The Dirt on Soil Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil How does soil form? Soil is a loose mixture of rock fragments, organic matter, water, and air that can support the growth of vegetation. Humans are directly and indirectly dependent on the soil for plants and their products. Soil is a product of chemical and mechanical weathering. The breaking apart of rock and the activities of living things also contribute to the formation of soil. Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil How does soil form? Soil begins when bedrock, AKA parent material is weathered Over time, organisms such as bacteria, fungi, and insects begin to live among the weathered pieces Organic waste from these organisms add nutrients to the weathered material, forming soil Soil is BOTH biotic and abiotic Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil How is soil biotic? Some living things break down, or decompose, the remains of dead organisms. This process forms humus, a substance that contains nutrients that plants need to grow. Larger animals, such as earthworms and moles, live in the soil. They loosen and mix the soil as the burrow, increasing the air and drainage of the soil. Some soils have greater organic content than others (ex: forests vs. deserts). Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil What is a soil horizon? Typically, the top layer of soil contains the most developed soil. Lower layers (closer to bedrock) are made of less-weathered materials. Soil that developed from the bedrock below it is residual soil, soil that has undergone erosion and deposition is called transported soil. Bedrock and soil may contain the same minerals, but in different ratios due to chemical weathering. Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil What is a soil horizon? A vertical section of soil layers from the top layer down is a soil profile. Each layer of soil in a soil profile is a soil horizon. Main horizons are: O, A, B, C Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil What is a soil horizon? The A horizon is the topmost layer of soil, often referred to as topsoil. It contains the most humus. The B horizon is under the A horizon. It has larger, less weathered rock fragments and less humus than the A horizon. The C horizon is under the B horizon. It is the least-developed layer, containing the largest rock fragments and no organic matter. Solid rock is under the C horizon. Copyright © Houghton Mifflin Harcourt Publishing Company

What are some soil characteristics? The Dirt on Soil What are some soil characteristics? Factors that affect weathering of soil include rock type, climate, topography, and the abundance and variety of living things. Soil texture describes the soil based on the relative amounts of soil particles. Soil particles can be classified as sand (the largest particle), silt, or clay (the smallest particle). Soil color depends on the minerals and organic matter that make up the soil. Copyright © Houghton Mifflin Harcourt Publishing Company

What are some soil characteristics? The Dirt on Soil What are some soil characteristics? Soil pH is a measure of how acidic or basic the soil is. Soil pH is depends on the minerals, sediment, and organic matter found in the soil. Pore space describes the spaces between soil particles. Water and air are found in the pore spaces of soils. Soil fertility describes how well a soil can support plant growth. Soil fertility depends on climate, the amount of humus, minerals, and nutrients, and the topography of the area. Copyright © Houghton Mifflin Harcourt Publishing Company

What are some soil characteristics? The Dirt on Soil What are some soil characteristics? How does the pore space of the soil on the left compare to that of the soil on the right? Copyright © Houghton Mifflin Harcourt Publishing Company

What are some soil characteristics? The Dirt on Soil What are some soil characteristics? Copyright © Houghton Mifflin Harcourt Publishing Company

The Dirt on Soil Copyright © Houghton Mifflin Harcourt Publishing Company