Organic Horizons Flashcards
Where do organic soils develop?
At the surface of the soil profile, above the A mineral horizon from the accumulation of leaves, twigs, coarse woody debris, usually not saturated for long time periods.
What are organic soils collectively reffered to as?
“Humus” form or “Forest floor”
What are the master organic horizons?
Litter
Fermentation
Humus
What composes the litter layer?
Accumulation of leaves needles and twigs as well as coarse woody debris that is easily discernible from its original form.
What composes the fermentation layer?
Accumulation of partially decomposed organic matter, mainly components of the litter layer but could be difficult to recognize. May be partly processed by soil fauna as in Moder soils, or may be in a partially decomposed mat permeated by fungal hyphae as in Mor soils.
What composes the H layer?
Accumulation of unrecognizably decomposed organic matter. Has greater humification than the F horizon due to action of soil organisms. May be sharply delineated in Mor soils where decomp. is dictated by fungal activity, or may be partially incorporated into the mineral soil as in Moder and Mull Ah.
What is the minimum requirement for a soil horizon to be “organic”?
By definition an organic horizon contains at least 17% organic carbon by weight - approximately 30% organic matter by volume.
Which type of ecosystems are O horizons found in?
Semi terrestrial ecosystems - wet sites since the humus form development is strongly influenced by the water table at or near the surface for significant periods of the year. They can also be found on poorly drained upland soils.
What are the 3 different classifications of the O horizon?
differentiate.
fibric, mesic or humic depending on the rate or extent of decomposition
Of fibric - material would be quite coarse and recognizable
Om mesic - material would be moderately decomposed and difficult to recognize
Oh humic - material is well decomposed, humified and unrecognizable.
Give some typical features of LFH horizons
Terrestrial Ecosystems
- sloping to level physiography
- very xeric to hydric SMR
- rapid to imperfect drainage
- generally absent water table
- presence of dissolved oxygen
- non hydrophytic vegetation
- predominantly aerobic soil organisms (fungal mycelia or actinomycetes)
Give some typical features of O horizons
semi-terrestrial ecosystems
- Generally depressional sites
- poor to very poor drainage
- water table at or near surface for large portion of year
- very little to no dissolved oxygen
- hydrophytic vegetation
- Anaerobic conditions (few bugs, bactera or fungi)
What are actinomycetes?
what environments are they most common in?
broad group of bacteria that grow as a hyphae like fungi. Actinomycetes are especially important in degrading tough compounds like cellulose.
Tend to be active in higher pH environments
List the soil biota groups and their respective sizes with some examples.
Megafauna: greater than 20mm. moles/rabbits/rodents
Macrofauna: 2-20mm. woodlice, earthworms, slugs etc, centipedes…
Mesofauna: 100um to 2mm. Tardigrades, mites and springtails.
Microfauna/flora: 1-100um. yeast, bacteria, fungi, protozoa…
How do soil organisms work together to improve soils?
Soil detrivores like earthworms ingest detritus and decompose it. Saprotrophs extract soluble nutrients from the detritus.
What name is given to soil bacteria that likve in close proximity to plant roots?
Rhizobacteria
Where do soil bacteria live? hint… not in soil
Soil water, including hygroscopic water that forms a film around soil particles.
Where would you expect to find aerobic soil bacteria?
most active in soils that are moist but not saturated with a close to neutral pH. Since they feed on carbohydrates and micronutrients from organic matter they are found where OM is present.
How many bacteria would you expect to find in a gram of soil?
100 million to 3 billion
how do bacteria reproduce?
binary fission when conditions are favourable
What is the collective term for soil flora and fauna?
Biomas
What is detritus?
A collective term for soil organic matter.
- litter above
- dead and dying plant root systems and soil organisms below
What is s more acceptable term for decomposition in soils?
Humification
describe the humification process
organic material is processed at the surface through bacterial/fungal action. The left over material (Humus) is transported downward into the soil by soil organisms and organic complexes known as ligans that form chelates with Fe and Al ions are moved down by water.
Name some vital functions of OM in the environment
- water retention
- improving soil structure as a bonding agent
- source of nutrients for plant growth
- increased aeration
- reduction of soil erosion
- source of nutrition for soil fauna
What are the typical CEC of humus?
of clay?
of sandy loam?
humus: 2.5 - 4.0 me/g
clay: 0.4 - 0.5 me/g
sandy loam: aprox 0.15 me/g
What is the typical OM content in podzols?
A horizon and B horizon seperate*
A: 3-5% OM
B: 1-2% OM
Describe mineralization
conversion of organic nutrients to inorganic forms, mostly by microbial decomposition.
what is enzymatic oxidation?
decay initiated by bacteria, producing C02, H20, heat and inorganic essential elements and compounds. Key process in mineralization.
name some decay resistant organic compounds?
Oils, fats, waxes, cellulose, lignin
what is humus composed of?
Carbon Hydrogen Oxygen
How would you determine soil organic matter?
-as a function of colour
or
-A method known as ashing where a known volume of soil is weighed, incinerated at 500 deg. and then reweighed to determine the relative proportion of OM.
differentiate between photoautotrophic bacteria and chemoautotrophic bacteria.
Photo - derive energy from sunlight
Chemo - derive energy from oxidation of inorganic minerals (nitrobacteria)
What sites would you expect to find algae and what is the importance of algae?
include diatoms and are common on richer sites that are well supplied with bases, available N and P.
They aid in solubilizing minerals and speed up the weathering process. They contribute to the humus content by increasing the amount of organic matter from inorganic material.
How do fungi improve plants nutrient uptake?
mycorrhizal fungi can act as root extensions of trees, giving the trees access to greater volumes of soil beyond the “nutrient depletion zone.”
differentiate ecto and endomycorrhizae
ecto - bulk of the tree-root hyphae form on the outside surface of root cortical cells
endo - tree-root hyphae develop within cortical cells of roots.
How does most usable nitrogen enter soils?
biological nitrogen fixation by living organisms and nitrogen fixing bacteria.
Lightning can also add plant soluble nitrogen to soils in the form of nitrate (NO3)
Describe Nitrification as it portends to soils
Conversion of organic nitrogen to its inorganic form in two steps
- Ammonia (NH3) oxidized by nitrosonomas to Nitrite (NO2)-
- Nitrate oxidized to Nitrate (NO3) by nitrobacter
- only happens in well aerated soils since nitrobacter and nitrosonomas are aerobic chemoautotrophs.
name in sequence, fastest to slowest decomposing OM types.
- sugars/starches/ simple proteins
- Crude proteins
- Hemicellulose
- Cellulose
- Fats, Waxes
- Lignins and phenolic compounds
What do site climatic conditions refer to?
- Drainage
- Temperature
- Moisture
- pH
What are some signs of nitrogen deficiency?
- yellowish tinge in leaves
- poor growth rate
- stunted needles
- purple tipping
What is a typical nitrogen per Ha production rate of Alders association with Frankia?
100 Kg/ha of nitrogen per year.
What is volatilization
loss of N to the atmosphere
NH4 + Hydroxyl —- H20 + NH3(gas)
How is nitrogen fixed?
when nitrogen is produced as NO3 or NH4 it is either absorbed by plants or becomes fixed to cation exchange sites of humus and clay colloids.
other than Humus, what are some other end products of decomposition?
how much organic carbon remains stored in these states?
Fulvic acids: light in colour (50 years to decay)
Humic acids: darker and last longer than fulvic acid
Humins: Black and can take centuries to decay.
10-40% of organic carbon held in these states
