Long‐term acidification of farmland in northern Idaho and Eastern Washington

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Abstract
The purpose of this study was to compile and generate data for the determination of the causes, rate and magnitude of the pH decline in the region. Soil pH records from the past 40 yr were collected from the soil testing laboratories at the University of Idaho and Washington State University. An intensive field pH survey was conducted in northern Idaho from 1982-1984 and virgin soil samples were acidified under laboratory conditions. Soils in northern Idaho and eastern Washington have undergone a rapid decline in pH in the past 25 years. These soils, which were near neutral in pH in their virgin state, have been acidified to the point that over 65% of the agricultural soils in the region have soil pH values less than 6.0. By 1984 over 45% of the agricultural soils in northern Idaho had pH less than 5.6. This pH decline is attributed to the long-term use of high rates of N fertilizers, as over 1250 kg N/ha have been applied to typical soils in the region since the virgin prairie sod was first broken for agricultural production. Most of the soil acidification has occurred in the last 25 yr. Field simulation in the laboratory was used to confirm the extent of soil acidification attributable to N fertilizer.