"We irrigate, fertilise, harvest — but where exactly do the costs go?" Without resource accounting, the answer is always approximate. Water and fertiliser are not just operational costs — they are variables that directly affect cost per unit of output, and they can be managed if they are measured.
Quick Glossary
- Drainage — the portion of irrigation water that passes through the substrate and exits the system; typical range: 10–30% of volume applied
- RO water — water purified by reverse osmosis; preparation cost includes membrane losses (typically 30–50% of input water is discharged as concentrate)
- Fertiliser efficiency coefficient — the fraction of fertiliser applied that is actually absorbed by the plant; the remainder remains in drainage or accumulates in the substrate
- Cost per unit — the resource cost allocated to one tray, one kilogram, or one harvest cycle
Water Consumption: What to Count
Actual water consumption in a system is not equal to irrigation volume. The full picture includes:
- Irrigation water — the volume applied to the crop
- Drainage losses — water that exits with drainage (10–30% of irrigation volume in drain-to-waste systems)
- Sanitation — washing trays, equipment, floors; often 20–40% of total water use in microgreens operations
- RO losses — if reverse osmosis is used, 30–50% of input water is discharged as concentrate and does not reach the crop
Example for a microgreens operation: 8 L of irrigation water per tray, 20% drainage = 1.6 L lost, plus 3 L sanitation per tray cycle = actual water cost of approximately 11–12 L per tray, not 8 L.
Fertiliser Efficiency: Recirculation vs Drain-to-Waste
The system type fundamentally determines fertiliser efficiency:
Recirculation systems: 80–90% of fertiliser is absorbed or recycled. Losses — primarily in blowdown (periodic drainage to prevent salt accumulation). Cost per unit of output is lower, but requires more precise EC and pH management.
Drain-to-waste systems: 70–75% efficiency is typical with good drainage management. With high drainage percentages (30%+) efficiency drops to 60–65%. The lost fertiliser in drainage represents a direct cost that should appear in the cost calculation.
The efficiency formula: if fertiliser cost per litre of solution is C and drainage fraction is D, then fertiliser cost per litre of water actually used by the plant = C ÷ (1 − D).
Cost Calculation: From Formula to Practice
Per-tray calculation for microgreens:
- Irrigation volume per tray cycle: 8 L
- Fertiliser concentration: 1.5 g/L (EC 1.8–2.0)
- Total fertiliser per tray: 8 × 1.5 = 12 g
- With 25% drainage: only 75% absorbed → effective fertiliser cost = 12 g ÷ 0.75 = 16 g equivalent per tray of crop produced
- At a fertiliser cost of 0.05 UAH/g: fertiliser component = 0.80 UAH/tray
- Water at 0.03 UAH/L × 11 L (including sanitation and RO losses) = 0.33 UAH/tray
Total resource cost: approximately 1.13 UAH/tray. Against a revenue of 80–120 UAH/tray, this is 1–1.5% — small in absolute terms, but the measurement habit reveals where efficiency is lost at scale.
Four Efficiency Levers
- Recirculation — switching from drain-to-waste to recirculation reduces fertiliser losses by 15–20 percentage points at scale
- Precise EC management — applying excess fertiliser "just in case" raises the cost per absorbed gram; target EC should be crop-specific, not uniform across all crops
- Drainage reuse — in drain-to-waste systems, drainage can be collected, tested for EC and pH, and blended back into irrigation at 20–30% of volume; effectively reclaims a portion of fertiliser losses
- Water cost accounting — treating water as a real input cost (not "almost free") changes irrigation decisions: shorter cycles, higher drainage percentages, and unnecessary sanitation become visible as cost drivers
Three Mistakes That Cost the Most
Counting only fertiliser purchase cost, not effective cost per absorbed gram. A cheaper fertiliser with lower solubility or higher drainage losses may cost more per gram of nutrient delivered to the plant than a premium product.
Not accounting for sanitation water in the cost calculation. In microgreens, sanitation water use per tray often exceeds irrigation water per tray. Ignoring it understates water costs by 30–50%.
Calculating RO water cost based on input volume rather than output volume. If your RO system rejects 40% of input water, your effective water cost is 1.67× the cost per litre of input water — not 1×. This matters when comparing RO water to tap water for a specific crop.
Signs That Resource Accounting Is Working
- You know the water consumption per tray or per kilogram of output, including all uses
- Fertiliser cost is calculated per gram of nutrient delivered, not per kilogram purchased
- Drainage percentage is measured and tracked, not estimated
- Cost per unit of output is recalculated when input prices or system parameters change