Soil Profile,TEXTURE BSC Notes | PDF

What is a soil profile? Soil Profile: Definition, Development & Types

What is Soil Texture?

Soil Structure & Texture | Northeast Beginning Farmer Project

Soil Profile Descriptions 
• Soil Profile - A vertical section of the soil extending vertically through all its horizons and into the parent material. 

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• Soil Horizon - A layer of soil, approximately parallel to the surface, with properties that differ from the horizons above or below it – the properties (characteristics) are produced by soil forming processes.
• Soil Layer - A layer in the soil deposited by a geologic force (wind, water, glaciers, oceans, etc.) and not relating to soil forming process.
Organic (O) Horizons • O horizons or layers: 
Layers dominated by organic material. 
• Identification Criteria
 – >20% organic matter 
– Dark color 
( – Feels ‘Squishy” – Identifiable dead leaves, grass, etc. accumulated at surface A Horizons 
• Referred to as topsoil 
• Typically ranging from 6-30 centimeters thick 
• Mineral horizon formed at the surface or below an O horizon.
 • Characterized by an accumulation of well decomposed organic matter intimately mixed with the mineral fraction. 
• Identification Criteria – Mineral soil material – Mix of well decomposed organic matter and mineral material – Surface mineral horizon – Typically dark in color-darker than underlying horizons 
E Horizons 
• Mineral horizon in the upper part of the soil typically underlying an O or A horizon. 
• Light colored, leached horizons ranging from not being present to several centimeters thick
Light color due to the natural color of the mineral grains. 
• Formed by weak organic acids that strip coatings from mineral grains. 
• Field Identification 
– Zone of eluviation 
- removal of clays, Fe, Al, and humus 
– Lighter in color than over or underlying horizon 
– Near surface, below O or A horizons and above a B horizon B Horizons 
• Referred to as subsoil. • The zone of accumulation (or illuviation) within the soil. 
• Field Identification 
– Subsurface horizon formed below an O, A, E horizon and above the C horizon 
– Formed as a result of soil forming processes 
– Expressed often by color 
– Illuvial concentration-zone of accumulation C Horizon 
• Referred to as parent material. 
• These horizons and layers are little affected by soil forming processes (unweathered geologic material). 
• Field Identification 
– Little affected by soil-forming processes 
– Geologic layering 
– Color of unweathered geologic material
R Horizon 
• Hard rock
• Field Criteria 
– Can not dig it with a shovel or backhoe Soil Texture 
 Soil texture is the single most important physical property of the soil. Knowing the soil texture alone will provide information about: 
 1) water flow potential, 
 2) water holding capacity, 
 3) fertility potential, 
 4) suitability for many urban uses like bearing capacity Particle Diameter Size 
 Soil particle diameters range over 6 orders of magnitude 
 2 m boulders 
 Coarse fragments > 2 mm 
 Sand < 2 mm to 0.05 mm 
 Silt < 0.05 mm to 0.002 mm
Clay < 0.002 m Coarse Fragment
 > 2 mm 
 Gravels,cobbles, boulders 
 Not considered part of fine earth fraction (soil texture refers only to the fine earth fraction or sand, silt & clay) 
 Boulders left in valley of Big Horn Mts.(Wy) by a glacier. 
Sand 
 < 2 mm to > 0.05 mm 
 Visible without microscope 
 Rounded or angular in shape 
 Sand grains usually quartz if sand looks white or many minerals if sand looks brown, 
 Some sands in soil will be brown, yellow, or red because of Fe and/or Al oxide coatings. 
 Feels gritty 
 Considered non-cohesive – does not stick together in a mass unless it is very wet. 
 Low specific surface area 
 Sand has less nutrients for plants than smaller particles 
 Voids between sand particles promote free drainage and entry of air
Holds little water and prone to drought 
Silt: 
 < 0.05 mm to > 0.002 mm 
 Not visible without microscope 
 Quartz often dominant mineral in silt since other minerals have weathered away. 
 Does not feel gritty 
 Floury feel–smooth like silly putty 
 Wet silt does not exhibit stickiness / plasticity / malleability 
 Smaller size allows rapid weathering of non quartz minerals 
 Smaller particles – retains more water for plants and have slower drainage than sand. 
 Easily washed away by flowing water – highly erosive. 
 Holds more plant nutrients than sand
. Clay: 
 Wet clay is very sticky and is plastic or it can be molded readily into a shape or rod.
 Easily formed into long ribbons 
 Shrink swell – none to considerable depending on the kind of clay. 
 Pores spaces are very small and convoluted Movement of water and air very slow 
 Water holding capacity Tremendous capacity to adsorb water- not all available for plants. 
 Soil strength- shrink/swell affects buildings, roads and walls. 
 Chemical adsorption is large
Soil Microbes
 What Are Microbes Doing For Plants? 
• Providing nutrition 
– Mycorrhiza 
– absorb water and nutrients
– Nitrogen fixation and siderophores
– rhizobacteria 
– Decomposition of dead material into plant usable 
– Store nutrients in their cells 
• Preventing pathogens 
– Bacteriocins and antibiotics 
– Competition 
– Molding the architecture of the soil 
– Aeration 

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