Sangrak Son
Major Professor: Dr. Krishna Nemali
PhD DEFENSE SEMINAR
December 1st @ 9:00 A.M.
Room: HORT 222
“HYDROPONIC CROPS GROWN WITH ORGANIC FERTILIZERS: YIELD, NUTRIENT RELEASE, AND STRATEGIES TO ENHANCE NUTRIENT AVAILABILITY”
Abstract: Hydroponic production is capital and resource intensive. Organic labels for hydroponic crops can potentially increase revenue as organic produce is sold at premium prices. However, organic production generally results in lower yields compared to conventional production, likely due to lower nutrient availability for plant uptake. The objectives of my research were to i) quantify the lettuce yield in hydroponic systems supplied with organic and conventional fertilizers containing similar amount of nutrients and identify the physiological reasons for yield loss, if any, with organic fertilizers; ii) study different organic fertilizer strategies, including increasing substrate incorporated organic fertilizer (SOF), supplementation of liquid organic fertilizer (LOF), and inoculation of plant growth promoting bacteria (PGPB) on nutrient availability, yield, and food safety levels in lettuce; iii) develop a growth-based automation system that uses both digital and analog sensors to optimize the delivery of LOF in hydroponic production. All studies were conducted in a glass-covered greenhouse and used organically lettuce seeds, peat-based soilless substrate, certified organic fertilizers, and recycling sub-irrigation systems with reverse osmosis water. No pesticides were applied to plants in any study. The results from my research indicated that hydroponic lettuce yields were lower with organic than conventional fertilizers (approximately 34%) due to a slower rate of nutrient release and lower nitrogen levels in the plant. The yield loss was greater in fast-growing green cultivars than slow-growing red cultivars due to increased nutrient demand in the former. The supplementation of LOF and PGPB to SOF increased yield and did not specifically enhance food-borne pathogens such as Salmonella and Listeria. However, they were not economically viable due to high cost and lower than expected yield gains (approximately 18%). Doubling the rate of SOF was the only strategy that that resulted in a comparable yield to that of conventional fertilizer. However, the substrate contained significant amount of residual organic fertilizer at harvest stage, requiring the reuse of substrate in subsequent production cycles. Growth-based automated fertilizer delivery system effectively reduced LOF wastage and maintained crop growth. However, the higher costs of LOF masked the observed benefits of growth-based automation system. We anticipate that such systems can be highly useful in the future when low-cost liquid supplements with increased nutrient release capability are developed.