The Effect of Soil pH on the Living Conditions
In Lumbricus terrestris
The difference in soil pH weighs a heavy measure on the appearance of the earthworm Lumbricus terrestris in different areas of soil. A highly acidic or highly alkaline soil may be the underlying reason for the absence of this earthworm in certain areas where pH plays a large role. In this experiment, soil pH was altered with Miracida soil acidifier and Agricultural Limestonea soil correctional for acidic soil. This was done to test the effects pH would have on the living conditions of Lumbricus terrestris over a twenty-one day period. The experiment shows the fluctuation of numbers on a daily and weekly basis verifying the theory that the earthworm prefers a soil with a pH between 6.0 and 7.0. In summary, this paper provides a three week synopsis of a closed experiment showing the preference of soil pH for the earthworm Lumbricus terrestris.
KEY WORDS: Lumbricus terrestris, earthworm, soil, pH, acid, alkaline, limestone, Miracid, Agricultural Limestone, living condition
Many earthworm species in North America existing today actually originated from Europe (Minard). One of these earthworms is known as Lumbricus terrestris, or the Night crawler. According to the University of California, the Night Crawler is of Palaearctic Origin and can grow to sizes from 90mm x 6mm to 300mm x 10mm. Lumbricus terrestris has a dark anterior with a lighter posterior. Contrary to most belief, the earthworm is not a symmetrical tube-like organism. This is because it has no proper top or bottom and in Lumbricus terrestris, the posterior end has the ability to flatten. The life span of this particular earthworm can be anywhere from 2.5 to 6 years, maturing around day 350 (University of California).
Lumbricus terrestris is common in cultivated soils where is builds vertical living tubes as deep as two meters into the subsoil (Graff). Lumbricus terrestris are detrivorous which means they eat leaf litter by taking it underground to the top layer of soil and by consuming soil. The earthworms take in food at the surface or in the op soil layer and deposit excrements along the lining of the tubes.
Earthworms are considered to exert significant direct and indirect positive effects on soil quality and fertility, and consequently, they are important organisms in ecotoxicity tests and in contaminated land assessments (Georgiev, 2004). Several factors, such as soil characteristics (pH, organic matter content, etc.), chemical properties of the contaminants and environmental conditions (precipitation, temperature) affect the exposure and potential hazard to biota (Barendregt, 2004). The soil characteristic concentrated on in this experiment was soil pHthe measure of acidity and alkalinity.
The pH of soil can range from very acidic, 1-6 on the pH scale, to very alkaline, 8-14 on the pH scale, with 7 being neutral. Worms prefer soil at a pH between 6.0 and 7.0; higher than 7.0 and lower than 6.0 can be potentially harmful to the earthworms (wormman). There are many ways to alter the soil pH. According to the Garden Helper, to make a soil more basic, the most commonly used product is powdered limestone. Also, it states that some natural products that can be used to make soil more acidic are sulfur, sawdust, composted leaves, wood chips, cottonseed meal, leaf mold, and peat moss. There are also other ways to change the pH of soil such as using man-made chemicals.
Earthworms take up organic compounds through their skin as well as from their food (Fleuren, 2003). In this study, the soil pH was altered and tested to find the preferred living conditions for Lumbricus terrestris over a three week time period.
Methods and Materials
To begin the experiment, 50 Lumbricus terrestris worms were obtained from a bait and tackle shop and then separated into groups of ten in separate containers with some soil in each. Next, potting soil was placed in a 74cm x 19cm x 15.5cm container. This was the main container used in the experiment. Once the soil level was equal throughout the container, it was partitioned off into five separate sectionseach 14.8cm in length. The sections were separated by four Plexiglas squares with four holes drilled in each. The soil in each section was treated individually.
The first section on the far left end was treated with Miracid, a soil acidifier. Approximately four tablespoons of Miracid was mixed with 3.79 liters of water. Before adding the solution to the soil, all the soil was placed in another container in order to keep the solution from mixing with the other sections; once added, the soil was mixed. The section directly to the right of the first treated section was also treated with Miracid. Approximately 1.5 tablespoons was added to 3.79 liters. The same procedure for adding the solution to the soil was followed for this section also. The middle section was the neutral section and water was the only liquid used to moisten the soil. The last two sections were treated with Agricultural Limestone. The soil in the section on the far right was placed in a separate container and had approximately twelve tablespoons added to the soil and then moistened with water until the limestone was fully saturated. Once the limestone was saturated, it was then mixed in with the soil. The soil in the section to the left of this (between the neutral soil and the very alkaline soil) was removed and placed in a separate container similar to the other sections. Approximately six tablespoons of limestone were then added to the soil and moistened. Once fully saturated, the limestone was mixed in with the soil. After all the soils had been treated, they were then placed in their designated sections in the large container in order from most acidic on the left to most alkaline on the right. The soil was then left to sit overnight so ensure the soil pH.
The following day the five groups of ten Lumbricus terrestris were added to each sectionten worms per section. Every day after the initial addition of the worms to the large container’s sections, the worms were counted and the data was recorded. When the worms were counted they were placed in separate containers per pH section until all worms were accounted for and then allowed to be replaced in the same section they were removed from. Every other day the soil was tested to see if the pH needed to be treated. On days six, ten, fifteen, and nineteen the soil was treated with the proper solution.
The number of Lumbricus terrestris worms varied in each section daily per pH level. Over the course of a twenty-one day period, the number fluctuation showed and enabled the calculation of Lumbricus terrestris average and percentage.
The numbers were broken down into the number of worms found each day in weeks 1, 2, and 3 (refer to tables 1-3). After this, the numbers were evaluated into the average number of worms per pH per week (refer to table 4). Continuing on, table 5 shows the percentage of worms found per pH per week out of a total 100 percent. After the numbers had been calculated by weeks, they were taken and put into total averages and percentages for the full twenty-one day period. These can be seen on Table 6 and Table 7. The number of Lumbricus terrestris worms in each section shows which soil pH is preferred. The following tables shown have references to graphs for easier comprehension of the numbers.
Table one shows the number variation of Lumbricus terrestris during week one of the experiment (Refers to Graph 1).
TABLE 1pH 4.0 – 5.0pH 5.0 – 6.5pH 6.6 – 7.9pH 8.0 – 8.9pH 9.0 – 9.9
Table two shows the number variation of Lumbricus terrestris during week two of the experiment (Refers to Graph 2).
TABLE 2pH 4.0 – 5.0pH 5.0 – 6.5pH 6.6 – 7.9pH 8.0 – 8.9pH 9.0 – 9.9
Table three shows the number variation of Lumbricus terrestris during week three of the experiment (Refers to Graph 3).
TABLE 3pH 4.0 – 5.0pH 5.0 – 6.5pH 6.6 – 7.9pH 8.0 – 8.9pH 9.0 – 9.9
The numbers from Tables 1, 2, and 3 are used to calculate the average number of worms per pH per week. This is shown in Table 4 (Refer to Graph 4).
TABLE 4pH 4.0 – 5.0pH 5.5 – 6.5pH 6.6 – 7.9pH 8.0 – 8.9pH 9.0 – 9.9
From these tables, the percent of Lumbricus terrestris per pH per week was calculated. This is shown in Table 5 (Refer to Graph 5).
TABLE 5pH 4.0 – 5.0pH 5.5 – 6.5pH 6.6 – 7.9pH 8.0 – 8.9pH 9.0 – 9.9
The average total of Lumbricus terrestris found over a three week period is found on Table 6:
TABLE 6Total Average
pH 4.0 – 5.08.059
pH 5.5 – 6.513.238
pH 6.6 – 7.914.905
pH 8.0 – 8.96.809
pH 9.0 – 9.96.905
The total percentage of worms per section was calculated from the numbers recorded in previously shown data. This can be seen on Table 7:
TABLE 7Total Percentage
pH 4.0 – 5.016.19%
pH 5.5 – 6.526.476%
pH 6.6 – 7.929.81%
pH 8.0 – 8.913.618%
pH 9.0 – 9.913.81%
The initial results at the beginning of the experiment contradict the theory that Lumbricus terrestris prefer soil with a pH of approximately 6.0 to 7.0. As seen in Graph 1, the soil with a pH of 4.0 – 5.0 contained the most number of earthworms. Since it is the very beginning of the experiment that indicated this, it is thought that the worms were in shock from being transferred from three different habitats (bait shop containers, new soil containers, altered soil sections) and did not absorb much of the soil. After a few days, the earthworms moved around more and numbers started to fluctuate.
Around days ten and eleven there was a major shift in the number of earthworms in certain sections. As seen in Graph 2, the major increase was in soils with a pH of 5.5 – 6.5 and 6.6 – 7.9. This information supports the theory of earthworm soil pH preference. Since the earthworm takes in nutrients from the soil, it is thought that the soil with the more neutral pH may offer more nutrients for Lumbricus terrestris which in turn enables it to survive. Also, the less acidic and less alkaline the soil, the less likely the earthworm is to absorb the chemicals into its body. Towards the end of the experiment, the largest decline in the alkaline soil as well as the most acidic soil is seen (Graph 3).
The overall average number of Lumbricus terrestris per soil pH section was highest in the soil with a pH of 6.6 – 7.9. This is the most neutral soil that was present for the earthworms to live in. The next largest average was the soil with a pH of 5.5 – 6.5. This soil was slightly more acidic, but it seemed to be a well enjoyed living condition. The majority of earthworms were found between these two sections. The soil with a pH of 4.0 – 5.0 started off the strongest, and its overall average was higher than both the alkaline soils.
Towards the end of the experiment, more earthworms preferred the most alkaline soil to the lesser alkaline soil. This may have been because the limestone in the lesser alkaline soil was not fully saturated before it was mixed in causing the earthworms pain (dry limestone on the skin of Lumbricus terrestris will burn). After this was noticed, more water was added to that section and the earthworm numbers seemed to regulate back to what they previously were (refer to Graph 3).
The overall conclusion of this experiment validated the theory that the earthworm Lumbricus terrestris preferred the soil with a pH between the levels of 5.0 – 7.9. The absence of other studies dealing with the relation of soil pH and the living conditions of Lumbricus terrestris is cause for further investigations. In further investigations, however, a larger study and a different set-up would be beneficial. If the set-up gave the worms in the two extreme soil pH conditions another direction choice instead of the only one that was offered in this experiment, the numbers may have varied and the results may have been different. Also, a lengthier study to verify these findings would validate the experiment more so than now.
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