ARS | Publication request: A soil quality assessment within the Iowa River South Fork Watershed

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 14, 2011
Publication Date: October 25, 2011
Citation: Stott, D.E., Cambardella, C.A., Wolf, R., Tomer, M.D., Karlen, D.L. 2011. A soil quality assessment within the Iowa River South Fork Watershed. Soil Science Society of America Journal. 75:2271-2282.

Interpretive Summary: Soil quality assessment is a proactive process for understanding the long-term effects of soil and crop management practices within agricultural watersheds. In the South Fork of the Iowa River Watershed, an aerial survey was conducted during the summer of 2006 to identify areas with both well-developed and poor (N deficient) canopy growth in fields that were planted to corn. Our primary objective was to quantify several soil quality characteristics (indicators), including the near-surface carbon content, in the watershed. Four areas were identified within 50 fields. Three sites represented the well-developed areas and one the poorly-developed area in each field. We collected the soil samples after grain harvest in the fall and analyzed for their physical, chemical and biological properties. It was difficult to find significant differences in these soil properties between normal and poor canopy areas due to changing soil types and textures. We used a soil quality assessment model (Soil Management Assessment Framework or SMAF) that accommodates changes in soil types and textures, and assigns different scores for different measured soil property values and produces an overall soil quality index. From our results, we could not find a single cause for the poor canopy development across all the fields, but if we looked at individual fields, we found low soil organic carbon (soil organic matter), poor nutrient cycling, high bulk density, and low water-filled pore space as factors causing the low soil quality scores in the poorly-developed canopy areas. Using the SMAF scoring system, a farmer would be able to identify causes for poorly-developed canopy (and resulting yield loss) and develop management schemes to improve poor performing areas of the fields.

Technical Abstract: Soil quality assessment is a proactive process for understanding the long-term effects of crop and soil management practices within agricultural watersheds. Fields with both well-developed and poor (N-deficient) corn (Zea mays L.) canopy growth were identified within the Iowa River’s South Fork Watershed. Our objectives were to quantify several soil quality indicators, including the near-surface soil organic carbon (SOC) content, and determine if the Soil Management Assessment Framework (SMAF) could distinguish between the well-developed and poor corn canopy areas. Four sites, three representing the major soil series in the well-developed canopy areas and one in the poor area, were identified and sampled (0–10 cm) within 50 fields. There were no significant differences between performance zones when analyzed collectively. Using SMAF indicator scores, SOC, bulk density (Db), water-filled pore space (WFPS), electrical conductivity (EC), and microbial biomass carbon (MBC) were significantly lower in the poor canopy areas; however, no single indicator scored significantly less across all 50 fields. When separated by landscape position (hilltop, sideslope, toeslope, or depression), only SOC was significantly different between performance zones across each position. Other indicators that differed in at least one slope position included Db, WFPS, MBC, EC, P, Fe, Cu, Zn, or potentially mineralizable C. A majority of fields had multiple indicators with SMAF ratings at least 0.10 lower in the poor areas than in the corresponding well-developed canopy areas. Soil quality assessment on a field-by-field basis thus provides an approach for identifying potential specific soil-based causes for the poor canopy development.