How does Soil Organic Matter affect Potato Productivity on Sandy Soil? Evidence from a Greenhouse Study

Soil organic matter (SOM) can contribute to crop productivity through nutrient availability and soil improvement. However, SOM is typically low (< 2%) in the coarse-textured soils used for potato production. The potato cropping system is irrigated, frequently tilled, and fertilized causing potential nitrate leaching and contamination. The effect of varying levels of SOM on potato productivity in sandy soil is unclear. This study aimed to estimate the effect of varying SOM on potato productivity in sandy soils and to understand if nitrogen (N) mineralization was a primary mediator of this effect. Soil from nine fields in Wisconsin, USA (SOM range of 1.1 to 3.8%) were collected for a greenhouse study. Red Norland was the variety planted and vine, tuber, and total biomass and N uptake were measured. In-situ ion exchange resin strips and potentially mineralizable N (PMN) measured at harvest were used as proxies for N mineralization. We found that SOM had a positive effect on plant productivity. The effect was statistically significant for four productivity metrics (fresh matter whole biomass, dry matter vine biomass, and total N uptake in the vines and whole biomass) and marginally significant for four more. We found that N mineralization (as PMN) was a partial mediator of SOM effect on productivity doing a formal mediation test. Hence, it is likely that SOM improved plant productivity through mechanisms beyond just N acquisition by plants. Our results suggest future efforts should explore other mechanisms through which SOM can affect productivity.