The significance of acid rain to agriculture in Eastern Canada
Precipitation falling on agricultural land in eastern Canada is contributing acid to soils and crops. In order to assess the significance of potential problems from an agricultural point of view, soil sensitivity criteria have been developed, and evidence for plant damage has been examined. Soils were classified into sensitive, moderately sensitive and non-sensitive classes according to the estimated degree of depletion of exchangeable bases in the plow layer resulting from 25 years of acid precipitation at input rates equivalent to the highest currently being observed in the region. About 70% of the agricultural soils of eastern Canada fall into the non-sensitive class
however, twenty-five percent of the agricultural soils of Quebec are sensitive to these criteria and in P.E.I. almost all soils are sensitive or moderately sensitive. There is little evidence of direct damage to crops from acid rain, although indirect effects resulting from soil acidification are known. There is concern, however, for potential chronic leaf tissue deterioration resulting from gaseous sulfur dioxide (a precursor of the acidity of rain) in polluted air. Synergistic interactions with other air pollutants may also result in leaf damage from SO2. Better crop and air monitoring data are needed in agricultural areas to assess these possible effects. The components of acid rain which affect the acidity of soils include sulfates, nitrates and ammonium. Estimates indicate that the net soil-affecting acidity reaching agricultural land in eastern Canada from atmospheric deposition (wet and dry) ranges from about 25 to 40 kilograms per hectare per year when expressed as calcium carbonate (limestone) equivalent. This rate of acid addition is exceeded by two to ten times as a result of acidity from normal nitrogen fertilizer applications to crop land. Natural soil processes also contribute to acidification and to the need for lime additions to some soils. Acidification will occur, even without acid rain, in soils which are sensitive and which are currently receiving nitrogen fertilizer without a regular lime application. In these soils the process may be somewhat accelerated by acid rain. In soils which are regularly and adequately limed, acid rain will have little effect. Constituents of air pollution which are not necessarily acid may be of greater long-term concern to soil quality and agriculture, but few data are available to assess their significance.