Most Important System Design Parameters
System Design Parameters
Fish to plant ratios( Feeding rate ratios / Grow bed calculators)
Every practitioner of aquaponics will have their own aquaponics ratios based on observable or calculable factors.
Aquaponics grow bed calculators define the ratio between the amount of fish and plants in your system. This ratio comes from the nutrient input (fish feed) from the aquaculture unit and the utilization of nutrients by the plants.
Fish feed is the most important input in aquaponics and there is a direct relationship between the feed going into the system and the growth of your plants. Think about it: You add feed into the system, the fish eat the feed and produce ammonia, bacteria convert ammonia into nitrates, the plants consume the nitrates and grow. Because of this relationship, balancing the proper amount of feed entering the system is essential to the overall success of the system.
There are 2 methods that have scientifically arrived at a way to determine this ratio, one byDr. Wilson Lennard and the other by Dr. James Rakocy. They both define the nutrient input as the daily amount of fish feed.
The difference between their approach is in defining the hydroponics component (plants) . Dr. Lennard’s practice bases itself on the number of plants and the plant species and also the fish being considered (this ratio is specific for each species of plant and fish ). Alternatively, Dr. James Rakocy relates the amount of daily fish feed to the growing area (general for most plant and fish species grown in aquaponics)
Before Calculating For The Ratio, It Is Important To First Identify Characteristics and Purpose Of Your System
In defining the aquaponics grow bed calculator, begin with identifying the following:
- The purpose and objective of your aquaponics system as a commercial setup or backyard setup. For a commercial setup, further, identify your market and main product
- Decide on the plant species, population, and corresponding spacing for the intended species.
- Outline the total growing area needed for the intended plant population( eg : Lettuce-30 plants can be grown as per square meter)
- Determine how much fish feed the fish need to eat to meet the nutrient requirements of the plant based on the species and total population of the plants.
- Get the amount of fish in terms of total biomass (weight) needed to consume the amount from the daily feeding requirement.
- Determine fish stocking density for your aquaculture component
- Determine the volume of water needed for the fish tank in liters for optimal growth of the fish species
From the information defined in these steps, you will be able to choose between the practices of Dr. Rakocy or Dr. Lennard. Personally, we believe that Dr. Rakocy’s system works well with backyard and hobbyist/small commercial scale aquaponics which use different plant species.
For Hobbyists and Small-Scale Systems With Various Plant Species, The Aquaponics Grow Bed Calculator Of UVI/Rakocy Sufficiently Provides Enough Nutrients Per Square Foot Of Growing Space.
Dr. James Rakocy defines his approach on the available amount of nutrients for the plants
per growing area coming from the daily input of feed .
The feeding rate ratio or the daily amount of feed per unit of plant growing area for raft type aquaponics grow beds( DWC) is 60-100 g/m2/day. If you are using a NFT system, you will only need 25% of what is needed in a raft system.
He emphasizes the need to supplement plant nutritional requirements with calcium, potassium, and chelated iron. In the case of the UVI approach, the rate of feeding depends on the plant species as well. Plants with low nutrient requirements can opt to go for a feeding ratio of 60 g per square meter daily.Alternatively, plants with larger nutritional requirements like tomatoes and fruiting crops should use 100g daily.
In this approach, you will get an excess of nitrogen in your system. This can easily be addressed through the removal of solid waste and dilution of water if required.
Large Commercial Setups For Aquaponics Should Follow The Approach Of Dr. Lennard For The High Specificity and Efficiency Of Production
Dr. Rakocy focuses on utilizing the fish feed to provide most of the nutrient requirements for the plants other than potassium, calcium, and iron. On the other hand, Dr. Lennard’s approach focuses on balancing the nitrogen required for plant growth and supplement nutrients .
We highly recommend additional reading to understand this aspect 🙁
http://www.aquaponic.com.au/Fish%20to%20plant%20ratios.pdf )
The result is a highly specific approach which requires research into the specific nutrient requirement of each plant species. This requires the supplementing of additional essential nutrients like phosphorus and others for proper plant growth ,specific to the combination of plant and fish species used . (Additional reading of page 67 :
https://www.aquaponic.com.au/The%20Symbioponics%20Aquaponics%20method%20-%20final%20draft%20211042021.pdf
)
A separate offline fish waste solids mineralization bed (can also be a media bed)is used for re-addition to the system for utilization of all available nutrients in the fish waste (for sizing you can refer to https://www.aquaponic.com.au/calculators.htm )
Overall this is a more economical approach for commercial systems.For a rough calculation of the ratio of this approach, follow these steps
- From daily feeding amount and protein content, get the amount of nitrogen available for your system
- Identify your plant species and the nitrogen requirement for the plant
- Compute for the amount of plants that could be grown with the available nitrogen in your system
This process can be worked backwards as well starting from the nitrogen requirement for the plants to the required daily feeding amount and protein content.
Comparison between the two methods with an example
The Dr Lenard feeding rate ratio that may be compared directly with the one of Dr Rackocy feeding ratio is a little difficult as both approachs state ratios in different units (Rakocy ratio-g/m2/ day, Lenard- Kg of fish feed per plant number) Rakocy is a generalised approach to meet all plant species requirements while Lenard’s approach is highly specific to each individual fish and plant species or variety.
However an example can be provided
Dr Lenards ratio states that for a fish species like tilapia 1kg of fish feed fed daily, will support approximately 1500 lettuce plants in a rotational planting/harvesting scheme
If you assume that the lettuces are planted at a density of 30 plants per square meter then this may be transplanted to an equivalant that may be compared directly to the Rakocy ratio. In these terms as a means of comparison , the Lenard feeding rate ratio is approximately 16 g of fish feed/m2/day .That is approximately 3.75 times less than the Rakocy ratio.If you assume the lettuces are planted at a density of 25 plants per square meter, then this translates to 13 g of fish feed /m2/day or approximately 4.5 times less than the Rakocy ratio.
This Lenard approach is very specific and so this ratio will not hold true for any other fish or plant species then the Tilapia or lettuce stated. If feeding ratios for one species of fish or plant are applied to others, then the system can easily go out of balance. This method depends on constant water testing and the use of highly specific buffer formulations.
CONCLUSION
Every practitioner will have their own feed or aquaponics grow bed calculator based on different variables. Before beginning to calculate your grow bed ratio, start by defining your system ’s goals. Then you can begin from either your plant’s nutritional requirements or the amount of nutrients from fish feeds. Dr. Rakocy and Dr. Lennard both approach their aquaponics grow bed calculators scientifically based on years of research and experimentation. Depending on your plant species, fish species, and purpose for aquaponics, there will be appropriate ratios and calculators.
Additional Resources :
We found this video to be particularly helpful in understanding :
