Soil Organic Matter

1. Soil Organic Matter Section C Soil Fertility and Plant Nutrition

2. 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

3. Major Soil Organisms Bacteria 108/gram Actinomycetes 107/gram Fungi 106/gram

4. Soil Microorganisms • Can be classified according to C and energy sources and their oxygen requirement: • photoautotrophs • Energy from sunlight & C from CO2 • Some bacteria and algae only • chemoautotrophs • Energy from oxidizing inorganic material, C from CO2 • Some bacteria only • chemoheterotrophs • Energy and C from oxidation of organic materials • Most bacteria, all fungi and actinomycetes

5. Soil Microorganisms • Oxygen requirement • aerobic • Require free O2 for respiration • All fungi and actinomycetes, most bacteria • anaerobic • Must use alternative electron acceptors instead of O2 • NO3 -, SO42-, Fe3+ , CO2 • Some bacteria are anaerobic • facultative • Can be aerobic or anaerobic. Some bacteria

6. Decomposition of Plant Residues(Under aerobic conditions) CO2 Plant Residues More microbial biomass + Dead Microorganisms NH4+, SO42-, etc. (inorganic waste) Humus (organic waste)

7. 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.

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

9. 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

10. 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.

11. Lignin

12. Humus Carbon Hydrogen Oxygen Nitrogen

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

14. 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 might contain 0.5% SOM by weight, a Mollisol 3-5% by weight An Aridisol with 0.5% SOM in the top 30 cm will contain 3000 m3/ha x 1500 kg/m3 x 0.005 = 22,500 kg/ha (top 30 cm) A Mollisol with 5.0% SOM in the top 30 cm will contain 3000 m3/ha x 1500 kg/m3 x 0.05 = 225,000 kg/ha (top 30 cm)

15. Decomposition of Humus • The rate of decomposition of humus is most strongly affected by soil moisture and temperature (<1 to >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.

16. Decomposition (Mineralization) of Humus • Releases N as NH4+ , 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 0.025 (% min) = 28 kg N/ha • Mollisol (from previous example) • 225,000 kg SOM/ha x 0.05 kg N/kg SOM x 0.025 (% min) = 280 kg N/ha

17. Decomposition of Plant Residues(Under aerobic conditions) CO2 Plant Residues More microbial biomass + Dead Microorganisms NH4+, SO42-, etc. (inorganic waste) Humus (organic waste)

18. What Happens to Residues? Chemically simple residues Chemically complex residues

19. 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

20. Chemical Composition of Plant Residues Sugars Complex proteins Hemicellulose Cellulose Lignin Simple proteins Waxes Starchs Increasing chemical complexity Increasing rate of decomposition

21. 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 CO2, 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

22. C:N Ratio decomposition 20 g as CO2 50 g C 10 g as waste 20 g as biomass 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) Microbial biomass has an average C:N of 10:1, therefore how much N is needed to balance the new biomass C? 2 g

23. 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 Ratios

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

25. Immobilization The conversion of inorganic (available) N (NH4+, NO3-) to microbial biomass N. Results from... C:N ratio of residues NH4+ and NO3-) CO2 release Time

26. Mineralization The conversion of organic (unavailable) N to NH4+ . Results from... C:N ratio of residues NH4+ CO2 release Time

27. 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.

28. Soil Organic Carbon

29. 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.

30. 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

31. Effects of Cropping on SOM - Oklahoma

32. 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

33. Effect of Cropping Practices

34. Effect of Fertilizers Manitoba Illinois

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

36. 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. P N

37. 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

38. 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.

39. “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