School of Civil and Environmental Engineering
Ph.D. Thesis Defense Announcement
Developing Robust Agricultural Kinetic Models:
The Hydroponic Cultivation of L. Sativa Lettuce in
Controlled Environment Agriculture
By Andrew Sharkey
Advisor:
Dr. Yongsheng Chen
Committee Members: 
                                                        Dr. Xing Xie (CEE)
                                                        Dr. Ching-Hua Huang (CEE)
                                                        Dr. Ameet Pinto (CEE)
                                                        Dr. Rhuanito Ferrarezi (UGA Dept. of Horticulture)

Date and Time: August 11, 2025.  1pm-3pm EST
Location: Price Gilbert Memorial Library #4222 (Dissertation Defense Room)
Teams Meeting ID:  279 728 293 764 0
Password:  Ui762w4Q

ABSTRACT
The lifecycle of fertilizers, as the foundational components of agriculture, is
riddled with inefficiencies including expensive and limited-resource supply,
centralized agricultural production, and many harmful environmental impacts.
Fortunately, hydroponics and Controlled Environment Agriculture (CEA) offer
potential solutions to issues of both centralized agricultural production and
harmful environmental impacts. To resolve the remaining issues, wastewater can be recovered to provide nutrients for new generations of agricultural crops
destined for human consumption.
However, in order to fully capitalize on these potential solutions, farmers
and engineers require improved models which track both nutrient uptake and
biomass growth. Such fundamental models, based on Michaelis-Menten and
Monod reaction kinetics, have been rigorously studied for microbial populations,
but fewer studies have applied these fundamentals to agricultural populations.
Furthermore, agricultural populations require unique modifications before models
are applicable to wastewater nutrient recovery efforts.
Hydroponic experiments cultivated Bibb lettuce with synthetic nutrient
solutions based on Modified Sonneveld’s Solution, with treatments focused on
individually varying each of three limiting nutrients: nitrogen (N), phosphorus (P),
and potassium (K). Researchers analyzed both plant tissue and water samples to
quantify changes in relative growth rates, nutrient uptake rate, and lettuce tissue
composition as plants matured in each treatment. These results were used to
incrementally address gaps to build a robust agricultural kinetic model.
This model pioneers the characterization of temporally dynamic agricultural
growth rates, incorporates multiple limiting nutrients, and incrementally builds
agricultural biomass to reflect the variable nutrient supply expected of reclaimed
domestic wastewater. Analysis of nutrient uptake identifies ideal nutrient
concentrations for hydroponic lettuce, and identifies minimum nutrient
concentrations required to grow compositionally healthy and nutritious lettuce.
Meanwhile, the resulting models explain transitions between life stages and
identify optimal harvest days for maximizing plant biomass. While these results
speak directly to lettuce grown in vertical hydroponics, the completed
methodology can readily apply to other vegetables, nutrients, and environmental
conditions.
The resulting models are finally validated against three treatments of
variable nutrient supply, analyzed against alternative nutrient-based agricultural
models, and evaluated against expected domestic wastewater nutrient
concentrations. This research allows and exemplifies practical application of
recycling nutrients from wastewater, significantly increasing prediction accuracy,
while relying solely on relatively non-invasive nutrient measurements from
hydroponic water samples.