Nutrient And ph lEVELS Of Wildwood Soil

Cathy Rabago, Tripp Wilkinson, and Brian Robey



Wildwood Park in Radford, Virginia is split by a stream called Connelly’s Run, which runs north/south. Therefore there are two sides to the park, the east (rocky, west-facing) side, and the west (non-rocky, east-facing) side. We noticed that the west side of the park had more visibly living things, such as flowers, on it than the east side. Our question was: What is the difference in pH, temperature, and plant nutrients (phosphate, nitrate, Potassium) on the east (rocky) side compared to the west (non-rocky) side of Wildwood Park?



We went to Wildwood Park on April 12, 2002. From 10:00AM to 12:00PM we sampled the west side, which was in direct sun at that time. At the top of the west side, we took five random samples of soil. One sample being one zip-lock bag filled with soil from about 3 to 4 inches down underneath all of the leaf litter. Each ample was labeled individually as collected. Also at each sample site, we measured the temperature right next to the hole where the sample was taken right through the leaf litter without removing it. We used a soil metal thermometer for this measurement. At the bottom of the west slope, we took five random samples of soil in the same way. Using the same procedure, we sampled the east side from 2:00PM to 4:00PM when it was in direct sun. Later in the laboratory, we used the LaMOTTE Soil NPK Kit #5880 to test for nitrogen, phosphorus and potassium. This kit used color differences to estimate low, medium, and high levels of each nutrient. We converted these to estimates of lbs. per 1000 square feet of 6 inches of soil using information provided in the kit. We used the Oakton Soil pH Test meter WD-35634-66 to measure soil pH. It measured to the nearest 0.1 pH.

Our constant variables were the depth we took the samples from and the way we measured the pH and plant nutrients. We also tried to keep the amount of direct sunlight exposure similar on the two sides. We took the west (east-facing) side in the morning, because the amount of direct sunlight would have been shining on that side for about a couple of hours as we took the samples. We took the east (west-facing) side in the afternoon, because the amount of direct sunlight would have been shining on that side for also about a couple of hours as we took the samples.

Our assumptions were that the test kits worked properly and were sufficiently precise, that our samples were a good representation of the east (rocky) side and west (non-rocky) side soil, that the clouds were covering the same amount both times we collected samples, and that that the amount of sunlight exposure was relatively the same on the two slopes when we collected samples.

Differences were considered statistically significant if margin of error bars around the means did not over lap.



Comparison of East Versus West Slopes

Average phosphorus levels of the east side (west-facing) were significantly higher(0.16 lbs) than on west side (0.08 lbs) (Fig. 1).Eight of 10 samples on the east had the lowest detectable amount of phosphorus(0.2 lbs), while only 4 of 10 samples had that amount of phosphorus on the west (Table 1).That trend was seen in both the top and bottom samples from the two slopes.

Nitrogen levels were very low and not significantly different on the two sides. Only 2 of10 samples on the east side had a detectable amount (0.9 lbs) and only one sample on the west had that much nitrogen (Table 1).

Average potassium levels of the whole east side (1.46 lbs) were not significantly different than the potassium levels of the whole west side (0.90 lbs), but very nearly so, because the error bars barely overlap (Fig. 2).The top of the east side averaged more than double the potassium on the top of the west side, a very significant difference. The samples varied in potassium content more than the other nutrients, from none detectable to high amounts(3.7lbs). The only 2 samples with high potassium content were from the eastside (Table 1).

Average pH on the east side (7.4) was significantly higher than pH on the west side(6.7) (Fig. 3).Samples on both sides ranged from slightly acidic to slightly alkaline, but only 1 out of 10 samples was acidic (less than 7.0) on the east side, while 5of 10 were acidic on the west (Table 1).

Mid-April soil temperatures the east side (west-facing slope) were significantly higher than on the west side (east-facing slope), in both the lower samples and the higher samples on the slopes (Fig. 4). Only2 samples on the west were as high as the lowest temperatures on the east (Table 1). The average soil temperature was 1.9 degrees Celsius (3.4 degrees F) higher on the east than on the west.


Comparison of Tops Versus Bottoms of Slopes

            If we compare combined samples taken from the top east and top west with combined samples of bottom east and bottom west, we can see if position on the slopes (top vs. bottom) has an effect on soil conditions.

            We found no significant difference in phosphorus, nitrogen, potassium levels, or temperature between tops and bottoms. Only pH showed a significant difference (Fig. 3). All 10samples from the bottom parts of the two slopes were slightly alkaline(>7.0), while 6 of the 10 samples from the tops of the slopes were slightly acidic (<7.0) (Table 1). The bottom samples averaged pH 7.6 (range 7.1-8.2); top samples averaged pH 6.6(range 5.9-7.6). That represents a 10 times difference in concentration of hydrogen ions in the soil.



            Both slopes were very low in nitrogen, most samples having no detectable amount.  Soils samples from the east side (west-facing) of Wildwood Park averaged significant richer in phosphorus and very nearly so in potassium as well.  Still, according to test kit information, the average phosphorus content was low, even on the east side. The potassium content ranged from low to high on the east side and none detectable to medium on the west side.

            The east slope averaged slightly acidic while the west slope averaged slightly alkaline. Also, the samples taken from higher on the two slopes were slightly acidic while the lower samples were slightly alkaline. The averages were significantly different and represented about a 10-fold difference in hydrogen ion concentration.

            Though we attempted to take our temperature measurements at times of the day when each slope had been exposed to about the same amount of direct sun, the east slope, which receives the afternoon sun, averaged about 2 degrees Celsius higher soil temperatures than the west side that got the morning sun. This is typical. The afternoon sun has a greater warming effect than the morning sun, so west-facing slopes are warmer and drier than their east-facing counterparts.

There are a few possible problems or errors that could have affected our results.  We employed a regular rule for sampling, but still the samples may have been non-representative of the area, especially since we had time to take only 5 samples per area. The soil NPK kit gives a rather crude qualitative result with varying color. It is often difficult to decide where a particular color falls on a quantitative scale, especially whether the amount is low or zero.  We would be more confident in the results if we had access to instruments that give a more precise, quantitative read-out. The pH meter is temperature-sensitive, but we measured pH in samples taken to the lab where they presumably were all the same temperature.  So we think the pH data is quite accurate.  We are much less confident of our temperature comparisons.  We didn't actually know if the amount of sun exposure was the same before we measured the two sides.  In the future, preliminary research is needed to determine when the sun begins to strike each slope directly. Also, choosing a completely cloudless day or completely cloudy day would eliminate the variation in cooling effect of clouds.

After doing this research, our suggestions for future research could be to find out exactly what kind of plants live on each of the two sides. Casual observations indicated quite a difference.  The east side had many more flowers in bloom and more pine trees.  Our results indicate that the difference in vegetation is not due to soil nitrogen or potassium.  Could it be due to the difference in phosphorus, pH, or temperature?  Or could the difference be due to a factor we did not measure, such as rockiness, other soil nutrients, or the history of the land?  These are questions that could be pursued by future investigators.



            Thanks to Dr. Kugler for his help with the equipment used in our research.