Soils

As noted in a previous section, direct evidence of cation depletion is limited to a few retrospective studies that were able to use archived soil samples and contemporary measurements of the exact field locations to determine changes in soil chemical properties (Bailey and others 2005, Lawrence and others 2005). These studies, in combination with a 9-year watershed acidification study in Maine that showed sizable reductions in exchangeable Ca and Mg compared to the control watershed (Fernandez and others 2003), and long-term watershed mass balance observations (Likens and others 1996), support the theoretical basis of cation depletion due to acid deposition (Reuss 1983). However, application of these results to larger landscapes is highly problematic because of the large spatial variability of soil properties. Plant-available Ca status within soils is primarily determined by the mineralogy of the parent material from which the soil was formed and the period of time that the rooting zone has been exposed to weathering. Thus, the underlying parent material and soil classification information are useful in identifying regions that may be at risk to Ca depletion, (e.g., areas of the White Mountains in New Hampshire or the Adirondacks in New York). However, within susceptible regions, soil properties are highly variable, both vertically and horizontally, which make it challenging to conduct site-specific evaluations. This spatial variability can be dealt with by using a large sample size and sampling the soil by genetic horizon (Bailey and others 2005); however, this requires the assistance of trained soil scientists and incurs high costs for analysis. Consequently, collection of soil nutrition data to support regional assessments would be expensive and requires the expenditure of considerable time and effort. In part to overcome the inherent difficulties in assessing soil nutrition, but also considering the theoretical value of assessing living organisms when considering biological deficiency thresholds, vegetation chemistry has also been monitored to test for Ca deficiencies.