Sample Size Calculations in Multi-Locational on-Farm Studies Focused on Improving Soils.

Modern technologies allow researchers to test different practices and inputs designed to improve soil properties in farmer’s fields. A common question raised by practical agronomists and consultants is how many sites and sampling locations within each site are needed to detect specific effects of interest. While not always intuitive, this is a slightly different question that many published research studies are focused on: is there statistically significant short or long-term effect of different treatments on soil properties? Power analysis can be used to estimate the probability of detecting a specific effect, at given sample size and at any specified level of confidence. This presentation will provide several examples of power analyses and sample size calculations from both short and long-term on-farm studies evaluating the effectiveness of cover crops, various soil amendments and soil health tests. Interpretations of results and practical considerations from the power analyses will be covered for field surveys and on-farm trials with different soil sampling schemes and treatment designs. Tools and resources currently available to address different questions related to sample size estimation at different scales will also be reviewed.