What it shows
Soils can be acidic, neutral or alkaline, and crops grow best in a suitable range. A homemade indicator sorts soils from different places by colour.
The science
Red cabbage contains a pigment that is red in acid, purple in neutral and blue-green in alkali. Soil water carries dissolved substances that make it a little acidic or alkaline. The pigment in the extract changes colour to show which.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Red cabbage leaves and warm water
- Eight clear cups or jars
- Soil samples from a garden, a farm, a roadside, a pot and a lawn
- Distilled or boiled and cooled water
- Coffee filters or cotton cloth
- Lemon juice and baking soda for colour reference
- A dropper or spoon, and a sheet of white paper
Safety
For this project: Wash hands after handling soil. An adult must chop the cabbage and heat water. Taste nothing used in the experiment.
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.
Step 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.
Chop cabbage, soak it in hot water and let it cool to make a purple indicator.
Make colour references: stir a little lemon juice into water in one cup and a little baking soda into water in another, and add the same few drops of indicator to each, plus a third cup of plain water with indicator as the neutral reference.
Mix one spoon of each soil with five spoons of the distilled or boiled and cooled water and stir for a minute.
Let the muddy water settle and filter it through a coffee filter or folded cloth into a clean cup.
Add the same few drops of indicator to each soil extract.
Hold each cup against the white paper, compare the colours with the reference cups and write down each result.
Rank the soils from most acidic to most alkaline.
Add your results to a map of where each sample came from.
What you should see
Each soil extract takes a colour from pink through purple to green-blue, letting you rank the samples. Roadside or lime-rich soil is often more alkaline than garden soil. Muddy extracts give faint colours, so treat the ranking as rough.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What does pH measure?
How acidic or alkaline something is.
02Why filter the soil water?
Muddy water hides the indicator colour.
03Why does soil pH matter to farmers?
It affects how well plants take up nutrients.
04What can be added to acidic soil?
Lime.
Take it further
Grow the same seed in the most acidic and the most alkaline soil to see which grows better.
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.
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