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A Spatial Model of Soil Parent Material


  Scott W. Bailey*
James W. Hornbeck*
John L. Campbell*

  *USDA Forest Service, Northeastern Research Station, Durham, NH

NUTRIENT CYCLING studies of forested watersheds in northern New England demonstrate a great range in nutrient loss and sensitivity to depletion of available nutrient pools. Much of this variation is due to spatial patterns in weathering flux. Due to structural and lithologic complexity of bedrock in this region, as well as the incorporation of transported materials in glacial deposits, spatial patterns in soil mineral or chemical content are not always apparent. Here we present a simple GIS-based model which uses bedrock geology, lithologic composition and direction of glacial transport to predict spatial variation of soil parent material for glacial till landscapes. Model performance was tested by conducting pebble counts and soil bulk chemical analyses for sites representing a range of predicted mineralogic compositions and landscape positions. This tool may have application in developing regional analyses of weathering flux. In addition, this model could facilitate scaling results from intensive small watershed studies to a landscape scale and in evaluating effects of forest management and atmospheric deposition.

Figure 1. The till source area for Cone Pond. The red area represents metapelites and metasandstones, which are low in calcium. The green and yellow areas represent granites, granite gneiss, and metavolcanics which are intermediate in calcium content. The green area is interesting -- it is Ammmonoosuc volcanics which contain high proportions of hornblende and plagioclase feldspars -- both of which have high calcium contents. Figure 2. The till source area for Hubbard Brook. It too contains some low calcium content bedrock as shown in red. But it contains more of the intermediate group, especially the green which again represents Ammonoosuc volcanics. The larger presence of Ammonoosuc volcanics is a probable explanation as to why Hubbard Brook has more calcium in streamwater than Cone Pond. Figure 3. The till source area for Sleeper's River presents a quite different picture. While there are some low calcium content bedrocks (red), and a significant amount of intermediate (yellow), over half of the area is of the high content (blue). This rock type in blue is the Waits River formation which consists largely of highly unstable calcium and magnesium carbonates -- an obvious source for the high calcium content in Sleeper River streams.
Cone Pond
Hubbard Brook
Sleepers River
Till source area: Cone Pond Hubbard Brook Sleepers River Ca content of rocks (%)
General Rock Type
Yellow square Granite and Granite gneiss





Red square Metapelites and metasandstones





Blue square Metalimestones





Green square Metavolcanic





Ca in C horizon (%):





1.8 + 0.1

1.4 + 0.1

6.1 + 0.2

Carbonate in C horizon (%):





Future Model Development

  1. Evaluate the ability of pebble counts to predict mineralogy of the fine earth (<2mm) fraction.
  2. Evaluate alternative weighting schemes to correct for local influences of landscape position, rock hardness, and proximity of each polygon to the study site.
  3. Run the GIS model to create derived coverages which depict spatial patterns in parent material chemistry and mineralogy across the region.
  4. Use the above derived coverages along with digital soils maps to drive spatial application of a soil chemistry model (eg. PROFILE) to produce a regional model of weathering rates.

Bailey, S.W. and Hornbeck, J.W., 1992. Lothologic composition and rock weathering potential of forested, glacial till soils. US For. Serv. Res. Pap., NE-RP-662, Radnor, PA, 7pp.

Hornbeck, J.W., Bailey, S.W., Buso, D.C. and Shanley, J.B.: 1997, Streamwater chemistry and nutrient budgets for forested watersheds in New England: variability and management implications, For. Ecol. Manage. 93, 73-89.