Did you know that a potted plant can lose mass even when some of the escaping water never passed through its leaves? The top of the soil may be drying too. A scale weighs the entire arrangement; it cannot name the route each escaping water molecule took.
Transpiration is the loss of water vapour from plants, mainly through leaf stomata. Soil evaporation is water vapour escaping directly from the soil. Their combined effect is called evapotranspiration. A decrease in the mass of a plant pot can reflect both processes, and potentially drainage or other losses. To interpret the result, students must define what is weighed, control inputs and outputs, and examine comparison groups rather than naming every gram ‘transpiration’.
What does the balance actually measure?
Imagine a class places a damp potted plant on a balance at 9 a.m. and again 24 hours later. The display is lower. Some water could have left through the leaves, some evaporated from uncovered soil, and some may have dripped away. A fallen label, splash of water or removed leaf could also change the mass. The observation is a net change in the weighed system, not automatic proof of its biological cause.
The Food and Agriculture Organization explains how evaporation from soil and transpiration from plants combine in evapotranspiration; it also explains why those processes are hard to separate. The US Geological Survey account describes how water enters plant roots, travels through tissues and escapes into the air. These scientific definitions are a useful starting point, but a classroom experiment still needs controls.
The educational task is to move from the tempting answer—’my plant transpired 40 grams’—to the more precise question: ‘What processes and objects could explain the 40-gram change, and what comparison would help distinguish them?’ This is evidence-based reasoning in everyday form.
Design the investigation before reading the number
A reasonable classroom question is, ‘How does covering the exposed soil change the net mass loss of a potted plant over one day?’ That question matches a possible intervention. A stronger question, ‘Exactly how much did the plant transpire?’ may require specialist methods, greater measurement precision and modelling that a simple kitchen scale cannot supply.
Use stable, healthy potted plants and waterproof trays. Weigh the entire pot-and-tray combination at both times so drainage captured by the tray is not lost from the measured system. Avoid pouring water, trimming foliage, moving labels or changing pots during the interval. If watering is unavoidable, measure it. Do not seal the leaves; any soil cover should be placed only across the exposed soil surface without constricting the stem. Adult supervision protects the plants and pupils.
Use a comparison with similar damp soil but no plant, and where possible several similar planted pots. Control light, temperature, airflow, size of exposed soil, weighing time and watering as best you can. A covering can itself alter humidity and soil conditions, so it is a helpful intervention, not an infallible switch that turns soil evaporation off.
Worked example: what four invented measurements actually prove
Invented teaching data, not a real class result. Assume each arrangement, including pot and tray, is weighed 24 hours apart; no water is added, no liquid escapes the tray, and no objects are removed. The four arrangements are separately measured and do not have exactly equal starting masses.
- Planted, soil uncovered: 1,250 g initially and 1,190 g finally; net mass loss = 60 g.
- Planted, soil covered: 1,240 g initially and 1,196 g finally; net mass loss = 44 g.
- Soil only, uncovered: 1,110 g initially and 1,094 g finally; net mass loss = 16 g.
- Soil only, covered: 1,090 g initially and 1,087 g finally; net mass loss = 3 g.
For each arrangement use initial mass − final mass. The planted uncovered pot loses 1,250 − 1,190 = 60 g in 24 hours. Its average rate is 60 ÷ 24 = 2.5 g per hour. The covered planted pot loses 44 ÷ 24 ≈ 1.83 g per hour. These describe the whole weighed system; they are not measured transpiration rates.
The uncovered soil-only pot loses 16 g, showing that a pot without a plant can lose mass during the same interval. The covered soil-only pot loses 3 g, reminding us that soil covering may reduce rather than perfectly eliminate evaporation or other losses. Such controls can strengthen the interpretation, but they do not make the four arrangements identical.
The difference between uncovered and covered planted pots is 60 − 44 = 16 g. It is reasonable to investigate whether exposed soil contributed to the difference. It is not justified to announce that the plant itself transpired exactly 44 g, or that 44 − 3 = 41 g is a precise measurement of transpiration. Plant size, surface wetness, soil condition and covering effects may differ.
A careful conclusion reads: ‘In this illustrative comparison, the uncovered planted arrangement lost more mass than the covered one. Both soil-only arrangements also lost mass. The observations are consistent with soil evaporation contributing to total system loss, but do not completely isolate leaf transpiration.’ Check every subtraction by adding the claimed loss to the final mass: 1,190 + 60 = 1,250, and similarly for the other arrangements.
Diagnose the learner’s mental model
Show a student this statement: ‘A pot went from 900 g to 880 g, so 20 g passed through the leaves.’ Ask whether it is proven, and why. A learner who agrees has confused an observation with a mechanism. A learner who disagrees but offers no alternative needs to trace the soil, plant, pot and tray with arrows. A learner who says only ‘evaporation’ may need to distinguish plant transpiration from direct surface evaporation.
Try a second question: ‘If an identical-looking empty pot of damp soil loses 5 g, may I subtract exactly 5 g from the planted pot’s loss?’ The better answer is ‘not automatically’. Roots, shading and water movement can change the soil conditions. Comparison groups help evaluate explanations, but a good student also states their limits.
How to record measurements so they can be checked
- Write the boundary: include the plant, soil, pot, tray, cover and labels that stay on the balance.
- Write start and finish: include the date, time, elapsed hours and the same balance resolution.
- Record water inputs: account for watering, rain, misting or accidental spillage.
- Capture drainage: keep any drainage in the weighed tray or measure it separately.
- Note conditions: light, airflow and soil-covering method influence the interpretation.
- Use honest units: report grams lost over the measured interval, with sensible precision.
For basic experiments, a table with arrangement, start mass (g), end mass (g), water added (g), duration (h), change (g) and observation notes is enough. If watering occurs, a simple conservation-style balance under stated assumptions is output mass = starting mass + input mass − final mass. This gives an overall output estimate only when other mass transfers are negligible or known; it does not tell how much left via leaves.
Progressive practice: build the method, then remove the hints
Guided calculation
A plant-pot system starts at 1,360 g and ends at 1,327 g after 24 hours. The answer is 33 g net mass loss. Ask the learner to reconstruct 1,327 + 33 = 1,360 g. The complete sentence should use ‘net loss from the weighed arrangement’ rather than claiming ’33 g transpiration’.
Supported comparison
A soil-only setup goes from 950 g to 938 g over the same period: it loses 12 g. The planted setup loses 33 g. Discuss why 33 − 12 = 21 g is a tempting but only approximate plant-associated difference under matched conditions, not an independently observed exact transpiration value. Ask the learner to name two differences between the arrangements that might matter.
Independent transfer
Now suppose another system starts at 1,200 g, receives 40 g of water during the trial and ends at 1,205 g. Under the explicit assumption that all other mass changes are negligible, estimated output is 1,200 + 40 − 1,205 = 35 g. The mass balance is correct. The pupil must still explain that drainage, soil evaporation and plant transpiration cannot be separated by this total alone.
Change the setting again to a tray of damp cloths or an uncovered bowl. Ask learners to draw a new system boundary and distinguish observed changes from inferred causes. This tests transfer, rather than memorising the word ‘evapotranspiration’.
Checking for observable progress
A good four-part exit check asks the student to: (1) use grams and an actual interval, (2) explain at least two possible causes of loss, (3) propose a relevant comparison or control, and (4) write a conclusion no stronger than the evidence. Repeat with a different numerical example days later. Track whether the student can do these steps independently, with a prompt, or only after a model answer.
Parents can ask the short question, ‘What else could change the scale?’ Teachers can let groups identify competing explanations before weighing starts, then compare their predictions with the observed result. Do not turn the task into a competition to dry out plants: avoid plant stress, unsuitable coverings or long, unmonitored experiments.
Limitations and safety
Kitchen balances may change by 1 g at a time; small changes near that resolution should not be overinterpreted. Air movements, a damp tray, a warm pot or a balance on an uneven surface can reduce consistency. Young plants can differ in leaf area and physiological state. In research, specialized weighing lysimeters and water-balance analyses can resolve components more rigorously, but even these require careful assumptions.
FAO’s discussion of evapotranspiration and measurement distinguishes rigorous field estimation from simplified classroom observation. This activity helps learners understand why controls matter; it is not a method for calculating exact agricultural irrigation needs.
Frequently asked questions
Does plastic covering remove all soil evaporation?
Not necessarily. The cover may leak, moisture can leave around the edges, and covering changes the environment. Treat the covered arrangement as a comparison, not a proof of zero evaporation.
Does every lost gram equal a millilitre of water?
Pure liquid water near room temperature has density close to 1 g/mL, but this is an approximation and the measured mass loss may include other changes. Grams are the directly observed quantity, so keep them in the main calculation.
Can a plant gain mass while losing water?
Yes, depending on watering, growth and the chosen system boundary. A net gain does not prove no water left; it may simply mean inputs exceeded outputs.
Why repeat rather than rely on one pot?
Plants and pots vary. Replication helps reveal variation and unexpected results, although it does not automatically remove every systematic difference.
Related learning and the next useful question
For another careful study of biological measurements, read How to Compare Seedling Growth Rates When Observation Days Are Uneven. The eduKate Education learning guide to formative assessment explains what a teacher should do after diagnosing the misconception. The immutable Clementi Mathematics teaching reference models the practice of explaining before drilling.
The core aim of this experiment is not to make a scale produce a dramatic result. It is to let a learner separate observation from interpretation, use mass conservation without inventing missing data, recognise alternative explanations, and carry that reasoning into a new investigation.