Soil pH Effects On Annual Bluegrass Growth, Quality, and Anthracnose Severity.

Soil pH affects nutrient availability, ion toxicity, and microbial activity.  Generally, annual bluegrass [ABG; Poa annua L. f. reptans (Hausskn) T. Koyama] is not considered tolerant of low soil pH; however, the soil pH levels that inhibit ABG growth and decrease turfgrass quality are not well documented.  The objectives of this field study were to 1) quantify the responses of annual bluegrass to a range of soil pH and determine a critical level for ABG growth and 2) determine the effect, if any, of soil pH on anthracnose severity of ABG turf.  The pH of the mat layer and underlying soil prior to treatment was 5.3 and 6.0, respectivley.  Five liming (CaCO₃) treatments were applied on 12 Dec 2011 at a rate of 118, 569, 1,184, 1,739, and 2,247 kg CaCO₃ ha^-1 based on target pH levels of 5.8, 6.3, 6.8, 7.3, and 7.8, respectively, in the mat-thatch layer.  Elemental sulfur was applied on the same date at a rate of 22 and 44 kg S ha^-1 to decrease pH.  By the end of August 2012, soil pH ranged from 5.8 (2,247 kg CaCO₃ ha^-1) to 4.3 (44 kg S ha^-1).  In both years, soil pH modification treatments had a significant impact on turfgrass color and quality.  Sulfur treatments resulted in decreased turfgrass quality and color throughout both years compared to limestone treatments.  Disease severity was more variable in 2012 than 2013; however, anthracnose severity generally increased as soil pH decreased. In 2013, the lowest disease severity was observed at the greatest soil pH levels (1,739 and 2,247 kg CaCO₃ ha^-1). It appears that soil pH below 5.0 strongly inhibits ABG growth which can results in decreased quality and increased disease incidence, especially during stressful periods of the growing season (mid-summer).