3910:
Calculating Average pH in Substrate Research: Should pH or [H+] Data Values Be Used?
Wednesday, August 4, 2010
Springs F & G
Eugene K. Blythe,
Coastal Research & Extension Center, Mississippi State University, Poplarville, MS
Donald J. Merhaut, Department of Botany and Plant Sciences, University of California, Riverside, CA
Data on pH is often collected during research with container-growing substrates using the pour-through method or other extraction methods. A measure of acidity/basicity, pH is approximately equal to the negative of the base 10 logarithm of the molar hydrogen ion concentration ([H+]) of a solution. When analyzing pH data, a question that often arises among researchers is: Should the pH data values be converted to [H+] before calculating a mean value, then calculating the mean pH from the mean [H+]? However, a more general question may be preferable: What are appropriate measures of average acidity for substrate extract? When pH was measured from pour-through extract collected weekly over 12 weeks from 100 2.8-L containers of a compositionally uniform and accurately irrigated pine bark/peat moss/sand substrate, the distribution of the pH values was shown to be symmetrical (near-normal) from week to week, whereas the distribution the [H+] values tended to be asymmetrical (skewed to the right). When a distribution is symmetrical, the (arithmetic) mean and the median are both acceptable measures of location, or average value. (With a perfectly symmetrical distribution, the mean and median coincide.) When a distribution is skewed, the median is typically a preferable measure of location since the median is less sensitive to extreme values than is the mean. Although the skewness of the [H+] values caused the distributions of the weekly data to show statistically significant deviations from a normal distribution, the deviations were not extreme. When values of mean pH calculated from pH values were compared with median pH calculated from pH values, mean pH calculated from mean [H+], and median pH calculated from mean [H+], all differences were within 0.02 pH units. Therefore, mean pH calculated directly from the pH data values is shown to be a valid summary measure of average acidity/basicity for container-substrate extract, with no data conversion from pH values to [H+] values being necessary. Median pH calculated directly from the pH data values was also a suitable summary measure of average extract acidity/basicity.
