Soil Science Lab Study Guide

which parent material releases more plant nutrients?

organic matter

which parent material weathers faster? (Ca & Mg or Al, Si & Fe)

Ca and Mg

understand the relationship between a mineral’s chemical composition, susceptibility to weathering, and soil composition (the presence of coarse and fine materials)

A mineral's chemical composition directly dictate its weathering susceptibility, which in turn determines the resulting soil composition. Resistant minerals become coarse sand, while weatherable minerals chemically transform into fine silt and clay.

disintegration (physical weathering) vs. decomposition (chemical weathering)

disintegration breaks rocks apart physically without changing what they are made of, while decomposition breaks them down chemically by transforming their actual mineral structure

soil =

f (climate, biota, parent material, relief/topography, time)

effects of hot climate on the weathering of rocks and decomposition of organic materials

accelerate chemical weathering, rapidly breaking rocks down into clay, and organic decomposition makes microbes quickly break down plant matter, often leaving thin topsoils

effects of cold climate on the weathering of rocks and decomposition of organic materials

slow chemical weathering, leaving physical weathering dominant, and severely slows organic decomposition allowing organic matter to accumulate

effects of management and topography on the colorations of the soil

topography and management control soil color by affecting drainage, oxygen, and organic matter. waterlogged lowlands produce gray, blue-gray, or mottled soils due to low oxygen and reduced/leached iron, but installing drainage introduces oxygen to waterlogged soils, shifting their color from gray back to brown

conclusions that can be drawn about soil drainage based on the presence of red/ yellow red colors

indicates well-drained, aerobic (oxygen-rich) soils

conclusions that can be drawn about soil drainage based on the presence of grey/ blue grey colors

indicates poorly drained, anaerobic (waterlogged) soils

conclusions that can be drawn about soil drainage based on the presence of grey/ reddish brown colors

indicates somewhat poorly drained or seasonally wet soils

hue

red, yellow-red, and yellow cover most soil colors

value

reflected light

chroma

hue intensity/purity

matrix color

dominant color

impacts of a toposequence on soil development

by driving how water, nutrients, and sediments move down a slope

how soil color changes in higher and lower elevations within a slope

due to the physical movement of particles by water erosion and deposition, soils shift from bright red/orange at the top (oxidized) —> lighter/paler on steep slopes (eroded) —> dark topsoils over grey subsoils at the bottom (organic accumulation and waterlogging)

how soil texture changes in higher and lower elevations within a slope

changes across a slope due to the physical movement of particles by water. mid slopes (erosion) —> water runoff strips away light, fine particles, leaving behind a coarser, more sandy or gravelly texture. lower elevations (deposition) —> eroded fine particles settle at the bottom of the slope, resulting in finer, heavier, clay-rich soils

how to properly characterize a soil pit wall

mark boundaries of the pit, identify different soil layers (horizons), characterize each horizon

which tends to have thicker soil profiles on a hillslope, summit vs toeslope

toeslope has a thicker soil profile because it acts as a deposition zone where eroded soil, sediments, and organic matter from higher up the slope continuously accumulate

particle size and its effects on nutrient and water holding capacity; sand (coarse particles)

large spaces (macropores) between grains allow water to drain quickly through gravity, nutrient holding capacity is low and lacks surface area and is chemically inert, meaning essential nutrients easily wash away (leach)

particle size and its effects on nutrient and water holding capacity; silt (medium particles)

smaller pores hold water better than one size while still allowing adequate drainage, nutrient holding capacity is moderate and offers more surface area than one but less than the other

particle size and its effects on nutrient and water holding capacity; clay (fine particles)

tiny microscopic pores hold onto water tightly (though some of this water can be held so tightly that plant roots struggle to absorb it), nutrient holding capacity is very high and clay particles have a massive surface area and a negative electrical charge, which acts like a magnet to attract and hold onto positively charged nutrient ions (like calcium, magnesium, and potassium)

sand

2mm-0.05mm

silt

0.05mm-0.002mm

clay

0.002mm>

relationship between size and specific surface area

particle size decreases, the specific surface area increases exponentially

order to characterize the soil horizons

top to bottom; color —> structure —> texture

importance of a stable soil aggregate

transform loose dust and particles into a structured, living sponge that resists erosion, breathes well, and manages water efficiently

factors affecting aggregate stability (disturbed or tilled versus undisturbed)

tillage acts like a reset button that physically smashes aggregates and starves the soil of its organic glue, while undisturbed management lets biology weave and protect a resilient, spongy soil structure

what is the commonly used particle density used when no Dp given

^{2.65 g/cm3}

relationships between soil compaction & bulk density & porosity

higher compaction means higher bulk density and lower porosity

what land use practices can impact bulk density and porosity?

intensive tillage, logging, and overgrazing

importance of bulk density and porosity from an agricultural standpoint?

high bulk density (compaction) creates mechanical resistance that physically blocks roots from expanding, stunting plant growth while low porosity causes waterlogging and suffocates roots.

importance of bulk density and porosity from an engineering standpoint?

need low porosity and high bulk density (compaction) beneath foundations, roads, runways, and parking lots to prevent structural settling or shifting under heavy weight

qualitative expressions

descriptive, sensory, and behavioral terms used in the field to characterize soil properties that cannot be easily or immediately measured with hard numbers

saturation

every single pore space (both macropores and micropores) is completely filled with water, and air is pushed out

field capacity

the amount of water remaining in the soil after excess water has drained away due to gravity (usually 2 to 3 days after a heavy rain or irrigation).

permanent wilting point (PWP)

the point at which the soil holds water so tightly that plant roots can no longer extract it. if water drops to this level, plants wilt and cannot recover even if the sun goes down or humidity rises

quantitative expressions

use precise mathematical measurements, physical formulas, and numerical values to evaluate soil properties

structureless

no observable peds (either single-grain like loose sand, or massive like a solid, uncracked block)

weak

peds are barely observable and mostly fall apart when handled

moderate

peds are well-formed, durable, and break out of the soil profile cleanly without destroying the whole mass

strong

peds are very durable, distinct, and clearly dominate the soil matrix even when disturbed

bulk density (Db)

the mass of dry soil relative to its total volume, g/cm³

porosity (f)

the fraction of total soil volume occupied by pore space, expressed as percentage

gravimetric water content

the mass of water divided by the mass of dry soil (expressed as a percentage or g/g)

volumetric water content

the volume of water contained in a given total volume of soil (expressed as a percentage or cm³/cm³

water thickness

the microscopic layer of water that coats the surfaces of soil particles, particularly clay and organic matter, expressed in cm