SOIL GEOCHEMISTRY OF EARLY HISTORIC NON-MECHANIZED AGRICULTURAL LAND USE AT PLUM GROVE HISTORICAL FARMSTEAD

Land use activities can have significant impacts on the physical, biological and chemical processes that form soil. In addition, over exploitation (or degradation) of soils could diminish their ability to perform critical ecological and economical services. Iowa is one of the most human modified landscapes in the Americas, with widespread alterations involving land clearing, grazing and intensive row-crop agriculture. Physical evidence of past land use activities are often subtle or masked by subsequent land use. Plum Grove Historical Farmstead (PGHF) in Iowa City, Iowa has a hundred year history as a working farm, from 1844 to 1943, and is on the National Register of Historic Places, thus protected from the physical and chemical overprinting from intensive row-crop agriculture.

This study evaluates the efficacy of an analytical framework which quantifies the geochemical effects of early historic land uses on characteristics of soils. The PGHF land use areas evaluated for this study are (1) a small barnyard; (2) a suspected corral; (3) an orchard “plum grove”; and (4) a pasture. The framework proved to be effective in discriminating differences in geochemical properties of soils under these land uses. The pXRF data identified variability of element concentrations both within soil profiles and between land use areas and were used to selectively sample for more costly, higher precision isotopic and elemental analyses. Whereas δ¹³C values for all use areas suggest mixed C₃/C₄ pre-settlement vegetation, the barnyard location had significantly more C₃ input. The orchard location has higher concentrations of Ni, Cu, and Zn near the surface, suggesting the application of amendments to increase productivity. Higher subsurface concentrations of P support this interpretation.

Identification of land use effects on geochemical characteristics of soils at PGHF enabled isolation of the imprint of early non-mechanized agricultural practices. This research has implications for topics such the role of soils in the carbon and nitrogen cycles, the effects of land use on sustainable use of soils, and augmentation of the historical record by revealing land use activities for areas that have limited records of past land use.