Ammonium-based fertilizers alter pH and cation concentrations in agricultural soils of Saskatchewan, Manitoba and Quebec

Soil pH is a key component of soil health, influencing chemical, biological and related processes such as nutrient cycling. Globally, it has been recognized for decades that fertilization with ammonium fertilizers (including ammonium nitrate, urea and ammonium phosphate) at high rates and/or over long time periods can decrease soil pH, and is of particular concern in soils with naturally neutral pH (6.5-7.0) due to reduced buffering capacity. To investigate acidification in Canadian agriculture, soils were sampled from the 0-5, 5-10 and 10-20 cm depths from long-term research plots in historically neutral-pH soils in Saskatchewan (SK) and Quebec (QC), and a shorter-term N fertilization trial in SK and Manitoba (MB), under a range of fertilization practices including with and without chemical N fertilization, organic management with no fertilizers (chemical or animal manure), and a range of management practices, including tillage and crop rotations. Soil pH ranged from 5.0 to 7.3 in SK and QC soils and up to 8.4 in MB soils, and was consistently lower in plots with long-term ammonium fertilization. For all fertilized plots, soil pH was lowest at the depth of fertilizer placement (5-10 cm) and highest at 10-20 cm, with the greatest pH differences between these depths in no-till plots. Acidification altered exchangeable cation concentrations, decreasing exchangeable calcium and increasing exchangeable aluminum; this indicates that buffering capacity was reduced in many of the studied soils. These results show that acidification is a concern for neutral-pH Canadian soils, warranting further investigation.