ssc - physical properties of soil

particles larger than 2 mm

coarse fragments

0.002 - 0.05 mm

silt

silt

medium sized, primarily mineral

0.05 - 2 mm

sand

clay

smallest, secondary minerals, carries surface charge, holds most nutrients

sand

largest, primarily mineral, doesn’t directly contribute to soil fertility

less than 0.002 mm

clay

properties influenced by texture

water-holding capacity, compactibility, pollutant leaching potential

properties influenced by surface area

texture, site of water/gas adsorption, mineral weathering, cation exchange, microbial colonization

adsorption

particles stick to the outer surface of a solid

shape of clay

plate-like, more surface area

shape of sand

round or cube like

methods of determining soil texture

feel method in the field, hydrometer, pipette method

feel method

based on grittiness, stickiness, length of ribbon

hydrometer method

measures suspended particles to determine amount of clay, silt, uses stokes law

sandy/coarse soil qualities

low OM and native fertility, very permeable, low water retention, nutrient leaching, low cation exchange and buffer capacities, good for road foundations and building sites

loamy/medium textured soil qualities

more fertile and OM, higher CEC and buffer capacity, less rapid movement of water, retains moisture and nutrients better

clay/finer textured soil qualities

hard to manage, too sticky when wet and too hard when dry, shrink-swell makes it bad for construction, high water-holding capacity but mostly unavailable to plants

soil structure

how soil particles stick together to form structures based on physical and chemical properties, primarily responsible for pore space

aggregates

clumps of soil separates

peds

aggregates bound into larger masses

structureless soil - single-grained

individual particles don’t stick together, common in sandy soils and c-horizon

structureless soil - massive

no visible structure, hard to break apart, found in very large clods and in c-horizon

structures poor for plant growth - platy

thin, flat plates of soil that lie horizontally, found in compacted soil

structures poor for plant growth - columnar

vertical columns with salt cap at the top, found in arid climates and subsoils (b-horizon)

structures poor for plant growth - prismatic

vertical columns a few cm long, found in b-horizon

structures good for plant growth - blocky

irregular blocks common in b-horizon

structures good for plant growth - sub-angular blocky

slightly more rounded, common in b-horizon

structures good for plant growth - granular

texture like cookie crumbs, small diameter, common in a-horizon with roots

effect of OM on stability

increases stability, heps soil keep structure when wet

effect of fungal hyphae on stability

surround aggregate like a net, protecting individual aggregates and binding them into peds

soil consistency

resistance to deformation and rupture, degree of cohesion and adhesion

determining moist soil consistence

gently squeeze ped until it falls apart

soil consistence: loose

hard to pick out a single ped, falls apart before handling

soil consistence: friable

breaks with small pressure

soil consistence: firm

requires good amount of presure, dents fingers before breaking

soil consistence: extremely firm

can’t be crushed with fingers

liquid limit

moisture content where a fine-grained soil no longer flows like a liquid

plastic limit

moisture content where a fine-grained soil can no longer remold without cracking

coloring agents

water, minerals, OM

properties inferred from soil color

OM content, water movement and content, aeration, vegetation, horizon development, heat transfer, biological activity

munsell color book components

hue, value, chroma

hue

dominant spectral color of the rainbow, 0-10

value

relative lightness/darkness of soil color, measures addition of grey, black (0) to white (10)

chroma

relative purity/strength of color, intensity/saturation of hue, 0-14 dull-bright

dark soil color meaning

OM goes from brown to black as it decomposes, soils high in OM look darker wet than dry

yellow soil color meaning

presence of iron oxides

red soil color meaning

intense weathering and aeration cause formation of red hematite

grey soil color meaning

comes from oxidation-reduction in the soil, red ferrous iron is water soluble and leaves behind grey silica color

white subsoil meaning

spodic conditions in acidic, sandy soils, caused by calcium and sodium carbonates

white soil from salt

high evaporation rates in arid areas leave salt residue

bulk density

mass over volume of undisturbed oven-dry soil

bulk density equation

(mass of oven-dry soil)/(volume of soil)

particle density

density of solid particles only, does not consider pore space, greater than bulk density

porosity

total pore space, measure of soil volume that holds air and water

porosity relation to bulk density

inverse

macropores

large, drain water rapidly

micropores

small, retain water tightly, unavailable to plants

mesopores

medium sized

porosity equation

(pore volume)/(soil volume)

permeability

ease of water, air, roots to move through soil

soil strength

ability to resist being moved by a force, comes from compaction

plow pan

plow breaks up higher level soil but compacts soil beneath, creating a shallow layer