3.2.2Unit 3 — Life on Earth
Measuring Abiotic Factors
5 min read 6 parts Lesson 8 of 31
Section 1 of 6Why Measure Abiotic Factors
Why Measure Abiotic Factors
Link to distribution
- To investigate how an abiotic factor affects distribution, it must be measured at every study site.
Specialised equipment
- Meters, probes and indicators give more precise values than estimates made by eye.
Reducing error
- Careful technique at every site makes the readings comparable from one place to another.
Fair comparison
- Measurements at different sites should be taken at the same time of day and year.
Section 2 of 6Methods of Measurement
Light intensity
- An electronic light meter is pointed at the region being assessed to give the light level.
Soil moisture
- Weigh a moist soil sample, dry it and reweigh it;
- the loss in mass is the water content.
- A moisture meter can also be used.
pH
- An indicator solution mixed with a soil or water sample, or an electronic pH meter, gives the pH.
Temperature
- A thermometer or an electronic temperature probe measures the temperature of the air, soil or water.
| Factor | Method |
|---|---|
| Light intensity | Light meter |
| Soil moisture | Dry and reweigh, or moisture meter |
| pH | Indicator or pH meter |
| Temperature | Thermometer or probe |
Section 3 of 6Minimising Error
1
For light, aim the meter at the exact area being studied and avoid shading it with your body or creating reflected light.
2
For moisture, take samples from a standard depth and clean any probe between uses.
3
For pH, keep the ratio of soil to water constant and wait the same time before each reading.
4
For temperature, keep the sensor out of direct sunlight when measuring the air.
5
For all electronic probes, wait until the value has stabilised before you record it.
Section 4 of 6Improving Reliability
Improving Reliability
Repeat readings
- Take several readings at each site and find the mean, which reduces the effect of any single odd value.
Mix samples
- For pH, mixing several soil samples from one site reduces local variation.
Same conditions
- Take all readings at the same time of day, because light and temperature change through the day.
Calibration
- Check that meters are working correctly before use so that readings are trustworthy.
Section 5 of 6Worked Example
Question
- A moist soil sample has a mass of 50.0 g.
- After drying its mass is 38.0 g.
- Calculate the percentage of the moist soil mass that was water.
- Mass of water = 50.0 − 38.0 = 12.0 g.
- Divide by the mass of moist soil: 12.0 ÷ 50.0 = 0.24.
- Multiply by 100 to give a percentage.
Answer 24% of the mass of the moist soil was water.
Section 6 of 6Exam Checklist & Tips
3.2.2 – Key Points for the Exam
- Abiotic factor A non-living factor that affects living organisms.
- Light meter An instrument that measures light intensity.
- pH A scale showing how acidic or alkaline a substance is.
- Reliability How consistently repeated measurements agree with each other.
Common Mistakes
- Casting a shadow over the light meter while taking the reading.
- Recording a value before the probe reading has stabilised.
- Taking soil samples from different depths at different sites.
- Measuring abiotic factors at only one site when several sites are being compared.