Exam 1 Soil Chemical Properties: Soil Organic Matter Flashcards
What is the difference between organic and inorganic molecules?
Organic: contain C-C or C-H bonds
Inorganic: do not contain either bond
Soil biomass
Organic Compounds present in live microbial tissues
Litter/Light Fraction
Undecayed plant/animal tissues and their partial decomposition products.
Litter: on surface
Light fraction: in soil
Soil organic matter or “humus”
Total of the non-living organic compounds except for undecayed organic matter or its recognizable partial decay products
Non-humic Substances
Chemically recognizable compounds dervied from plant or microbial tissue (category of SOM)
Humic Substances
High molecular weight (complex) brown to black colored substances formed in the soil and distinctive to the soil environment. Dissimilar to the biomolecules of microorganisms and higher plants.
Carbohydrates
Energy storage; structural molecules
Cellulose
Carbohydrate: polysaccharides; most abundant bipolymer on earth
Proteins
Linked chains of amino acids; major source of organic N in soils; perform critical functions
Lipids
Energy storage; structural components of cell membrane; fat and water soluble components
Nucleic Acids
Cellular information storage and energy currency; DNA, RNA, ADP ATP; contains N and P
Ligin
Structural component of woody plant tissues; 2nd most abundant bipolymer
Tissue Death
Entry of organic compounds into soils as non-living organic matter
Determining Organic Matter (%) by Loss On Ignition (LOI)
- Weigh oven dried soil
- Burn in muffle furnace at 375-400 C
- Reweigh sample
- Difference in weight (mass) in the OM
C:N:P:S ratio
100:10:1:1 (140:10:1.3:1.3)
Percent C in soil organic matter
50%
Other 50% of soil organic matter
O and H (but N,P,S are important as well)
Percent microbial Biomass of total soil organic matter
1-12% (avg 5%)
Percent Humus and non-humic compounds
Humus: 40-80% (avg 50%)
Non Humic: 50%; mostly light fraction and microbial biomass
Organic matter tends to accumulate and “stick around” longer in soils when:
- Microbial activity is low (cold/wet)
- OM is closely associated with mineral surfaces of fine particles
- OM exists as complex, difficult to degrade molecules
Source of reds, browns, yellows, and oranges in soils
Iron Oxides
Source of dark browns to black in soils
Organic matter
Source of gray to colorless soils
Mineral Particles
Properties and Effects of OM in Soils
- Water retention (can hold 20x its weight in h20)
- Structure/Aggregation (promotes both granular and macropores)
- Negative charge (>4 pH, weakly acidic)
- Fertility (P, N, S availability)
Soils with 1-10% OM
Mineral Soils
Soils with >30% OM
Organic Soils
