SOIL

Learning objectives Recall that soil is the result of weathering of rocks and includes weathered particles: sand, silt and clay

What is soil? Soil is the loose covering of broken rock fragments (regolith) overlying the bedrock of the Earth’s surface Soil is the result of mechanical and chemical weathering over long periods of time and consists of four major components: Mineral matter (broken down rock) Humus (decayed remains of organisms) Water Air During soil development, layers form… Horizon A Horizon B Horizon C Parent Material / Bedrock

Composition by volume good quantity soil Most soil is composed of mineral matter, organic matter, water, and air Usually, the mineral and organic matter exist mostly as particles, with the water and air existing in pores between the particles A soil's texture is determined by the coarseness or fineness of its particles 5% 25% 25% 45%

SOIL CHARACTERISTICS Soil textures are classified by the size of the particle, from very coarse sand to fine clay Soil fertility is a measure of how well a soil can support the growth of plants Soil color is determinate of the climate in which it develops as well as the composition of the soil. Color alone is unreliable in determination of fertility of the soil

SOIL TEXTURE Soil texture refers to the proportions of different particle sizes Sand (large and coarse) Silt Clay (small and fine) Loam (a mixture of all three sizes) is most suitable for plant life Soil particles “clump” together to give soil it’s ultimate structure

Soil Development A Step by Step Process The primary factors involved in soil development are parent material, time, climate, organisms, and slope The soil forming process begins when weathering breaks solid bedrock (parent material) into smaller pieces Pieces then continue to undergo weathering and break down into smaller and smaller pieces Many organisms (bacteria included) begin to live in these weathered materials Over time, the organisms die, decay, and add nutrients to the weathered materials to form soil, such as dead fungi, bacteria, and protozoa In general, soil formation occurs over a long period of time. It can take hundreds of years for only a centimeter of soil to form

Soil Composition While the parent bedrock determines the type of minerals in a soil, the proportion of minerals may not be the same Differences may occur as the result of chemical weathering, which changes the chemical composition of the soil The length of time it takes for soil to form also depends on the type of parent rock, as well as the climatic conditions of an area. The longer a soil has been forming, the thicker it becomes. Soil located above it’s parent material is called residual soil Soil that has been moved to a location away from its parent bedrock is called transported soil

Soil Profiles Soil Profile: Vertical sequence of soil layers Soil Horizon: A distinct layer within a soil profile Horizon A - Lots of organic matter, darkest and richest… “topsoil” Horizon B - Subsoils enriched with clay minerals, red or brown, washed out from Horizon A Horizon C - Contains weathered parent materials, just above the bedrock Soils tend to be thin, coarse, and infertile on slopes, and tend to be thick and fertile in valleys

Types of soil SOIL TYPES LOCATION CONTENTS Pedalfer Under Forest Vegetation Iron oxides and aluminum-rich clays in Horizon B Pedocal Drier Grasslands Calcium Carbonate Laterite Hot, wet, tropical Climates Intense chemical weathering to the top layers of soil

Soil erosion Water erodes soil Rates of Erosion Sediment Deposition Human activities that remove or alter natural vegetation speed up the erosion process Farming Logging Construction Sediment Deposition Reservoirs fills with sediment Sediments are then contaminated with pesticides and fertilizers

Controlling Soil erosion Soil erosion can be lessened by implementing different strategies: Planting Windbreaks Terracing Hillsides Contour Plowing along hills Crop Rotation