This picture of Pleasant View farmhouse shows a farm in New Hampshire.  Pleasant View lies on a flat, sandy glacial outwash plain that is very similar to the location of North Haven pitch pine site.  Like North Haven,  this field was cleared of forest, probably white pine forest (note trees behind house) 200 years ago and plowed for a century or more.  This page will show images of soil pits taken from this field to explain the general soil patterns found at the pitch pine site.

Soil scientists have found that certain distinctive layers occur in soils in different locations.  By giving names to these layers and then discussing the differences between these layers in soils found in different environments, they are able to discuss the characteristics of different soils and the processes occurring in these ecosystems.  These layers occur because percolating ground water transports organic matter, fine mineral matter and various dissolved and colloidal 'elements" downward and over many decades creates layers or "horizons" distinguished by the presence or absence of particular elements.
 

The illustration below of a 1 meter deep soil pit in the field in front of Pleasant View illustrates the sharpness with which the Ap horizon is demarked in the profile.

In the Northeast, a typical cross section of plowed soil looks like this soil pit from Pleasant View farm.  This image shows an Ap (plowing) horizon, overlaying a B horizon, grading down to the lighter C horizon, the quartz sandy parent material.

Before plowing, this site was the location of a forest, with typical pit-and-mound topography.  (When a tree falls over, the root base comes out of the ground, creating a mound and an empty pit.)  The uneven terrain meant that soil horizons were not planar but undulated with the bumpy terrain.  The white pocket near the pole is a relic E horizon that was very deep, probably at the bottom of the pit of a pit and mound.   When plowing occurred, the plow truncated the profile leaving the deep relic remnant of an ancient forest soil.

 
Scientists initially divide the ground layer into two parts -- the forest floor and the mineral soil -- and then subdivide these into horizons.  The forest floor is absent from this mowed field. However in a forested setting the leaf litter accumulates in various states of decomposition as a layer over the mineral soil.  It is distinguished from the mineral soil by the amount of organic matter present.  Within the mineral soil, soil scientists divide the ground layer ("soil") into horizons - typically as follows for New England:

Because these layers occur as a result of differences in mineral distribution, the chemical processes differ throughout the soil profile.  These differences can affect the movement of other elements.

To study the movement of lead and other pollutants without these complicating factors, Marsh and Siccama (1997) proposed studying the soil in areas where the soil was plowed in the past.  Plowing churns the soil and homogenizes the top 20 cm (8 inches -- the depth of the plow blade) of soil.  This homogenous layer is
called an Ap layer.

These illustrations show the differences between the upper horizons of a plowed and unplowed soil.  This comparison is from a soil on similar geology and under a white pine forest in Litchfield, CT. The forest in this setting on the plowed soil is about 80 years old and there is the beginning of a visual expression of the redevelopment of a "new" E horizon in the top of the Ap under the forest floor.
 
 

The image on the left shows the clear divisions between the forest floor, the grayish E horizon and the Bhs horizon.  An Ap layer is shown in the image on the right.  Since the field is no longer being plowed, leaf litter has created a forest floor layer above the relic Ap, and an E horizon is beginning to form.  However, approximately 20 cm. of soil remains visually homogenous.  [For full size images, click on either picture.]

The North Haven pitch pine site was plowed until approximately 125 years ago. The oldest known tree  in the stand is about 125  years (based on ring counts on tree cores).  As in the picture, the old Ap is now covered by a thick forest floor, predominantly pine needles.  An incipient E horizon can also be found in some places, but the Ap layer appears homogenous.  One question of this study is to what extent the underlying Ap layer has begun segregating into horizons chemically.

More importantly, though, is that this still relatively homogenous soil stratum allows us to consider the movement of particular elements without the complicating effects of highly spatially variable soil environments in unplowed ecosystems.  This layer is especially valuable for observing the movement of lead, since the input of lead occurred after the plowing was ended in the mid 1800's.  Very little lead was present in the environment until gasoline began to contain lead in the 1920's.  Therefore, this soil layer is virtually a "blank slate" which lead has slowly been moving down into.  Because very low levels are present naturally, and because lead seems to be moving downward as a front, it is fairly easy to perceive how deep it has permeated.  By observing changes in lead amounts and concentrations over time, we hope to draw conclusions about the rate of lead movement.

Similarly, most minerals of anthropogenic origin were not being released in large quantities until after the field was returned to farmland, and therefore their situation is similar to lead.  In the case of some elements, high amounts are naturally present.  But in any case, the amounts should be relatively constant throughout the horizon, allowing for easy discernment between background levels of the element and the levels of anthropogenic inputs.  By gathering data points at different points in time, we hope to track the changes and illuminate the underground processes of mineral movement and soil organization.
 

 

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