Fresh Residues. Up to 15% of organic matter is fresh residue Comprised mainly of litter fall Much can be recognized as plant residue

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1 Organic matter encompasses all organic components of a soil: Fresh residues Decomposing organic matter Stable organic matter Living organisms Soil Organic Matter

2 Fresh Residues Up to 15% of organic matter is fresh residue Comprised mainly of litter fall Much can be recognized as plant residue

3 Decomposing Organic Matter Plant material is transformed from one organic compound to another mainly by organisms in the soil Organisms create byproducts, wastes, and cell tissue Compounds released as waste by one organisms can often be used as food by another

4 Soil Organic Matter =SOM SOM is labile -it can decline rapidly if the soil environment changes and renewable -it can be replenished by inputs of organic material to the soil. Adequate levels of SOM can be maintained with proper fertilization, crop rotations, and tillage practices if crop residues are returned to the soil.

5 Soil Organisms A cubic meter of healthy soil = home to billions of bacteria and fungi, millions of nematodes, tens of thousands of spring tails and mites and several hundred earthworms These "critters" are nature's recyclers, converting plant residue and animal manures into usable nutrients and soil organic matter.

6 Soil Organisms The soil microflora (bacteria, fungi and algae) are responsible for 90% of the decomposition of organic material. Plant residue is the main fuel for sustaining the broad spectrum of organisms residing in the soil.

7 Soil Organisms The microbial population can increase rapidly to take advantage of a favorable change in the soil environment, doubling in a few hours. The size of the microbial population is usually controlled by soil moisture, aeration, temperature and their distribution in the soil.

8 Active Fraction 10 to 30% of the soil organic matter (active fraction) is responsible for maintaining soil microorganisms. The active fraction of organic matter is most susceptible to soil management practices. (Inactive = humus)

9 Adding Fresh OM In a soil which at first has no readily decomposable materials, adding fresh tissue under favorable conditions: 1) immediately starts rapid multiplication of bacteria, fungi, and actinomycetes, 2) which are soon actively decomposing the fresh tissue.

10 Fresh SOM as most readily available energy sources are used up, microorganisms again become relatively inactive, leaving behind a dark mixture usually referred to as humus a stable organic compound

11 Stable Organic Matter Soil organic compounds become stabilized and resistant to further changes by microorganisms Bound inside soil aggregates Stabilized organic matter acts like a sponge and can absorb six times its weight in water

12 HUMUS Newly-formed humus - a) combination of resistant materials from the original plant tissue, b) compounds synthesized as part of the microorganisms' tissue which remain as the organisms die. (Fluvic and Humic Acid) humus is resistant to further microbial attack- N and P are protected from ready solubility.

13 Function of Humus holds water and nutrients; it sticks together & helps establish and maintain a strong crumb structure & thus reduce soil erosion it provides some nutrients (N & P) as it is slowly decayed by microbial activity, Buffers effects of pesticides humus decomposes at the rate of 2.5% per year

14 SOM Maintains soil Tilth aiding infiltration of air and water promoting water retention reducing erosion

15 SOM = SOIL HEALTH Measuring SOM is one step in assessing overall soil quality or soil health - measuring various key attributes of soil organic matter quantity and quality will give an indication of the health of the soil.

16 Soil Food Web The community of organisms living all or part of their lives in the soil Use sun s energy to fix carbon dioxide plants, lichens, moss, photosynthetic from atmosphere bacteria, and algae Get energy/carbon by consuming organic compounds found in plants, other organisms, and waste by-products Fueled by primary producers

17 Soil Food Web Diagram

18 Microbial Biomass Around roots In litter On humus On the surface of soil aggregates In spaces between soil aggregates