The bulk salt price decision is one variable: the stock discount d that clears the green above-20%-savings line. The optimizer prices nutrient liters as reservoir volume x EC x concentration factor, then applies the discount to the whole-cycle mix bill (C_bulk = C_mix x (1 – d/100)) — at 25% off, a $2.80/kg retail part costs $2.10/kg, a $0.70/kg spread that compounds across every top-up the evaporation line forces. Bulk salt price per kilo only earns its keep when the net cost per liter stays under the $0.25/L red line after labor and recycling credits are folded in.
Key takeaways
- Let evaporation compound across the cycle and apply the recycling rate: make-up water = V × (1 + evap × days/100) × (1 − recycling/100).
- Read the green—yellow—red band off savings % (of mix cost) and net cost per liter — green above 20% savings, red under 10% or past $0.25/L.
- Nutrient liters scale with EC: a 1.8 mS lettuce feed mixes more concentrate than a 1.0 mS one at the same volume (the GH Flora 20-gallon 1.4 EC archive anchor).
- Evaporation is the quiet line: 5%/day over a 90-day cycle replaces volume × 5.5 in make-up water before recycling ever trims it.
How Nutrient Cost Optimizer — Bulk Mixing & Recycling Blueprint Helps You Master This
- Enter the reservoir volume, EC target and price per liter — nutrient liters = V × EC × concentration factor × (1 + waste/100).
- Let evaporation compound across the cycle and apply the recycling rate: make-up water = V × (1 + evap × days/100) × (1 − recycling/100).
- Build the mix bill: nutrient liters × price, water liters × water cost, plus pH adjustment on the standing volume.
- Apply the bulk discount off the mix bill and the 80% recycling recovery credit to get bulk savings and recycling savings.
- Add labor as a real line, subtract total savings from the mix bill, then divide by volume and days for cost per liter and cost per day.
- Read the green—yellow—red band off savings % (of mix cost) and net cost per liter — green above 20% savings, red under 10% or past $0.25/L.
The Formula Behind the Result
Nutrient solution to mix — volume × EC × concentration factor → N = V cdot EC cdot f_c cdot left(1 + frac{w}{100}right)Make-up water — evaporation compounds, recycling trims it → W = V cdot left(1 + frac{e cdot D}{100}right) cdot left(1 - frac{R}{100}right)Mix bill — nutrient liters, water liters, pH adjustment → C_{mix} = N cdot p_N + W cdot p_W + V cdot p_{pH}Bulk cost — stock discount off the mix bill → C_{bulk} = C_{mix} cdot left(1 - frac{d}{100}right)Bulk savings — the discount line → S_{bulk} = C_{mix} cdot frac{d}{100}Recycling savings — 80% recovery credit → S_{rec} = C_{mix} cdot frac{R}{100} cdot 0.8Total savings — bulk plus recycling → S = S_{bulk} + S_{rec}Net cost — labor on top, savings off the mix → C_{net} = C_{mix} + C_{labor} - S, qquad frac{C_{net}}{V}, frac{C_{net}}{D}
Target Ranges: What the Numbers Should Be
- Dose to net reservoir volume, not the stamped tank — the archived reservoir-volume engine tied every top-up to actual current depth.
- Nutrient liters scale with EC: a 1.8 mS lettuce feed mixes more concentrate than a 1.0 mS one at the same volume (the GH Flora 20-gallon 1.4 EC archive anchor).
- Evaporation is the quiet line: 5%/day over a 90-day cycle replaces volume × 5.5 in make-up water before recycling ever trims it.
- Bulk mixing uses the concentrated masterblend 4-18-38 stock rate and the C1 V1 = C2 V2 dilution rule — the discount applies to the whole-cycle concentrate order.
- Recycling savings carry an 80% credit: pumps, filtration and flushing leak about a fifth of what a recirculating loop recovers.
Use the free Climate Controller & HVAC Sizer — VPD, Dehumidifier & CO2 Blueprint to run the numbers on your own setup.
Use the free EC / PPM / CF Converter Hub to run the numbers on your own setup.
Use the free Dissolved Oxygen Calculator — Saturation & Aeration Blueprint to run the numbers on your own setup.
Compare every free calculator side-by-side on the MistCulture tools hub — each one keeps your workspace in sync.
Related guides
Worked Example: a 25% Bulk Discount on a 4-18-38 Concentrate Order
Price the concentrate that runs the room: retail 4-18-38 stock at $2.80/kg. A 50 kg order fills the mix-bill line at 50 x $2.80 = $140 (the C_mix base). Order the same stock in bulk at a 25% stock discount and C_bulk = 140 x (1 – 0.25) = $105, a bulk savings of $35 that reads straight off the green above-20% band.
Per kilo the gap is $2.80 retail versus $2.10 bulk — a $0.70/kg spread — and that spread multiplies through every top-up the volume line asks for, including the make-up water that 5%/day evaporation forces across the cycle. Add the recycling leg from the same bill: at 30% recycling with the 80% recovery credit, S_rec = 140 x 0.30 x 0.8 = $33.60, so total monthly savings run $68.60 before the net-cost-per-liter check. Hold the result under the $0.25/L red line and the bulk order is the call.
- $2.10/kg — bulk price at a 25% stock discount versus $2.80/kg retail, a $0.70/kg spread
- $35/month — bulk savings at d = 25% on a $140 mix bill, inside the green above-20% band
- $0.25/L — red line for net cost per liter; the bulk order only wins underneath it
Frequently Asked Questions
References & Science
- NCSU CALS — Vapor Pressure Deficit resources
- Cornell CALS — hydroponic research manual
- Purdue Extension — hydroponics guide
Frequently asked questions
How do I benchmark bulk salt prices per kilo?
Compare delivered price per kilo on the whole-cycle concentrate order, then apply the band: bulk savings above 20% of the mix bill reads green, under 10% or a net cost past $0.25/L reads red. At a 25% stock discount a $2.80/kg retail part lands at $2.10/kg, and that spread has to survive the net-cost check after labor and recycling credits before you commit.
What is the biggest mistake when buying salts in bulk?
Buying on the per-kilo sticker alone. The discount only wins if net cost per liter stays under the $0.25/L red line once labor and the recycling recovery credit are folded in, and evaporation quietly inflates every volume — 5%/day over a 90-day cycle is about 5.5 full reservoir volumes of make-up water before recycling trims any of it. Price the whole cycle, not the bag.
Which tools help me compare bulk salt prices?
The embedded Nutrient Cost Optimizer prices nutrient liters by reservoir volume x EC x concentration factor, applies the bulk discount and the 80% recycling credit, and returns the net-cost-per-liter number that decides the order. Use the EC / PPM / CF converter to confirm the EC feed before the concentrate quantities land in the bill.
References
- Resh, H. M. — Hydroponic Food Production (7th ed.), CRC Press
- ISHS Acta Horticulturae — nutrient film technique (NFT) system publications
- Cooper, A. (1979) — The ABC of NFT, Grower Books
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