Estimating Hydroponic Crop Production and Transplant Cycles
Avoiding Common Mistakes in Calculating Production Potential
Yeah, I know, this post is slightly more technical, and far less fun than some of the others, but these simple calculations are important, and can help you to avoid some common mistakes that people often make when designing a hydroponic or aquaponic system.
When setting out to construct a hydroponic or aquaponic growing system, determining the volume of crops that the system you have designed could grow, also referred to as the production potential of the system, is an important step in assessing the feasibility of the facility. If you intend on constructing a growing facility for commercial production, properly projecting harvest volumes is key to determining revenue potentials, market viability, and return on capital for various off-take scenarios, crop types, or system configurations. In this post, we are going to walk through some simple formulas that can be used to estimate harvest volumes for common crops, and to establish a stable crop rotation.
The first step in estimating crop production potential is to calculate the size of the system that you intend to build; for these calculations, system size is noted by the total number of plants that the system can hold. Determining the total number of plant sites involves laying out the growing equipment within your greenhouse or warehouse space, and then counting up the total number of plant sites. On smaller systems that do not require multiple transplants, this can be a simple task. For example, if a system has a single, 4x8ft Deep Water Culture (DWC) float bed, which holds four 2x4ft rafts, each with 18 plant sites, this bed would hold 72 plants.
On larger facilities, which utilize multiple transplants to maximize productivity per square foot, counting the total number of plant sites can be complicated by the different classes of plant sites: seeding, nursery, and growout sites. As an example, we will look at a commercial greenhouse for cultivating leafy greens, in which plants are transplanted twice during their growth cycle: from a dense seeding tray into a four-inch-on-center nursery site for juvenile growth, and then into an eight-inch-on-center growout site.
We will assume that the greenhouse contains two 40x80ft Deep Water Culture (DWC) float beds. In the preliminary designs, one float bed contains nursery sites, and one is designated for plant growout. Given that each bed is 3,200 square feet, they both contain four-hundred 2x4ft rafts. Assuming 18 plants per growout raft, and 36 plants per nursery raft, the beds have a combined 21,600 plant sites (7,200 growout sites and 14,400 nursery sites). In addition to the DWC float beds, the facility is equipped with three vertical seeding racks. Each rack is capable of holding 12 sheets of 98-site rockwool media, adding an additional 3,528 plant sites to the facility, bringing the total number of plant sites to 25,128.
Once the total number of plant sites in a system has been calculated, the next step is to determine the days to harvest for crops that you are interested in growing. By dividing the total number of plant sites by the days to harvest for a given crop, will project the estimated number of heads of that crop that can be harvested per day. Continuing with the above greenhouse scenario, if we were looking to cultivate a variety of lettuce that takes 47 days to reach harvest size, we would divide the 25,128 plant sites by 47 days to harvest, which results in a harvest of roughly 535 heads per day, or 16,039 heads per month. Assuming an average head weight of 8 ounces, this would equate to ~8,020 pounds of lettuce per month.
Before accepting these theoretical projections as achievable in a real-world growing facility, there is a critical check that must be conducted to ensure that the system contains the proper ratio of seeding, nursery, and growout sites to establish a stable transplanting rotation. This is one of the most common mistakes I see people make when working to determine crop production potential.
To check that we have a proper ratio of different plant site types, we can use a similar formula to the one used above, rearranged to the configuration: total number of a given plant site, divided by the heads harvested per day (which was determined in the previous section), equals the estimated number of days that the plants will spend occupying that type of plant site.
Continuing with the greenhouse scenario in the previous section, the preliminary design contains 3,528 seeding sites, 14,400 nursery sites, and 7,200 growout sites. As calculated in the previous section, this number of total plant sites will produce an estimated 535 harvested heads of lettuce per day. Dividing the 3,528 seeding sites by 535 harvested heads per day, we find that the plants in this facility would spend 7 days in the dense seeding trays before being transplanted. By dividing the 14,400 nursery sites by 535 harvested heads per day, we find that the plants in this scenario would spend 27 days in nursery sites before being transplanted into growout sites. Finally, by dividing the 7,200 growout sites by 535 harvested heads per day, we find that the plants would spend their final 13 days in growout sites. Running a total age check, where we add up the calculated days in each site (7+27+13), we get confirmation that the age of the plants in the rotation equates to the harvest age assumed in the previous section.
For certain varieties of leafy greens, a 7, 27, 13-day rotation is suitable for healthy crop development, and so we know that based on the total number of plant sites in the facility, and the numbers of different types of plant sites, it is feasible to stably produce roughly 535 heads of leafy greens per day, from the above greenhouse configuration. If we are looking to cultivate a crop that would require more time in growout sites, relative to nursery sites, we can allocate 600 of the 800 rafts in the float beds to be used for plant growout, and use only 200 of the rafts for nursery sites. It is important to note that the new total plant site count would fall to 21,528, and the new production potential would be roughly 458 heads per day.
Given the production volume of 458 heads per day, and the adjusted numbers of nursery and growout sites (7,200 and 10,800 respectively), the plants would spend the same 7 days in seeding sites, but would now spend 16 days in nursery sites, and 24 days in growout sites prior to being harvested at 47 days of growth. It is important to note that whenever you make changes to the number of different types of sites within a growing environment, or if assessing a crop with a different harvest-age, that the first formula be used to determine the estimated yield per day, as this is a critical variable for calculating transplant timelines using the second formula.
Over the years, I have reviewed a number of system proposals and implementation plans that accurately calculate the estimated number of heads produced based on the total number of sites, but do not properly run calculations on the rotation scheme. This can lead to over-estimating the production potential for a given footprint. Often, problems arise when people aggressively increase the number of seeding sites or nursery sites in their design, as the plant densities of these types of sites allows them to drastically increase the total number of plants without increasing the footprint of growing equipment.
To demonstrate this, we can quickly increase the total number of plant sites in the original greenhouse scenario above by drilling 720 of the 800 rafts for nursery sites, and only maintaining 80 rafts for growout sites. This increases the total number of plant sites in the facility from 25,128 to 30,888, without adding any additional equipment. While such a tweak might seem appealing when running the first production calculation, which shows that the system can now potentially produce 657 heads per day, rather than the previous 535, the model falls apart when run through the plant rotation calculator that confirms whether a functional transplanting cycle can be established.
With the above shift in the ratio of nursery sites to growout sites, plants would spend 5 days in seeding sites, then 40 days in nursery sites, before being transplanted into growout sites for only 2 days. This transplant cycles is not feasible in practice, as seedlings would be transplanted from germination trays prematurely, then would spend the majority of their time in dense nursery sites, where they would quickly become overcrowded, leading to poor plant health, decreased growth rates, and increases in incidences of pests and mildew infections.
When designing an aquaponic or hydroponic growing system, it is critical that time be taken to properly determine the estimated total production potential of the facility, and to run the necessary follow-up calculations related to crop rotations, ensuring that the system contains the proper ratios of seeding, nursery, and growout sites to allow for stable production.
