Investigating the response of worms to soil improvers
Class practical
This protocol assesses the reaction of worms to compost or soil with and without an added soil improver to see if they avoid, prefer, or are indifferent to its presence in the soil. It is a simple procedure with scope for more thorough statistical analysis of the results.
Biochar has been chosen as the soil improver to work with because its use has recently been proposed as a contributor to reducing atmospheric carbon dioxide. See the background information below.
Lesson organisation
Explain in outline what the value of biochar might be. Explain that worms are essential to healthy soil, and hence to healthy crops. Therefore, it is important to know if biochar (or any other soil improver) is harmful to worms before adding it to large areas of land.
Set up containers with compost or soil on one side of a separator, and compost or soil containing biochar on the other. Remove the separator and add worms. Cover the containers and leave them until the next lesson. Replace the separator carefully, and count the worms in each section of the container. Analyse the data to find out if the worms are influenced by the presence of the biochar.
Apparatus and Chemicals
For each group of students:
Worms, 10
Soil:biochar mix, two concentrations of biochar, enough of each type to fill one side of the container
Container, 1
Separator, 1
Lid, 1
Marker pens
Labels
For the class – set up by technician/ teacher:
Worms (Notes 1 and 2)
Biochar, ground to fine granules (1-2 mm), (Note 3)
Mixtures of soil (or garden compost) with differing amounts of biochar, for example, adding at 4% and 2% by mass or none added.
Containers, flat and round, approx 10 cm across and 10 cm deep
Separators, cardboard or plastic sheet cut to size to fit snugly into the container and divide it in half.
Lids for the containers, with holes to allow ventilation (Note 4)
Health & Safety and Technical notes
Take appropriate good hygiene precautions after handling soil and earthworms. Make sure soap and hot water are available in the room for hand-washing. Avoid inhaling the dust when grinding the biochar. Moisten after weighing and before grinding to reduce dust in the air.
1 The worm species Eisenia fetida and Eisenia andrei, commonly known as redworms, brandling worms, tiger worms, and red wigglers can be used. Both species are used for vermi-composting and can be obtained from various suppliers, or from a nearby worm compost bin. Suppliers include Wiggly Wigglers and VermiSell. A common type of worm used for this test is the white worm (Enchytraeus albidus). It is widely used as a live aquarium fish food, and can be bought where aquarium supplies are sold, or on the internet.
2 Handling worms safely (from the worms’ perspective): keep the worms in a humid, dark container, with some sphagnum moss (not sphagnum moss peat).
Handling worms safely (from a student perspective): ensure hand-washing facilities are available, and that students wash their hands thoroughly before leaving the lab after handling worms and soil.
3 Use any locally sourced charcoal made from timber (available from most good DIY stores) as biochar. Grind in a mortar and pestle or coffee grinder (not later used for coffee) to make 1-3 mm granules before mixing with your compost or soil. Refer to CLEAPSS Hazcard 21 for hazards of powdered carbon. Finely powdered charcoal is a fire hazard. Dampen after weighing and before grinding to reduce the risk, and avoid inhalation of the dust. If you, or any member of your school team, are gardeners who regularly make bonfires of garden waste material, you could try gathering charcoal from ash heaps for comparison.
4 If you do not have lids to fit the containers, cover each container with plastic film or a plastic bag – again with small holes for ventilation.
5 Do not water the soil after removing the separator, as the materials might mix together.
Ethical issues
Teachers should be careful to introduce these animals in a way that promotes a good ethical attitude towards them, and not a simply instrumental one. Although they are simple organisms that may not 'suffer' in the same way as higher animals, they still deserve respect. Animals should be returned promptly to their original environment after the investigation. Composting worms from a supplier can be added to any compost bin or to garden soil. Whiteworms from aquarists can also be returned to any soil if surplus is not wanted as fish food. This supports ethical approaches that are appropriate to field work where collected animals are returned to their habitat after observations have been made, although worms returned to soil in winter may not survive.
Procedure
SAFETY:
After handling soil or worms, wash hands with soap and hot water and dry thoroughly. Avoid inhaling the dust when grinding the biochar. Moisten after weighing and before grinding to reduce dust in the air.
Preparation
a Collect containers and make suitable separators.
b Mix biochar into soil (or compost) to make different mixtures with different amounts of biochar. Try adding 4% or 2% biochar by mass, or no biochar.
c Collect worms to use in the investigation (Notes 1 and 2).
Investigation
a Place the separator in the container. Mark the position of the separator on the edge of the container, so you can insert it later in exactly the same position.
b On one side of the separator place soil (or compost) only, and on the other a soil:biochar mixture. Use the same amount of soil as soil: biochar mixture. The soil on both sides should be equally moist but not saturated (Note 5).
c Remove the separator, and place 10 worms along the line where the separator was.
d Cover with a lid to prevent drying out (Note 4), and place the container in an area where normal room temperature is maintained for 48 hours.
e After 48 hours, replace the separator in the same position as before. Make careful observations of the soil or soil:biochar mixture. Count the number of worms on each side of the separator.
f Collate the class results. Perform a statistical test (such as X2) to demonstrate significance in the results if desired.
Teaching notes
This procedure is described in the International Biochar Initiative Guide to conducting biochar trials as the ‘Worm avoidance’ test. It would be interesting to run this test in addition to a soil fertility test, to find out if the results support or contradict one another.
If the worms have avoided the side of the container where biochar was applied, it should not be used on a large scale without further investigation. If the worms are indifferent to the biochar, it is probably safe to proceed with its use and investigate its effects further. Discuss whether the results of this one investigation are sufficient to make a final conclusion about biochar, and how you could investigate the worms' reaction further. Is it possible that the particular charcoal you have used is a problem for the worms, but that other kinds would not be? Is it possible that the soil or compost you have used is so comfortable for the worms that adding biochar doesn’t make much difference? Is it possible that you have added too much, or too little biochar? Is it possible that what happens in the container is not a good model of what might happen in the open soil?
Background information
Biochar is a fine-grained, highly-porous charcoal produced from carbon-rich biomass feedstocks, including forestry waste, animal manures and agricultural waste products such as husks, shells and stover. (Stover is the dry stalks of crops such as corn, sorghum or soybean that remain in a field after harvest.)
Biochar is produced by pyrolysis or gasification (heating the biomass with little or no air) in a process similar to the production of charcoal.
However, the primary use of biochar is as a soil improver rather than a fuel. Biochar has been shown to add value to soils in terms of fertility, particularly to acidic soils. The exact details of the mechanism of action are not fully established, but it may act by filtering out and retaining nutrients from percolating water in the soil or by changing crumb structure.
Biochar shows a mean residence time in the soil in the range of hundreds to thousands of years. This makes it a long-term carbon store (compared with uncharred biomass) and so a long-term contributor to climate change mitigation.
Some wastes proposed as feedstock for making biochar are currently burned or composted. This quickly returns part or all of their carbon to the atmosphere. In the case of anaerobic composting or landfilling, methane is produced which is a more potent greenhouse gas than carbon dioxide.
Clearly, however valuable it is as a carbon store, biochar would not be a good soil improver if it causes a significant negative disturbance to the soil ecosystem.
Health & Safety checked, October 2009
Web links
www.biochar-international.org
The website of the International Biochar Initiative – a registered, non-profit organization supporting those committed to supporting sustainable biochar production and utilisation systems that remove carbon from the atmosphere and enhance the earth’s soils. From here you can download the Guide to conducting biochar trials from which the practical idea above has been adapted.
www.iso.org/iso/catalogue_detail.htm?csnumber=38402
Note: A standardized methodology for this test is available from the International Organization for Standardization (ISO 17512-1:2008), and can be downloaded from the internet for a fee.
www.bioregionalhomegrown.co.uk
This is the website of BioRegional, a commercial company that produces locally grown biochar. It also explains in simple terms the thinking behind the advantages (environmental and in terms of sustainability) claimed for locally grown charcoal over other kinds.
(Websites accessed October 2011)