Establishment of Soil and Tissue Potassium Sufficiency Ranges for Annual Bluegrass Turf.

Potassium (K) is an important soil nutrient that strongly influences the tolerance of turfgrasses to drought, cold, high temperature, wear, and salinity stress; however, soil and tissue K sufficiency ranges for annual bluegrass [Poa annua L. f. reptans (Hausskn) T. Koyama; ABG] turf are lacking.  The objectives of this 3-yr study were to (1) evaluate ABG turf response to a range of soil test K (STK) levels and (2) determine critical soil and tissue K levels for this host.  A study was initiated in 2012 in North Brunswick, NJ, on a modified native soil rootzone with a 60 mm sand topdressing layer.  The study was arranged as a factorial with four K fertilization levels, ranging from 0 to 262 kg K ha^-1 yr^-1, applied as potassium chloride or potassium sulfate in a RCBD with four replications.  Potassium nitrate, and potassium carbonate were included at the 262 kg K ha^-1 yr^-1 rate. Mehlich-3 STK level, tissue K content, clipping yield, anthracnose severity, and turfgrass quality and color were quantified periodically for three growing seasons.  Nonlinear regression analysis was used to calculate STK and tissue K content critical levels.  Anthracnose severity was responsive to both STK and tissue K content and provided clear critical values using the Cate-Nelson model. Soil test K and tissue K content critical levels for anthracnose on ABG ranged from 43 to 49 mg kg^-1 and 19.5 to 19.6 g kg^-1, respectively.  Quadratic plateau analysis of STK and tissue K content indicated that maximum tissue K content for ABG ranged from 27.3 to 29.2 g kg^-1 at STK levels of 78.2 to 103.2 mg kg^-1.  Soil test K levels above 100 mg kg^-1 did not provide increased anthracnose suppression compared to values within the sufficiency range; therefore, there may be no benefit to increasing STK above this level.