U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


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Great Plains Soil Fertility Conference, March 5-6, 2002.


Decreases in soil pH in agricultural soils can affect plant nutrient availability and crop yield. For soils possessing high levels of calcareous minerals, such as those found throughout much of the Northern Great Plains, decreases in soil pH can also enhance C loss to the atmosphere due to acid decomposition of CaCO3. We evaluated changes in soil pH over a 16 yr period for a long-term cropping systems experiment established on calcareous parent material near Mandan, ND. Management variables included in the experiment were crop sequence [spring wheat-fallow (SW-F) and spring wheat-winter wheat-sunflower (SW-WW-SF)], tillage (conventional, minimum, no-till), and N fertilization (0, 22, and 45 kg N ha-1 for SW-F and 34, 67, and 101 kg N ha-1 for SW-WW-SF). Management effects on soil pH were modest over the 16 yr period. Nitrogen fertilization resulted in acidification, with decreases in soil pH greatest in the HIGH N treatment (-0.67), followed by the MED (-0.33) and LOW (-0.15) N treatments. While soil acidification did occur, it was limited to the surface 7.6 cm where pH values were less than 7.2. Consequently, C loss by acid decomposition of CaCO3 was highly unlikely in this long-term cropping systems experiment. Below 15.2 cm, soil pH increased over the 16 yr period. The exact mechanism for the increase is unknown, though may be the result cations leaching from the acidified 0-7.6 cm depth to lower depths. Further evaluation into exchangeable cations levels is necessary to better understand the observed trends in soil pH over depths.