Impact of Amendments in Combination with Fertilizers on Turfgrass and Soil Quality

Soil amendments are widely promoted to improve turfgrass performance and soil properties in suboptimal soil; however, their effectiveness under field-managed conditions remains unclear. Two concurrent field experiments (i.e., turfgrass and soil and reduced nitrogen) were conducted from August 2022 to November 2023 in Gainesville, FL, using a randomized complete block design to evaluate organic and biological amendments under standard and reduced nitrogen (N) fertilization. In the turfgrass and soil portion, treatments included granular humic + fertilizer, liquid humic + fertilizer, biochar + fertilizer, microbial inoculant + fertilizer, compost, natural fertilizer, fertilizer, and a non-treated control. In the reduced N experiment, fertilizer rates for all amendment combinations and the natural fertilizer were applied at 50% (12.2 kg N ha−1), while the full-rate fertilizer (24.4 kg N ha−1) and non-treated control were included for comparison. Treatments were applied to St. Augustinegrass (Stenotaphrum secundatum (Walt.) Kuntze) and zoysiagrass (Zoysia spp. Willd.) established on a sand-based root zone. Turfgrass performance was assessed using visual quality, normalized difference vegetation index, and percent green via digital image analysis. Soil properties were evaluated using physical, chemical, and biological parameters. Treatment responses varied by amendment type and N rate. All treatments improved turfgrass performance relative to the non-treated control, with compost producing the greatest improvements in turfgrass quality and soil properties, including organic matter, pH, and plant-available P, K, and Fe. Humic substances, biochar, and microbial inoculants primarily increased potentially mineralizable N but provided limited improvements in turfgrass performance compared with fertilizer alone. Nitrogen rate was the primary determinant of turfgrass performance, with full N treatments producing the highest quality. Although reduced N treatments improved turfgrass quality relative to the control, amendment additions did not consistently enhance turfgrass performance under reduced N conditions.