Effect of Earthworms (Lumbricus terrestris) on the Amount of Phosphorus in Soil
Vatsana Dethleuxay & Therese Fahres
November 25, 2009
The effects of earthworms on the level of phosphorus were tested to see whether soil with earthworms (Lumbricus terrestris) would contain higher level of phosphorus than soil with no earthworms. We tested a total of 16 samples that were divided into a group of 8 samples in each treatment (soil with no earthworm and soil with 5 earthworms). We measured the level of soil moisture and pH along with the level of phosphorus. Results yield a significant difference (P=0.00). Results revealed that soil with earthworms have a higher amount of phosphorus (Mean=3.67, S.D=0.52) than soil with no earthworms (Mean=3.07, S.D=0.66). However, we cannot fully confirm the findings due to the potential misreading of data collected.
Keywords: Earthworms (Lumbricus terrestris), phosphorus, soil moisture, burrowing, casting.
Earthworms play a major role in the ecology of soil, by altering biological, chemical, and physical properties. The specific activity of burrowing by earthworms speeds up the decomposition of leaf litter, increases air and water infiltration, and affects the diffusion of nutrients in the soil over time (Savin, Görres, and Amador, 2004). This evidence contributes to the well known and commonly used strategy of introducing earthworms to home gardens on the assumption that earthworms increase the fertility of the soil. This has been tested often in laboratories, but further experimentation continues to be done to discover which nutrients are most enhanced by the presence of earthworms. There is evidence that earthworms have an impact on the mineralization of phosphorus by making it more available for plants through their casts. Previous studies have shown higher levels of useable phosphorus in soil laden with earthworm casts than in control soil (Jiménez, et.al, 2003).
We chose to conduct a similar experiment in which we would test the phosphorus level of soil containing earthworms and soil without earthworms. Based on what we already know about earthworm ecology, we hypothesized that the soil containing earthworms would have higher levels of phosphorus than the soil without earthworms after a period of four weeks.
Materials and Methods
Beginning on October 2, 2009 we set out 16 small plastic tote basket (Volume: 2,340 cubic cm) and put 1”x3” white labels on each of the basket. Eight of the baskets had a label indicating that there are five worms in the basket. The other eight were labeled with “no worms”. After we were done labeling the baskets we poured loam soil (potting mixture soil of peat, humus, compost, sand, perlite, ¾ cu. ft., 21L) into the basket. We took a 12” ruler and measured the depth of the soil at 10 cm and we did that for each of the baskets. We measured the soil depth in the center of the baskets. On October 8, 2009, we recorded our first data on soil moisture, pH and phosphorus level. The 16 baskets contained no earthworms when we took our initial readings.
We stuck the 4-way analyzer meter (Rapidest Electronic 4-Way Analyzer, No. 1880) 8 cm into the soil to test the level of moisture and pH for each of the 16 baskets. The 4-way analyzer meter was stuck in the center of the basket every time. Soil moisture and pH was measured for each of the baskets. We then used the soil test kit (LaMotte Combination Soil Outfit #1, Model EM, Code 5934) to measure the level of phosphorus. We then filled the first test tube up to line 6 with the phosphorus extracting solution. With the 0.5g spoon (Code: 0698) we dipped it in the center of the basket 7.5cm into the soil and added three dippers of soil sample into test tube 1. We capped it and gently shook it by hand for 1 minute. We removed the cap and allow for the soil particles to settle and then we used pipet (Code: 0364) to transfer the clear liquid to a second test tube up to line 3. We then added 6 drops of phosphorous indicator (Code: 5705) into the second test tube. We capped it and shook it for approximately 3 seconds to mix the solution. Lastly, we added 1 phosphorus test tablet (Code: 5706) into the second test tube, capped and shook the test tube until the tablet dissolved and a blue color developed. We then placed the test tube in a white container with the phosphorus color chart (Code: 1372) placed next to it to match the test color. We repeated the procedure for each of the 16 baskets.
After we were done, we inoculated 5 earthworms into each of the 8 baskets labeled 5 worms (Lumbricus terrestris) (DMF Bait Co., 20 Canadian Nightcrawlers) and we put a handful of leaf litters on the surface of the soil for each of the 16 baskets. We collected data every Thursday at 2:00pm for 4 weeks following the same procedure. We placed the 16 baskets in an incubator in room TL 214 at Alverno College, set at 17ºC.
Buckets Labeled “No Earthworms” Buckets labeled “5 Earthworms”
Buckets Labeled “No Earthworms”
Buckets labeled “5 Earthworms”
Setup was on separate tables across from one another in an incubator set at 17 ºC.
Microsoft Excel (2007) was used to calculate the average, standard deviation and 1 tailed type 3 t-test results. There was a significant difference in the amount of phosphorus between soil with no earthworms (Lumbricus terrestris) and soil with earthworms (P=0.00). Results revealed that soil with earthworms had a higher amount of phosphorus (Mean=3.67, S.D=0.52) than soil with no earthworms (Mean=3.07, S.D=0.66).
Figure 1. Graph revealed that soil with earthworms had a higher amount of phosphorus (Mean=3.67, S.D=0.52) than soil with no earthworms (Mean=3.07, S.D=0.66).
The hypothesis that soil with earthworms (Lumbricus terrestris) will contain higher phosphorus than soil with no earthworms was supported. However, despite the significant difference between the levels of phosphorus in the “5 earthworm” treatment and the “no earthworm” treatment, we cannot fully confirm that there is a significant difference. We had some difficulty in reading the results when matching the color with the color card of the soil toolkit. We felt that color is subjective, and so a few disagreements occurred in reading the result of the soil solution that may have affected the outcome of the overall results of the experiment.
Aside from the level of phosphorus reading, there were some odd readings of the level of soil moisture. Soil moisture dropped from 2% to .5% (very dry). This was not what we expected, but makes sense when looking at what we know about earthworms. We learned through research that soil containing the earthworm species L. terrestris may dry out more quickly because their burrowing accelerates the exchange of water vapor from the soil to the air (Ernst, et al., 2009). However it also seemed like the variance of soil moisture for both treatments was dependent upon where the bucket is placed. Some soil was drier than others. We believe that it may be the result of the ventilation of air in the incubator because the soil in the first row seemed to be drier than soil in the second row. If we were to repeat the experiment again we would try keeping the measures for soil moisture consistent in terms of giving it a measuring time of 1 minute before moving on and measuring the next basket. This way there would be some certainty that the meter in the 4-way analyzer isn’t still moving down. By doing this we will avoid recording inaccurate results.
The earthworm specie Lumbricus terrestris prefer cool moist soil environment, but does not do too well in warm dry soil environment (Lowe & Butt, 2005). Lumbricus terrestris prefer moisture of loam soil around 14% (Lowe & Butt, 2005). However, the soil moisture has decreased to a minimum of .5% (very dry). We believe that the soil dryness may have an effect on the functioning of the earthworms, since we have found one earthworm in Sample 2 dead. In future experiments maybe the effect of soil moisture on Lumbricus terrestris casting of phosphorus can be tested. Lumbricus terrestris is unable to tolerate soil dryness and we predict that may affect its processing of organic materials if its environmental condition is unsuitable for it to thrive (Samuel, 1991). If we are to repeat this experiment again, we would add water into the soil to keep it moist for the earthworms. However, despite the dryness of the soil, the organic materials in the soil for the “5 earthworms” treatment have a higher rate of decomposition of organic materials then the “no earthworm” treatment. This is shown in its higher level of phosphorus. Again, we cannot confirm this because we believe that the soil tool kit is ineffective in testing color. Maybe in the future biologist should consider finding a more technological advance color readings similar so that it gives a numerical value for the colors. There is no variation in pH for both soil treatments. The soil pH for both treatments is neutral.
However, despite the problems that have occurred in our experiment that led to an unconfirmed result, past studies have demonstrated that earthworms helps to speed up the rate of decomposition by processing organic materials into smaller particle size for microbes. Earthworms aerate the soil by burrowing and casting (Samuel, 1991). They act as a distributor of nutrients by spreading out nutrients throughout the soil. The casting of earthworms plays a very important role in the availability of phosphorus by providing phosphorus to microbes. Microbes then uses the phosphorus to aid them in the nitrogen fixation process and in turn supply plants with nitrogen for plant growth in addition to the phosphorus supplied by the earthworms (Samuel, 1991 and Wurst et al, 2003).
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Jiménez, Juan J., Cepeda, Alex, Decaëns, Oberson, Astrid, and Friesen, Dennis K. 2003. Phosphorus fractions and dynamics in surface earthworm casts under native and improved grasslands in a Colombian savanna Oxisol. Soil Biology and Biochemistry, Volume 35, Issue 5, Pages 715-727. Retrieved October 4, 2009 from ScienceDirect College Edition.
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