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Title: A Preliminary Watershed Scale Soil Quality Assessment in North Central Iowa USA

item Karlen, Douglas
item Tomer, Mark
item Cambardella, Cynthia

Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2008
Publication Date: 6/2/2008
Citation: Karlen, D.L., Tomer, M.D., Neppel, J., Cambardella, C.A. 2008. A Preliminary Watershed Scale Soil Quality Assessment in North Central Iowa USA. Soil & Tillage Research. 99:291-299.

Interpretive Summary: Public and private investment in conservation practices has reached a level where many people are asking what is being accomplished for our investment. To address this question, the Conservation Effects Assessment Project (CEAP) was established with one goal being to determine how different soil management practices affect soil quality. This study conducted in the South Fork Watershed of the Iowa River in the U.S. Corn/Soybean Belt used both the Natural Resources Conservation Service (NRCS) Soil Conditioning Index (SCI) and the Soil Management Assessment Framework (SMAF) to show that soil quality was relatively good for the management practices being used throughout the watershed. Samplings from hilltop positions did have lower soil organic matter levels and a slightly higher risk of nitrate nitrogen leaching. One way to increase soil carbon (organic matter) content would be to switch to reduced or no-tillage practices. The sampling showed that if this were done, producers should closely monitior levels of the essential plant nutrient potassium. These results will be useful for producers in the watershed, soil and crop management consultants, and NRCS field office personnel who are responsible for developing conservation plans. As one of the first watershed-scale assessments of soil quality, this study will also be useful to other research scientists who are responsible for developing soil quality assessment programs.

Technical Abstract: Soil quality assessment has been recognized as an important step toward understanding the long-term effects of tillage, cropping system, landscape position, and conservation practices within agricultural watersheds. Our objective is to provide an initial assessment of various soil quality indicators and assessment approaches within the South Fork Watershed of the Iowa River in North Central Iowa, USA. Soil samples were collected during autumn 2003 and spring 2004 from 29, 65-ha areas along two transects traversing the South Fork Watershed. Several soil quality indicators were measured and four (pH, electrical conductivity [EC], total organic carbon [TOC], and soil-test P [phorphorus]) were used to compute a soil quality index (SQI) using the Soil Management Assessment Framework (SMAF). The RUSLE2 erosion model was used to compute Soil Tillage Intensity Rating (STIR), soil loss, nitrogen (N) leaching potential, and the Soil Conditioning Index (SCI) for each sampling area. Soil-test P levels were often high or very high with regard to crop needs, but not to the point where the SMAF scoring functions would indicate an environmental risk. Low soil-test potassium (K) could be an agronomic concern if no-tillage is adopted throughout the watershed. Soils in upper landscape positions had lower organic carbon (C) and C:N ratios indicating an increased risk for nitrate leaching. The SCI and SQI indices were positively correlated and indicate that soil quality is relatively consistent for the management practices being used throughout the watershed.