Borgmann Aquaponics Hydroponics
Nitrogen Balance in Aquaponics: From Ammonium to Plant Nutrition
Question: If a system produces 6.49 g NH₄⁺/day, how much of it is converted to nitrate for the plants or remains?
1. Basic Nitrification Process
In a well-functioning biofilter, the following reaction takes place:
- Step 1: NH₄⁺ → NO₂⁻ (nitrite) by Nitrosomonas bacteria
- Step 2: NO₂⁻ → NO₃⁻ (nitrate) by Nitrobacter bacteria
Overall Reaction:
2. Conversion for 6.49 g NH₄⁺/Day
A) Conversion to NH₄-N (Nitrogen Basis)
Since plants and bacteria utilize nitrogen, we convert to an N-basis:
B) Theoretical Nitrate Production (100% Efficiency)
With ideal nitrification:
Or on a nitrogen basis:
3. Realistic Estimates (Practical Factors)
Factor 1: Biofilter Efficiency
- Good Biofilter: 85-95% conversion
- New/Uneven Biofilter: 70-85%
- Assumed 90% Efficiency:
Factor 2: Plant Uptake
- Typical Uptake Efficiency: 60-80% of available nitrate
- Plants use N primarily as NO₃⁻, but also some NH₄⁺ (approx. 10-20%)
- Assumed 70% Uptake:
Of this as nitrate: ≈ 2.86 g N/Day (90% of 3.18)
Factor 3: Other Losses/Processes
- Denitrification (anaerobic zones): 5-15% N loss as N₂ gas
- Algae/Biofilm Growth: 5-10%
- NH₃ Volatilization: 1-5% (pH dependent)
- Sludge/Sediment: 2-8%
4. Practical Balance for 6.49 g NH₄⁺/Day
| Process | N-Share (g/Day) | % of Input | Remark |
|---|---|---|---|
| Input: NH₄⁺ from Fish | 5.04 g N | 100% | 6.49 g NH₄⁺ |
| Nitrification (90%) | 4.54 g N | 90% | Converted to NO₃⁻ |
| Plant Uptake (70%) | 3.18 g N | 63% | Actually utilized |
| Denitrification (10%) | 0.50 g N | 10% | Loss as N₂ |
| Algae/Biofilm (5%) | 0.25 g N | 5% | Unwanted growth |
| NH₃ Volatilization (2%) | 0.10 g N | 2% | pH/temp dependent |
| Accumulation in System | 1.01 g N | 20% | Increasing nitrate concentration |
5. What This Means for Your Plants
A) Nitrate Availability Per Day:
B) Amount of Plants That Can Be Supported:
Lettuce: Requires ≈ 0.15 g N/plant/growth period (6 weeks)
→ Daily requirement: ≈ 0.0036 g N/plant/day
→ Possible Number of Plants:
Tomatoes: Require ≈ 3.5 g N/plant/season
→ Daily requirement: ≈ 0.028 g N/plant/day (at 125 days)
→ Possible Number of Plants:
6. Important Parameters
Recommended Calculation Steps:
- NH₄⁺ Input → Convert to NH₄-N
- Apply biofilter efficiency (user can set: 70-95%)
- Plant uptake efficiency (60-80%)
- Subtract losses (denitrification, algae, etc.)
- Calculate Nitrate Accumulation:
Warning at accumulation > 50 mg/L NO₃-N (approx. 220 mg/L NO₃⁻)
Summary
From 6.49 g NH₄⁺/Day you obtain about 3.2 g N/Day for your plants, which corresponds to approx. 20 g NO₃⁻/Day. The actual amount depends heavily on biofilter efficiency and plant uptake!
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