Introduction to Soils

Weathering Formation of soil by processes such as weathering, organic matter accumulation ,and leaching (soil as a system)

Organic and Inorganic Organic – dead plants and animals, waste products decompose (process of once-living material into dark-colored organic matter) Inroganic – mechanical and chemical weathering of rocks (boulders, cobbles, gravel, sand, silt, or clay)

Soil Formation Parent material (starting material of soil) – rock or sediment that weathers and forms soil Climate – average weather of an area – precipitation, daily/average temps – affects weathering rates Topography – shape and steepness of a landscape – water flow affects sedimentation Biota – living things in a region – air and water pores formed by organisms speeds up weathering and decomposition or organic matter Time – slow / constant process – thousands of years for a mature soil to form

Pores Pore spaces provide air and water for plants Pores determine drainage and water storage Sand > Silt > Clay Which soil would swell the greatest amount after a rain – sand/silt size particles or silt/clay?

As soils age, they form distinct soil horizons and a soil profile topsoil subsoil parent material

Soils are dynamic, they change over time Parent material Young soil Mature Old Time

HORIZONS A – contains most of the organic matter in the soil – excellent for plant growth B – clay, other materials – water carries clay particles (small) and deposits below the upper layer C – layered parent material – rocks or sediments

The Role of Climate The balance between moisture and temperature is particularly important to the development of soils. MOISTURE how much when TEMPERATURE season length range of highs/lows

Houston, TX

High rainfall during warm temperatures Water used by plants Lots of plant growth Darker topsoil Houston soil

Gainsville, AL

High rainfall during cooler temperatures Few plants to use water Excess water leaches through the soil Red/orange subsoil Colors as a result of oxidation Bama soil

Thin, light colored topsoil Little water reaches subsoil Low rainfall Little vegetation Low plant growth Thin, light colored topsoil Little water reaches subsoil Salts accumulate White subsoil (Orovada soil from Nevada) (Casa Grande soil From Arizona)

The Role of Parent Material Material from which soil forms: bedrock colluviam (sediments transported via gravity to the base of a slope) alluvium (loose, eroded soil) loess (windblown material) Parent material influences mineral composition and particle size of soil. (Arizona)

SOIL TEXTURE The particles that make up soil are categorized into three groups by size – sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture.

Particle size Smaller, finer particles (like clay) hold water and nutrients, including carbon Coarser particles don’t hold water or nutrients, but they promote oxidation (Arizona)

A clay loam texture soil, for example, has nearly equal parts of sand, slit, and clay. Each texture corresponds to specific percentages of sand, silt, or clay. Knowing the texture helps us manage the soil. There are 12 soil textural classes represented on the soil texture triangle. This triangle is used so that terms like “clay” or “loam” always have the same meaning.

Services by the Soil System Food Fiber Filtration of gas & water Water storage Carbon Storage

Threats to Soils Include: Erosion Contamination Sealing Compaction Loss of biodiversity Salinization Floods and Landslides Sealing - destruction or covering of soils by buildings, constructions and layers of completely or partly impermeable artificial material

Soil Quotes “How can I stand on the ground every day and not feel its power? How can I live my life stepping on this stuff and not wonder at it?” - William Bryant Logan from Dirt-The Ecstatic Skin of the Earth “We abuse land because we regard it as a commodity belonging to us. When we see land as a community to which we belong, we may begin to use it with love and respect.” - Aldo Leopold, 1949. A Sand County almanac. “We might say that the earth has the spirit of growth; that its flesh is the soil.” - Leonardo da Vinci