolved salts in mS/cm. The conversion depends on the meter: EC × 500 = PPM (Hanna/DiST), EC × 640 = PPM (Eutech), EC × 700 = PPM (Truncheon). Most US growers use the ×500 conversion. An EC of 1.5 mS/cm ≈ 750 PPM total salts.
The constant 75.2 comes from unit conversion: 1 oz of a 1% fertilizer dissolved in 1 gallon of water yields 75.2 PPM of that nutrient. It accounts for the density of water (128 oz/gal) and the PPM conversion factor (mg/L ÷ weight fraction).
For practical purposes in fertilizer mixing, yes — 1 PPM = 1 mg/L in dilute aqueous solutions. The two terms are interchangeable for greenhouse fertigation concentrations.