NRES 201 Lecture 14 Outline of Last Lecture I Soil Management and Bulk Density II Soil Porosity III Aggregate formation and stabilization Outline of Current Lecture IV What is soil organic matter SOM V Functions of soil organic matter VI Soil organic matter management VII Decomposition in soils VIII Factors affecting soil content and distribution of organic matter IX The global carbon cycle Current Lecture Soil Organic Matter The organic component of soil consisting of Living organisms biomass Organismal remains residues Decay products Humus Estimated from soil organic C SOC o SOM SOC x 1 8 o Excludes inorganic C in carbonates Functions Nutritional functions o Energy and C source for heterotrophic microbes o Supplies N P and S to plants and microbes o Increases nutrient holding capacity Physical functions o Improves soil structure aggregation and macro porosity Critical for temperate soils o Enhances water infiltration and aeration o Increases water holding capacity o Reduces erosion and compaction Biological functions o Essential for all heterotrophs including most microbes macro and micro fauna Ecological functions o SOM is the Earth s major surface C reservoir These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Soil Organic Matter Management Need to balance opposing factors o SOM decreases when residue decomposition is greater than residue input o SOM increases when residue input is greater than residue decomposition Residue inputs increased by o High crop yields with Improved varieties high planting rates optimal fertilizer use and adequate crop production practices Decomposition decreased by o Reduced tillage o Less decomposable residues having A higher C N ratio or a higher lignin content o Avoiding excessive N or P fertilization o Fine textured soil Decomposition in Soils Organic constituents and their ease of decomposition o Most decomposable Sugars starches crude protein fats lignin Dynamics of decomposition o For residues with a high C N ratio corn stalks Decomposition is prolonged N is tied up immobilized Plant N availability is depressed o For residues with a low C N ratio alfalfa tops Decomposition is rapid N is liberated mineralized Plant N availability increases o Rate of decomposition Faster with a low C N ratio Slower with a high C N ratio Factors affecting soil content and distribution of organic matter Pedologic factors temperature and moisture regime type of vegetation Soil texture o More organic matter in fine textured soils because a higher water content reduces aeration clay protects organic matter Management practices Tillage and drainage promote the loss of soil organic matter by stimulating microbial decomposition
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