The response of Kentucky bluegrass (Poa pratensis L.) to potassium (K) fertilization has been inconsistent. The objective of this research was to determine the effects of K fertilization across varying nitrogen (N) rates and clipping management on Kentucky bluegrass clipping yields, quality, tissue K concentrations, apparent N recovery, and N use efficiency. A 2 × 4 × 4 factorial was arranged in a split‐plot design and repeated across two years. Main plots were clipping treatments (returned vs. removed) and subplots were N rates (0, 98, 196, and 294 kg ha−1 yr−1) in combination with K rates (0, 81, 162, and 243 kg ha−1 yr−1). There was no positive effect of K on clipping yields and quality even though soil extractable K levels tested low. Higher K rates, however, increased N recovery and use efficiency for all but the highest N rate. Tissue K response to K fertilization was nonlinear. Yield and quality responses were not correlated to tissue K concentration. Nonexchangeable K levels were high in the native soil, and may have provided an additional source of K for bluegrass. The results suggest that extractable K values alone may not adequately predict available K to Kentucky bluegrass in this sandy loam soil.

Objective approaches to guide N fertilization of turfgrass sod crops are lacking. This study was conducted to determine the relationships among normalized difference vegetative index (NDVI), frequently‐measured soil NO3–N concentrations, and peak‐shear force of predominately Kentucky bluegrass (Poa pratensis L.) sod, and to evaluate if those relationships could help guide N fertilization. Ramp calibration strips (RCS) with varying N rates were established within production sod fields in Rhode Island across three consecutive years. At 2‐wk intervals during each growing season, soil NO3–N concentrations and NDVI readings were recorded and correlated. Biweekly relative NDVI readings plateaued when soil NO3–N concentrations ranged between 5 and 12 mg kg–1. Mean relative NDVI plateaued at 196 kg N ha–1 yr–1. Relative sod peak‐shear force was negatively correlated to soil NO3–N concentrations and the total amount of N applied per year. Relative peak‐shear force was maximized when mean relative NDVI readings ranged from 0.969 to 0.982, but declined as mean relative NDVI increased towards 1.0. The results suggest that frequently measured NDVI and soil NO3–N concentrations show promise as guides for N fertilization of predominately Kentucky bluegrass sod. Further research will be required to validate these approaches on larger‐scale sod fields.

Adequate turfgrass sod strength for harvesting and handling is typically determined by the producer’s past experience and subjective appraisal. This study was conducted to determine the relationship between producer subjective sod-strength assessments and quantitative shear-strength measurements with predominantly kentucky bluegrass (Poa pratensis L.) turf. Across three consecutive growing seasons, 93 samples were collected from sod fields in Rhode Island and assessed for sod strength by subjective and quantitative methods. Producer subjective ratings of sod strength were significantly (P < 0.0001) associated with quantitative measurements of peak force required to shear a sod strip. Minimally acceptable strength occurred most frequently when peak shear force was between 55 and 85 kg·m−1 width of sod; whereas preferred sod strength occurred most frequently when peak shear force was between 70 and 140 kg·m−1 width of sod. Once peak force exceeded 58 and 86 kg·m−1, there was a > 50% probability that sod strength would be judged at least adequate and at preferred strength, respectively, up to a peak force of 140 kg·m−1. The results suggest that quantitative measurements of shear strength can be related to producer subjective assessments, and provide unbiased benchmark values to guide management decisions for kentucky bluegrass sod production.