Soil Organic MatterCornerstone of organic agricultureWhy important?OutlineSOM can naturally vary from <1 to 47%What is soil organic matter (SOM)?Plant tissueDecompositionOrganic decay process through timeSlide 11Factors controlling decomposition and mineralization: Environmental conditions and litter qualityWhy is C/N ratio importantC/N ratioHigher C/N ratio as cover crop plants matureLower C/N ratio less nitrate depression C/N of 20 optimal for plant nutrient uptakeN release of nematodes feeding at higher C/N ratio is equivalent to a lower C/N ratioPolyphenols decrease decomposition rateQuality depends both of C/N and lignin and polyphenolsHow SOM influences soil properties!Composting at various scalesSlide 23Aerobic composting processBenefits of compostingClopyralid (pyradine herbicide) especially used in grasses for thistle controlFig 12.12 Three stages of compostSlide 28Most of C released as CO2Fig 12.16 Three pools of C after the CENTURY modelWhat determines soil organic carbon (SOC) of soil?Native organic carbon differs in different soil typeSoil texture: Clay holds SOMFig 12.21 Native US SOMThurston Co soilsWA State Soil Profile: AndisolLoss of active and slow pools from native to cultivatedEffects of ManagementFig 12.20 Temperature and access to O2 determine accumulation of SOCPoor drainage leads to accumulation of SOCHistosols can be oxidized when drainedFig 12.21 Native US SOM: More precipitation greater SOCMaintaining/increasing SOM with proper mgmtLess tillage more SOM especially on surfaceSOM is a flow- additions increase, oxidation decreasesRecommendations for managing SOMCan you have too much organic matter?Histosols (peat and muck soils) 20-30% OM, mined for potting mixesNRCS, formerly Soil Conservation Service, is concerned with decreasing annual erosional soil loss in USSoil Conditioning IndexBudgets and Soil Condition Index- models to determine >SOMTrends in US SOM management 1982-1997Ecol. Econ.: What is topsoil worth?Study QuestionsSoil Organic MatterMartha RosemeyerJanuary 20, 2006Ecological AgricultureCornerstone of organic agricultureOrganic matter content of soil is the most single importantRodale vs. Sir Albert HowardIs adding organic matter is sufficient or need to pay attention to minerals as well?Why important?Soil quality (the capacity of the soil to function to sustain plant and animal productivity, maintain or enhance water and air quality and support human health and habitation) depends on quality and quantity of soil organic matterThree times more C in soil than in world’s vegetationImportant in global warmingOutlineThe C cycleC in soil poolsManagement of SOCQuestions BW CH 12SOM can naturally vary from <1 to 47%What is soil organic matter (SOM)?Originates from plant tissue primarily and animal secondarily (soil and above ground) as well as microbialThree parts:1) Living plant, animal and soil organisms2) Dead roots and other identifiable residuesdetritus3) non-identifiable amorphous and colloidal materials = humusContains carbonFigure 12.2 Plant tissueRapid decomposition Sugar, starch and simple proteins Hemicellulose Cellulose Fats and waxes Lignins and phenolicsSlow decompositionElementsDecompositionCarbon compounds oxidized to CO2Essential plant nutrients mineralized/immobilized depending on each elementResistant compounds formed (fulvic and humic acids)Figure 12.3 Organic decay process through timeMicrobial respiration peaks as microbes use up easily degradable substratesSmall resident pop’n of active organisms (autochthonous)Fresh material stimulates group of inactive opportunistic (zymogenous) organismsMicrobial pop’n at peak is 1/6 of SOMPriming effect: stimulates breakdown of resistant microorganismsMineralization due to death of microbial pop’n due to lack of substrate and predationN, S from protein breakdownCarbon can be chemically protected (humus) or physical protection with clayFactors controlling decomposition and mineralization: Environmental conditions and litter qualityEnvironmental conditionstemperaturemoistureoxygenLitter qualityC/N ratiocontent of resistant compounds: lignins and phenolsWhy is C/N ratio importantPlants have higher C/N ratio in tissues than bacteria and fungi needPlant tissue 10C:1N to 600:1 but bacteria and fungi ratio 5:1 to 10:1 (but 2/3 of C is respired so optimum is 25-30:1)Microbes are fed first, then whatever is left is available to plantsMicrobes need more N than plants do so there is a competition for N which may lead to a “nitrate depression period” where scare N not available to plantsC/N ratioFigure 12.4 Higher C/N ratio as cover crop plants matureLegumes with lower C/N;faster decay; more netmineralization of NFigure 12.6 Lower C/N ratio less nitrate depressionC/N of 20 optimal for plant nutrient uptakeHigh C/NLow C/NFigure 12.7 N release of nematodes feeding at higher C/N ratio is equivalent to a lower C/N ratioPolyphenols decrease decomposition rateLeucaenaGliricidiaBrady and Weil Table 12.3Figure 12.8 Quality depends both of C/N and lignin and polyphenolsFigure 12.15 How SOM influences soil properties!Figure 12.13 (a)(b)Composting at various scalesComposting: practice of creating humus-like organic material outside of the soilFigure 12.11Aerobic composting processThree steps:Mesophyllic- pre-peak and less than 40º CThermophyllic- peak of microbial activity and heat 50-75º CMesophyllic or curing stage-less than 40º C actinomycetes and fungi dominate, recolonization by thermophyllic organisms- plant growth stimulating, orgs antagonistic to plant pathogensBenefits of compostingSafe storageEasier handlingN competition avoidanceN stabilization: co-composting of high and low C/N ratio materialsPartial sterilization- weed seeds and pathogensDetoxification, but see chlopyralid Disease suppressionClopyralid (pyradine herbicide) especially used in grasses for thistle control50-0 ppb symptoms on pinto bean50 ppbwsu.gov/compostAcute toxicity not available, not likely carcinogen, potential ground water contaminant, much not knownFig 12.12 Three stages of compostFigure 12.9Figure 12.10 Most of C released as CO2Humus consist of:Non-humic substances (20-30% of SOM): synthesized by microbesHumic substances (60-80% of SOM) fulvic acids: half life is 10-50 yearshumic acids: half life is centurieshumin: highest mw, most resistant to decayFig 12.16