The Relationship Between pH and Nutrients in the Soil

 

Alysia Sorby & Kathryn Knecht

 

 

Abstract

 

            In this experiment, we tested the pH level and the amount of nitrate nitrogen in the soil.  The purpose of testing these two variables was to see if they were correlated.  We collected ten soil samples from ten different areas and tested them for their pH value and nitrate nitrogen level.   There was a negative correlation (correlation coefficient =  -0.3284).  An r2 value was calculated in order to determine whether a strong or weak correlation was found.  A weak correlation (r2 = 0.1078) was found between pH and nitrate nitrogen, therefore not supporting our hypothesis.

Introduction

            Our experiment was based off of the cation exchange capacity of soil.  This states that if a soil is basic (has a pH value greater than 7) it will retain positively charged nutrients and this retention will decrease leaching.  For example, ammonium (NH4+) is a positively charged ion and is attracted to the bases (such as OH-) in the soil.  This attraction reduces leaching.  Nitrate ions (NO3-) are negatively charged and are repelled by the negatively charged bases.  This increases leaching (Pancholy & Rice 1972, Kellman 1985).      

            The purpose of this experiment was to test whether a correlation in pH level and nitrate nitrogen could be found.  Using the information that basic soils retain positively charged nutrients and negatively charged nutrients are repelled, we hypothesized that a greater pH value will result in lower concentrations of the negatively charged ion NO3-.

 

Methods

Our samples for this experiment were collected on three different days.  The first four samples were taken on October 9th, 2007 at the University of Wisconsin Milwaukee Field Station located near Saukville, Wisconsin. A pedometer was used to record the location of each sample at the field station.  The steps were later converted to meters.  All samples were collected using labeled plastic bags.  The first sample was 349.2 meters from the UWM Field Station sign off Blue Goose road in the old field area (Fig 1). The second sample was taken 849.6 meters into the bog located in the West Island area (Fig 1). Sample three was taken 891 meters in the bog located within the swamp hardwoods (Fig 1). Located in the East Island within the bog, the fourth sample was taken at 1205.4 meters (Fig. 1). 

 

 

 

 

             349.2 meters

 

 

 

 

 

                                                                  849.6 meters

                                                                                       891 meters

                                                                                  

                                                                                                              1,205.4 meters

 

 

 

       Figure 1: UWM Field Station Bog and surrounding vegetation (Reinartz 1985)

 

Another set of samples were taken on October 18th, 2007 at Mt. Olivet cemetery in Milwaukee, Wisconsin. The fifth sample was taken by the woods 100 meters 270 from the entrance of the Mt. Olivet Cemetery. Sample six was taken by the stream 50 meters 225 from the entrance of Mt. Olivet Cemetery.     

The last four samples were taken October 19th, 2007 at the Alverno College campus. The seventh sample was taken 5.8 meters 280 from the South East corner of Christopher Hall. Sample eight was taken 35 meters 200 from the corner of 3250 South 43rd Street and Whitehouse Drive. The ninth sample was taken 25.8 meters 175 from the entrance of the Alverno Parking Structure. Sample ten was taken 22 meters 5 from the center of the west side of the TL building.

On October 19th, 2007, we performed both a pH test and a nitrate nitrogen test. For both tests we used a soil kit called Lamotte Soil Test Kit. Within this kit, directions were provided on how to execute each test as needed. In order to determine the range of the pH, we first performed a duplex test. The range told us which chemical to use in order to obtain a specific pH value. The only two chemicals that we used to find a specific pH value were Bromothymol Blue and Phenol Red; this was due to the duplex ranges being basic. We used the universal soil extracting solution in order to obtain filtered samples of the soil for the nitrate nitrogen test. Once this was performed we used the chemicals as the directions stated to help us determine the level of nitrate nitrogen. All nitrate nitrogen was recorded as pounds per acre.  Using an Excel spreadsheet, we performed a correlation test in order to determine our relationship between pH and nitrate nitrogen. 

 

 

Results

            We found the relationship between the pH and nitrate nitrogen of the soils to be weakly negatively correlated (correlation coefficient = -0.3284; r2 = 0.1078; Fig. 2). For samples 3 and 9 we had a neutral pH (Table 1). Sample 1 was slightly acidic with a pH of 6.8. The rest of the samples all had a basic pH. The highest nitrate sample was 4 which had 44.83 kilograms per hectare of nitrate nitrogen. Samples 1, 3, and 9 had the second highest nitrate nitrogen reading of 22.42 kilograms per hectare.  The rest of the samples were all 11.21 kilograms per hectare of nitrate nitrogen (Table 1).

 

 

 

Table 1: pH values and amount of nitrate nitrogen in samples 1-10

 

Sample

pH

Nitrate (Kg/hectare)

1

6.8

22.42

2

7.8

11.21

3

7

22.42

4

8.2

44.83

5

8.4

11.21

6

8.4

11.21

7

8.6

11.21

8

8.4

11.21

9

7

22.42

10

8.4

11.21

 

                                          

 

 

 

 

 

 

 

 

 

 

 

Figure 2: Relationship between pH and Nitrate nitrogen

 

 

Discussion

            Our hypothesis was not supported.  A correlation was found but it was very weak.  The correlation that we found (although weak) can be explained by the cation exchange capacity of the soil.  The majority of our soil samples were relatively basic.  The negative bases (OH-) most likely repelled the negative nitrate ions (NO3-) and this increased the amount of leaching in the soil, which may have been the reason for the low concentration of nitrate nitrogen (Pancholy & Rice 1972).

            Because a strong correlation was not found, other factors may have affected the amount of nitrate nitrogen in the soil.  Sample 4 had the highest amount of nitrate nitrogen (44.83 kilograms per hectare) and had a basic pH value of 8.2.  The site where this sample was taken appeared to have been burned at an earlier time.  When vegetation is burned, the nutrients within the vegetation are deposited back into the soil (Christensen 1973).  The soil at this site may have had an abundance of nitrate because of the recent burning activity.

            If we were to do this experiment differently, we would collect more soil samples because a greater sample size would give us more representative results.  Our soil samples would also have a greater variety of land type.  We would also use a new soil testing kit so that we could be sure that no contamination had occurred in the chemicals.         

 

 

References Cited

 

 

Christensen, N. (1973). Fire and the Nitrogen Cycle in California Chaparral, Science,

181, 66-68.  Retrieved October 29, 2007 from JSTOR database.

 

 

Kellman, M. (1985). Nutrient Retention by Savanna Ecosystems: III. Response to

Artificial Loading, Journal of Ecology, 73, 963-972.  Retrieved October 28, 2007 from JSTOR database.

 

Pancholy, S., Rice, E. (1972). Inhibition of Nitrification by Climax Ecosystems,

American Journal of Botany, 59, 1033-1040. Retrieved October 28, 2007 from JSTOR database.

 

 

Reinartz, J. (1985). A Guide to the Natural History of the Cedarburg Bog Part I. The

University of Wisconsin Milwaukee Field Station Bulletin. Vol. 18. No. 2:12.