Soil Science Exam 2.0

Macrofauna/flora

mammals, most insects, plants, mosses

Mesofauna

nematodes, protozoa

Microorganisms

nematodes, protozoa

Mircroorganisms

fungi, bacteria, actinomycetes, algae

Soil Organism abundance depends on?

quantity & quality of food, environment, predators/competition

Autotrophs

get energy from photosynthesis

Heterotrophs

get energy from consumption of C

Earthworms

segmented worms, burrows, casts; chem/phys properties of soil

Termites

ant-like insects that eat wood/residues; extensive mouths that can alter soil development

Nematodes

• unsegmented worms

• most prevalent in sandy soils

• predators of nematodes, fungi, bacteria, algae, protozoa, insect larvae

• plant parasites & plant biological control agents

Protozoa

• single-celled, mobile

• moist well-drained soils

• prey on bacteria

Algae

photosynthesize, some form association with fungi (lichen) that help colonize rock

Fungi

extremely diverse, like acid soils (low pH); includes yeasts, molds, mushrooms

• prefer pH 4-5.5

Yeasts

single-celled, often in anaerobic soils

Molds

important in OM breakdown

Mushrooms

high moisture & residue

Mycorrhizae

special type of fungi-plant root association, known to improve P (also Zn) uptake

Actinomycetes

• type of bacteria that looks like a fungi

• OM degradation (particularly resistant compounds)

• produces antibiotics

• prefers high pH (7-8.5)

Bacteria

• extremely diverse, rapid reproduction

• many roles

  • N reactions

  • OM & hydrocarbon breakdown

• prefer pH 6-7

What are the factors affecting MO growth?

food, environment, predators/competition

What are the effects of organisms on soil?

organic matter decomposition, breakdown of toxic substances, inorganic transformations, N fixation, plant protection/plant pests, physical mixing of soil, competition for O₂ when supply is limited

OM Decomposition

C and nutrient mineralization

• Residue + O₂ = CO₂ + humus

• termites and other insects involved as well as bacteria, fungi, and actionmycetes

What are the effects of carbon on soil?

soil darkening, CEC, slow release of nutrients, water holding capacity, stabilization of aggregates, infiltration, food for organisms, reduced plasticity of soils (improved manipulation of clayey soils), stabilizes pH, alleviates some toxicities, improves availability of some nutrients

Rate of Degradation

carbohydrates, hemicellulose/cellulose, protein, fats/waxes/oils, lignin, polyphenols/tannins

High C/N Ratio

competition for N = immobilization

Low C/N Ratio

plentiful N = mineralization and plant availability of N

Effects of Lignin on Degradation

• reduces immobilization when C/N is high

• delays mineralization when C/N is low

Biomass

living

Detritus

dead, but identifiable

Humic Substances

cannot define, but not inorganic

Non-Humic Substances

cannot identify source, but can classify chemically

What are the characteristics of humus?

colloidal, negatively charged, high water holding capacity, dark colored

What happens when you add C?

• add residue, compost, manure, green manures

• increase plant biomass

How can you reduce losses of C?

• reduce harvested biomass

• reduce tillage

• reduce erosion

Factors affecting Soil C

warm temps, water, vegetation type, soil texture, and poor drainage

• cool/wet is HIGH SOM

• clay can reduce decomposition and promote fertility

Matric Potential

micropores

Osmotic

water moves toward salt

Gravitational

macropores

Positive soil water potential

moves freely with gravity

0kPa or bars

saturation, water moves primarily via gravity

-10 to -30 kPa (-0.1 to -1/3 bars)

field capacity, water moves primarily via capillary

-1500 kPa (-15 bars)

permanent wilting point, water move primarily via capillary

< 1500 kPa

hygroscopic water, water doesn’t move

Saturated Water Flow

gravitational

Unsaturated Water Flow

capillary

Preferential Water Flow

gravitational