Soil Color and Redox Chemistry

Soil Society of America/Earth Science Week Soil Society of America/Earth Science Week
Type Category
Instructional Materials
Experiment/Lab Activity
This resource, vetted by NSTA curators, is provided to teachers along with suggested modifications to make it more in line with the vision of the NGSS. While not considered to be "fully aligned," the resources and expert recommendations provide teachers with concrete examples and expert guidance using the EQuIP rubric to adapted existing resources. Read more here.




Soil color is often an indication of the presence of iron (Fe) oxides or organic matter on soil particles, not the soil itself. The minerals beneath are often quartz or feldspar, which are grey. In saturated soil (wet), Fe3+ is reduced to Fe2+ due to a microbial mediated redux reaction.  If all the oxygen is removed from the soil/water (by the microbes), the soil becomes anaerobic. The red color of the iron oxides is removed in the reaction and the soil becomes grey. Soil scientists can identify where the water table is even when the soil is not saturated. The presence of grey colors indicate height of the water table. In this activity students will compare jars of different colored soil that are wet to observe the anaerobic reduction of the iron compounds in the soil (the color fades to grey). This is compared with a jar of sterilized wet soil. Teachers will need to procure soil samples for the students in advance of the lesson.

Intended Audience

Educator and learner
Educational Level
  • High School
Access Restrictions

Free access - The right to view and/or download material without financial, registration, or excessive advertising barriers.

Performance Expectations

HS-ESS2-5 Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.

Clarification Statement: Emphasis is on mechanical and chemical investigations with water and a variety of solid materials to provide the evidence for connections between the hydrologic cycle and system interactions commonly known as the rock cycle. Examples of mechanical investigations include stream transportation and deposition using a stream table, erosion using variations in soil moisture content, or frost wedging by the expansion of water as it freezes. Examples of chemical investigations include chemical weathering and recrystallization (by testing the solubility of different materials) or melt generation (by examining how water lowers the melting temperature of most solids).

Assessment Boundary: none

This resource is explicitly designed to build towards this performance expectation.

Comments about Including the Performance Expectation
There are many opportunities for students to modify this activity in order to better include the full practice. Students could decide on how many types of soil to observe or could keep some soil dry for comparison with the wet soil. They could perform the activity as written, then refine the design in order to get better data.

Science and Engineering Practices

This resource appears to be designed to build towards this science and engineering practice, though the resource developer has not explicitly stated so.

Comments about Including the Science and Engineering Practice
There are many opportunities for students to modify this activity in order to better include this practice. After gathering data students should argue from the evidence in the data to explain why the colored iron compounds changed color. This would be easier with the modification in the activity of including soil samples that are dry/not heated and dry/heated.

Disciplinary Core Ideas

This resource appears to be designed to build towards this disciplinary core idea, though the resource developer has not explicitly stated so.

Comments about Including the Disciplinary Core Idea
This activity allows students to see the effect of water on chemical compounds in soil. Further discussion could be conducted with students centered around the idea of what else might water effect in a similar way on the surface of the Earth. Students would have to research what other compounds/elements water effects in the same way as the color of the chemical compounds in the soil.

Crosscutting Concepts

Resource Quality

  • Alignment to the Dimensions of the NGSS: There is alignment to the 3 dimensions. Students are asked to analyze data (Practice), observe changes over time related to phenomena (Crosscutting Concept and Core Ideas) in order to come up with an explanation based on their evidence (Practice). Explanations of terminology mentioned during the lesson would enhance the link to the standards.

  • Instructional Supports: The lesson engages students in authentic science as they view data and use the data to make sense of phenomena. Student data sheets are not provided. This lesson does not provide any rubrics for student achievement. Differentiation for students varying educational levels is not included in this activity. Teachers could have advanced students record data from several different soil types to see if they all resulted in the same color soil after sufficient time or if all soil samples lost color at the same rate.

  • Monitoring Student Progress: The lesson does not have any built in methods for monitoring student progress. The teacher can introduce several ‘Arguing from Evidence’ sections for students to explain what they have developed up to that point. After collecting all data as described in the activity, Arguing from Evidence can be used to explain why the soil lost color. Students could then modify the activity by adding soil samples that are dry and heated/not heated. This would allow them to collect data to support their argument that the water was involved in changing the color of the soil compounds.

  • Quality of Technological Interactivity: This activity requires no technology.