Depth Matters: Soil pH and Dilution Effects in the Northern Great Plains

Abstract

Core Ideas Suggested soil testing depths in the northern Great Plains are 0–15 or 0–20 cm. Soil acidification is often most pronounced in near‐surface depths. Soil pH change was evaluated in two long‐term dryland cropping studies. Soil acidification at 0–7.6 cm was notably diluted at both 0–15.2 and 0–30.5 cm. We suggest sampling soils at <8 cm for testing pH in the northern Great Plains. In the northern Great Plains (NGP) of North America, surface sampling depths of 0 to 15 or 0 to 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 to 7.6 cm was notably diluted at both 0 to 15.2 and 0 to 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 depths <8 cm for testing pH in the NGP.