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Comparison of Soil pH and Buffering Capacity Between an
Agricultural Field and a Wooded Lot
July 16, 2001

Introduction:
       The pH of a substance is a measure of its acidity or alkalinity.  The pH scale ranges from 0 to 14 with a value of 7 considered neutral, below 7 acidic, and above 7 basic.  In soils, the pH affects the availability of plant nutrients, activity of microorganisms, and the solubility of soil minerals.  A soil pH of between 6 and 8 is considered best since most nutrients are available in this range, although it should be noted that certain nutrients become more available at higher or lower pH values.  Soil acidity is determined by both naturally occurring and anthropogenic (human caused) processes.  The main natural causes of soil acidification are long term leaching from carbon cycle processes, plant nutrient uptake, and nitrification.  For instance, pine and oak leaf decomposition is known to cause an increase in soil acidity.  The main anthropogenic causes include excessive use of soil amendments, such as inorganic nitrogen fertilizers and lime, needle-leaf afforestation (converting hardwood forests to pines), and acid deposition.  
       The buffering capacity of a soil is the ability of the soil to resist change in pH upon the addition of substances that are either more acidic or more basic than the soil itself.  Soil make-up and soil amendments play major roles in the buffering capacity of a soil.  For example, soils high in calcium and magnesium content have a greater ability to buffer against added acidic components.
      Our group, with the help of our determined leader, set out to compare the pH and buffering capacity of two fields to see if  they varied in the two properties, where any difference could possibly be traced back to previous human activities each site was subjected to.  Field 1 was formerly used for growing feed crops, such as corn and had been fallow for a period of one year.  Field 2 was a wooded lot, having a tree canopy with about 80% coverage.  The dominant tree species was American beech.  This lot was adjacent to field 1 and had also seen human impact in the form of a ropes course, which no longer exists.
       According to the Mercer County soil survey map, the sites tested are part of the Matapeake Series.  These soils are described as, "deep, well-drained, nearly level to sloping soils.  They are formed in a silty mantle that is underlain by sandy and gravelly materials of the Pensaukin Formation of the Coastal Plain.....The surface layer of a typical Matapeake soil is dark-brown loam about 15 inches thick, with a strong brown subsoil about 26 inches thick....with the lower part considered a sandy loam"  The survey went on to say that almost all the areas of the Matapeake soils have been cleared and used for agriculture.

Method:
       We took surface samples from each field by randomly choosing sites within the study area.  Field 1 had previously been demarcated  by our mentor and was a (100 m) x (100 m) square.  Our group then demarcated field 2 which became a (50 m) x (50 m) square.  We also sampled from the top two layers (horizons) of the soil profile.  Pits were dug to a depth of approximately 1 meter in order to expose the soil profile and thus the different soil horizons.  We actually sampled from 3 different horizons at each site but, because of time constraints, we only managed to do the analysis on the top two layers for each along with the surface samples noted.  Once the samples were air dried, we followed the procedures outlined in the section of this web site to determine the pH, buffering capacity, bulk density, particle size, and texture of each sample.  A Fisher pH meter was used to do the testing.
        The profile horizons were determined based on differences in color, structure, and textures that we observed at each site.  

Results and Interpretations:
      Our results show that the pH values in the agricultural field were consistently higher than those found in the wooded lot.  This was true for initial pH, as-well-as the pH measured upon the addition of the different concentrations of the base potassium hydroxide, KOH, used to check buffering capacity.  In the samples from the surface sites, there was very little difference between the range in pH change of the two sites.  The field had a net pH change of 4.71 while the wooded lot changed by 4.66 pH units.  This was in stark contrast when comparing the net change of the pit samples.  The A-horizon in the field changed by 4.23 pH units while the wooded lot A-horizon changed by 6.39 pH units.  A similar change occurred in the B-horizons with a 4.04 pH change in the field compared to a 7.01 pH change in the wooded lot.  When the data is plotted as Meq KOH/100 g soil -vs- pH and a linear regression line drawn to determine slope, where slope = buffering capacity, similar results appear.  The slope for each agricultural field graph is greater than the corresponding graph for the wooded lot, thus indicating the field has a greater buffering capacity.  The equations for the regression lines can be found on each graph below and have the form,  y = mx + b,  where "m" is equal to the slope/buffering capacity.
Links to surface graphs,  A-horizon graphs,  B-horizon graphs.
       The table below shows the "m" (slope) values for the samples tested.  Teachers should note that the steeper the slope (e.g. a greater value for "m"), the greater the buffering capacity.

       Our observations at the fallow field seemed to be consistent with the Mercer County soil survey indicating a Matapeake soil type.  Our A-horizon was a dark brown loam followed by a B1 horizon which tested out to be a loamy sand which contained a significant amount of mottling between a reddish-brown and  mustard yellow color.  The B2 horizon was a consistent reddish-brown.   The A and B2 horizons did not show the same mottling characteristics as the B1 horizon.  The horizons we found in the wooded lot appeared to be quite different than those found in the field.  The horizon boundaries were not as clear as in the field and the colors seemed to be more yellowish-brown than the brown to reddish-brown found in the field.  The table below shows the results of our bulk density and particle size (texture) tests.

Discussion:
       The pH values from the agricultural field were within an acceptable range, very close to 7.   This is probably due to the addition of soil amendments in order to maintain a balanced pH for optimum plant growth.  The wooded lot had very low initial pH readings for each sample, indicating poor soil quality by this measure.  Leaf decomposition is one possible cause of this, although a thick leaf-litter layer was not noted.  Another source could be the wood chips that were spread over the surface when the lot was used as a ropes course.  Further analysis may be needed to determine a more specific cause.  According to the graphs, the field also has a greater buffering capacity.  This is most likely due to soil amendments being added.  The buffering capacity of the field remained constant between the first two horizons, while the wooded lot showed a decrease in buffering capacity when moving from the "A" to "B" horizon.  The fact that the buffering capacity did not change in the field horizons may indicate that the agricultural impact penetrates below the surface, while the impact on the wooded lot was greater at  the surface and did not penetrate as much to lower levels.  It is clear than that the soils in the two sites, the agricultural field and the wooded lot, varied significantly according to the two major components measured in this analysis, pH and buffering capacity.  We expected this given the significant differences in the human activities formerly found at both sites.  In conclusion, more samples and further analysis would be required to more accurately determine the extent of the human impact.

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