Potassium Fertilization In Perennial Ryegrass Turf: Effects of N and Year On Plant Response.

Conflicting reports exists in turfgrass regarding the effects of potassium (K) on growth and tissue K. Plant response to K is confounded by the amount of nitrogen (N) applied, the species, the season, and year to year variation. The objective of this study was to assess perennial ryegrass (Lolium perenne) shoot growth (yield), soil available K and leaf-tissue K in response to 15 N-K combinations over a five-year period (2003 to 2007). Field studies evaluated five rate levels of N (49, 147, 245, 343, and 441 kg ha-1 yr-1) with three rate levels of K (49, 245, and 441 kg ha-1 yr-1) with 4 replications as a RCB. Urea (45-0-0) and potassium sulfate (0-0-41.5) were used as the sole sources of N and K. Plots (1.5 by 3.0 m) were established on silt loam soil in South Deerfield MA and were mown twice per week at 3.2 cm height of cut with clippings returned. No supplemental P and K were needed; soil tests were high (23 and 123 mg kg-1, respectively). Soil K was determined in each year of the test from samples taken from each N-K treatment plot in April prior to fertilization. Clipping yields were oven dried following collections made in May and June and averaged. Tissue K was determined from analysis of dried leaf-clippings collected in April. Soil K (modified Morgan), plant yield and tissue K responses to N and K rates were linear. Significant regressions were only observed with yield in response to soil K at 343 and 441 kg N; no response was observed with other N rates with soil K as low as 66 mg kg-1. Tissue K in response to soil K was significant at all N rates except 49 kg. As such, luxury consumption of K (LCK) was detected (averaged across all years) at 147 and 245 kg N. Moreover, LCK at these same N rates was 4 times greater after year 3 compared to all other years. At later years of the test (year 4 and 5) and at typical N rates (49 to 245 kg N ha-1 yr-1), leaf tissue K (g kg-1) and shoot growth (g dry wt m-2 d-1) were independent (uncorrelated) while at all other years and N rates these responses co-varied significantly with shoot growth increasing with tissue K. Current soil test K levels appear too high at N rates routinely applied for optimal growth but are justified in association with LCK because of the potential for better stress tolerance.