Long-term effects of nitrogen and potassium fertilization on perennial ryegrass turf

Research on the effects of potassium (K) fertilization on turfgrass growth and its relationship to tissue K and soil K have been inconsistent due to the many factors affecting tissue K such as nitrogen (N) fertilization. This 5-yr study was initiated in 2004 to better understand N (as urea) and K (as K₂SO₄) effects on soil and plant response of perennial ryegrass (Lolium perenne L. 'Brightstar'). A field study was conducted to evaluate five rate levels of N (49, 147, 245, 343, or 441 kg ha^-1 yr^-1) with three rate levels of K (49, 245, or 441 kg ha^-1 yr^-1). Clipping yield (CY), leaf tissue K, soil pH, rooting (in 2004 and 2005 only), and soil K were assessed in spring of each year. Leaf K and soil K increased linearly with K applied, whereas leaf K and CY regression slopes with soil K increased with greater N applied. Leaf K and CY increased linearly with N, whereas rooting, soil pH, and soil K decreased linearly with increased N. Rooting was inversely related to CY. Luxury consumption of K was observed in the latter years of the test (2006) at N rates of 245 kg N ha^-1 yr^-1 and lower. The strength of the regression (r²) and covariation between CY and leaf K increased with N rate and decreased with year of the test. There were no observed changes in shoot and root growth in response to K fertilization even at low soil test K levels (50 mg kg^-1).