Children are delighted and amazed by soil. Some can while away hours making sand castles at the beach or scooping up sand by the bucketful at the neighborhood sandbox. Others turn ordinary spoons into instruments of adventure as they dig holes as big as craters in their own backyard. Many children inspect the soil for the treasures it yields--worms, insects, fossils, rocks. They might grow flowers in rich garden soil or squeeze wet clay into fanciful shapes. And, after a pouring rain, they enjoy the simple pleasure of splashing through mud puddles.

Soils, a 16-lesson unit for second-graders, deepens children's awareness and appreciation of soil. Using simple tests, students learn to identify sand, clay, and humus in soil. They also study how water affects different kinds of soil. Through long-term experiments, they explore how roots and plants grow in various soils and how, with the help of worms, old plants decompose and become part of soil. Then, applying what they have learned, they investigate their own local soil.

Students begin by discussing what they already know about soil and what they want to find out. After learning the difference between predictions and observations, they predict what a sample of garden soil might contain. Examining the sample with a hand lens reveals particles of different sizes, along with bits of plants and tiny animals. This sets the stage for Lesson 2, when students create compost bags using garden soil, plant matter, and redworms. For the next five weeks they watch what happens as dead plants decay in soil.

Lessons 3 through 7 introduce students to sand, clay, and humus, three of the basic components in soil. First they explore and compare dry samples. Then they perform classic soil tests used by real soil scientists: They touch the wet sample, try to roll it into a ball, smear it on paper, and observe how long it takes to settle in water. Using these simple tests, students discover how to identify each component by its unique characteristics. For instance, wet clay rolls into a smooth ball, while a clump of wet sand crumbles in their hands. They also discover that soil can revert from wet to dry to wet again. A reading selection explores some of the many uses people have found for sand and clay.

Lesson 8--an embedded assessment--challenges students to analyze an unfamiliar mixture of soil components. Applying the same soil tests as in previous lessons, they figure out what their "mystery mixture" contains.

Next they explore the connection between plants, roots, and soil. In Lesson 9 they plant seeds in samples of sand, clay, humus, and their local soil. By tending these seedlings until the end of the unit, students find out that different soils can affect plant growth. Since a plant draws water and minerals from the soil through its roots, in Lesson 10 students plant seeds in glass test tubes to observe root growth in sand, clay, and humus. Over the next three lessons they compare the roots in each tube and discover what roots look like underground. To visualize further what goes on beneath the soil surface, students read about earthworms and their vital contributions to soil.

Lessons 11 and 12 focus on how water percolates through soil. Pouring water over humus in Lesson 11 enables students to see that soil can hold water at least for a while. In Lesson 12, by pouring water simultaneously over sand and clay, students find out that different components absorb water differently: Water passes rapidly through sand but puddles on top of clay. Students begin to recognize that the amount of water in a particular soil affects how well plants may grow there.

In Lesson 13, students learn some fundamental principles of composting as they revisit the compost bags begun in Lesson 2. As they observe that the plants in their bags have decomposed and become part of the soil, they discover nature's cycle of decay and regeneration. By comparing their compost bags with worms to one they set up without worms, they discover that worms greatly improve the texture of soil. A reading selection connects this activity to the more complex composting process that occurs outdoors. Students also complete their root experiment by comparing how successfully roots have grown in sand, clay, and humus.

Students conclude this unit by investigating their own local soil. This entire activity is an embedded assessment. In Lessons 14 and 15, students conduct soil tests learned in previous lessons to acquire data about their local soil. They pick up more clues in Lesson 16 by comparing the plants grown in various samples, including local soil. Recording the height of all 30 plants on a classroom chart enables students to compare how well each sample has sustained plant growth. Pooling their data, students draw final conclusions about what their local soil contains.

This unit motivates students to observe carefully, keep accurate records, and synthesize information from multiple experiments. Through hands-on activities and class discussions, students answer many of their original questions about soil and pose new questions as well.

The post-unit assessment after Lesson 16 is matched to the pre-unit assessment in Lesson 1. Additional assessments provide four more strategies for evaluating student progress.