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THE INFLUENCE OF TILLAGE METHODS, CROPPING SEQUENCE, AND N RATES ON THE ACIDIFICATION OF A NORTHERN IDAHO SOIL

Soils in northern Idaho are exhibiting a rapid decline in pH. In undertaking this study, we sought to determine the relative effects of common northern Idaho cultural practices—tillage methods, crop rotations, and N fertilizer rates—on the acidification of a northern Idaho soil. The plots were established north of Moscow, Idaho, in 1974 and evaluated no-till, minimum tillage, and conventional tillage treatments and three-crop rotation systems: (1) a 2-year winter wheat (Triticum aestivum L. cv. Nugaines)-spring pea (Pisum sativum cv. Alaska) rotation; (2) a 3-year winter wheat-spring wheat (Triticum aestivum L. cv. Fieldwin)-spring pea rotation; and (3) a 3-year winter wheat-alfalfa (Medicago sativa L. cv. Ladak) red clover (Trifolium pratense cv. Kenland) peaalfalfa red clover rotation. The plots were split by N application rates and sampled by depth. When sampled by 300-mm increments, depth was the only factor that affected soil pH. Soil pH increased with increasing depth. There was a statistically unique calcium chloride pH at each sampling depth down to 1500 mm. When the surface 300 mm was sampled by 75-mm increments, the interaction of crop rotation x tillage x N rate x depth was significant. It was apparent that N rates were the major influence on pH differences. Crop rotations influenced the amount of N fertilizer required for crop production and consequently affected the magnitude of the pH decline. Tillage influenced the placement of N fertilizer and consequently the location and depth of acidification in the soil profile. The greatest acidification under no-till management occurred in the surface 75 mm, under minimum tillage at the 75-to 150-mm depth, and under conventional tillage in the surface 250 mm. A significant decrease in pH was observed in all treatments over the 8-year period.