Radish dry mass at five fertiliser levels · Concept Studio

The idea

Plants make sugars by photosynthesis but need nitrogen from the soil to build proteins, phosphorus to build nucleic acids and ATP, and potassium for growth, so dry mass rises with nutrient supply until something else limits growth.

What you need

radish seed, about 150
20 identical small pots filled with a 50:50 mix of peat (or coir) and vermiculite, which is very low in minerals
slow-release N:P:K 14:14:14 fertiliser pellets (the SAPS specification), 60 (0 + 1 + 2 + 4 + 8 pellets for each set of five pots, four sets)
light bank for constant illumination, in a warm room
balance reading to 0.01 g, paper bags, drying oven set to 90–95 °C (the SAPS method)
gloves and P2 dust masks

How to do it

Set up four replicate pots at each of five levels: 0, 1, 2, 4 and 8 fertiliser pellets per pot, pushed into the mix.
Weigh 10 pellets to find the mean pellet mass and read the N:P:K analysis on the label, then express each level as grams of nitrogen per pot (pellets x mean pellet mass x %N / 100).
Sow five seeds per pot and thin to three seedlings after emergence.
Arrange the pots in a random order under the light bank (draw pot numbers from a hat) and rotate them each day.
Water every pot with the same volume whenever the mix surface dries.
Harvest whole plants after 18 to 21 days, wash the roots, blot dry, and dry each pot's plants in a labelled bag at 90–95 °C for 2 to 3 days until the mass stops changing.
Calculate mean and standard deviation of dry mass per plant for each fertiliser level and plot mean dry mass against pellets per pot.

What you should see

Plants with no pellets stay small and pale and have the lowest dry mass. Dry mass rises with the number of pellets at the lower levels, and the gain from each extra pellet is expected to shrink at the highest levels once something else limits growth. Dry mass is used rather than fresh mass because water content is large and variable, as the SAPS notes explain. The learner knows it worked when the four replicate pots at each level show the same trend and the standard deviations are small compared with the differences between the lowest and highest levels.

What changes

What you change: fertiliser pellets per pot (0, 1, 2, 4, 8)
What you measure: dry mass per plant (g)
What you keep the same: pot size and growing mix
three plants per pot
light and temperature
water volume
harvest day

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Fertiliser is plant food.
More fertiliser always gives more growth.
Fresh mass is the best measure of how much a plant has grown.

Safety card

Low riskLearners carry it out

Hazards

fertiliser pellets irritate skin and eyes
dust from peat and vermiculite
hot drying oven

Controls

gloves and eye protection when handling fertiliser
dampen the mix and wear a P2 mask
teacher manages the oven

Note

Fertilisers: NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agricultural Technology Years 7–10 Syllabus (2019)AG5-6 AG5-11 AG5-12
Agriculture Technology 7–10 Syllabus (2024), new syllabus, implementation from 2027AGT5-APG-01
Australian Curriculum v9AC9TDE10K04AC9S9I02

Sources

The pages the author read to write this activity.