Brenda Medina-Privatt, Florence Sessoms, Dominic Petrella, Steven Lyon, David Gardner, Cale Bigelow, Jada Powlen, Ed Nangle
Introduction
Previous research has shown that soy flour can be used as a source of nitrogen to improve turf quality in an organic way. Greenhouse data indicated that the rate of 0.60 lb. N/M of soy flour was the breakpoint for achieving acceptable turfgrass quality with this soy-based mineral nutrient source. However, field results demonstrated that soy flour provides nitrogen to turfgrass for only 2-3 weeks, after which a decline in color and growth was observed (VanLandingham et al., 2024). Because of this limitation, we have been exploring new delivery methods for applying soy-based fertilizer. Pelletizing the material using only water appears to be the first step of a promising way to slow the release of mineral nutrients into the soil.
Moreover, since the motivation for applying organic fertilizers to lawns is an eco-friendly matter, developing a soy-based slow-release fertilizer should follow the same principles. It is well known that the use of slow-release granular fertilizers poses problems since their coating compounds contain microplastics, which contaminate soil and watercourses. To avoid this issue, we are using polylactic acid, a biopolymer, to coat these soy-based pellets. Thus, this material can break down slowly in landscape settings, releasing mineral nutrients from the soy-based pellets over time.
Results
Image of perennial ryegrass pots two weeks after soy flour application.
Figure of mean clipping biomass accumulation as effected by soy flour treatments applied to perennial ryegrass growing on sand or field soil
Current thoughts / Future work
At the second week of application, NDVI results indicate a stronger vegetative response from soy flour treatments compared to the pelletized forms. The 1 lb N soy flour treatment showed the highest NDVI (0.48), followed closely by the 0.75 lb rate (0.46), suggesting effective and readily available nitrogen release. In contrast, both coated and uncoated pellets resulted in lower NDVI values (0.35–0.40), with the 1 lb coated pellets underperforming even the control. These early results align with expectations for soy flour as a faster-releasing nitrogen source, while the pellets, particularly the coated form, may require more time to mineralize and become plant-available.
Normalized Difference Vegetation Index (NDVI) Response to Soy-Based Nitrogen Sources at Two Application Rates