Fungicides in Kentucky Bluegrass Turf: Effects on Thatch and pH 1

Management of thatch and pH are important aspects of turfgrass culture. The use of disease‐preventive fungicides is a standard practice on highly‐maintained turfgrasses, and fungicides may be toxic to many thatch‐decomposing microorganisms. However, little information exists concerning the effects of long‐term fungicide programs on thatch accumulation. The influence of 14 fungicides, one nematicide, and of five mixed fungicide programs, applied over a 3‐year period, on thatch decomposition and the resultant pH of underlying soil was investigated on a blended Kentucky bluegrass ( Poa pratensis L.) sod. The physical depth of noncompressed thatch was measured after samples were removed from plots with a Noer Soil Profile Sampler. Cores of 2.54 cm diam were used to determine pH characteristics of the soil profile. Thatch depths among fungicide treatments ranged from 2.8 to 22.0 mm, and the pH (saturated paste in 10 mM CaCl 2 ) of the surface 3 cm ranged from 5.6 to 6.5. Thatch depth and pH of the surface 3 cm of thatch plus soil were inversely related (r = −0.797; significant at p = 0.001), although differences in pH among treatments were associated with the mineral soil fraction and not with the thatch. Fungicide‐induced reductions in pH occurred to depths exceeding 20 cm. Acidification of soil was apparently not the result of organic acids leaching from decomposing thatch, but was correlated (r = −0.564; significant at p = 0.01) with the contributions of sulfur (up to 491 g S/are/year) from the fungicides. Oxidation of sulfur from decomposing fungicides reduced the soil pH which, in turn, reduced the activity of microorganisms responsible for degradation of thatch. Thatch accumulation was not considered to be due to inhibitory effects of fungicides toward earthworms.