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.
