Effect of Soil pH on Annual Bluegrass Quality and Color.

The effect of soil pH on turfgrass growth varies depending species; however, most turfgrasses are adapted to slightly acid soil conditions (pH 6 – 7).  In general, annual bluegrass [Poa annua L. f. reptans (Hausskn) T. Koyama; ABG] is considered intolerant of low or high soil pH; however, it is unclear what the optimum soil pH is for ABG growth and quality under field conditions.  The objectives of this field study were to quantify the responses of annual bluegrass over a range of soil pH and determine a critical level for ABG growth and quality.  The trial was initiated in 2011 on ABG turf that had an initial pH value of 5.3 in the 0- to 60-mm mat layer.  Five limestone (CaCO₃) rates were applied on 12 Dec. 2011 and 1 Apr. 2014 based on target pH levels of 5.8, 6.3, 6.8, 7.3, and 7.8 in the mat-thatch layer.  Elemental sulfur was applied at a rate of 24 and 49 kg S ha^-1 on 12 Dec. 2011 and at a rate of 12 and 24 kg S ha^-1 on 3 Apr. 2014 to decrease pH.  Linear plateau regression analysis was used to determine a critical soil pH level with respect to turfgrass quality, color, NDVI, and chlorophyll index.  Turfgrass quality and color data from 2013 and 2014 suggest a critical value for soil pH of 5.8 (R² = 0.70 and 0.71, respectively).  Additionally, few differences in quality and color were seen between different soil pH levels during the spring of each year; however, low soil pH (< 5.8) caused a significant decrease in turfgrass quality and color during the summer.  It is likely that environmental stresses such as heat and drought are required for ABG to exhibit a negative response to low soil pH.