A PROPOSED METHODOLOGY FOR ASSESSING THE RELATIVE IMPACT OF ACID RAIN AND NITROGEN FERTILIZERS ON ACIDITY OF AGRICULTURAL SOILS IN CANADA

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Abstract
Acid rain and N fertilizers both contribute to soil acidity, but no method has been available to compare their relative impacts. A simple model (SOLACID) is presented to assess quantitatively the acidifying effects of precipitation and N fertilizers on agricultural soils. Acid rain has been treated as a dilute solution of NH4NO3, (NH4)2SO4 and associated acids. Soil and plant pathways are considered for NH4+,NO3- and SO4= by way of leaching, gaseous losses from microbial reduction, plant uptake and removal, and organic immobilization and mineralization. Leaching of NO3- was the factor to which the model was most sensitive. A relationship between base saturation and base cation leaching is described. Field data reported from 21 treatments at six experimental sites were used to test the model, which provided reliable estimates of final pH (r2 = 0.92**) and of changes in base saturation (r2 = 0.86**). Compared with previously published methods, the model provided the best estimates of lime requirements as computed from field measurements (r2 = 0.87**).