Soil Science Exam III

Moisture Conditions (suborders)

(from wet to dry) Aquic- poor aeration, low O2, reduced Fe Udic- dry, sufficient for plant needs Ustic- some moisture, but some drougtht period Aridic- moist Xeric- dry long drought periods

soil nematodes

- non-segmented worms that feed primarily on bacteria and fungi - also release N into soil (C:N ratio lower than bacteria or fungi)

Soil Protozoa

- single-celled animals that feed primarily on bacteria - release N into the soil (C:N ratio lower than bacteria) - bacteria consumed by protozoa contain too much nitrogen for the carbon needed. - the excess is released as NH4+

when a base like CaCO3 is added to a soil

- the pH increases - adsorbed H+ cations can be displaced by a cationic component of the base (e.g. Ca2+) - the soil will tend to buffer the pH

soil bacteria

-billions to trillions per gram of soil - single0celled organisms - small (<5um) - most are decomposers that consume simple organic compounds and fresh plant litter - extremely important to form and abundance of various chemical elements in soils including N, Fe, and S - immobilize (retain) nutrients in their cells - stabilize aggregates

Soil actinomycetes

-filamentous bacteria, branched liked fungi, internal structure of bacteria -can break down highly resistant compounds like cellulose and chitin often dominant in the latter stages of decay -can tolerate high salinity and temps -can fix atmospheric N to plant-available forms - can produce antibiotic compounds (1943- Streptomycin)

Types of soil acidity

1. Active 2. Exchangeable 3. Residual

Components of buffering capacity

1. CEC (determines # of negative sites) 2. % BS (determines # of negative sites occupied by acid cations)

Sources of H+

1. CO2 from microbial respiration/atmosphere (CO2 dissolved in H2O produces carbonic acid) 2. Anthropogenic acid rainfall (gaseous N and S oxides) 3. Acid functional groups on OM 4. Plant root exudation of H+ 5. Hydrolysis of aluminum

principles of soil taxonomy

1. classify soils on the basis of soil properties 2. soil properties should be readily obsservable/measurable

Consequences of low alkalinity

1. generally higher CEC 2. less weathering; more 2:1 clays 3. Greater pH dependent negative charge 4. Bad pH for plants (limits micronutrients)

diagnostic components for classification

1. surface horizons (epipedons) 2. subsurface horizons

factors affecting rate and degree of carbon cycling

1. the nature or quality of the substrate 2. Carbon:Nitrogen (C:N) ratio 3. environmental factors: temp, moisture, pH, O2

Measuring salinity

1. total dissolved solids (salt) -> dry a solution extract, and determine the weight of solids 2. Electrical conductivity (proportional to salt content of the solution) -> pure water conducts poorly; conductivity increases with more salt; measured in dS/m

Soil fungi

10-100 billion/m^2 -cell with nuclear member and cell wall -most versatile and more active in acid forest soils -tolerates extremes in pH can convert recalcitrant OM to forms other organisms can use -hyphae stabalize aggregates -important to development of soil humus

BS% Ultisol

<35%

BS% Alfisol

>35%

threshold for what are considered saline soils

>4 dSm^-1 (electrical conductivity)

Osmotic adjustment

A biochemical mechanism that helps plants acclimate to dry and saline conditions.

Diagnostic Subsurface Horizons

Accumulation or loss - OM (dark colors, metals) - Clays (smectites, kaolinite) - Oxides (iron, aluminum)

Exchangeable acidity

Acidity associated with cation exchange on mineral or organic colloids

Active acidity

Acidity associated with soil solution

Liming (to increase pH)

Adding CaCO3 to soils However, when it reacts with water, you produce both Ca2+ and CO3^2-. Ca2+ displaces H+ from exchange pool, and the pH decreases. So your soil's pH does not increase as much as you think it would. More CaCO3 must be added than would be required to neutralize acidity in the active pool alone.

Sampling Depth and Entisols

At 2 meters (200 cm). If you dig deeper, you may find Bt, not C Horizon, which would mean that it's not an Entisol. So there are a lot of misdiagnosed soils in FL.

Percent base saturation

Base cations: Na, K, Mg, Ca Charge basis Exchangeable bases (cmolc/kg)/CEC (cmolc/kg)

Great group

Based on diagnostic horizons and their arrangements or other features like age, color, texture Arg- argillic horizon Pale- old Kand- kandic horizon Quartzi- quartz sand Hum- humid Alb- leached

Factors that influence carbon cycling: C:N ratio

C:N ranges from 5- >600 insufficient N can slow decomposition/cyclingC:

Florida Entisols

Dry, sandy uplands Turkey oak, wiregrass Excessively well drained deep sands, no water restriciton 2nd most dominant soil order in FL

Entisol (Soil Order)

Ent = recent Young soils Weakly devleoped Sandy or clayey A-C profiles Found by: Recent Alluvium, Fresh lava , Arid regions

Buffering

Exchangeable acidity resists changes in pH

Spodosols

Formed under wet acidic conditions Subsoil accumulation of OM with Al and/or Fe (Bh- spodic) Poorly or very poorly drained Florida pine flatwoods Fluctuating acidic high water tables dissolve OM and translocate it to the subsurface Most dominant soil order in Florida

Suborder

Genetic similarity: Moisture regime, specific diagnostic horizons

What are the acids in soils?

H+ and aluminum

Salinity

High salt concentrations due to lack of leaching. Inhibits water takeup by plants.

Oxic Horizon (Diagnostic Subsurface)

Highly weathered (high temps, high rainfall) High in Fe, Al oxides Contain low-activity clays (kaolinite) <10% weatherable primary minerals Disintegration of secondary minerals by strong weathering produces Fe and Al

Soil Series

Horizon number, order, thickness, texture, structure, color, OM, pH, temp, CEC

Spodic horizon (Diagnostic Subsurface)

Illuvial accumualtion of OM and Al (+/- Fe) Dark (value, chroma <3) Formed under humid acid conditions Low BS%

Argillic Horizon (Diagnostic Subsurface)

Illuvial accumulation of silicate clays (or can form in place) Illuviation based on clay content of overlying horizon Clay bridges Clay coatings Bt or Btg B horizon, which has an increase in clay content at least 20% greater than the elluvial horizon above

How are soils given names based on their taxonomy?

It goes from greater specificity to lower specificity. Great group->Suborder->order Ex) paleudult (old, somewhat dry, ultisol)

Albic Horizon (Diagnostic Subsurface)

Light colored (value>6) Elluvial (E Horizon) Low in clay, Fe and Al oxides Dominated by quartz sand Low chemical reactivity (low CEC) Typically overlies Bh or Bt horizons Albic horizons are E horizons but not all E horizons are albic horizons

Umbric Epipedon

Meets all mollic criteria except is has <50% BS% Develop in regions with somewhat higher rainfall, and parent material with lower Ca and Mg contents

Subgroup

Moisture, sandiness, depth. Typic- shallow to the Bt (<50 cm) Arenic- deeper to Bt (50-100 cm) Grossarenic- deep to the Bt (>100 cm) Aquic- wet Entic- weakly developed Ex) typic paleudult, an ultisol with shallow depth to Bt

Types of Organisms Present in Soil

Most present is bacteria Fungi has most biomass in soil

State Soil of Florida

Myakka fine sand

Histic Epipedon

Organic horizon (peat or muck) Formed in wet areas Black to dark brown Low bulk density 20-60 cm thick overlying a mineral soil Organic = >20-35% OM

Histosols (Soil Order)

Peat or muck 20-30% OM Agriculturally valuable Sometimes used as fuel Often over impermeable material Southeast Florida has this soil, biggest agricultural area

Plaggen Epipedon

Produced by long-term (100s yrs.) manuring Old, human-made surface horizon Absent in US >50 cm

Soil Families

Properties important to the growth of plant roots. 1. Particle size (sandy, loamy, fine loamy, clayey) 2. Mineralogy (siliceous, kaolinitic, smectitic, oxidic) 3. Temp C (frigid <8, mesic, thermic, hyperthermic >22)

Hydrolysis of water (aluminum)

Self-ionization of water Adding aluminum to H2O can release 3 H+ per aluminum

Melanic Epipedon

Similar to Mollic Formed in volcanic ash Lightweight, fluffy, low BD >6% organic carbon and >30 cm thick

Soil Order

Soil forming processes, presence or absence of major diagnostic horizons

Increasing pH in Soils- Bases

Substances which decrease H+ concentration (OH-, CO3^2-) Calcium carbonate CaCO3 is commonly used to increase pH. When it reacts with water, it produces Ca2+ and CO3^2-.

Reducing pH in Soils - Acids

Substances which increase the H+ concentration (elemental sulfur 3-4 months; iron sulfate 2-3 weeks) When you introduce H+ into the solution, you disrupt equilibrium. Cation exchange will occur to try and remedy that, so your pH won't decrease as much as you think it would.

Mollic Epipedon

Thickness: >25 cm Dark color: value, chroma<3.5 Soft Organic Carbon: >0.6% BS%: >50% (mostly Ca and Mg) Temp: >5C to 50 cm Some of the world's most productive soils Typical of grasslands

Ochric Epipedon

Too: thin, light, low in OM Ochric = pale Extremely common

Alfisols/Ultisols (Soil Orders)

Translocation dominant Umbric/Ochric epipedon Bt is diagnostic of these orders Humid native deciduous hardwood forest BS% Alfisol >35% BS% Ultisol <35%

A soil has a high CEC and a low BS. Is it difficult to raise the pH of this soil?

Yes. Sites are occupied by cations and it's difficult to neutralize all those acids.

Is CEC always a good indicator of of fertility?

You can check with cation concentration. H and Al concentration = acid concentration. The rest is base saturation.

the loss of organic soils in the Everglades was because

aerobic heterotrophs broke down organic material faster than it was being added

the decomposition of OM is slowed under

anaerobic conditions

Acid

any substance that increases the H+ concentration of a solution

you have a soil that is a very wet mollisol with an argillic horizon. it is a

argiaquoll (great group= argillic, suborder= aquic, order= mollisol)

an umbric epipedon differs from a mollic epipedon due to

base saturation

all organisms on earth are

carbon based

Soils are resistant to

changes in pH

Factors that influence carbon cycling: environmental factors

decomposition proceeds most quickly when pH is neutral, temp is warm, and both O and moisture are available temp: 25-35C or 77-95F O2, moisture: 60% of pore space filled with water Near neutral pH

epipedons

diagnostic horizons that occur at the surface roughly correspondent with A horizons but can extend to the E or B horizons if colored by OM based on divergence from the mollic

R-NH2 denotes a mineral, plant available form of nitrogen (t/f)

false

exchangeable acidity can be measured with a pH meter

false

when limestone is added to soils, the increase in pH is accomplished by Ca2+ (t/f)

false

aluminum is acidic in soils because it removes H+ from solution by hydrolysis (t/f)

false (H+ is added to solutions)

an argillic horizon is a subsurface accumulation of organic matter (t/f)

false (accumulation of clay)

during aerobic decomposition of organic matter, electrons are transferred from oxygen to carbon in the substrate (t/f)

false (electrons are transferred to oxygen)

active acidity has no impact on exchangeable acidity (t/f)

false (it directly impacts it)

when nitrogen is taken up by a plant, the process is called mineralization (t/f)

false (it's immobilization)

the principle cause of soil alkalinity is high rainfall (t/f)

false (low rainfall leads to alkalinity)

it is easy to increase the pH of a soil with a high CEC and a low base saturation (t/f)

false (you need to neutralize all the acids)

the main difference between an alfisol and an ultisol is the base saturation in the epipedon (t/f)

false (I think it's because they're soil *orders*)

exchangeable acidity is relatively unimportant to soil pH buffering (t/f)

false (buffering occurs due to exchangeable acidity)

a common material used to raise the pH of soils is calcium chloride (t/f)

false (calcium *carbonate* is used)

you have a soil designated as paleudult. this is an old, dry, utilsol (t/f)

false (pale= old, ud= pretty wet, ult= ultisol)

under flooded conditions, oxygen levels tend to be

low this means that there are anaerobic conditions and slow decomposition of OM

Causes of alkalinity

low rainfall (rainfall is acidic and weather minerals)

anaerobic heterotrophs use ___ to accept electrons

other elements such as nitrogen, sulfur, or iron (not O, of course) can use energy stored in complex compounds in the absence of free oxygen; energy is obtained by exchanging electrons with elements other than oxygen

aerobic heterotrophs use ___ to accept electrons

oxygen the energy is obtained by exchanging electrons between carbon and oxygen

Factors that influence carbon cycling: nature/quality of substrates

rapid: sugars, straches, proteins v. slow: waxes, polyphenols, lignins (most recalcitrant)

a soil order that forms under wet, acid conditions in pine flatwoods

spodosols

carbon must be continually cycled to maintain

the availability of carbon for cell growth, development, and energy

pedon

the unit of classification for soils. it is the smallest three-dimensional unit at the surface of th eearth that is considered as a soil. expresses the full range of properties characteristic of a particular soil

both aerobic and anaerobic bacteria produce energy by

transferring electrons from carbon to an inorganic electron acceptor

NH4+ is converted to NO3- under aerobic contiditions (t/f)

true

a Btg horizon indicates an accumulation of clay sunder reduced conditions (color, depletion of iron) (t/f)

true

adding organic amendments with a high C:N can reduce nitrogen levels (t/f)

true

anaerobic respiration is slower, less efficient and produces less energy than aerobic respiration (t/f)

true

gleying occurs under low oxygen conditions (t/f)

true

most soil organisms are aerobic

true

soil organisms prefer a C:N ratio of about 24:1 in their food (t/f)

true

the presence of a Bh subsurface horizon is characteristic of a spodosol (t/f)

true

to increase the pH of a soil, it would be necessary to add more limestone than would be indicated by a measurement of the active acidity (t/f)

true

under conditions of low oxygen content, nitrogen is generally less mobile in the soil environment (t/f)

true