What Produces Nutrient-Rich Soil? An Investigation of Earthworms and Their Diet

 

Introduction


This fall when I was helping my family clean up our 6,000-square-foot organic garden for the winter, I saw some earthworms wiggling in the soil. My family depends on earthworms to nourish our garden soil. The worms ingest decaying matter, and their castings, the by-products of their metabolism, enrich the soil. They are nature’s great recyclers because they turn discarded and decaying matter into something useful—fertile soil. I then decided to investigate if one type of earthworm called red wigglers contributes more nutrients to the soil than another type of earthworm called night crawlers. In addition, I decided to investigate whether specific earthworm diets would affect the nutrients found in the soil around the worms. What combination of worms and food supplements would create the most fertile soil? 

This question interested me because the answer would help my family learn how to increase the nutrients in our garden soil. The answer would inform other gardeners about organic ways to enrich soil. The answer would be timely, as organic gardening is on the rise. One source claims a 500% increase in sales of seeds and plants for organic gardening (Mercola). As a matter of fact, my findings are of interest to a local nature reserve that teaches organic gardening practices.

 

Hypothesis

My hypothesis is that if the physically larger worms—night crawlers—are given leaves and grass to eat, then they will improve the nutrient levels in the soil more than any other combination of worms and food supplements. I think night crawlers will contribute more nutrition to the soil than red wigglers because night crawlers are larger, their castings are bigger, and worm castings are what enrich soil. I think that leaves and grass will be the most helpful food supplement because they contain nitrogen, potash, and phosphorus (Starbuck, “Composting”).

 

Background

I am studying two types of earthworms: red wigglers and night crawlers. Red wigglers can eat one-third of their body weight each day (Pocock).They consume any organic material but readily eat plant matter (“What to Feed Red Wiggler Worms”). Their average length is three to four inches long, so they are smaller than night crawlers. They live about one year in the wild, and they usually dig horizontal tunnels (“Red Worms”). Night crawlers can eat half their body weight in a day (“Tell Me—The Nightcrawler”). They eat any organic material but readily consume plant matter.Night crawlers are both longer and fatter than red wigglers. They have been known to grow up to 12 inches in length, but they are on average five inches long. They live about five years in the wild, and they usually dig vertical tunnels (“Tell Me—The Nightcrawler”). Both worms prefer a temperature between 60˚F–70˚F (“Earthworm Facts”). Both of these worms can reproduce when they are about two months old (Pocock) and can produce cocoons that hatch in about three weeks (“Earthworm Facts”).

I am also studying what food supplements result in the best worm castings to nourish the soil. Three types of food supplements are involved in this experiment. The first food supplement is a mix of green grass and brown leaves that would be found naturally on the ground, the earthworms’ habitat. Grass and leaves contain high levels of nitrogen, potash, and phosphorus (“Phosphorus Concerns with Grass Clippings and Leaves”). Another food supplement is a mix of fruits and vegetables that would fall on the ground naturally if they were growing near the earthworms’ habitat. Fruits and vegetables contain high levels of nitrogen and phosphorus (“Phosphorous Concerns with Grass Clippings and Leaves”). Finally, the third food supplement is a mix of eggshells and coffee grounds, which are common ingredients in compost. Eggshells do not contain nitrogen, but they do contain high levels of phosphorus and potash (“Composting”). Coffee grounds contain a high level of nitrogen (“Composting).

Both types of worms breathe through their skin. This kind of respiration requires their skin to be moist. No oily food will be introduced because it could coat their skin and block respiration. No salty food will be introduced because it could dehydrate their skin and prevent respiration.

 

Materials

  • 12 five-gallon water cooler bottles

    Red wigglers


  • Pea gravel
  • Nylon screen
  • Nine large rubber bands
  • Soil
  • Soil testing kit
  • 200 red wigglers
  • 200 night crawlers
  • Water
  • Watering can
  • Measuring cup
  • Kitchen scale
  • Thermometer
  • Three food supplement mixtures:
    • leaves & grass
    • fruits & vegetables
    • eggshells & coffee grounds
  • Four Rubbermaid storage boxes
 

Procedure

5-gallon water cooler bottles filled with soil.


1. I obtained 12 five-gallon water cooler bottles and drilled drainage holes in the bottom of each of them.

2. I cut off the top of each bottle with a jigsaw.

3. I put a thin nylon screen inside the bottle to cover the drainage holes so that the worms could not escape by crawling through the bottom drainage holes.

4. Then I put 7 cm of clean pea gravel into each bottle on top of the nylon screen.

5. I gathered soil and mixed it thoroughly to ensure uniformity. I then tested two samples of this soil for pH, nitrogen, phosphorus, and potassium. I recorded the average of these two sets of test results.

Room where the experiment was housed.


6. I then filled each bottle with this soil.

7. I put 50 red wigglers each in four of the bottles and 50 night crawlers each in four other bottles. I weighed each bundle of 50 worms before I deposited them into the soil. I recorded the weight of each bundle. I also kept four bottles of soil free of any worms at all. The total number of bottles in the experiment numbered 12. The worms that I used were bought from a commercial worm farm.

8. After the worms were safely housed in the bottles, I put a nylon screen over the top of each bottle so the worms could not crawl out of the top of any bottle. These screens were anchored by rubber bands.

9. After the worms made their way under the surface of the soil, which took about an hour, I gently poured four cups of water into each pot to provide the moisture necessary for the worms’ survival.

10. I chose transparent water bottles, so I could observe the worms and their tunnels.

Worm tunnels


However, worms like the dark. Consequently, I drilled ventilation holes in the bottom of four large Rubbermaid boxes and covered the bottles with these large boxes to ensure that the worms would have a comfortably darkened environment. When I wanted to observe the worms, I would lift the boxes off of the bottles and flip on the light in the room. The room in which the worms were housed was kept at a steady 68˚F. A thermometer monitored the room’s temperature.

11. The worms were alternately fed or watered every week. 

12. Each food supplement was fed to the worms in 150 ml quantities. Food supplements were prepared in three different ways. The first kind of food supplement was cut-up leaves and grass that would naturally fall onto the ground. I gathered the leaves and the grass from the area around my garden.

 The second kind of food supplement was crushed-up fruit and vegetables that would fall to the ground if they grew by the earthworms’ habitat. I gathered these fruits and vegetables from the area around my garden.

The third kind of food supplement was smashed-up eggshells and coffee grounds, which are common composting ingredients.

The three food supplements.

The three food supplements.


Walter recording nitrogen, potash, phosphorous and pH levels.

Walter recording nitrogen, potash, phosphorous and pH levels.


13. The water was provided in two-cup quantities, following the initial four-cup soaking. This two-cup amount provided a suitably moist habitat for the worms, but did not drown them. A two-cup watering resulted in a small to moderate amount of clear drainage from the bottles.

14. After 105 days, I removed the top 1.5 inches of soil from each of the 12 pots and discarded it. No worms were found in this discarded topsoil. I then took two independent soil samples from three inches below the new surface of each pot. I took the samples from three inches below the surface because I was interested in studying the nutrients found at root level. I tested the samples for levels of nitrogen, potash, phosphorus, and pH. I recorded the average values of nitrogen, potash, phosphorus, and pH for each pot.

The worms from each bottle are counted and weighed.


15. I then sifted carefully through the soil of each pot and collected the worms. I kept each bundle of worms from each bottle separate from the other bottles’ bundles. I counted and weighed each collection of recovered worms. I recorded these worm measurements for each bottle.

16. After this data was collected, I put the worms in a suitable container with appropriate food and water. They will be maintained in a healthy environment until spring of 2014, when they will be released into my school’s organic garden. 

 

Variables, Sample Size, and Controls

Independent variables: Food supplements and types of worms

Sample size: 50 worms per pot

Controls: Pot with water, night crawlers, and no food supplements; pot with water, red wigglers, and no food supplements; pot with water, eggshells, coffee grounds, and no worms of either kind; pot with water, fruits, vegetables, and no worms of either kind; pot with water, leaves, grass, and no worms of either kind; pot with water, no food supplements of any kind, and no worms of either kind

 

Note: Worms live in non-supplemented soil throughout much of the Earth. The two pots with water, worms, and no food supplements did not endanger the worms. These two pots represented worms living in soil not enriched by additional organic matter.

 

Discussion of Data

I discovered the following facts about the worms:

The pot with the most worms contained eggshells, coffee grounds and night crawlers.


The pot containing fruits and vegetables had the most red wigglers.


1.The pot with the most worms was the pot containing eggshells, coffee grounds, and night crawlers (332 worms). This pot also contained the heaviest worm weight (69 g). One might expect this pot to contain the most fertile soil; however, it was not the pot that held the most fertile soil. This pot’s soil had only an adequate amount of phosphorus and no nitrogen or potash. This surprising discovery may be because the worms may have used some of the nitrogen and potash for their biological needs.

2.The pot with the most abundant population of red wigglers was the pot containing them, fruits, and vegetables (206 worms). This pot also held the heaviest red wiggler weight (43 g). One might expect this pot to contain the most fertile soil with red wigglers; however, it was not the pot that held the most fertile soil. While this pot’s soil had a high level of nitrogen, it had low levels of phosphorus and potash. This surprising discovery may be because the worms may have used some of the phosphorus and potash for their biological needs.

Photo 41

Photo 40

3. The pots that housed no worms but were supplemented with organic matter—leaves, grass, fruits, vegetables, eggshells, and coffee grounds—tended to form mold on their surface soil. These pots also attracted fruit flies. The pots that contained worms never produced mold and were never infested with fruit flies. The absence of mold and flies in the worm pots may be because of the worms’ metabolic activity. The worms may have eaten the food supplement so fast that mold did not have time to grow on the food supplement and there was nothing to attract the flies.

4. Castings from both red wigglers and night crawlers were visible on the soil shortly after food supplements were added.

Worm castings


I discovered the following facts about the soil:

  1. The most fertile soil was found to be in the pot with leaves, grass, and red wigglers. This pot’s soil had high levels of phosphorus, potash, and nitrogen.
  2. Observations about each nutrient follow:

Phosphorus

1. Phosphorus levels increased most significantly in two pots containing worms:

  • The pot with leaves, grass, and night crawlers
  • The pot with leaves, grass and red wigglers

However, the pot with only leaves and grass (no worms of either kind) did not increase in its phosphorus level. This observation suggests that worms are necessary for the release of phosphorus into the soil from leaves and grass.

2. The phosphorus level increased significantly in one pot containing no worms:

  • The pot with fruit, vegetables, and no worms

The pot with only fruit, vegetables, and no worms had more phosphorus in its soil than the pots with fruit, vegetables, and worms of either kind. The worms may have used the phosphorus released from the fruits and vegetables for their biological needs.

3. The untouched soil showed a slight increase in phosphorus from the soil’s initial reading. This increase may have occurred because the untouched soil continued its process of slow decay.

4. Plants that would benefit from the phosphorus-rich soil produced by the leaves, grass, and worms of either kind, or by the fruits and vegetables without worms of either kind, are citrus trees, tulips, and daffodils (Bales, Brickell).

Photo 36

Nitrogen

1. Nitrogen levels increased significantly in eight pots:

  • The pot containing fruits, vegetables, and no worms
  • The pot containing leaves, grass, and no worms
  • The pot containing fruits, vegetables, and night crawlers
  • The pot containing leaves, grass, and night crawlers
  • The pot containing night crawlers without any food supplements
  • The pot containing fruits, vegetables, and red wigglers
  • The pot containing leaves, grass, and red wigglers
  • The pot containing red wigglers without any food supplements

    Nitrogen levels increased the least in three pots:

  • The pot containing eggshells, coffee grounds, and no worms
  • The pot containing eggshells, coffee grounds, and night crawlers
  • The pot containing eggshells, coffee grounds, and red wigglers

These findings may have resulted from the fact that while coffee grounds do contain nitrogen, the other food supplements released their nitrogen into the soil more readily.

2. The untouched soil showed a slight increase in nitrogen from the soil’s initial reading. This increase may have occurred because the untouched soil was exposed to air.

Photo 37

3. Plants that would benefit from nitrogen-rich soil produced in the above pots include lettuce and tomatoes. High levels of nitrogen in the soil can inhibit blooming in some plants, such as nasturtium (Bales, Brickell).

Potash

1. Potash levels increased significantly in four pots:

  • The pot containing fruits, vegetables, and night crawlers
  • The pot containing leaves, grass, and night crawlers
  • The pot containing fruits, vegetables, and red wigglers
  • The pot containing leaves, grass, and red wigglers

The potash levels had a marginal increase in two pots:

  • The pot containing fruits, vegetables, and no worms
  • The pot containing eggshells, coffee grounds, and red wigglers

All other pots showed no increase of potash. These findings suggest that worms are necessary for the easy release of potash into the soil from plant matter such as leaves, grass, fruits, and vegetables.

2. Potash levels in all the pots containing eggshells—an organic matter rich in potash—were low. This surprising observation may be because the potash in the eggshells did not penetrate beyond the surface soil or did not release well into the soil given the time frame of the experiment.

3. Plants that would benefit from potash-rich soil produced in the pots containing leaves, grass, and both kinds of worms and in the pots containing fruits, vegetables, and both kinds of worms include leafy plants like spinach and arugula (Bales, Brickell).

Photo 38

pH

1. pH levels became more acidic in four pots:

  • The pot containing eggshells, coffee grounds, and no worms
  • The pot containing fruits, vegetables, and no worms
  • The pot containing fruits, vegetables, and night crawlers
  • The pot containing fruits, vegetables, and red wigglers

Otherwise, no other pot showed much alteration in the pH level. pH was the variable that changed the least. This finding may be because a change in pH level requires more time than the experiment allowed.

2. Plants that would benefit from more acidic soil produced in all three pots containing fruits and vegetables (with and without worms) and the pot containing eggshells, coffee grounds, and no worms include heathers, foxglove, bleeding hearts, and blue hydrangea (Bales, Brickell).

Photo 39

I discovered the following facts about food supplements:

1. Pots containing leaves and grass had the greatest overall elevation in nutrients.

2. Pots containing eggshells and coffee grounds had the least overall elevation in nutrients.

3. The pot that held pure soil, without either type of worm or any food supplements, had the lowest levels of nutrients. It was deficient in phosphorus and nitrogen, and it was depleted of potash.


Conclusion

My hypothesis was that the night crawlers that were fed leaves and grass would produce the most fertile soil because night crawlers are large worms that would produce large castings. Leaves and grass are rich in phosphorus, potash, and nitrogen, which would produce nutrient-rich castings. However, the most nutrient-rich soil was produced by red wigglers—not night crawlers—that were fed leaves and grass. This finding is based on the greatest relative change in phosphorus, nitrogen, and potash readings compared to the control pot. Night crawlers that were fed leaves and grass also created fertile soil, just not to the same degree as the red wigglers. The combination of worms—either red wigglers or night crawlers—and leaves and grass produced fertile soil. The high nitrogen reading of the leaf-grass-worm combination may be attributed to the worms’ consuming leaves and grass and then producing nitrogen-rich castings that fertilized the soil.

Overall, this experiment demonstrates that any organic matter—be it living worms, decaying leaves and grass, decaying fruit and vegetables, or decomposing eggshells and coffee grounds—will enrich soil when added to it. Healthy soil encourages a healthy worm population and increases plant growth.

 

Addendum

All the worms remain healthy and alive. They will continue to be fed and watered throughout the remaining winter months. Come spring, I will donate the ever-increasing worm population to my school’s organic garden.

 

Possible Errors and Further Research

One limitation could be that I overlooked some juvenile worms during the final count when I was sifting through the soil by hand.

Because this experiment studies organic matter with life cycles, biological systems, and decomposition timelines, a longer study would yield more dramatic changes in all the variables—nutrient levels in the soil samples, worm population, and worm weight. A longer time frame may be especially pertinent to changes in pH.

Further research could include planting seeds in the soil of each bottle to explore whether the most nutrient-rich soil leads to better plant growth. 

 

Bibliography

Bales, Suzanne. Burpee American Gardening Series. New York: Prentice Hall, 1991. Print.

Brickell, Christopher. The American Horticultural Society Encyclopedia of Garden Plants. New York: Macmillan, 1989. Print. 

“Composting.” Eartheasy: Solutions for Sustainable Living, n.p. n.d. Web. 25 Sept 2013.

“Earthworm Facts.” Biology Junction, n.p. n.d. Web. 7 Nov. 2013.

Mercola, Joseph. “The Rise of Organic Gardening.” Mercola.com, n.p., 8 Feb 2014. Web. 9 Feb 2014.

“Phosphorus Concerns with Grass Clippings and Leaves,” NYC Environmental Protection, n.p., n.d. Web. 3 Oct 2013.         

Pocock, Jennifer. “How Vermicomposting Works.” How Stuff Works, n.p., March 2011. Web. 27 Nov 2013.

“Red Worms.” Worm Farm Facts, n.p., 2012. Web. 1 Oct 2013.

Riotte, Louise. Carrots Love Tomatoes. Maine: Story Publishing, 1998. Print.

Starbuck, Chris. “Grass Clippings, Compost and Mulch: Frequently Asked Questions.” University of Missouri Extension, Department of Horticulture, n.d. Web. 3 Oct.    2013.

Starcher, Allison Mia. Good Bugs for Your Garden. Chapel Hill: Algonquin, 1995. Print.

“Tell Me—The Nightcrawler.” Discovery Kids, Discovery.com., n.d. Web. 2 Oct 2013.

“Vermicomposting.” National Institute of Environmental Health Sciences, n.p. n.d. Web. 30 Nov 2013.

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