Organic Matter Flashcards
What is organic matter?
Consists of plant and animal residues at various stages of decomposition, cells and tissues of soil organisms and substances synthesized by soil organisms
What is organic matter
Living organisms <10%
Stabilized organic matter (humus) 33%-50%
Decomposing organic matter (active fraction) 33%-50%
Living Organisms
bacteria, fungi, nematodes, protozoa, earthworms, arthropods and living roots
Dead plant material; organic material; detritus; surface residue
refer to plant or other organic substances that have recently been added to the soil and have only begun to show signs of decay
Detritivores
organisms that feed on such material
Active fraction organic matter
organic compounds that can be used as food by microorganisms.
The active fraction changes more quickly than total OM in response to management changes
Labile Organic matter
organic matter that is easily decomposed
Root exudates
soluble sugars, amino acids and other compounds secreted by roots
Particulate organic matter (POM) or light fraction (LF) organic matter
POM and LF have precise size and weight definitions.
Thought to represent the active fraction of OM which is more difficult to define.
Larger and lighter than other types of soil OM (can be separated using a sieve)
Lignin
A hard to degrade compound that is part of the fibers of older plants. Fungi can use the carbon ring structures in lignin as energy source
Recalcitrant organic matter
OM such as humus or lignin containing material that few soil organisms can decompose
Humus or humified organic matter
stable long lasting remnant of decaying OM (a complex, dark-brown, amorphous-heterogeneous [non-crystalline, non-uniform] structure that no longer resembles the decaying matter of origin
Resistant to further microbial decay
has chemical and physical properties of great importance to soils and plants
Humus
not readily decomposed
physically protected inside of aggregates or chemically too complex to be used by most organisms
important in binding tiny soil aggreagates
improves water and nutrient holding capacity
Bacteria
100 million to 1 billion per gram
decomposition in turf soils and compost heaps
Rhizobia- N fixers
food for other members of the food web
Bacteria and their wastes have CEC
Actinomycetes
are a type of bacteria that look like fungi because they form hyhae
they decompose hard to decay substances like chitin and cellulose
White mycelia in dry compost heaps
earthy smell of soil comes from these
Fungi
Dominate in forest soils
Aerobic
Different roles for different species: Mycorrhizae, Decomposers, Pathogenic
Hyphae, mycelia
Protozoa
Single-celled bacteria that feed primarily on bacteria, releasing N as ammonium (bacteria have a lower C:N that protozoa require, therefore excess N is excreted)
Need a water film to move in
1000 to 1 mill per teaspoon
Nematodes
Non-segmented round worms
1mm length
Most are beneficial but some eat plant roots
Operate on several trophic levels: feed on plants and algae, feed on bacteria and fungi, feed on other nematodes
Arthropods
many that influence soil in many ways
Earthworms
Major decomposers; dramatically alter soil
Stimulate microbial activity through their cast (feces)
Mix aggregate soil
increase infiltration
improve water holding capacity (through aggregates)
bury and shred plant residue
Earthworms
Derive their nutrition from fungi and bacteria
eaten by some flatworms and one species of parasitic fly
main predators are birds and mammals
Favored by low-till agriculture
Types of Earthworms
Epigeic: Surface and soil litter
Endogeic: upper soil species
Anecic: deep-burrowing species
What does organic matter do?
of the components…..OM is arguably the most important; also the most misunderstood
serves as a reservoir of nutrients and water
aids in reducing compaction and surface crusting
increases water infiltration
often ignored or neglected
compaction
drier the soil less compaction, the wetter the soil the deeper compaction goes (But not really sig. different)
Surface Crusting
tilling causes small particles, when it rains the small particles are moved and packed together and then dry into hard cement like layer
Organic material is not organic matter
leaves, manure, or plants parts…..not OM
What is the difference between organic material and OM
Organic material is anything that was alive and is now in or on the soil
will eventually become OM through decomposition
Needs to be converted by microorganisms to a resistant state of decomposition
Decomposition
Organic materials to inorganic
biological process that includes physical breakdown and biochemical transformation of complex organic molecules of dead material into simpler organic and inorganic molecules
(C6H10O5)n + O + F&B –> CO2+ H2O + Heat
Organic Material is unstable in soil
changes from mass readily as decomposition takes place
as much as 90 % disappears as a result of decomposition
Organic matter (OM)
is stable (for the most part)
usually only ~5% of OM mineralizes each year
Rate increases based on temperature, oxygen, and moisture: occurs with excessive tillage, stable OM is analyzes in soil test
How much OM is in the soil?
an acre of soil (6in depth) weighs ~2,000,000lbs
if we have ~1% OM weight=~20,000 lbs per acre: takes at least 10lbs of organic material to decompose into 1lb of OM, 100 tons of organic material applied or returned to soil to add 1% stable OM under favorable conditions
How much OM is in soil
Soils formed under prairie vegetation are generally high in OM (top growth and roots)
ex. 1.4 tons of organic material per acre from shoot growth….4 tons acre from root yield
How much OM soil
soils formed under forest vegetation usually have comparably low OM: trees produce a smaller root mass per acre than grass plants and trees are perennials
Much of the OM in a forest is tied up in the tree itself instead of being returned to the soils
Prairie soils have ~twice OM than forest derived soils
Benefits of OM
Nutrient supply: OM is a reservoir of nutrients to be released to the soil, each % of OM in the soil releases ~20-30 lbs of N, ~4.5-6.6lbs of P2O5, and ~2-3lbs of sulfur per year
Nutrient release occurs primarily in spring/summer
Available Phosphate
P2O5 equivalent of fertilizer soluble in water plus neutral ammonium citrate, and considered readily available to growing plants (however it is never found in fertilizer)
Benefits of OM
Water holding capacity:
OM behaves like a sponge: can absorb and hold up to 90% of its weight in water
OM will release most of this water to plants
in contrast clay holds approx. same amount of water, but this is mostly unavailable to plants
Benefits of OM
Soil structure aggregation:
OM causes soil to clump and for soil aggregates
improves soil structure, improved soil structure improves permeability which in turn improves the ability to take up and hold water
Benefits of OM
Erosion prevention:
increases from 1-3% OM can reduce erosion 20-35% due to increased infiltration and stability of soil aggregates
Management practices to maintain OM in soils
Reduce or eliminate tillage: tillage aerates the soil causing a flush of microbial activity. This speeds up the decomposition process. Increases erosion
addition of organic material
cover crops
field rotation (fallow)
