Soil Organic Matter Section C Soil Fertility and Plant Nutrition

Review - Soil organisms Bacteria –Most numerous, smallest –Aerobic and anaerobic Actinomycetes –Share characteristics of bacteria and fungi –Active in degradation of resistant compounds Fungi –Aerobic only, filamentous –Active in degradation of resistant compounds

Major Soil Organisms Bacteria 10 8 /gram Actinomycetes 10 7 /gram Fungi 10 6 /gram

Soil Microorganisms Can be classified according to C and energy sources and their oxygen requirement: –photoautotrophs Energy from sunlight & C from CO 2 Some bacteria and algae only –chemoautotrophs Energy from oxidizing inorganic material, C from CO 2 Some bacteria only –chemoheterotrophs Energy and C from oxidation of organic materials Most bacteria, all fungi and actinomycetes

Soil Microorganisms Oxygen requirement –aerobic Require free O 2 for respiration All fungi and actinomycetes, most bacteria –anaerobic Must use alternative electron acceptors instead of O 2 –NO 3 -, SO 4 2-, Fe 3+, CO 2 Some bacteria are anaerobic –facultative Can be aerobic or anaerobic. Some bacteria

Decomposition of Plant Residues (Under aerobic conditions) Plant Residues CO 2 NH 4 +, SO 4 2-, etc. (inorganic waste) Humus (organic waste) + Dead Microorganisms More microbial biomass

Soil Organic Matter Soil organic matter: all organic matter in the soil, including humus, microbial biomass, and plant and animal residues in various stages of decomposition. –Composed of a wide range of organic materials, from highly decomposable to resistant to decomposition.

Roles of Soil Organic Matter Microbial substrate Nutrient reserve (esp. N, P, S) CEC Water-Holding capacity Soil structure

Humus The stable portion of soil organic matter that results from microbial degradation of residues. –Dark colored –About 58% C, 5% N –Complex chemical structure, aromatic plus aliphatic functional groups –Difficult to break down because of structure –high CEC

Humus The major organic “waste” by-product of OM degradation. The percentage of a residue that will become humus is approx. proportional to its lignin content.

Lignin

Humus Carbon Hydrogen Oxygen Nitrogen

Decomposition of Organic Matter Organic materials are decomposed by heterotrophic microorganisms. The organic matter is a source of _______, __________, and _____________ to these organisms. carbon energynutrients

Humus and Nutrients Humus contains about 58% C, 5%N, 0.6% P, and 0.6% S How much humus in soils? How much OM does this represent? An Aridisol with 0.5% SOM in the top 30 cm will contain 3000 m 3 /ha x 1500 kg/m 3 x = 22,500 kg/ha (top 30 cm) A Mollisol with 5.0% SOM in the top 30 cm will contain 3000 m 3 /ha x 1500 kg/m 3 x 0.05 = 225,000 kg/ha (top 30 cm) An Aridisol might contain 0.5% SOM by weight, a Mollisol 3-5% by weight

Decomposition of Humus The rate of decomposition of humus is most strongly affected by soil moisture and temperature ( 5%/yr). Humus is chemically complex and has a C:N ratio of about 11:1 High soil temperatures, abundant (but not excessive) moisture encourages “rapid” humus breakdown In soils where OM content is not decreasing, synthesis of “new” humus approximately equals decomposition of “old” humus.

Decomposition (Mineralization) of Humus Releases N as NH 4 +, available for plants If 2.5% of the N in SOM is mineralized each year, how much N would be released for plant uptake? Aridisol (from previous example) –22,500 kg SOM/ha x 0.05 kg N/kg SOM x (% min) = 28 kg N/ha Mollisol (from previous example) –225,000 kg SOM/ha x 0.05 kg N/kg SOM x (% min) = 280 kg N/ha

Decomposition of Plant Residues (Under aerobic conditions) Plant Residues CO 2 NH 4 +, SO 4 2-, etc. (inorganic waste) Humus (organic waste) + Dead Microorganisms More microbial biomass

What Happens to Residues? Chemically simple residues Chemically complex residues

Decomposition of Plant Material The rate of decomposition of plant residues is governed mostly by: –Chemical makeup of the residue –C:N ratio –Available soil N –Temperature, moisture, oxygen, and other environmental conditions that affect microbial growth

Chemical Composition of Plant Residues Sugars Complex proteinsHemicelluloseCellulose Lignin Simple proteinsWaxes Starchs Increasing chemical complexity Increasing rate of decomposition

C:N Ratio Why is the C:N ratio important? –Microorganisms need C and N in fixed ratios, because C and N are used to synthesize proteins, nucleic acids, etc. –Bacterial cell C:N is 5:1 to 8:1. Since about 50% of the C in an organic material is converted to CO 2, they need roughly a C:N of 10:1 to 16:1 in the residue they consume. –Fungi need a C:N of about 40:1 in their diet

decomposition C:N Ratio 50 g C 20 g as CO 2 20 g as biomass Microbial biomass has an average C:N of 10:1, therefore how much N is needed to balance the new biomass C? 10 g as waste 2 g Therefore, if the residue containing 50 g of C contains < 2 g of N (C:N>25:1), it will have insufficient N for microbial needs. What about >2 g N (C:N <25:1)

C:N Ratios High C:N material: –Woody –Grain crop residue –Mature plant tissues Low C:N material: –Green –Young plant tissues –Legume residues –Composts –Manures

C:N Ratio and Residue Mgmt. What are the implications of the C:N ratio of crop residues for nutrient management?

Immobilization The conversion of inorganic (available) N (NH 4 +, NO 3 - ) to microbial biomass N. Results from... NH 4 + and NO 3 - ) Time CO 2 release C:N ratio of residues

Mineralization The conversion of organic (unavailable) N to NH 4 +. Results from... NH 4 + Time CO 2 release C:N ratio of residues

Why are we talking about organic matter? SOM contributes to soil aggregation, drainage, aeration, structure SOM is the major substrate for microbial growth in the soil SOM is key for good “soil quality” SOM is the major reservoir of N in the soil Soil SOM is the major storage reservoir of C in the environment.

Soil Organic Matter Content In “undisturbed” soils: SOM = f (I, O) –Inputs = plant residues –Outputs = decomposition, erosion In managed soils: SOM = f (I, O, M) –M = management practices such as tillage, cultivation,residue management, etc.

Soil Organic Carbon

Soil Organic Matter Content The amount of organic matter in a soil tends to be difficult to change, and reflects an equilibrium between additions and losses over long periods of time. In the absence of changes in management or climate, soil organic matter content tends to remain relatively constant (steady state). In this case, the low amounts broken down each year are replaced by new humus.

Additions and Losses Additions of organic matter to soils: Losses of organic matter from soils:

Management Effects on SOM Agricultural management of soils usually _____________ amounts of SOM (compared to undisturbed soils) because: – tillage increases aeration and aerobic microbial activity – liming, where practiced, increases microbial activity – irrigation may increase microbial activity – erosion decreases

Effects of Cropping on SOM - Oklahoma

Conserving SOM Management practices that can help conserve or build SOM: –Reduced (minimum) tillage –Cover crops –Growing high residue crops –Adding organic materials to soils –Practicing crop rotation

Effect of Cropping Practices

Effect of Fertilizers Manitoba Illinois

Organic Materials Animal Manures –Solids, liquids Human Manures –Solids (sewage sludge, biosolids) –Liquids (effluent) Composts Reasons for applying to soils: –

Animal Manures Were a major source of plant nutrients (especially ____ and _____) before widespread use of commercial fertilizers Manures average 0.5 to 1% N, 0.25 to 0.5% P Significant environmental problems are associated with storage and disposal of animal manures. N P

Human Waste In some parts of the world, have historically been an important fertilizer source Average 4% N, 3% P, 0.3% K Soil disposal is one of the few options for disposal Use is becoming more common

Composting Compost is formed from the aerobic breakdown of organic materials which results in a mass of partly decomposed organic matter. Can be a valuable soil amendment. Most valuable for organic-matter building in soils. Not nutrient-rich.

“Sustainable Agriculture” A general term that is often applied to agricultural practices deemed “organic”. Usually means that organic fertilizer sources are emphasized. “Organic” agriculture means that only organic fertilizer sources are used. In organic agriculture, the proper use and management of organic inputs is critical