Surface Soil Quality Response to Irrigation Method in a Western US CEAP Watershed.

The 820 km² Twin Falls (Idaho) irrigation tract is part of the Natural Resources Conservation Service (NRCS) - Conservation Effects Assessment Project (CEAP).  Furrow irrigation was initiated in the early 1900s but, since the 1990s, the NRCS Environmental Quality Incentives Program (EQIP) and other specialty projects have resulted in conversion of approximately 40% of the irrigation tract area to sprinkler irrigation. Most CEAP research focused on water quantity and quality, effects of conservation practices, and specifically shifting from furrow to sprinkler irrigation, but little was done to quantify soil quality/health effects. Eleven soil quality indicators were therefore measured within the 0 to 5 and 5 to 15 cm depth increments of soil samples obtained from paired producer fields; the data was then scored using the Soil Management Assessment Framework (SMAF).  Soil indicators (organic C, pH, EC, Olsen-P, extractable K, beta-glucosidase, potentially mineralizable N) tended to be greater under furrow as compared to sprinkler irrigation, yet this did not always translate into higher SMAF indicator scores. According to the SMAF, the bottom of furrow irrigated fields always had greater soil quality indices scores likely due to long-term erosional deposition leading to greater parameter concentrations.  Differences in soil quality indices were almost lacking between sprinkler irrigation field tops or bottoms, suggesting that this irrigation practice may be able to positively alter soil constituents over time and reduce the negative effects of previous furrow irrigation, at least within the tops of fields.  Sprinkler irrigation always had greater soil quality indices in field tops, while furrow irrigation had greater soil quality indices in field bottoms, yet soil quality was similar between both irrigation types when all sampling locations and positions were combined.  Sprinkler irrigation may certainly be considered a proper conservation practice, yet the overall results suggest that soil management and irrigation practice effects need to be carefully considered when identifying soil quality changes and comparisons between furrow and sprinkler irrigation.