Sampling depth confounds soil acidification outcomes

Authors: Liebig, Mark

Submitted to: CSA News
Publication Type: Popular Publication
Publication Acceptance Date: 10/11/2016
Publication Date: 10/28/2016
Citation: Liebig, M.A. 2016. Sampling depth confounds soil acidification outcomes. CSA News. 61:11 p.13.

Interpretive Summary: Low soil pH can affect herbicide persistence, decrease nutrient availability, and contribute to metal toxicity, all of which can compromise crop production. In the northern Great Plains of North America, surface sampling depths of 0-6” or 0-8” are suggested for testing soil pH. Soil acidification, however, is often most pronounced nearer to the soil surface. Researchers at the USDA-ARS Northern Great Plains Research Laboratory quantified soil pH change in two long-term dryland cropping studies near Mandan, North Dakota. Soils were sampled at multiple depths in both studies, allowing for soil pH evaluation at surface (0-3”) as well as deeper (0-6” and 0-12”) depths. Sampling depth was found to be an important confounding factor affecting pH outcomes. Significant differences existed between sampling depths for both final soil pH and pH change in both studies. Final pH values were higher (and pH changes smaller) as sampling depth increased. Findings from this evaluation suggest the regionally-recommended sampling depths of up to 8” may be too deep for early detection of surface acidification. Adoption of surface sampling depths less than 3” is recommended for testing soil pH in the northern Great Plains.

Technical Abstract: In the northern Great Plains (NGP) of North America, surface sampling depths of 0-15 or 0-20 cm are suggested for testing soil characteristics such as pH. However, acidification is often most pronounced near the soil surface. Thus, sampling deeper can potentially dilute (increase) pH measurements and therefore change management recommendations. Here, we show evidence from two long-term (16-19 yr) dryland cropping experiments that soil acidification at 0-7.6 cm was notably diluted at both 0-15.2 and 0-30.5 cm. There were significant differences among 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 relatively young, highly buffered NGP soils, acidification can occur, and sampling depth for testing pH could be an important confounding factor. We suggest sampling soils at surface depths <8 cm for testing pH in the NGP.