- Vegetables: greens (lettuces)
- Production Systems: aquaponics
A proposal was submitted to quantify nitrate levels in aquaponic systems to determine recommendations to producers interested in this emerging industry.
A total of 58,020 and 50,595 readings were collected from HD and LD, respectively. Data was collected for 199 days at the HD system and for 177 days at the LD system.
Results indicate that actual nitrate levels were very different from the perceived readings obtained by the 2 producers. While the HD system regularly measured at least 80 ppm, our measurements indicate that the nitrate level was 35.76 ppm on average. Similarly, while the LD system was rated at 5 ppm or less (as measured by the producer), our probe measured a 13.9 ppm for an average of 50,595 measurements.
The ‘sticks’ are not accurate and producers need to use better tools to quantify their nitrate levels.
Water temperatures were almost identical over the duration of this project (about 180 days) in both a greenhouse and a high tunnel. This is good news for aquaponics in Texas as there's no need to invest in an expensive greenhouse.
The HD system had an average of 5,253 ppm nitrate for an average weight of 108 g. At 39% absorption, this is equal to 221 mg of nitrate per head of lettuce. The LD system had an average of 7,321 ppm nitrate for an average weight of 224 g. this is equal to 640 mg of nitrate ingested. Both systems exceed the recommended daily allowance of nitrate, according to FAO/WHO.
A proposal was submitted to quantify nitrate levels in aquaponic systems to determine recommendations to producers interested in this emerging industry. As of today, growers rely on hearsay or recommendations viewed on youtube on the ideal nitrate level for aquaponics. Of course, nitrate level is a direct result of fish stocking density and two contradictory schools of thought address this point. The high-density (HD) concept relies on high fish stocking density to achieve high nitrate levels of 80 ppm or higher. The low-density (HD) concept relies on low fish stocking density with a goal of 5 ppm or lower. Each school of thought decries the other and aquaponics beginning growers enthusiasts are not sure which to choose.
To complicate the situation, fish feeding rate is not accurately measured or tailored to actual fish weight. Instead, a grower relies on visual observation of the fish feeding habit and quits adding feed when the fish stop feeding. This results in either over- or under-feeding. More importantly, this results in a fluctuating nitrate level in the water which is a waste of money.
Finally, most producers rely on ‘calibration sticks’ to measure pH, nitrate, nitrite, and ammonia. These ‘sticks’, although cheap and fast, are not accurate or reliable by any means. Producers regularly over- or underestimate these parameters and base the feeding amounts based on those unreliable readings.
This proposed research aimed at conducting an experiment at Sand Creek Farm to evaluate 3 nitrate concentrations in an aquaponic system grown with ‘Rex’ lettuce. The current nitrate level is 5 ppm, as measured by the producer. A complete system (about 1,000 sq ft production area) housed in one tunnel (about 3,000 sq ft) was to be used for each rate. The two other treatments planned were 20 ppm and 80 ppm. Increasing the nitrate levels was to be achieved by increasing the fish stocking density and/or spiking with nitrate fertilizer. However, due to the unwillingness of the producer to adjust nitrate rates to higher levels which required additional filtration and settling tanks, two other producers were sought with nitrate rates suitable for this research proposal. Sustainable Harvesters in Hockley measures nitrate levels at 80 ppm at least, and Ag Farm in Bryan measures nitrates at about 20 ppm.
Therefore, one digital probe (Hanna Instruments) was installed at each location to conduct this research. Producers were asked to record seeding and harvest dates, and time and amount of fish feed used. The probe is an independent unit calibrated to take a reading every 30 minutes and can measure simultaneously nitrate, electrical conductivity (EC), water temperature, and dissolved oxygen (DO). Growers were asked to continue their daily nitrate level measurements to compare to the probe readings.
With one probe per location/nitrate rate, our vision was to quantify actual nitrate levels, daily fluctuations of nitrate, final harvest size, and leaf nitrate content.
From these results, our goal was to:
- Demonstrate the actual nitrate levels vs. what the producers expected to have using their ‘sticks’.
- Demonstrate that nitrate fluctuates during the day and recommend increasing the feeding frequency, using the same total amount, in order to reduce this daily fluctuation
- Determine which nitrate rate – low or 5 ppm, medium or 20 ppm, and high or 80 ppm – is most productive in terms of lettuce yield and time to harvest.
With these goals in mind, we expect to save producers money and increase their profitability through one of these decisions:
- Increasing feeding frequency will reduce large nitrate fluctuations, which is beneficial to the plants
- If 20 ppm is just as productive as 80 ppm, then the grower will save a lot of money by reducing his/her fish stocking density and feeding rate.
- If 20 ppm is better than 5 ppm, then the grower will increase his profits by upping his nitrate levels and produce a better quality crop in a shorter interval.
- If 80 ppm is the best rate to achiever most productivity and profit, then producers can determine the cost and profitability of increasing their fish stocking density to achieve this rate.