Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2016
Publication Date: 10/13/2016
Publication URL: http://handle.nal.usda.gov/10113/63305
Citation: Reeves, J.L., Liebig, M.A. 2016. Depth matters: Soil pH and dilution effects in the northern Great Plains. Soil Science Society of America Journal. 80:1424-1427.

Interpretive Summary: In the northern Great Plans, surface soil sampling depths of 0-6 or 0-8 inches are suggested for testing soil properties such as pH. However, acidification is usually strongest closer to the surface (less than 4 inches). Because of this, sampling deeper than 4 inches could result in higher (more basic) pH values that could change management recommendations such as lime application rate. In this study, we use two long-term (16-19 years) dryland cropping experiments that show that soil acidification at 0-3 inches was significantly greater than at both 0-6 and 0-12 inches. For both studies, there were significant differences between depths for both final pH and pH change over time, with final pH being higher and pH change smaller at the deeper depths. Even in the young northern Great Plains soils that are naturally resistant to pH changes, acidification can occur, and sampling depth for testing pH can clearly affect pH readings. We suggest sampling soils at 0-3 inches for testing pH in the northern Great Plains.

Technical Abstract: In the northern Great Plans (NGP), surface sampling depths of 0-15.2 cm or 0-20.3 cm are suggested for testing soil characteristics such as pH. However, acidification is often most pronounced near-surface (e.g., <10 cm). Thus, sampling deeper can potentially dilute (increase) pH measurements and therefore change management recommendations such as lime application rate. Here, we show evidence from two long-term (16-19 yrs) dryland cropping experiments that soil acidification at 0-7.6 cm was notably diluted at both 0-15.2 and 0-30.5 cm. Across studies and treatments, there were significant differences between depths for both final pH and pH change over time, with final pH being progressively higher and pH change smaller at deeper depths. Even in the young, highly buffered NGP soils, acidification can occur, and sampling depth for testing pH could be a consequential confound. We suggest sampling soils at 0-7.6 cm for testing pH in the NGP.