Soils Final Exam

Starts off with Ch. 7.4 ( Salinity ) - Soil classification

what is sodicity ?

Sodicity refers to the presence of high levels of sodium ions in soil, which can negatively affect soil structure, water infiltration, and plant growth. It is typically measured by the sodium adsorption ratio (SAR) and is an important factor in soil health.

How is exchangeable sodium percentage (ESP) calculated ?

ESP is calculated by dividing the amount of exchangeable sodium in the soil by the total cation exchange capacity (CEC) and then multiplying by 100 to get a percentage.

• levels > 15 are an issue

What is the sodium adsorption ratio (SAR)?

The sodium adsorption ratio (SAR) is a measure of the proportion of sodium to other cations in the soil, calculated by the ratio of sodium concentration to the square root of the average concentration of calcium and magnesium. SAR is used to assess soil sodicity and its potential impact on plant growth.

Na+/ [ 0.5 (Ca) + 0.5 (Mg)] ^1/2

How are salt affected soils classified ?

by EC, SAR (ESP) & soil pH into either saline, saline-sodic, & sodic soil categories

Describe saline soils.

• Electrical Conductivity (EC): Typically EC > 4 dS/m (a standard threshold used to classify a soil as saline).

• dominant salts: Ca & Mg

• pH: usually neutral to slightly alkaline ( 7-8.5)

• clay dispersion not problematic

  • white crust/ alkali: white salt crust on soil surface

Describe saline-sodic soils.

• Saline–sodic soils are soils that contain both high levels of soluble salts and high levels of exchangeable sodium (Na⁺).

• possible colloidal dispersion if leached ( removal of salts can cause Na to disperse clay particles, destroying soil structure

Describe sodic soils.

• sodium dominated (SAR > 13 )

• pH- higher than 8.5

  • high pH causes OM to disperse/ dissolve & migrate up by capillarity > black crust ; clay dispersion w/ poor permeability (puddled condition ) & areation > commonly devoid of plant life

• commonly forms a thin A horizon over a clay layer with columnar structure

what are the 2 causes of sodic soil dispersion?

1. High Na+

2. low salt

Describe cause 1) High Na+

large ions & positive charge result in loose bonds w/ colloid surfaces

Describe cause 2) Low tot. salt concentration.

strong ionic gradient causes cations to diffuse away from clay which contributes to a thick ionic swarm

In what 2 ways do plants respond to salt affected soils, in addition to nutrient deficiencies?

1. osmotic effects

2. specific ion effects

what do soluble salts do to osmotic potential in soil water?

expend energy to accumulate solutes inside cells

• more difficult for plant roots

• young plants are most susceptible

what are 3 ions that can be toxic to some plants?

• Na

• Cl

• HCO3

  • Na competes with uptake for useful ions (e.g., K+)

what are some signs of salinity in plants?

small, bluish , dulll leaves ; scorching of outer tips of older leaves

What are 2 types of plants that can grow in salty soils ?

1. halophytes ( salt loving )

2. salt tolerant

why are soil managers in certain regions required to understand salt balance ?

for the equilibrium of salt inputs (Irrigation) & outputs (drainage)

• field drainage is always more saline than inputs ( flushing & releasing out salts )

• drainange can cause downstream environment issues > disposal issues & costs

what is soil reclamation ?

the restoration of chemical & physical properities conducive to high productivity > leaching with high quality irrigation

what is leaching requirment (LR) ?

the excess amount of water needed to remove salts

• ratio of irrigation salinity to max salinity specific to individual crop requirments

  • desirable to lower LR & amount of drainage water to be disposed

what 2 things do management attempt to do?

1, protect root zone from salt

2. lower drainage for water conservation

• High quality, immediate irrigation water when plants are young; switch later

• alternate furrow irrigation & roots grown adjacent to water furrow ( narrow channel in a field that drains water)

what affects spatial distribution of irrigation salinity?

soil texture

What is saline-sodic soil management ?

ractices that remove excess soluble salts and replace sodium (Na⁺) on soil colloids with calcium (Ca²⁺)—because these soils have both high salinity and high sodium,

• Na can be removed by Gypsum (CaSO4 * H2O) application

• replaced Na forms soluble salt, Na2SO4 > subsequently leached ( soil dissolves salts, carries them downward out of root zone0

What are the three categories of organisms that live in soil for part or all of their lives?

1. Fauna (animals) & flora ( plant roots, algae, non animals )

2. herbivores, parasites, predators, detrivores, fungivores, etc.

3. Heterotrophs & autotrophs

what are the different sizes of Fauna?

macro- ( > 2 mm )

meso & micro ( > 0.1 mm)

What is high species diversity commonly associated with ?

functional diversity

what is meant by functional redundancy?

multiple organisms carry out similar biological / enzymatic processes

What is meant by stability?

the ability soil to cycle nutrients, assimilate organic wastes, maintain physical structure, etc.

what is meant by resilience?

ability to bound back after severe environmental disturbance

what are keystone species ?

species whose primary responsibility for accomplishing important processes ( nitrification, burrows, etc)

What is global biodiversity ?

where below ground species are possible greater than above ground, on land species

what are primary producers?

organisms that make organic molecules; base for food web

what are primary consumers?

consumers that use energy from plant residues; macro & meso-fauna devour into smaller pieces; microbes decompose dead tissues

Describe the 3 types of primary consumers.

• herbivores - eat live plants (e.g., insect larvae, termites, ants, mice )

• detritivores - most numerous ; plant detritus

• saproptrophic fungi/ bacteria - most decmposition of dead organic remains

what type of primary consumer greatly outnumbers autotrophs in soil?

heterotrophs

what are secondary consumers?

• some are microbes & carnivores (e.g., centipedes, mites); eat microbes

what are tertiary consumers?

• predatory carnivores (e.g., ants, birds, moles )

  • releaase nutrients tied up in living cells > microbes in digestive tracts facilitate digestion of resistant OM & eventually decompose feces/ dead animal bodies.

what are the different factors that influence soil organisms ?

• amount & quality of food , also physical, biotic, & chemical factors

  • undisturbed native soils usually > cultivated fields

what is soil biomass?

• living fraction of the soil

• usually 1-5% of total soil organic matter

• importance of specific groups of organisms

  • i. # of individuals

  • ii. biomass per unit are or volume

  • iii. metabolic activity ( CO2 respiration )

• microbes dominate biological activity in most soils

what are 3 hotspots for microbes to concentrate in ?

• detrital tissues

• fecal pellets

• plant roots

why are earthworms important?

• perhaps most important macrofauna in humid, temperate regions

  • eat deitritus, organic matter, & microorganisms

  • don’t eat living plants or roots

What are epigeic earthworms ?

• small

• live in the litter layer of organic rich surfaces

• hasten decomp but don’t mix OM into the mineral soil

what are endogeic earthworms?

lives in upper 10-30 cm of soil w/ shallow horizontal burrows

what are anecic earthworms ?

forms vertical, durable burrows that are meters deep > emerge to forage on surface litter & bring back to burrows

what are burrow systems ?

plant root pathways; increased aeration & water infiltration into soil

what are casts ?

• they ingest slil globules

• OM mixed w/ mineral soil > enhances aggregate stability > enhanced soil fertility & productivity

  • higher in bacteria, OM, & available nutrients

what can middens (burrow cover ) do ?

• can leave surface bare of residues

what kind of environment do earthworms prefer ?

• prefer cool, moist, well-aerated soils w/ decomposable OM ( e.g., mulch)

• most active in spring & fall in temperate regions

  • dont live in anaerobic conditions & don’t prefer sandy, saline, or low pH ( <5.5 ) soils

what are predators negatively affect earthworm populations?

• predators, ammonia fertilizer, insecticed application, & tillage

What purpose do ants serve?

• improve soil aeration

• water infiltration

• modify pH

  • associated microbes stimulate N cycling

What purpose do termites serve?

• form a symbiotic relationship w/ anearobic microbes in gut to digest plant cellulose

• produce CH4 (methane) > in tropics & semi-arid zones, remove lots of decaying logs, grasses, & leaves

what does termite saliva do to soil & OM?

they glue soil particles & OM together

what type of environment do soil microanimals live in? Describe nematodes.

• aquatic habitats - similar to lakes & streams

Nematodes:

• unsegmented roundworms > huge diversity > coil into a cryptobiotic (non-respiratory) state during dry conditions

what do nematodes feed on ?

• fungi, bacteria, algae, or are predatory

what 3 roles do nematodes play?

• stimulate nitrogen cycling

  • bacteria have more N than nematodes use

• sold as biological control agent

  • attack insect larvae by bacterial disease

• some species infest plant roots & stunt their growth

  • control measures include rotation of non-host crops or fumigation w/ toxic nematicides > less toxic options increasingly available (e.g., glucosinolates from mustards )

what are protozoa ?

• mobile, single celled organisms ( amoebas, ciliates, flagellates)

• capture & engulf bacteria around plant roots near surface > most varied & numerous of soil microfauna

what’s the purpose of plants ?

• primary producers of OM

  • contribute carbon & energy

  • compete for oxygen

  • roots ( meso or micro-scale) commonly occupy 1% of soil volume ( sometimes more )

what purpose do root hairs serve in plants?

they anchor roots & absorb water & nutrients

• root growth - cells at meristem ( behind cap lubricated w/ mucigel)

Describe fine roots

proliferate in areas of high nutrient concentration

Scarce soil water is a plant response caused by what?

decreasing shoot to root ratio

Describe growth along paths of least reistance (e.g., ped surfaces)

• expansion enlarges pores

• moisture removal stabilizes chemical bonds & increases aggregate stability

Roots exuding organic compounds & decompose support what 2 things?

• support microoganisms & humus formation ( especially in grasslands)

what is the rhizosphere?

• zone of soil significantly influenced by living roots (usuall ~2-3 mm from root )

• deepest in hot & dry areas

• soil pH & nutrient availability commonly different

what is the endorhizosphere ?

loosely packed root cortical cells w/ soil solution & soil microbes in intracellular space

what is the rhizoplane?

outer surface & root hairs contacting soil

what is the exorhizosphere?

soil in proximity to major root influence

what is rhizodeposition ?

• old root decary & root-cap cells secrete high weight mucigel compounds that:

  • lubricates root movement

  • improves root-soil contact

  • stabilizes soil structure

  • protects from toxins

  • provides growth medium for microbes (2-10x bulk soil )

• typically 2% - 30% of dry matter production in young plants

• decreases w/ age but increases w/ soil stress (e.g. soil compaction )

Describe algae

• mostly photoautotrophic eukaryotes near the ground surface

• they swim around in pore water

• add OM & secrete polysaccharides that enahnce soil aggregation a=

• can form lichens with fungi & microbiotic crusts in dry environments

Describe fungi

• there’s immense diveristy

• biomass typically 1000- 15,000 kg/ha in upper 15 cm of soil

• dominate many soils

  • aerobic heterotrophic eukaryotes ( some are tolerant of O2)

what are filamentous fungi?

• molds & mushrooms w/ long, threadlike, branching chains of cells

  • hyphae are individual filaments

what role do molds play?

major role in OM breakdow ; can thive in acidic soils ( & higher pH)

describe mushrooms

• forest & grassland areas w/. moisture & organic residue

• visible, aboveground body only small part of extensive hyphae network in soil

• important for breakdown of woody tissue & mycorihizal symbiosis w/ plant roots

what are the most versatile & persistent decomposer of resistant compounds ? How?

fungi

• efficient at utilizing metabolized OM ( up to 50% becomes fungal tissue)

What benefits do fungi provide ?

• soil fertility benefits - nutrient cycling by decomposing complex OM that bacteria aren’t able to do it

• tilth benefits - soil structure maintained by hyphae

• produce compounds lethal to antagonistic organisms (biological control agents ); also mycotoxins toxin to plants & animals ( disease, root root, etc)

what is mycorrhizae ?

• symbiotic relationship b/w soil fungi & plant roots

  • most higher plants require this

  • fungi get sugars from plant cells photosynthetic

  • hyphae extend 5-15cm from root & permits improed uptake of immoble nutrients (e.g., P) & water

  • also can protect plant from toxic Al, other metals, diseases, & nematodes

Describe the 2 soil prokaryotes?

Bacteria & Archea:'

• cells lack a nucleus surrounded by a membrane

• smaller than fungal hyphae ( some can be clay particle size)

• can be in form of soil solution, on surfaces, or as biofilms

Prokaryotes can be either ___ or ____.

either autotrophic ( sunlight or chemical oxidation ) or heterotrophic

• most soil bacteria are heterotrophic & decompose low reistance OM

Prokaryotes participate in what ?

in almost all organic transactions in a healthy soil

• scientific endeavors to harness / facilitate microbe abilities to remediate soils

How is oxidation/ reduction of chemicals controlled by prokaryotes?

nitrificatio; sulfur oxidation to sulfate ( & damaging sulfuric acid ); Fe / Mn & soil color ; nitrogen fixation

Describe the prokaryote: Cyanobacteria

• photosynthesize

• common in wetlands

• fix nitrogen

• form microbiotic crusts in desert soils

Describe the prokaryote: Actinomycetes

• filamentous / branched

• fix nitrogen in mildly acidic to neutral pH soils

• secrete antibiotics that kill other microbes

What do soil microorganisms respond to ?

• infusion of energy rich organic substances

  • fungi dominant in surface litter; bacteria play role when OM is mixed below the surface

  • bacteria communicate w/ organic compounds for coordination

when is soil microbial activity optimal ?

at water potentials near or slightly lower than field capacity

• most aerobic, some anaerobic or facultative (hybrid)

when is microbial activity the greatest?

b/w temps of 20 and 40 degrees C

• < 5 degrees > biological zero ( except for psychrophilic species )

What 2 factors favor bacterial activity ?

high exchangeable Ca & neutral to alkaline pH

• low pH > fungi become dominant

What are the 5 beneficial effects of plant growth & soil ecology?

1. decomposition of plant detritus & assimilation of animal wastes > mineralization of nutrients

2. toxic organic compounds brokwn down by soil prokaryotes & fungi →

   most occurs in the upper layers of soil

3. Oxidation of Fe, Mn, Se, Cr to non-toxic levels

4. Nitrogen gas (N2) fixation (actinomycetes) in root nodules; cyanobacteria

5.  Rhizobacteria (root-associated) → enhances nutrient uptake & suppresses

   some diseases

what are biocrusts ?

• undisturbed arid & semi-arid regions → mutualistic

associations of algae, cyanobacteria (particularly important),

fungi, mosses, bacteria, &/or liverworts

• Binds soil & protects from wind / water erosion; traps nutrient-rich dust

How do biocrusts improve ecosystem productivity ?

• Conserve & cycle

  nutrients

  ➢ Increase nitrogen by

  cyanobacteria fixation

  ➢ Increase infiltration &

  reduce evaporation

  ➢ Net OM production by

  crust photosynthesis

what are 3 common damages to plant growth by soil organisms?

➢ Herbiverous fauna → notably nematodes & insect larvae

➢ Fungi responsible for many plant diseases (root rots, wilts, etc.)

➢ Deleterious rhizobacteria → noninvasive chemical interactions can

cause stunted plant growt

what are disease suppressive soils?

disease fails to develop despite

presence of virulent pathogen &

susceptible host → pathogenic

organism inhibited by antagonism

from other bacteria & fungi

what is general suprression soil?

high levels of microbial activity → competition,

antibiotic production, & mycorrhizae

what is specific suppression soil ?

a single species or group that inhibits or kills pathogen

what is meant by induced systemic resistance ?

signaling chemical in plant shoots

derived from rhizobacteria → defends against pathogen

what type of soil management is used to control plant disease?

quarantine systems,

crop rotation w/ non-susceptible

plants, mulching

What are different factors that could contribute to plant disease?

➢ Soil pH regulation → dependent on

type of fungal or microbial disease

➢ Nutrient balance to ensure mature,

healthy plants → too much N can

increase plant susceptibility to fungal

diseases

➢ Controlled introduction of organic

toxins (e.g., broccoli residues)

➢ Soil compaction can aggravate

fungal diseases

➢ Solarization (clear, plastic sheeting)

or chemicals to sterilize undesirable

microbes → but also kills off

beneficial mycorrhizal fungi & others

How do monocultures effect the soil ecosystem?

by reducing the functional diversity in soil ecosystems

how does tillage affect the soil ecosystem?

greatly disrupts the soil ecosystem

➢ Fungal hyphae networks, earthworm burrows, speeds loss of OM

how do pesticides affect the soil ecosystem?

fumigants / nematicides sharply & temporarily

reduce organism numbers → might result in a dominant microbe

Increases in nutrients & pH lead to what ?

generally greater microbial activity with OM

additions

(9.1) Describe soil organic matter

•  key to soil health

  • (i.e., water retention, CEC,

    soil structure, nutrient supply)

  • is about half carbon (C) by mass → foundation

    of all life → tissues as C compounds of chains / rings

what happens when there is disruption of the global carbon cycle ?

• it can be disastrous to life

Describe the global carbon cycle.

➢ Plants take up CO2 from

atmosphere → some returned by

respiration (roots) → some

photosynthesized as organic

molecules (tissue)

➢ Plant litter → soil → CO2 returns

➢ Carbonates / bicarbonates