Testing Nitrogen and Phosphorous Levels in Soil on the Tops of Hills and the Bottoms of Hills 



December 1, 2007

By Mary Siegel


            I tested whether or not elevation (tops and bottoms of hills) had an effect on the leaching of nitrogen and phosphorous out of the soil. The levels of nitrogen and phosphorous found in the soil showed that there was not a significant difference between the tops and the bottoms of the hills.


Farmland is an important part of our environment and how the farmland is treated can change the soil. Something that is often done to the soil on farmland is that it is fertilized. This increases the amount of nitrogen and phosphorous in the soil. This is important to the farmland because nitrogen and phosphorous can change how well the plants grow and produce (Syers, Powlson, Sanchez, and et. al., 1997).

One of the issues that farmers face is being able to keep nitrogen and phosphorous levels up to support their crops. The challenge of deciding how much fertilizer to put on the soil is figured out by testing the soil for nitrogen and phosphorous levels. This is how the farmer decides what kind of fertilizer and how much fertilizer the farmer needs to put on the soil. If the farmer adds too much fertilizer in the soil, the possibility of the nutrients, nitrogen and phosphorous leaching into ground water becomes an issue. Even though it is important that these nutrients are present, it is also just as important that they are not present in excess because of the leaching and the dangers of the nutrients getting in the ground water.

This particular experiment has been designed to explore if leaching is affected by elevation (hills versus bottoms of hills). I hypothesized that because if leaching of soil does occur and can pull nitrogen and phosphorous out of the soil (Smil, 1991), that there will be higher levels of nitrogen and phosphorous at the bottom of the hill.


Materials and Methods

            On October 14, 2007 around 12:00 p.m. soil samples were collected off of Robert Poblockiís Berwick farm on Rural Route 2, located in Illinois Warren County. A total of 20 samples were collected. Ten of these samples were collected by locating areas on a topical map that were elevated. The other ten samples were collected by finding a low area in the vicinity of each elevated area located. Once there was a total of 20 designated areas to sample from the samples were collected all in the same way.

            How I collected samples was by locating one of the designated areas and digging a hole .15 m depth to collect a soil sample from. The depth of the hole that was dug was figured out by using a Kenson 50 meter tape measure model OTR50M to measure. Then around the hole I first dug and sampled from I dug four additional holes at .15 m depth to sample from using the same method of measuring for the first hole. These four additional holes were dug at a distance of 1.21 m, which was determined by the usage of Kenson 50 meter tape measure model OTR50M. In addition, these four holes were located in the cardinal directions with respect to the first hole dug and sampled. How the cardinal directions were determined was through the usage of a compass. Once all five holes were dug and sampled from the soil was mixed to make a composite of the soil and put in a glade zip block bag to be brought back to the lab. This same procedure was repeated for the 19 remaining designated areas.




Figure 1: This is how samples were collected for each of the 20 samples

            Two weeks later after the samples have been collected I tested the samples for levels of nitrogen and phosphorous. These tests were conducted through the usage of a Lamott brand model STH series combination soil outfit. The protocol for extraction procedure was followed along with the protocol for the nitrate nitrogen test and phosphorous test.

            In order to analyze data it was inserted into Microsoft Excel to get means, standard deviations, produce graphs, and to obtain p-values through using a paired T-test with one tail.  



            The nitrogen and phosphorous tests that were conducted on the soil samples from both the tops and the bottoms of the hills indicated through statistics that there was no significant difference between the levels of nitrogen (Bottom of hill: Mean = 30.03 kg, S.D. = 14.92, Top of hill: Mean = 35.94, S.D. = 11.39) (Fig. 2). Furthermore the p-value for hill tops and hill bottoms nitrogen levels is .1 this indicates that the nitrogen levels for the tops and bottoms of hills does not significantly differ.

Figure 2: Nitrogen levels for hill tops and


                Like the nitrogen levels at the top and bottoms of hills the phosphorous levels also do not significantly differ at the tops or bottoms of hills. This is not only indicated by the p-value of .17, but also is indicated by the mean and standard deviation (Bottom of hill: Mean 88.63, S.D. 7.18, Top of hill Mean 90.9, S.D. 0) (fig. 3) indicating that the levels of phosphorous do not significantly differ from the hill tops and bottoms.


Figure 3: Phosphorous levels for hill tops and hill bottoms


                Both the results of the nitrogen and phosphorous tests indicate that the levels of nitrogen and phosphorous do not differ significantly. This refutes the hypothesis that I would find higher levels of nitrogen and phosphorous at the bottoms of hills due to leaching.



            According to the results there was no difference in the levels of nitrogen and phosphorous at the bottoms and tops of hills. This was opposite of what was expected. However, an explanation for the results could be due to the fact that rain fall was so small that leaching in the soil did not cause a significant difference in nitrogen and phosphorous levels (Wagenet, Nye, Nowland, and et. al., 1990). Another explanation for why leaching may have not caused a difference in the levels of nitrogen and phosphorous levels is because the soil has been used for farmland for at least the last 9 years, which may cause the soil to be so saturated with phosphorous and nitrogen that leaching doesnít cause a significant change in the soil.

            These above mentioned factors that were not considered before doing the experiment would be a definite improvement and consideration for a follow up experiment.


Literature Cited

Syers K. J.; Powlson S.D.; Sanchez A.P.; Greenland J.D.; Ingram J. (1997, July). Managing Soils for Long-Term Productivity [and Discussion], Philosophical Transactions: Biological Sciences, 1011-1021. Rtrieved October 1, 2007 from Jstor database.


Smil V. (1991, December). Population Growth Nitrogen: An Exploration of a Critical Existential Link. Population and Development Review, 569-601. Retrieved October 17, 2007 from Jstor database.


Wagenet R.J.; Nye P.H.; Nowland J.; Burns I.G.; Greenwood D.J.; Addiscott T. M.; Graham-Bryce I.J. (1990, September). Quantitative Theory is Soil Productivity and Environmental Pollution, Philosophical Transactions: Biological Sciences, 321-330. Retrieved November 28, 2007 from Jstor database.