Crop temporal diversity is negatively associated with inherent soil quality across the Central U.S.: A novel high-resolution spatial analysis

Background/Question/Methods: Crop temporal diversity (i.e. crop rotation) is a fundamental practice of sustainable agricultural systems and improves soils in ways that boost crop yields. Yet analyses at the county level show declines in rotational complexity in the Central U.S., likely due to economies of scale, government policies, and established infrastructure that push farmers towards more frequent corn cropping. Fine-scale knowledge of where these declines occur are needed to understand their causes and tailor policies aimed at restoring rotational complexity. These declines may be most prevalent on high-quality soils, which can tolerate the degradative effects of corn monocultures in the short term.

We examine field-level rotational complexity and its drivers, hypothesizing that farmers on the best land will reduce rotational complexity the most. We use the USDA’s Cropland Data Layer, which shows remotely-sensed crop type in cropland pixels, to quantify crop rotational complexity in the Central U.S. We define a metric of crop rotation, the rotational complexity index (RCI), as a function of the number of crop species and turnover events present in a six-year window. Combining this index with the National Commodity Crop Productivity Index (NCCPI from SSURGO), we examine RCI’s spatial patterning and its correlation with inherent soil quality.

Results/Conclusions: RCI values calculated for 2012-2017 range from 0-5.2 (median = 2.2), and are positively skewed. Corn monoculture accounts for 2% of the study area. Both RCI (Moran’s I = 0.21, pseudo p-value = 0.001) and soil quality (Moran’s I = 0.35, pseudo p-value = 0.001) show significant spatial clustering. We find a significant negative correlation between NCCPI and RCI using spatial error regression (p < 0.01). These results show that areas of high rotational complexity tend to cluster together, as do areas of low soil quality, and that the two tend to overlap.

Since NCCPI reflects inherent soil properties and climate, we expect soil quality to drive rotational complexity via farmers’ decisions, rather than vice versa. Farmers with marginal soils may choose to retain diverse crop rotations partly as a means of improving soil and stabilizing crop yields. Though crop rotation can also boost yields on high-quality soils, farmers have strong market and policy incentives to reduce rotational complexity at the expense of accruing soil benefits. By identifying a paradox wherein the highest quality agricultural soils are most prone to degradation, we hope to encourage policies that better align market incentives with long-term soil health in America’s heartland.