Comparing the effects of sewage sludge ash and other recycled phosphorus fertilizers on soil quality; a two-way design plus control with several repeated measures

In this study, I worked with researchers who were studying options for recycled phosphorus fertilizers, with particular interest in sewage sludge ash from the St. Paul Wastewater Treatment Plant. This ash contains significant amounts of phosphorus, which is essential to healthy plant development and is regularly applied in agronomic settings to ensure maximum crop yields. This study was originally developed as a two-way factorial design, with four phosphorus sources and five application rates. Various soil quality parameters were measured multiple times throughout the growing season for three years, and the crop was rotated between corn and soybeans. The initial complication was to realize that because one of the application rates was zero, the design was actually a two-way plus control design. This was made additionally complex because the effect of application rate on soil phosphorus was expected to be linear and so the rate of increase in soil for a given application rate was of interest. Finally, various random effects for repeated measures within each growing season and across years had to be considered.