Degree Type


Date of Award


Degree Name

Doctor of Philosophy





First Advisor

Adam W Thoms


Adequate amounts of nitrogen (N) fertilizer and irrigation are required for maintaining high quality turfgrass. However, excess applications of N can cause N losses to the environment by leaching and volatilization. Thus, environmentally sustainable management practices have garnered considerable interest within the turfgrass industry. Enhanced efficiency fertilizers (EEF) have been used on turfgrass to increase nutrient availability and decrease potential nutrient losses to the environment. Furthermore, management practices that enhance soil health and reduce N inputs are gaining popularity. Humic substances are complex heterogeneous mixtures of materials that can affect root growth and architecture, nutrient uptake, stress tolerance, and photosynthesis. Thus, incorporating humic substances with fertilizers has the potential to decrease N fertilizers rates by improving turfgrass efficiency.Several objectives were focused on in these studies to determine if there was a benefit of incorporating humic substances with fertilizers on turfgrass. The first objective was to determine if the addition of humic products to fertilizers would enhance turfgrass rooting. The second objective was to evaluate if incorporating humic substances with fertilizers could improve turfgrass performance, tolerance, and recovery under simulated traffic. The third objective was to determine if reduced N fertilizer applied with humic substances could provide comparable turfgrass quality as full N rates, and if humic fertilizers could improve soil biological activity. The last objective was to evaluate if the addition of humic substances to fertilizers could be classified as an EEF. In a greenhouse study, starter fertilizer and starter fertilizer + humic dispersing granules (HDG) increased Kentucky bluegrass (KBG; Poa pratensis L.) total root length (105 and 112%) and root biomass (232 and 244%) compared to urea. Poly-coated humic-coated (PCHCU) and urea + HDG increased KBG total root length (37 and 20%) compared to urea. On bermudagrass [Cynodon dactylon (L.) Pers.], starter fertilizer and starter fertilizer + HDG increased total root length (241 and 248%) and root biomass (672 and 707%) relative to urea. Adding humic products to the starter fertilizer did not increase turfgrass rooting. In the absence of a starter fertilizer, PCHCU and urea + HDG improved KBG total root length compared to urea. In field experiments, the addition of humic substances to fertilizer treatments did not improve KBG traffic tolerance and performance. In 2020, four applications of fertilizers increased turfgrass recovery by 136% relative to the nontreated. Additionally, incorporating humic substances improved turfgrass recovery by 9% compared to fertilizers alone. On a native soil KBG, reduced rates of N with humic substances maintained turfgrass quality and cover, and reduced clipping biomass compared to full N rates. Fertilizer treatments had minimal effect on microbial biomass and other plant-available nutrients. However, PCHCU increased potentially mineralizable carbon (PMC) and potentially mineralizable N (PMN) by 68% and 59%, respectively, compared to the nontreated control. On a sand-based creeping bentgrass (Agrostis stolonifera L.) putting green, the incorporation of humic substances with reduced rates of N provided similar turfgrass quality and cover compared to full N rates. Urea, humic-coated urea (HCU) +/- HDG and black gypsum (BG) treatments increased microbial biomass from year one to year two. Minimal differences were seen in microbial activity besides HCU + BG. The HCU + BG had 60% greater potentially net N mineralization relative to HDG and the nontreated. Over the growing season, PCHCU had 25% greater inorganic N concentrations on ion exchange membranes compared to all other treatments. Both PCHCU and poly-coated sulfur-coated urea show extended nitrogen release curves with PCHCU having a greater release period. Results from these studies indicate humic products have variable responses and more research is needed.


Copyright Owner

Alex Lindsey



File Format


File Size

134 pages