Acid rain is considered by some environmentalists to be the nation’s most serious environmental problem. It is formed by the combination of water vapor in clouds with nitrogen oxide and sulfuric dioxide emissions from the burning of coal, oil, and natural gas. The acidity of rain in central and northern Florida consistently ranges from 4.5 to 5 on the pH scale, a decidedly acid condition. To determine the effects of acid rain on the acidity of soils in a natural ecosystem, engineers at the University of Florida’s Institute of Food and Agricultural Sciences irrigated experimental plots near Gainesville, Florida, with acid rain at two pH levels, 3.7 and 4.5. The acidity of the soil was then measured at three different depths, 0–15, 15–30, and 30–46 centimeters. Tests were conducted during three different time periods. The resulting soil pH values are shown in the table above. Treat the experiment as a 2 × 3 factorial laid out in three blocks, where the factors are acid rain at two pH levels and soil depth at three levels, and the blocks are the three time periods.

a. Is there evidence of an interaction between pH level of acid rain and soil depth? Test using α = .05.

b. Conduct a test to determine whether blocking over time was effective in removing an extraneous source of variation. Use α = .05.

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ACIDRAIN April 3 Acid Rain pH June 16 Acid Rain pH June 30 Acid Rain pH 3.7 4.5 3.7 4.5 3.7 4.5 0-15 cm 5.33 5.33 5.47 5.47 5.20 5.13 Soil Depth 15-30 ст 5.27 5.03 5.50 5.53 5.33 5.20 30-46 cm 5.37 5.40 5.80 5.60 5.33 5.17