Five-Year Evaluation of Soil and Tissue K in Response to N and K Fertilization in Perennial Ryegrass.

Long term evaluations of soil and plant response to major nutrients such as nitrogen (N) and potassium (K) are limited. The objective of this study was to assess shoot growth, soil-exchangeable 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 were 1.5 by 3.0 m and were mown twice per week at 3.2 cm height of cut with clippings returned. No supplemental P and K were recommended prior to fertilizer treatment. Soil K was determined in each year of the test from samples taken from each N-K treatment plot in April prior to fertilization. Clippings were oven dried following collections made in April and May and averaged. Tissue K was determined from analysis of dried leaf-clippings collected in April. Spring clipping yields (YD) decreased from 2003 to 2007 and year-to-year variation accounted for 43% of the total variation followed by N (33%) and K (1%). Spring YD increased with N, with 97% of the N main effect linear. Total variation in soil K was due principally to N-K treatment (70%) with 80% of the N-K variation due to K and 19% due to N. N and K effects on soil K were linear (99% of N and K main effects) with soil K increasing with K and decreasing with N. N-K variation was the principal source of total variation in tissue-K (42%) with N main effects accounting for twice the N-K variation than K (61% vs. 32%) with N and K effects linear in response to fertilization.