Do Plot Scale Studies Yield Useful Data When Assessing Field Scale Practices?.

Plot scale data has been used to develop models used to assess field and watershed scale nutrient losses. The objective of this study was to determine if phosphorus (P) loss results from plot scale rainfall simulation studies are “directionally correct” when compared to field scale P losses. Two fields (2.2 and 2.7 ha) have been monitored for edge of field nutrient and pesticide losses since 2004. Both fields had corn-soybean annual rotation with a long term history of no-till. In 2004, the 2.7 ha field was converted to rotational tillage, with tillage prior to corn. During the 2004 and 2006 growing seasons, plots were constructed in these fields, but just outside of the monitored area. The plots did have the same management as the monitored fields. Rainfall was simulated at 50 mm hr^-1 for 50 min and 75 mm hr^-1 for 15 min, and runoff samples were collected for analysis of soluble and total P (SP and TP). From the plot scale data, a 95% confidence interval was calculated for comparison of field scale P losses. In 2004, SP concentration data from plots were roughly 1 to 2 orders of magnitude greater in the no-till field than the rotational till field, which agreed with the field scale observations thus providing directionally correct data. This occurred for all comparisons, except those with overlap of confidence intervals, for which data from the fields were within 10% of each other. Based on these results, plot scale data at this site was directionally correct.