Jonathan A. Sandor

Contribution of organic matter to cation exchange capacity and specific surface area of fractionated soil materials

To investigate the contribution of organic matter to the cation exchange capacity (CEC) and the specific surface area (SSA) of organomineral complexes in soils, we studied A horizons of four soils (two Hapludalfs and two Argiudolls) in central Iowa. The effect of mineralogy on CEC and SSA was held nearly constant by sampling soils developed in the same parent material (loess) and landscape position (summit). A range of organic matter contents was obtained by variations in native vegetation, the effects of cultivation, and horizon depth. The CEC, SSA, and organic C content of unfractionated samples were determined. Soil samples were also separated into coarse silt, fine silt, coarse clay, and fine clay size fractions after dispersion by sonification. The organic C content of each fraction was determined, as were CEC and SSA, before and after peroxide treatment. Multiple linear regression equations were developed to relate CEC and SSA to organic C and peroxidized SSA. Partial regression coefficients suggested that the net contribution of organic matter to the CEC of unfractionated soil materials was 184 cmol(+) kg−1 of organic C. In the coarse silt, fine silt, and coarse clay fractions, organic matter was estimated to contribute approximately 559 cmol(+) kg−1 of organic C to the CEC and 7.22 x 105 m2 kg−1 of organic C to the SSA of the fractionated materials. On average, organic matter was calculated to contribute 49% of the CEC and 19% of the SSA of the fractionated materials. Because “independent” variables were themselves highly correlated, principal components regression analysis was used to improve the precision of estimates of organic matter contributions derived from the partial regression coefficients.

Anthropogenic Soil Change in Ancient and Traditional Agricultural Fields in Arid to Semiarid Regions of the Americas

Soils form the foundation for agriculture and are changed by farming through active management and unintentionally. Soil change from agriculture ranges from wholesale transformation to ephemeral and subtle modification. The archaeological record of early agricultural systems holds information about soil change on centurial to millennial scales, with important implications for long-term soil condition and land use sustainability. Knowledge of early agricultural management can also be inferred from soils, including farming strategies in dynamic, challenging environments. This paper discusses soil change processes and outcomes mainly using studies of ancient and traditional agriculture in arid regions of the Americas. The potential and limitations of soil change research methods in ancient agriculture are also considered. Soil anthropogenic change involves complex and interactive physical, chemical, and biological processes across a wide range of spatial and time scales. Soil change outcomes in early agriculture relating to soil health and productivity vary from improvement to degradation. Soil productivity improvement commonly resulting from management includes soil-landscape physical stabilization for crops, increased topsoil thickness and plant-available water capacity, and improved soil tilth and fertility. Degradation commonly results from unintentional soil-geomorphic-ecosystem changes that cause accelerated erosion, destabilization of soil structure and compaction, and decreases in plant nutrients and soil fertility. Soil change outcomes vary across space and time in response to complex environmental, agricultural, and social factors.

A Maize Experiment in a Traditional Zuni Agroecosystem

Maize has sustained the Zuni and other people in the arid American Southwest for many generations. In the traditional Zuni dryland agricultural system, fields are carefully placed on valley-edge landforms to tap into watershed hydrologic and ecosystem processes. In these geomorphic positions, field soils are managed to receive supplemental water and nutrients for crops by retaining storm runoff transported from adjoining uplands. Crop experiments were conducted to examine the effects of runoff on maize (Zea mays) productivity. Productivity of a Zuni maize cultivar and modern hybrid maize was evaluated with five treatment combinations of water and nutrient input sources in two traditional agricultural areas that have been cultivated for at least 1000 years. During the first year of the two-year experiment (1997–1998), one field received inputs from four runoff events, while the other field, with a larger watershed, received no runoff. In year two, the one remaining field (the other field was disrupted) had inputs from one runoff event. Growing season precipitation was above average for both years of the experiment. All treatments, including those receiving only precipitation, produced grain yields ranging from 852 to 3467 kg ha−1 for Zuni maize. Grain and biomass productivity tended to be greater in the irrigation-plus-fertilizer control treatment. Productivity differences among treatments are attributed primarily to differences in water inputs rather than nutrient supply. Although the more densely populated hybrid maize out-yielded Zuni maize on a land area basis, Zuni maize produced greater yields per plant and more biomass than did the hybrid maize.