Plant Indicators

Part of Acids and Alkalis

Without laboratory pH meters or synthetic indicator strips, plant-derived indicators are your primary tool for measuring acidity and alkalinity. Several common plants contain pigments that change color predictably across the pH spectrum, enabling everything from testing lye strength to checking soil health.

How Plant Indicators Work

Many plant pigments belong to a family of compounds called anthocyanins. These molecules change their chemical structure in response to hydrogen ion concentration (pH), and each structural form absorbs different wavelengths of light — producing visible color changes.

The beauty of anthocyanin indicators is that they often provide a continuous color gradient across the full pH range, not just a single color flip. This gives you semi-quantitative pH measurement using nothing more than plant extract and observation.

Red Cabbage: The Master Indicator

Red cabbage is the most versatile natural pH indicator. It covers nearly the entire pH scale with distinct, easily distinguishable colors.

Preparing Red Cabbage Indicator

1. Chop one-quarter of a red cabbage head into small pieces
2. Place in a pot and cover with water
3. Bring to a gentle boil and simmer for 15-20 minutes
4. The water turns deep purple-blue
5. Strain out the cabbage pieces
6. Allow to cool — the liquid is your indicator solution

Color-pH Reference Chart

pH Range	Color	Examples
1-2 (strong acid)	Bright red	Battery acid, stomach acid
3-4 (moderate acid)	Pink-magenta	Vinegar, lemon juice
5-6 (weak acid)	Purple-violet	Most rainwater, coffee
7 (neutral)	Blue-purple	Pure water
8-9 (weak alkali)	Blue-green	Baking soda solution
10-11 (moderate alkali)	Green	Soap solution, weak lye
12-13 (strong alkali)	Yellow-green	Lime water, strong lye
14 (very strong alkali)	Yellow	Concentrated lye

Make a reference chart before you need it. Prepare small cups of known solutions (vinegar, pure water, baking soda water, lime water, lye) and add a few drops of cabbage indicator to each. Label the resulting colors and keep the chart posted in your workshop.

Making Test Strips

For portable testing:

1. Cut absorbent paper or cloth into strips (2 x 10 cm)
2. Soak in concentrated cabbage juice for 5 minutes
3. Hang to dry in a shaded, well-ventilated area
4. Store dried strips in a sealed container away from moisture
5. To use: wet the strip with water, then touch to the substance being tested

Dried strips last 2-6 months depending on storage conditions.

Other Indicator Plants

Red cabbage is not available everywhere. Many other plants work as pH indicators.

Excellent Indicators

Plant	Part Used	Acid Color	Neutral	Alkali Color
Red cabbage	Leaves	Red/pink	Purple	Green/yellow
Red rose petals	Petals	Red	Pink	Green
Blueberries	Fruit	Red	Purple	Blue-green
Elderberries	Fruit	Red	Purple	Green
Hibiscus flowers	Petals	Red	Purple	Green
Beetroot	Root	Red	Pink	Yellow

Moderate Indicators

Plant	Part Used	Acid Color	Alkali Color	Notes
Turmeric	Root	Yellow	Orange-red	Only detects alkalis well
Red onion skins	Outer skins	Red	Green	Works well for strong alkalis
Grape skins (red)	Skins	Red	Green	Similar to cabbage but less range
Cherry juice	Fruit	Red	Blue-green	Seasonal
Blackberry juice	Fruit	Red	Blue-green	Works fresh or frozen
Marigold flowers	Petals	Yellow	Slight orange shift	Limited range

Simple Two-Color Indicators

Some plants show only a clear color change at a single pH point — useful as pass/fail tests.

Plant	Change Point	Below =	Above =
Litmus (lichen)	pH ~7	Red	Blue
Turmeric	pH ~8.5	Yellow	Red-brown
Cornflower petals	pH ~6	Blue	Pink

Turmeric is exceptionally useful as an alkali detector. Dip a turmeric-stained strip into a solution — if it turns red-brown, the solution is alkaline. This is the quickest way to confirm that your lye is actually lye and not just dirty water.

Making Litmus

True litmus is extracted from lichens (particularly Roccella tinctoria and Lecanora tartarea). It has been the standard laboratory indicator for centuries.

Lichen Litmus Preparation

1. Collect lichens from rocks or trees (gray, crusty species work best)
2. Grind or chop finely
3. Soak in a mixture of stale urine (which provides ammonia) or ammonia water and water (1:1) for 2-3 weeks
4. The liquid develops a deep purple-blue color
5. Strain and use as liquid indicator, or soak paper strips for test papers

Litmus behavior:

• Acid (below pH 4.5) — turns red
• Neutral (pH 4.5-8.3) — stays purple
• Alkaline (above pH 8.3) — turns blue

Litmus is less precise than red cabbage (narrower range of color change) but extremely reliable and well-documented.

Practical Applications

Testing Lye Strength for Soap Making

1. Dip a cabbage indicator strip into your lye solution
2. Yellow-green (pH 12-13) — good strength for soap making
3. Green (pH 10-11) — too weak; concentrate further
4. Bright yellow (pH 14) — dangerously strong; dilute
5. Combine with the egg float test for confirmation

Testing Soil pH

1. Take a soil sample from the root zone (5-15 cm depth)
2. Mix 1 part soil with 2 parts distilled or rainwater
3. Stir and let settle for 30 minutes
4. Strain the water into a clean container
5. Add cabbage indicator drops

Interpreting results:

Indicator Color	Soil pH	Meaning
Pink	Below 5.5	Very acidic — add lime
Light purple	5.5-6.5	Mildly acidic — most crops tolerate this
Blue-purple	6.5-7.5	Neutral — ideal for most crops
Blue-green	7.5-8.5	Alkaline — add compost or sulfur
Green	Above 8.5	Very alkaline — significant amendment needed

Testing Water Quality

• Drinking water should test near neutral (purple on cabbage indicator)
• Lime-treated water should show blue-green (pH 8-9) — the lime is working
• Fermentation should show pink (acid forming — normal for vinegar, sauerkraut)

Monitoring Fermentation

Cabbage indicator can track fermentation progress:

• Start — near neutral (purple)
• Active fermentation — turns pink/red as lactic or acetic acid develops
• Complete — stabilizes at pink

Indicator Preservation

Fresh plant indicator extracts last only 3-7 days before mold and oxidation degrade them.

Extending Shelf Life

Method	Shelf Life	How
Refrigeration (if available)	2-4 weeks	Store sealed in cool, dark location
Adding alcohol	2-3 months	Mix 1:1 with vodka or other clear spirit
Adding vinegar (small amount)	1-2 weeks	Acidifies to slow bacterial growth
Drying on paper strips	2-6 months	Most practical for long-term storage
Freezing (if available)	6-12 months	Freeze in ice cube trays for portioned use

The most reliable long-term method is dried paper strips. Make a large batch whenever indicator plants are in season, dry thoroughly, and store sealed. You can make a year's supply in an afternoon.

Seasonal Planning

Not all indicator plants are available year-round. Plan accordingly:

• Spring: Violets, cherry blossoms, young beet greens
• Summer: Rose petals, elderberries, blueberries, hibiscus
• Autumn: Red cabbage, grape skins, elderberries, red onion skins
• Winter: Stored cabbage (fresh if root-cellared), dried strips, turmeric (dried root stores indefinitely)

Building a pH Reference Kit

Assemble a permanent testing kit for your workshop or laboratory:

1. 10 small clay pots or glass vials with stoppers
2. Reference solutions at known pH:
   • Vinegar (pH ~3)
   • Lemon juice (pH ~2)
   • Rainwater (pH ~5.5)
   • Clean creek water (pH ~7)
   • Baking soda solution (1 tsp/cup — pH ~8.5)
   • Lime water (pH ~12)
3. Add indicator drops to each and label the color
4. Stock of dried test strips sealed in a jar
5. Fresh indicator liquid prepared monthly during growing season

Common Mistakes

1. Using old, oxidized indicator — fresh purple cabbage juice that has turned brown has lost its sensitivity. Prepare fresh batches regularly or use dried strips.
2. Testing colored solutions — the sample’s own color can mask the indicator color change. Dilute deeply colored samples with water before testing.
3. Confusing indicator species — not all red/purple plants work as indicators. Test any new plant against known acid/base solutions before relying on it.
4. Only checking one end of the scale — turmeric only detects alkalis; some plant dyes only detect acids. Use cabbage or another full-range indicator for general testing.
5. Ignoring temperature effects — very hot solutions can alter indicator colors temporarily. Cool the sample to room temperature before testing for accurate results.

Summary

Plant Indicators — At a Glance

• Red cabbage is the best natural pH indicator — covers pH 1-14 with distinct colors from red (acid) to yellow (strong alkali)
• Make indicator by simmering chopped red cabbage in water; store as liquid, dried paper strips, or preserved with alcohol
• Other good indicators: blueberries, elderberries, red roses, hibiscus, beetroot, and turmeric
• Litmus from lichens provides the classic red/blue acid/base test
• Practical uses: testing lye strength, soil pH, water quality, and fermentation progress
• Dried paper strips are the best storage method — 2-6 months shelf life, portable, instant results
• Build a reference chart with known solutions for comparison during testing
• Prepare and stockpile indicator strips when plants are in season for year-round availability