A hay infusion is a jar of water with some dried grass or hay soaked in it. Within days it fills with bacteria, and then with the single-celled organisms that eat them. If you’ve ever wanted to see a Paramecium swimming, a Vorticella snapping on its stalk, or a whole microscopic food chain develop week by week, it’s one of the easiest and cheapest projects in amateur microscopy.
Why it works
Dried hay carries huge numbers of dormant microbes: bacterial spores, and the resting cysts of protozoa that dried out in the field. When you soak the hay, nutrients leach into the water. Bacteria feed on them and multiply fast. Protozoa wake from their cysts and feed on the bacteria. Larger protozoa, rotifers and small animals then feed on the smaller ones. The result is a small, changing ecosystem you can sample with a pipette.
What you need
- A clean glass jar, about 0.5–1 litre
- A handful of dry hay or dried grass. Pet-shop timothy hay works, as does long grass dried in the sun. Avoid anything treated with weedkiller or pesticide.
- Water: rainwater, bottled spring water, pond water, or tap water left to stand for a day or two
- Optional: a little pond, ditch or puddle water to add extra organisms
- Optional: a few grains of uncooked rice or wheat
- A loose cover such as a saucer, cloth or a piece of mesh
- A pipette, slides and coverslips, and a microscope with roughly 40x–400x magnification
Step-by-step recipe
- Prepare the hay. Cut or tear a small handful, enough to loosely fill a few centimetres at the bottom of the jar, into short pieces.
- Make the infusion. There are two common methods:
- Boiled method: simmer the hay in about half a litre of water for 10–15 minutes, then let it cool completely. Boiling releases nutrients quickly. Hardy bacterial spores, such as those of Bacillus subtilis, the “hay bacillus”, survive, but many protozoan cysts may not. That’s why the next step matters.
- Unboiled method: simply soak the hay in cool water. It’s slower to get going, but more of the original organisms in the hay survive.
- Add a starter of living organisms. Pour in a little pond, ditch or puddle water, especially water from near plants or sediment. This adds protozoa and makes success much more likely, particularly with the boiled method.
- Cover loosely. Use a saucer or cloth to keep out dust and insects while still letting air in. Don’t seal the jar tight: many of the organisms you want need oxygen.
- Put it somewhere sensible. Room temperature is ideal, somewhere bright but out of direct sunlight, which can overheat the jar and encourage algae to take over. A shelf or windowsill that gets no direct sun works well.
- Wait, and look regularly. Check every couple of days at first, then every few days.
Variations
- Rice or wheat grains. A few grains of uncooked rice or wheat added to pond water make a slower-releasing food source. Many people use this to keep cultures going for longer.
- Lettuce or leaf infusion. Dried lettuce leaves or other dried plant material can be used in the same way.
- Pond water alone. Pond water left to stand with a little plant material often develops a similar community.
- Several jars. Set up two or three jars with different water sources or methods and compare them. It’s a good school or family experiment.
What happens over the following weeks
Every infusion is different. Timings depend on temperature, the hay, the water and the starter you used, so treat these as rough guides, not a schedule.
| Roughly | What you may notice | What’s in the water |
|---|---|---|
| First 1–3 days | Water turns cloudy and yellowish; a thin film forms on the surface; it starts to smell | Bacteria multiplying rapidly; the surface film is largely bacteria |
| About days 2–7 | Cloudiness may continue | Small flagellates and small ciliates such as Colpoda feeding on bacteria |
| About week 1–2 | Water may start to clear a little | Larger ciliates such as Paramecium, often in large numbers |
| About week 2–4 and beyond | Water clearer; green tinge if light reaches it | Stalked ciliates like Vorticella, amoebae, rotifers, predatory ciliates, algae; the mix keeps changing |
This change over time is called succession. A popular project is to sample on the same days each week and record what’s most common. Scientists use similar methods to study how microbial communities develop.
How to sample and view
Different organisms live in different parts of the jar, so sample in several places:
- The surface film. Touch a slide or coverslip flat onto the surface, or skim it with a pipette. It’s often rich in bacteria and the protozoa grazing on them.
- Around the hay. Take a drop close to a piece of hay, or squeeze a strand onto a slide. Attached organisms such as Vorticella live here.
- The bottom. Draw up a little sediment for amoebae and crawling organisms.
- Mid-water. Free-swimming ciliates are found throughout.
Then make a wet mount: one drop on a slide and a coverslip lowered at an angle. Our guide to making a wet mount without air bubbles explains how. Start at 40x to find things, and move to 100x and 400x for detail.
To slow fast swimmers down:
- Add a few wisps of cotton wool to the drop. The fibres form a maze that traps them.
- Let the drop partly evaporate so they’re confined.
- Use a commercial slowing agent such as methyl cellulose, sold for this purpose.
To see internal structures, try very dilute stains. A trace of methylene blue can make nuclei and other structures stand out. See our methylene blue staining guide. Strong stain will kill the organisms.
What you might see
| Organism | Approximate appearance | Movement |
|---|---|---|
| Bacteria | Tiny dots, rods or spirals at 400x; detail needs 1000x | Jiggling or darting |
| Small flagellates | Very small cells with one or more whip-like flagella | Jerky, darting swimming |
| Colpoda | Kidney-shaped ciliate | Tumbling, spinning |
| Paramecium | Slipper-shaped ciliate covered in cilia, often roughly 0.1–0.3 mm long | Fast, spiralling swim; backs up when it bumps into things |
| Vorticella | Bell-shaped cell on a thin stalk attached to debris | Stalk coils up suddenly, then slowly extends |
| Amoebae | Irregular, shape-changing cells | Slow creeping with flowing extensions (pseudopodia) |
| Euglena and other algae | Green cells; Euglena is spindle-shaped with a red eyespot | Swimming or gliding |
| Rotifers | Multicellular animals with a spinning crown of cilia | Swimming, or creeping like an inchworm |
For more on identifying what you find, see viewing microorganisms under a microscope. Some things that look like organisms are actually bubbles, fibres or debris; our guide to microscopy artifacts helps you tell the difference.
Keeping the culture going
A single jar usually peaks and then declines as food runs out and conditions change. To keep things going:
- Feed it lightly. Add a few more rice or wheat grains, or a small pinch of fresh hay, every week or two. Overfeeding causes a bacterial bloom, bad smells and low oxygen, which can crash the protozoa.
- Top up with stood water to replace what evaporates.
- Subculture. When you have lots of Paramecium or another favourite, transfer a few pipettes of that water into a fresh jar of cooled, prepared infusion. This is how many classrooms keep cultures running.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| Only bacteria, no protozoa after a week or more | Boiling killed most cysts and no starter was added; or it’s too cold | Add a little pond or puddle water; move somewhere warmer (normal room temperature) |
| Very strong, rotten smell | Too much hay or food; low oxygen | Use less hay; keep the cover loose; start a new jar with less material |
| Water turned bright green | Algae, from too much light | Move out of direct light; algae are interesting to look at too |
| Mould on the surface | Too much plant material at the surface | Remove the mould; push hay under the water; use less next time |
| Lots of organisms, then suddenly none | Food ran out, or a predator bloomed | Subculture earlier; feed lightly |
| Mosquito larvae wriggling in the jar | Jar left uncovered outdoors or near an open window | Cover with fine mesh or a cloth; dispose of the contents if larvae appear |
Safety and disposal
A hay infusion is a culture of whatever microbes happened to be in the hay and water. Most are harmless, but treat it sensibly:
- Don’t drink it or touch your face while working with it. Wash your hands afterwards, and cover any cuts.
- Use a pipette bulb, never your mouth, to draw up samples.
- Keep it away from food areas, and out of reach of young children and pets.
- People with weakened immune systems may prefer to stick to prepared slides or ask someone else to handle cultures.
- Cover the jar so it doesn’t breed mosquitoes or attract flies.
- Dispose of it carefully. When you’re finished, many people add a little household bleach, leave it for a while, then pour it down a toilet or drain with plenty of water. Wash the jar thoroughly with hot, soapy water. Follow any rules your school or local area has.
For more living specimens to explore, try hunting for tardigrades in moss or watching planaria under a microscope. A hay infusion also makes a great project for children; see what kids can look at under a microscope for more ideas.