Understanding Soil

What is soil? Top layer of the Earth’s surface Created when rocks, dead animals and plants break down into tiny pieces Rocks, animals & plants in an area determine the composition of the soil Weathering turns mineral particles into topsoil

Soils Perform Vital Functions Sustaining plant and animal life below and above the surface Rain Runoff Soil Infiltration Regulating and partitioning water and solute flow Filtering, buffering, degrading, immobilizing, and detoxifying Storing and cycling nutrients Providing support to structures

Soil is the Basis of the Ecosystem The living systems occurring above and below the ground surface are determined by the properties of the soil. We often ignore the soil because it is hard to observe.

Soils Support Life Organism Types Roles & Benefits bacteria fungi protozoa nematodes arthropods earthworms Roles & Benefits decomposition release nutrients create pores stabilize soils

Pedology, the study of soil, is a unique discipline. Soils Have Unique Physical, Chemical, and Biological Properties Important to Their Use color texture structure consistence roots pores other features Soil is a natural body of solids, liquid, and gases, with either horizons, or layers or the ability to support rooted plants. Pedology, the study of soil, is a unique discipline.

Soil-Forming Factors Determine the Location and Kind of Soil There are 23,000 soil types in various combinations with different slopes and surface textures in the U.S. Soil Forming Factors: Parent Material Climate Living Organisms Topography Time

Soil Formation The A horizon is the topsoil, which is rich in organic matter. The dark color of the topsoil is due to the carbon in the decomposing organic material. The B horizon is a subsoil containing clay and soluble minerals. The C horizon comprises weathered parent material and rock fragments. A B C

Disintegrating parent rock Soil Formation Disintegrating parent rock Bedrock Stage 1 Weathered parent rock (C horizon) Bedrock Stage 2 C horizon A horizon Bedrock Stage 3 C horizon A horizon B horizon Bedrock Stage 4

Climatic Influence on Soil Photo: USDA Photo: USDA ARID Desert soils These are alkaline mineral soils with variable amounts of clay, low levels of organic matter, and poorly developed vertical profiles. Horizon A Horizon A Horizon B Horizon B HUMID Tropical soils Leaching and chemical weathering make these soils acidic. Aluminum and iron oxides accumulate in the deep B horizon.

Climatic Influence on Soil Photo: USDA Photo: USDA MID-LATITUDE Grassland soils Mature, alkaline, deep, well drained soils. They are typically nutrient-rich and productive with a high organic content. Horizon A Horizon A Horizon B Horizon B POLAR Very low temperatures slow the decomposition of organic matter and maintain the permafrost layer in these frozen soils.

Climatic Influence on Soil Photo: USDA Photo: USDA Horizon A Horizon B TEMPERATE Weathered forest soils Well developed soils with a deep organic layer and accumulated clay at lower levels. Horizon A Horizon B SEASONALLY WET Swelling soils Marked seasonal rainfall results in deep cracks as the soil alternately swells and shrinks.

SOIL Component definition: Mixture of mineral matter, organic matter, water, and air. Example: Air 25% Mineral Matter 45% Water 25% Organic Matter 5%

Soil Texture Soil texture depends upon the amount of sand, silt and clay present. Percentage clay Percentage silt Percentage sand 90 80 70 60 50 40 30 20 10 Read silt in this direction The optimal soil texture is called a loam and is around 40% sand, 40% silt and 20% clay. Clay Sandy clay Silty Sandy clay loam Clay loam Loam Sandy loam Silty clay Loamy sand Sand Silt Other loam exist but are named after their various components. Read clay in this direction Read sand in this direction

Examples of Soil Structure

Porosity Porosity is the amount of pore space in a rock (the open spaces between the grains) and is independent of grain size. Depends on: Sorting of the grains (or uniformity of grain size) Are the grains all the same size (well sorted), or are a variety of grain sizes present, with finer grains filling the spaces between the larger grains (poorly sorted)? Shape of the grains Packing and arrangement of grains. Cubic packing. Porosity = 47.64% Rhombohedral packing. Porosity = 25.95% Examples of rock/soil with high porosity: sands Examples of rock/soil with low porosity: well-cemented sandstone, shale, mudstone, very poorly sorted material

Microscopic structure of shale, sandstone, and limestone with water in pore spaces. Note differences in scale among views of each rock type. %Porosity = V1(volume of pores or average volume water) x 100 Total sediment volume

Permeability Permeability is the ease with which fluids flow through a rock or sediment. A rock/soil is permeable if fluids pass through it, and impermeable if fluid flow through the rock/soil is negligible Depends on: Grain size Coarser-grained sediments are more permeable than fine-grained sediments because the pores between the grains are larger Sorting Grain shape Packing (controls pore size)