What it shows
A layer of mulch slows evaporation from the soil surface. Weighing identical pots each day shows how much water each mulch saves, which helps farmers during dry spells.
The science
Water leaves the soil by evaporating from the surface, and wind and sun speed that up. A layer of mulch shades the soil, slows the wind at the surface and keeps the air above the soil humid. The pot with the least loss of mass has the best mulch.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Five identical small pots or cut bottles with soil
- Dry straw, dry leaves, shredded paper and gravel
- A kitchen scale
- A measuring cup
- A shaded sunny windowsill or balcony
- Labels and a notebook
Safety
For this project: Wash hands after touching soil and mulch. Keep pots on a stable ledge so they do not fall.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill all pots with the same mass of soil and water each with exactly 100 ml.
Cover pot 1 with 2 cm of straw, pot 2 with leaves, pot 3 with paper and pot 4 with gravel.
Leave pot 5 bare as the control.
Weigh each pot and record the starting mass.
Place all the pots together in the same sunny spot.
Weigh each pot at the same time daily for two weeks.
Work out the mass lost since day one for each pot.
Plot mass lost against days and rank the mulches.
What you should see
The bare pot loses water fastest and the mulched pots lose less. The graph lets you rank straw, leaf, paper and gravel by how much water they save.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is mulch?
A covering of material on the soil surface.
2How does mulch save water?
It shades the soil and slows evaporation.
3Why have a bare pot?
It is the control to compare against.
4What other benefits does mulch give?
It reduces weeds and adds humus as it rots.
Take it further
Plant a bean in each pot and see which mulch grows the tallest plant.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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