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Preparation
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You can help your students investigate some of the chemicals in soil that affect plant growth.
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For each group of four students:
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This investigation is considered generally safe to do with students, but they all must wear goggles and wash their hands when they finish. As always, please review the investigation for your specific setting, materials, students, and conventional safety precautions.
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Bring in a bag of commercial potting soil and ask students to examine some of it briefly. Its texture is the result of a variety of particle sizes, just right for making sure that plants get what they need to grow. Tell the students that the soil has been mixed to include many of the chemicals that plants need. Read the names of a few of these chemicals from the label on the bag (potassium-K, nitrogen-N, phosphorus-P) and pH (a measure of how acidic or basic the soil is on a scale of 0-14). Since it would be very expensive to grow lots of plants in potting soil, farmers and gardeners test the soil they have for the chemicals that plants need. When they find that the soil is low in one chemical, they buy a supply of that chemical and work it into the soil.
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After the scene is set, introduce your students to the investigation question: “How can you tell what chemicals are in soil?” Tell your students that they will be investigating this question and that at the end of their investigations, they will be able to provide reliable answers.
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At the elementary level, chemical explanations are best kept at the macro or observational level. At this stage, it is appropriate to talk about the fact that whether a material is an acid or a base is an important property of the material. Operational definitions constructed from observations are useful. For example, “An acid is a material that makes a cabbage juice indicator solution turn more ‘orange-ish’.” is a reasonable definition for a student to construct based on immediate experience. Most soil testing kits include an acid-base indicator with a range of pH levels to reflect and sort out those most likely to be found in samples. Be sure to introduce pH as a number scale that expresses how acidic or how basic a material is. |
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