title: "Soil Colour as Physics — Teacher's Guide"
subtitle: "Earth Pigments from the Soils of Müllrose"
author: "Michel Garand"
date: "February 2026"
version: "1.0"
lang: en
license: "CC BY-NC-SA 4.0"
project: "Erdpuls Müllrose OER Collection"
Soil Colour as Physics — Teacher's Guide
Earth Pigments from the Soils of Müllrose
Erdpuls Müllrose — Living Laboratory & Makerspace Garden
Part of the "Brücken bauen durch Boden — 13 Questions to the Soil" project
Purpose of This Guide
This Teacher's Guide accompanies three differentiated lesson-plan documents (Classes 1–4, Classes 5–8, and Classes 9–12) and the accompanying Pedagogical Framework. It provides everything you need to deliver the Soil Colour as Physics unit confidently: subject-matter preparation, safety guidance, materials management, assessment approaches, and logistical support for the Erdpuls campus visits.
You do not need to be a physics or chemistry specialist to deliver this unit. The "Body first, then instrument" principle that runs through all grade levels means that your students' hands, eyes, and questions do most of the teaching. Your role is to hold the space for discovery, ask good follow-up questions, and connect what students notice to the concepts in this guide.
How This Unit Fits the Erdpuls Project
The "Brücken bauen durch Boden" project asks 104 questions to farmers across three partner farms (conventional, organic, biodynamic). Each of the 13 questions receives one painted answer per farmer — that is 104 paintings, all made with earth pigments produced from local Müllrose soils.
Students in this unit do not produce samples. They produce the actual pigment medium for a real artwork that will travel. This is the difference between a school exercise and genuine participation in a project. Keep that in mind throughout: the work matters beyond the classroom.
The three farms give students something rare — a direct, side-by-side comparison of how a farmer's relationship with the land shows up in the colour of the soil itself. That is both a scientific question and a human one.
Subject-Matter Preparation
The Core Physics in Plain Language
Why does soil have colour?
Soil colour comes primarily from iron. Iron makes up about 5% of the Earth's crust and exists in soils in several mineral forms, each with a different colour. The key ones in the Müllrose region are:
- Goethite (iron oxyhydroxide): yellow to yellow-brown. This is the most common iron mineral in Brandenburg soils.
- Lepidocrocite: orange-red. Forms where iron is alternately wet and dry.
- Haematite (iron oxide): red to red-brown. Dominates in warmer-climate soils; can form here where soil has been disturbed or heated.
- Ferrihydrite: freshly precipitated, red-orange. Often seen near springs or drainage channels.
- Organic matter (humus): dark brown to black. More organic matter = darker soil. Biodynamically managed farms in this project typically show more humus.
- Quartz: white to pale grey. The sandy soils of the Oder-Spree region are rich in quartz.
Why does colour change with wetness?
Dry soil has air in its pores. Light bouncing off many air-mineral interfaces scatters strongly in all directions — this is called Mie scattering — and makes the soil appear lighter. When water fills those pores, there are fewer interfaces, less scattering, and the soil looks darker. This can be observed and measured by students at every grade level.
Why does grinding change the colour?
Smaller particles have more surface area per unit of mass. More surface = more light interaction. Finely ground iron oxide pigments appear richer and more saturated in colour than coarsely ground versions of the same soil. But grind too fine (below about 5 μm), and the pigment becomes semi-transparent — covering power actually drops again.
Why does earth paint lighten as it dries?
As the water or oil binder evaporates, the pigment particles gradually become surrounded by air rather than liquid. Air has a lower refractive index than binder materials. The result: more Mie scattering at the surfaces, which lightens the appearance. The same physical mechanism as the wet-dry effect in the field.
The Key Minerals at a Glance
| Mineral | Formula | Colour | Where found locally |
|---|---|---|---|
| Goethite | α-FeOOH | Yellow–yellow-brown | Agricultural brown earths |
| Lepidocrocite | γ-FeOOH | Orange-red | Gley soils near lakes and streams |
| Haematite | α-Fe₂O₃ | Red–red-brown | Podzol ortstein layer |
| Ferrihydrite | Fe₅O₈H·4H₂O | Red-orange | Near drainage channels |
| Humus | Organic compounds | Dark brown–black | Fen soils, biodynamic farm fields |
| Quartz | SiO₂ | White–pale grey | Sandy raw soils, sandpits |
Connections to Curriculum Areas
| Grade band | Primary curriculum area | Secondary connections |
|---|---|---|
| 1–4 | Science (Sachunterricht) | Art, Geography |
| 5–8 | Physics, Biology | Art, Geography, Chemistry |
| 9–10 | Physics, Chemistry | Mathematics, Biology |
| 11–12 | Physics, Chemistry | Mathematics, Ecology, Research methods |
Safety Guidelines
This unit involves working with soil, fine dust, and simple chemical binders. All activities are safe when the following protocols are followed.
Dust Safety (All Grade Levels)
Fine quartz dust (particles below 10 μm) is a lung hazard with prolonged repeated exposure — this is the cause of silicosis in miners and quarry workers. In a school context with occasional grinding sessions the actual risk is very low, but precautionary practice is required:
- FFP2 dust masks must be worn during all dry grinding and sieving.
- Safety goggles must be worn during grinding.
- Good ventilation is required — open windows or work outdoors during dry processing.
- Wet-route processing (stirring soil into water) eliminates the dust risk entirely and is the preferred method for younger students (Classes 1–4 and 5–8).
Binder Safety
- Linseed oil: Non-toxic. Rags soaked in linseed oil can ignite spontaneously — spread flat to dry, never ball up.
- Gum arabic: Non-toxic, water-soluble.
- Egg yolk (tempera): Standard food item. Perishable — do not store prepared paint for more than a few days.
- Casein (from quark): Non-toxic.
- All binders are safe for students at all grade levels.
Soil Handling
- Wash hands after all soil handling.
- Do not eat or drink in the workspace.
- Soils from partner farms are agricultural soils with no unusual contamination. Use site description sheets to confirm this before collecting.
For Classes 1–4 Specifically
- Wet route only — no dry grinding or sieving.
- Pre-sieved soil samples from the teacher (remove stones and debris before class).
- Supervision ratio of at least 1:8 during hands-on activities.
Materials Management
Before the Unit
- Contact the Erdpuls team to confirm your visit dates and which partner farms will be included in the sampling.
- Request the site description form and Munsell chart reference (or simplified photographic version) from the project documentation.
- Review the sample collection map in Section 2.3 of the concept document to identify your sampling sites.
- Pre-dry soil samples at 50°C for 24 hours if preparing lab materials in advance (do NOT exceed 50°C — goethite converts to haematite above 250°C, changing the colour permanently).
Consumables Checklist (per class group)
| Item | Quantity needed | Notes |
|---|---|---|
| Pre-collected soil samples (min. 3 sites) | 200–500 g each | Dry at 50°C beforehand |
| FFP2 dust masks | 1 per student | Required for dry grinding |
| Safety goggles | 1 per student | Required for grinding |
| Mortar and pestle (porcelain) | 2–4 | Wash between samples |
| Sieves: 500/250/100/63 μm | 1 set | For grades 5–12 only |
| Digital balance (1 mg) | 1 | For grades 5–12 only |
| Watercolour paper 300 g/m² | 3 sheets per student | Heavier paper holds pigment better |
| Linseed oil | 100 ml | |
| Gum arabic (10% solution) | 100 ml | Mix 10 g in 100 ml warm water |
| Egg yolks | 4–6 | Fresh on the day |
| Small glass jars (50 ml) | 10–15 | For storing pigment batches |
| Water in containers | — | For wet processing |
| Paintbrushes (flat, 10–20 mm) | 1 per student | Flat brushes work best |
| Disposable gloves | 1 pair per student | Optional but useful |
Digital Tools (Free)
- Spectroid (iOS / Android): Reflection spectroscopy app for smartphone
- GIMP: Image editing for RGB colour analysis (desktop)
- Physics Toolbox Sensor Suite: Alternative spectroscopy approach
- RRUFF Project (rruff.info): Reference spectra for mineral identification
- SoilGrids (soilgrids.org): Online soil maps for regional comparison
The 4A-Pathway in Your Lessons
Every lesson sequence at Erdpuls follows the 4A-Pathway: Awareness -> Acknowledgment -> Attitude -> Action. You do not need to announce this structure to students — it is a teacher's map, not a student handout.
| Stage | What it looks like in your lesson |
|---|---|
| Awareness | Students encounter soil and colour directly, in the field or with samples in hand, before any explanation is given. "What do you notice? What surprises you?" |
| Acknowledgment | Students measure, compare, and see that their sensory impression is confirmed by data. "The wetter sample really is darker — and here is the measurement to prove it." |
| Attitude | Students form their own view of the connection between farming practice and soil colour. This is not a fact to memorise — it is a question to hold. |
| Action | Students produce real pigments for the Erdpuls WP4 exhibition. Their work enters the world. |
Make sure every unit, even a single 45-minute session, has a moment from each stage. The Action stage does not require actual pigment production — it can be as simple as: "What would you do with this colour if you were making a painting about the soil you collected it from?"
Assessment Guidance
This unit is primarily assessed through process documentation rather than written tests. Suggested approaches:
All Grade Levels — Soil Colour Journal
Students keep a running journal of their samples, observations, and questions. This functions as both a learning tool and an assessment artifact. Prompts to include:
- A colour swatch from each soil sample (painted or photographed)
- Three observations in words
- One question the sample raises
Classes 5–8 — Lab Report (Light)
A simple one-page document: what did we do, what did we measure, what do we notice? Focus on the wet–dry experiment and the binder comparison as the core data to record.
Classes 9–12 — Research Report or Presentation
Students design and conduct their own mini-investigation (see the Classes 9–12 lesson plan) and present findings. Assessment criteria: clarity of research question, quality of measurement, interpretation of data, connection to the broader project thesis.
Self-Assessment Question (All Levels)
At the end of the unit, ask: "What does this soil's colour tell you about its history?" The quality of a student's answer to this question is a good proxy for depth of understanding across all grade levels.
Frequently Asked Questions
What if we can't visit the partner farms?
Pre-collected samples from the farms can be provided by the Erdpuls team. Contact Michel Garand at the project office. The field-visit lessons can be adapted to work with sample kits brought to the classroom.
What if students have no prior knowledge of optics or chemistry?
The unit is designed to build from direct sensory experience. No prior knowledge is required. The concept document's Section 3 (Physical Principles) provides the background you need — you can introduce as much or as little as your class is ready for.
The wet-dry experiment — how wet should the sample be?
Fully saturated (dripping). The contrast with the completely dry version is what makes the effect clearly visible. Use the same sample mass (5–10 g) for both states.
Can we use soils from our school garden instead of the farm sites?
Yes, for most experiments. The three-farm comparison is the most educationally powerful aspect of this unit, but local school garden soil works well for the grinding, spectroscopy, and binder experiments.
How do we store finished pigments?
In tightly sealed glass jars, away from light. Dry earth pigments store indefinitely. Mixed paints (with egg yolk or gum arabic) should be used within a week or refrigerated.
Connecting to the Exhibition
The finished pigments from your class join the Müllrose Earth Colour Library — the material source for the 104 exhibition paintings. If possible:
- Label each jar with the site name, date, farm, depth, and your class name.
- Photograph the jar against a white background for the colour library record.
- Share the photograph with the Erdpuls project office for inclusion in the project archive.
Students who see their labelled jar on the table in the exhibition room have a different relationship to the finished artwork than any other audience member. That is worth the extra five minutes of labelling.
Further Reading
- Schwertmann, U. & Cornell, R.M. (2000): Iron Oxides in the Laboratory. Wiley-VCH. [Schwertmann was Professor at TU München — the definitive European reference on iron oxide mineralogy and soil colour]
- Blume, H.-P. et al. (2018): Scheffer/Schachtschabel: Lehrbuch der Bodenkunde. 17th ed. Springer Spektrum. [The standard German soil science textbook; covers iron oxide colour, humus, and soil formation in depth. English edition: Soil Science, Springer 2016]
- AG Boden / BGR (2024): Bodenkundliche Kartieranleitung (KA6). 6th ed. E. Schweizerbart, Stuttgart. [The official German field guide for soil description, sampling, and classification — essential background for the fieldwork units. In German]
- Doerner, M. / Hoppe, T. (2011): Malmaterial und seine Verwendung im Bilde. 24th ed. Maier, Ravensburg. [Classic German reference on painting materials, pigments, and binders; covers earth pigments and binder chemistry in detail. In German]
- Delamare, F. & Guineau, B. (2000): Colour: Making and Using Dyes and Pigments. Thames & Hudson. [French authors; accessible introduction to the science and history of pigments]
- Canti, M. (2003): Earthen pigments and their analysis. English Heritage Research Transactions.
- Toland, A., Noller, J.S. & Wessolek, G. (Eds.) (2019): Field to Palette — Dialogues on Soil and Art in the Anthropocene. CRC Press / Taylor & Francis. 681 pp. [The central reference for the science-art bridge this unit is built on. Over 100 contributors including soil scientists, artists, and educators; includes DIY experiments and soil recipes. Wessolek is Emeritus at TU Berlin — the direct institutional neighbour of the Erdpuls project]
- Feller, C., Landa, E.R., Toland, A. & Wessolek, G. (2015): Case studies of soil in art. SOIL 1: 543–559. DOI: 10.5194/soil-1-543-2015 [Open access; shorter and classroom-accessible; covers earth pigments and soil art across cultures and history. Freely downloadable]
- Wessolek, G. (2021): Böden in Kunst und Gesellschaft neu positionieren. Handbuch der Bodenkunde. Wiley. DOI: 10.1002/9783527678495.hbbk2021001 [In German; theoretical grounding of soil science's role in public education and society — direct BNE relevance]
- Public Lab (publiclab.org/wiki/spectrometry): DIY spectrometer build instructions [Note: SpectralWorkbench.org closed in August 2022; for classroom spectroscopy use the Spectroid app listed in the Digital Tools section above]
License and Attribution
© 2025–2026 Michel Garand | Erdpuls Müllrose — Center for Sustainability Literacy, Citizen Science & Reciprocal Economics
Müllrose, Brandenburg, Germany
Licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0)
You are free to share and adapt this material for non-commercial purposes, provided you give appropriate credit, provide a link to the license, indicate if changes were made, and distribute any adaptations under the same license.
All software components referenced in this document are licensed under the GNU Affero General Public License v3.0 (AGPL-3.0)
This document and its translations were developed with assistance from Claude (Anthropic PBC). All strategic decisions, philosophical positions, and project commitments are those of the author.
Contact: erdpuls@ubec.network · https://erdpuls.ubec.network
Alle Dokumente und ihre Übersetzungen / All documents and their translations.
Müllrose, Brandenburg — February 2026