Biology IA · C · Interaction and interdependence · topic 20 of 25

Tea bags: how much soil moisture do decomposers need? · IB Biology IA idea: guide, sample graph and step-by-step plan

slow, honest fieldwork. examiners like it.

Bury a tea bag. Come back in four weeks. The soil has eaten part of it, and the missing mass is your data.

Difficulty●●○ Medium
KitFieldwork: tea bags, balance, drying oven or similar, trowel, flags
TimeAbout 10 hours, plus 4 weeks of burial
FitsSL and HL
  • ✓ you like fieldwork that runs by itself while you wait
  • ✓ you can get to the same plot every few days
  • ✓ you want to measure what decomposers do, without handling animals

The big idea

← make it a fair test!

Decomposers, mostly bacteria and fungi, break down dead organic matter and return nutrients to the soil. They need water, but too much water starves them of oxygen. So moisture may help up to a point, then hurt: a hump-shaped relationship you can test. A tea bag is a standard, tiny package of dead plant material, so you can compare plots fairly. Your task is to make the comparison fair and replicate it, because one plot is one data point, not a trend.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Shade versus sun

Choose pairs of plots in the same soil, one shaded and one sunny, and measure the soil moisture as well.

→ Your independent variable is the setting, but you still measure moisture as the mechanism; the analysis compares two groups, not a trend, and needs a suitable test.
Two kinds of tea

Bury green tea (breaks down quickly) and rooibos (slowly) side by side.

→ You get two dependent variables; you can compare how moisture affects each, and the discussion can link to how easily the material is digested.
Vary depth

Bury bags at three depths in one plot.

→ The independent variable is depth; you have to measure moisture and temperature at each depth and avoid disturbing the soil layers.
Use a watering gradient

Choose neighbouring plots on one site and water them at different, recorded amounts, with permission.

→ You control moisture rather than finding it, so cause is easier to argue; you must check the soil's real moisture, not only the volume of water you added.
Follow it through time

Dig up a few bags every week, not all at the end.

→ You get mass remaining against time, a decay curve you can fit; you need more bags and a bigger spreadsheet.
Change the context

Compare a compost heap, a lawn and a flower bed, or link to farming or landfill.

→ Gives your introduction a real purpose; the method stays the same, but the plots now differ in more than moisture, which you must discuss.

Your Biology IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable and says how soil moisture is measured (and when, because it changes over four weeks).
  • Names the dependent variable as a percentage of the starting dry mass, and says how the starting dry mass is found.
  • Gives the number of plots or sites per condition and the number of bags in each, with how they are spread out.
  • Names what stays the same: tea type, bag type, depth, burial time, drying temperature and time.
  • Is small enough to bury, retrieve, dry and weigh everything within the time you have, with a spare for lost bags.

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The set-up

One way to lay it out. Draw your own, with your site, your depths and your plot sizes, for the Method section.

dplan viewdrymoderatewetflag so every bagis found againsoil sample: weigh,oven-dry, weighbag at the same depth d everywhere5+ bags per plot,spread out, not in a row

How your Biology IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why decomposition matters for nutrient cycling; why you expect moisture to matter and in which direction.
Background biology~400
Decomposers and what they need; how water and oxygen change in wet soil; how a tea bag stands in for dead plant material (cite the method).
Variablestable
Independent, dependent and controlled variables; how each is measured or held constant.
Method~500
Site and permission; how plots were chosen; tea type and starting mass; depth; how many bags per plot; how soil moisture was measured; how the bags were dried and cleaned before weighing.
Safety, ethics, environment~100
Permission for the site; digging safety; gloves and hand-washing after handling soil, with cuts covered; leaving no bags behind; what you did about the bag material, which may be plastic.
Raw datatable
Starting dry mass and final dry mass for every bag, soil moisture for every plot, with units and balance uncertainty.
Processing~300
Percentage mass lost for each bag, then the mean and spread per condition; one worked example; the correction for the tea's own moisture.
Graphs and analysis~350
Mass lost against soil moisture with SD error bars; the model you choose (line? curve with a peak?); Spearman's rank or a regression on plot means, with plots (not bags) as the replicates.
Conclusion~400
Answer the question with values and spread; compare with published decomposition rates and say whether your range of moisture is wide enough to claim a peak.
Evaluation~600, the biggest part!
Weaknesses ranked by their effect on the mass loss: spread between bags, soil on the outside, drying, moisture changing during the four weeks, site differences besides moisture.
Referencesnot counted
Method, theory and any decomposition data you compare with.

What your Biology IA graph could look like

replicate spread, not just the mean

Made-up data, to show you the form of a strong graph, not the answer. Yours may be a peak, a slope or nothing at all. example only

051015202530354045051015202530354045Soil moisture / % water by mass (mean of 3 samples, ± 1 %)Mass lost after 4 weeks / % (plot mean ± SD of 5 bags)curve with a peakstraight line (does it fit?)one plot per point: mean of its bags, with SDa peak, or just scatter?check the error barsILLUSTRATIVE · MADE-UP DATA
  • Axes name the quantity, the unit and what the uncertainty is.
  • Error bars are the spread of the replicate bags, not the balance resolution.
  • Moisture is the mean of samples across the burial period, because it changes.
  • A curve with a peak is a bigger claim than a line. Say what the data can support, and what range.
  • Another graph to consider: the soil moisture for each plot at each visit, to show how much it moved.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result, the percentage of mass lost, not on the raw weighings. Numbers here are for a made-up bag that started at 1.80 g of dry tea.

Spread between bags (SD, n = 5)4 ppSoil left on the outside (±0.05 g)2.8 ppNot fully dry (±0.04 g)2.2 ppStarting dry mass (±0.02 g)1.1 ppBalance (±0.01 g, two weighings)0.8 ppbiggest effect: discuss this first!uncertainty in % mass lost, in percentage points (pp)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictFind the standard tea bag method and what is known about moisture and decomposers. Write down the shape you expect: rising, falling, or a hump, and why.
  2. Trial run (1 week)Bury a few bags and dig them up. Can you clean and dry them reliably? Do they tear? How much soil sticks? Can you find them again? How long to reach constant mass?
  3. Choose the site and the conditionsGet permission. Decide how you will get a range of moisture and how many separate plots you can have in each condition. Write down why.
  4. Decide the detailsTea type, starting mass, depth, number of bags per plot, drying temperature and time. Plan to bury a few extra: some will be lost.
  5. Weigh and buryDry and weigh the starting bags (or a matching set for the dry mass), label, flag and record the position and moisture of each plot.
  6. Monitor the moistureTake soil samples at several visits. Weigh, oven-dry, weigh again. Moisture changes over four weeks, and your RQ depends on it.
  7. Retrieve, clean, dry, weighDig up all bags, remove soil from the outside, dry to constant mass (weigh twice until it stops changing) and record everything.
  8. Process and graphPercentage mass lost per bag, then mean and SD per condition. Plot against moisture with error bars and choose a model and test with reasons.
  9. Conclude and evaluateAnswer with values, compare with published data, and rank weaknesses by their effect on your result.
  10. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Biology you'll need

Traps to avoid in this Biology IA

  • One plot per condition: one data point. The spread between bags in one plot is not the spread between sites.
  • Weighing damp bags, or bags with soil on the outside, so the mass lost includes water and dirt.
  • Measuring moisture once, on the day of burial, when it changes over four weeks.
  • Comparing a shaded plot and a sunny plot and calling the difference moisture, when temperature and soil also differ.
  • Not counting how many bags you lost, and quietly dropping them.
  • Burying bags with plastic mesh that you cannot find and collect again.
  • Treating the starting mass as dry when the tea contains moisture.

More Biology IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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