Effects of Soil pH on Bacterial Growth

By: Chiara Richter and Natalie George

November 22, 2010



We tested how soil pH would affect the growth of bacteria in various locations across southeastern Wisconsin. After testing the pH of a slurry or mixture of water and soil, the mixture was plated to allow us to count the bacterial colonies able to grow in the soil samples. Our hypothesis was that more acidic soil would decrease bacterial growth. The results did not support a significant correlation between the pH values of the samples and bacterial growth. This was indicated by receiving an r-squared value of 0.0039, which did not indicate a correlation.

Keywords: soil, pH, bacteria, colonies


pH values have been shown to have significant influences over the ability of organisms to function. Bacteria are organisms that can be affected by pH values. In stream studies the acidity and alkalinity of water running through monitored decomposition bags affected the bacteria colonies present (Chamier, 1987). Other studies showed that acidity of streams greatly influenced the levels of nutrients present for bacteria to use for important metabolic processes (Peters, et.al, 1989). These studies led us to want to study whether the pH in soil could affect bacteria colonies in the same way. Our hypothesis was that pH levels would significantly influence the bacterial growth with more acidic soils allowing less growth than more alkaline soils.

Methods and Materials

Soil samples were taken from various locations in Milwaukee and Racine counties located in Southeastern Wisconsin from October to November 2010, (Table 1). These samples were taken from horizon A, topsoil, of the various sites using gardening tools and plastic spoons. Samples were separated and labeled in Ziploc bags then transported to Alverno College in Milwaukee, WI, for testing.

We tested the soil samples using basic pH test strips then plated the samples using Petrifilm plates from 3M Microbiology Products, lot 2012-01 KB, with gridlines to count bacterial colonies. Before plating and testing the pH, 10 ml of distilled water was added to about 10 g of soil in small glass dishes to help form a slurry to test and plate. Each soil sample was plated twice by adding five drops of the slurry to the plates, slowly pulling the cover over the plates, and pushing out any air bubbles. The plates were then labeled with the site locations and stored at 32°C for 48 hours before results were read and recorded.

Bacterial counts were taken by counting the number of colonies present in two of the squares, eliminating any squares with soil clumps on them or that were not completely covered by the sample slurry. The counts from each plate were averaged .This number was then multiplied by 20, the number of squares on the plate. Since two plates were used for each location we added the number of colonies calculated for each plate and again used the average of the two.

Microsoft Excel 2007 software was used to analyze the data and form a graph. Analysis was done by running a correlation statistical test.


We found no correlation between the soil pH and bacterial counts present in the samples we tested. The r2- value calculated using Microsoft Excel was only 0.0039, showing the variables did not correlate with one another.


Table 1: Record of locations, their pH and bacterial growth found on plates



Bacterial colonies

2400 Mitchell St, Racine, WI back porch



2400 Mitchell St., Racine, WI under pine bushes



Root River bank, Island Park, Racine, WI



1806 Carlisle Ave, Racine, WI under pine tree



1806 Carlisle Ave, Racine, WI, by pond



1806 Carlisle Ave, Racine, WI middle of back yard



Jackson Park, Milwaukee, WI, Bench near lagoon close to boat house between run off area



Jackson Park, Milwaukee, WI, Near base of an oak tree in the wooded area past the monument



Jackson Park, Milwaukee, WI, Far corner between softball field and tennis courts near the rail road tracks. 



Jackson Park, Milwaukee, WI, Near base of a tree across from the pool and within site of the play area



Jackson Park, Milwaukee, WI, At the base of the runoff site but before the concrete starts for the river



Fig. 1 Bacterial Colonies Present based on pH, R2 value of 0.0039.



Our results indicate that pH values do not significantly influence bacterial growth in soil. This analysis does not support our original hypothesis that pH would influence bacterial growth.

Influences which may have contributed to our results are varied. The first factor is that samples were taken from the same region. Being from the same region could have limited the variation of bacteria able to colonize. These bacteria may have also adapted to the varying pH levels or may live in the lower horizons which were not tested. Because samples were taken in the fall months which tend to have cooler conditions than summer months, the metabolic processes of the bacteria may have been limited. The moisture of the soil may also have influenced the bacterial growth.

 Living organisms can also influence the way the soil is structured chemically, physically and biologically (Angers and Caron 1998). For example, plants and trees in a specific area can influence the type of soil in that area. Deciduous areas tend to have a more alkaline pH than coniferous areas due to the acidity of the pine needles (Skyllberg et al. 2001). Soils with a less acidic pH tend to have a higher and more diverse bacterial count.  Studies have shown that the pH of soil in a specific area may predict the bacterial growth present in that area (Fierer and Jackson 2005). These factors can all influence soil pH and bacterial growth.

If we had to do the project over our emphasis would have been put on finding samples from more random locations and possibly spreading the counts out through different seasonal and climatic variations.  By using more random locations our data would be more representative of the pH values and bacteria present in the Wisconsin area. We would also use different means of collection and transportation to reduce the possibility of contamination.


Works Cited

Angers, D. A. and Caron, J.  (1998). Plant induced changes in soil structure: Processes and feedbacks.  Biogeochemistry, 42, 55-72.  Retrieved November 9, 2010, from JSTOR database. 

Chamier, A. (1987). Effect of pH on Microbial Degradation of Leaf Litter in Seven Streams of the English Lake District. Oecologia, 71(4), 74-84. Retrieved September 21, 2010, from JSTOR database.

Fierer, N. and Jackson, R.B.  (2005). The diversity and biogeography of soil bacterial communities.  Proceedings of the national academy of sciences of the United States of America, 103, 3, 626-631.  Retrieved November 9, 2010, from JSTOR database. 

Peters, G., Benfield, E. & Webster, J. (1989). Chemical Composition and Microbial Activity of Seston in Southern Appalachian Headwater. Journal of North American Benthological Society, 8(1). 74-84. Retrieved September 21, 2010, from JSTOR database.

Skyllberg, U., Raulund-Rasmussen, K., and Borggaard, O.K. (2001). pH buffering in acidic soils developed under Picea abies  and Quercus robur –effects of soil organic matter, absorbed cations and  soil solution ionic strength.  Biogeochemistry, 56,1, 51-74.  Retrieved November 7, 2010, from JSTOR database.