The Effects of Potassium Fertilization and Sand Topdressing on Creeping Bentgrass.

Turfgrass managers often apply large doses of potassium (>300 kg K₂O ha^-1 yr^-1) to putting greens. The conventional wisdom is that high levels of potassium confer stress tolerances to the plant. However, potassium applications have been associated with increased winter diseases. The objective of this long-term research project was to evaluate putting green quality, growth, and disease incidence over a range of soil and tissue potassium levels.

This project was initiated in 2011 on a USGA putting green with ‘A4’ creeping bentgrass (Agrostis stolonifera L.). The experiment was a randomized complete block design with four replications. The treatments included four levels of liquid potassium sulfate, ranging from 0 to 300 kg K₂O ha^-1 yr^-1 and one level of granular gypsum at a rate of 100 kg Ca ha^-1 yr^-1.

There were no significant differences in chlorophyll index, visual turfgrass quality, or clipping yield due to treatment over the entire course of the research. Tissue K, exchangeable soil K, and Michrodochium patch infection were all significantly affected by treatment over the six-year study, with high-K treatments being significantly higher in all three instances.

Turfgrass removed a greater amount of K through daily clipping collection than was available in the exchangeable K pool identified by Mehlich-3 and 1 M HNO₃ testing. This indicates K was becoming plant-available from a non-exchangeable pool. Hence, traditional soil testing methods may be inadequate for determining plant-available K in sand-based root zones.

Total soil K analysis of the 0-K treatment indicated an increase in soil K during the six-year study. With top-dressing sand being the only input on this treatment, the increase in potassium was likely from the weathering of potassium feldspar, a mineral commonly found in sand. The potassium derived from the sand was in sufficient levels to compensate for the lack of K fertilizer.