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Abstract This study explores how scientists can support on‐farm experiments using analytical methods that align with farmers’ endogenous learning processes to inform their management decision. Four maize ( Zea mays L.) farmers across 10 site‐years in New York participated in this study to evaluate the effectiveness of a nitrogen‐fixing inoculant (NFI) applied with a reduced side‐dress nitrogen rate. Farmers designed and implemented their own experiments using a range of layouts, including side‐by‐side comparisons and strip trials. Two analytical approaches were compared: a quantitative yield analysis using spatial regression, and a causal pathway analysis based on mechanistic steps informed by field sampling (e.g., quantitative polymerase chain reaction detection of NFI organisms, nitrogen nutrition index, and yield). While yield data suggested positive or neutral treatment effects at all sites when simply comparing yield average, the spatial regression analysis and causal pathway analysis identified positive outcomes in only seven or four of 10 site‐years, respectively, reflecting a more conservative interpretation of efficacy. Both methods provided consistent conclusions at four out of 10 site‐years, demonstrating the contribution of metrics other than yield in the interpretation process. Findings suggest that simple causal diagrams can structure data collection and interpretation in ways aligned with farmers' goals. Supporting farmer experiments with digital agronomy, mechanistic reasoning, and site‐specific data enhances learning outcomes and scientific rigor without requiring formal replication. This work contributes to the development of collaborative, scalable methodologies that integrate farmer knowledge and scientific analysis in on‐farm experimentation.

The effects of N fertilization on growth and root colonization of preinoculated onion (Allium cepa L. cv. Improved Autumn Spice) were studied. Onion transplants, inoculated with either Glomus intraradices, G. versiforme or nothing at sowing, were grown under three levels of N in soils which had either been irradiated, irradiated and amended with nonmycorrhizal microflora, or not irradiated. Interactions between inoculation and soil treatment had a significant effect on dry biomass and final bulb diameter. Control plants cultivated in non-irradiated natural soil grew normally because of the presence of indigenous arbuscular mycorrhizae, but control plants in irradiated soils were stunted. There was no such difference among inoculated plants. In non-irradiated natural soil, bulbs of onions inoculated with G. intraradices or G. versiforme were significantly firmer than bulbs of control plants. Bulb firmness decreased as N fertilization level increased. In non-irradiated natural soil, tissue P concentration of onion plants preinoculated with either fungus was significantly higher than that of control plants. In all soil types, N, P, and Zn concentrations were higher in onion plants colonized by G. versiforme than in those colonized by G. intraradices. The opposite was true of Mn tissue concentration.