USE OF EDAPHIC VARIABLES TO CONTROL EXPERIMENTAL ERROR - A CASE-STUDYON BLOCKING AND USE OF COVARIANCE

Field studies that extend over large areas are commonly blocked in an attempt to minimize experimental error. In an experiment that was desi gned to determine the effects of exposure to acidic rain and ozone on seedling loblolly pines through three years of growth, principal compo nent and cluster analyses were conducted on the pretreatment soil chem istry of each plot in an attempt to make the plots in each block as ho mogeneous as possible. After one growing season, a series of edaphic m easurements were taken in each plot to quantify soil drainage and aera tion. Analyses of covariance were then conducted using the edaphic mea sures as covariates to determine their effectiveness in controlling ex perimental error. Even though the chosen blocking pattern was effectiv e in reducing experimental error, the edaphic variable, MOTTLING (dept h to mottling clay), was an effective covariate. MOTTLING reduced the experimental error by an additional 25 to 46%. Block sums of squares w ere also reduced by as much as 94% but still retained significance, su ggesting that though there was a strong relationship between the block ing pattern and the MOTTLING pattern, blocking still accounted substan tially for variability due to other unknown factors.