Deep water culture keeps roots in nutrient solution. A commercial installation needs oxygenation, circulation, nutrient management and plant support suited to the crop. Those requirements define the system; its name does not establish a yield multiple, disease reduction or investment return.
Cornell’s hydroponic lettuce handbook describes a floating-pond production system and its monitoring routines. It is useful evidence for that crop and system. Its operating settings should not be transferred unchanged to every DWC project, especially one intended for a different crop or growing cycle.
What the root zone and circuit must provide
Ask for a drawing that shows the solution volume, flow route, oxygenation equipment, measurement points, drains and maintenance access. Identify how the system will operate with the intended crop load and how the team will respond when a component stops. Reservoir volume and aeration duty require a design basis; a generic litres-per-plant rule cannot settle either decision.
Keep these operating responsibilities visible:
| Part of the system | What the operator needs to know | Handover evidence |
|---|---|---|
| Pond or reservoir and circulation | Normal level, flow route, access, leak response and cleaning method | Drawing, volume basis and equipment instructions |
| Oxygenation | Equipment state, measurement locations and the agreed response to low readings | Duty basis, monitoring procedure and response plan |
| Nutrient management | Water source, crop program, solution checks and permitted adjustments | Water analysis, instrument instructions and crop-adviser limits |
| Plant support and access | How plants are supported through the growing cycle and reached for work | Layout, access clearances and support details |
An air pump running is an equipment observation. A dissolved-oxygen reading at a representative location is a separate observation. Keep both in the operating record so a stopped pump, a questionable measurement and a genuine root-zone problem are investigated on their own evidence.
Strawberries and herbs: check crop fit before scaling
Define the crop and cultivar, propagation material, growing period, support needs and harvest method. A lettuce pond is not, by itself, a validated strawberry or herb production plan. Ask the crop adviser which parts of a proposed reference system apply, which require a trial, and what would rule the design out.
Judge the crop through a representative harvest. Record marketable output, rejected material, labor and operating interruptions using the same area definition and grading rules. Larger-looking roots or a clean-looking harvest photograph cannot establish commercial performance. Keep crop protection and sanitation responsibilities in the plan; water culture does not remove the need for them.
Make the dissolved-oxygen record comparable
Write down the displayed unit with every result. mg/L is a concentration; percent saturation expresses the reading relative to the saturation concentration for stated conditions. A number in mg/L cannot be compared directly with a number in % saturation. The conversion depends on the relevant temperature, pressure and salinity basis, so do not silently change a unit label to make two displays agree.
The USGS 2020 chapter A6.2, Dissolved Oxygen covers optical sensors, calibration, measurement and reporting. It supersedes the earlier USGS version 3.0. The guidance concerns water-quality measurement, primarily in natural waters; it does not set oxygen targets for lettuce, strawberries, herbs or nutrient solutions.
For a measurement, follow the instrument manual and the agreed site procedure. Allow the sensor to equilibrate with the solution temperature and wait for a stable reading under that procedure. Record repeated observations if the display keeps changing; do not select a convenient value from a drifting display. Use the planned measurement location and depth, and record equipment state at the same time. A sample that changes during handling may no longer represent the root zone.
| Record item | Enter or check |
|---|---|
| Location and time | Pond or circuit ID, point, depth and timestamp |
| Instrument | Meter and sensor identity, sensor type, maintenance and calibration/check record |
| DO result | Value and unit; keep mg/L and % saturation in separate labelled fields |
| Temperature and stability | Solution temperature, equilibration completed and the agreed stability check |
| Pressure basis | Local ambient pressure used, its source and how the instrument applies it |
| Salinity compensation | Measured or entered value, unit, source and whether compensation is automatic or manual |
| Operating context | Crop stage, circulation/aeration state, unusual events and follow-up action |
Check compensation before adjusting equipment. The USGS chapter distinguishes local ambient pressure from weather pressure adjusted to sea level. It also explains that salinity correction depends on the instrument and method. Confirm that the meter’s temperature measurement is sound, that the appropriate pressure is used, and that a salinity correction is neither missing nor applied twice. For a nutrient solution, ask the instrument supplier whether its conductivity-to-salinity conversion and operating range suit the solution; do not assume an unverified conversion is exact.
The record should answer two questions separately: is the measurement reliable, and does it meet the crop’s agreed operating limit? Set the latter with the responsible crop adviser and system designer. This article supplies no universal DO threshold, calibration tolerance or fixed stabilization time for a commercial DWC crop.
Test the production and financial claims
Request the record behind each forecast. Keep measured performance, design capacity and expected future performance in separate columns.
| Claim used in the proposal | Evidence needed before relying on it |
|---|---|
| More marketable crop per area | Comparable cultivar, planted area, harvest interval, crop conditions and grading records |
| Faster production | Defined starting and finishing stages and a complete planting-to-harvest calendar |
| Less disease | Identified disease, observation method, comparison conditions and management differences |
| A higher selling price | Evidence from the intended buyer and grade, with volume, timing and rejected crop accounted for |
| Lower operating cost | Power, labor, nutrients, water, monitoring, maintenance and replacement assumptions for both systems |
Calculate marketable harvest per planted square metre over the same interval. Keep the absolute weight or count alongside that result. If growing cycles differ, show the actual calendar rather than multiplying a short trial into an annual forecast. A trial establishes what happened under its conditions; assess its transferability before using it for a different crop or site.
For an investment comparison, use the saleable quantity, price basis and cost records together. Include startup periods, maintenance interruptions and the buyer’s route to market. If a premium price or production claim has no suitable evidence, leave it identified as an unresolved assumption rather than a confirmed benefit.
What to have ready before committing
Collect the crop plan, circuit drawing, equipment duties, measurement procedure, maintenance responsibilities and evidence for the proposed output. Specify who responds to an alarm and how the crop is protected during an interruption. Use the agreed controlled acceptance procedure to verify equipment response and recorded conditions without improvising faults in a working crop.
Compare the system boundary with the NFT, DFT, DWC and drip overview. Keep wider facility assumptions in the commercial greenhouse buyer’s guide. For a project discussion through the CFGET contact page, bring the crop plan, layout and unresolved evidence questions. These records also support decisions made with your own crop adviser or another supplier.
