Soil Organic Matter As A Soil Health Indicator: Sampling, Testing, And .
OREGON STATE UNIVERSIT Y EXTENSION SERVICESoil organic matter as a soil health indicator:Sampling, testing, and interpretationD.M. Sullivan, A.D. Moore, and L.J. BrewerSoil organic matter (SOM) includes a wide variety of compoundscontaining carbon, hydrogen and oxygen. These compounds vary instructure, age and resistance to decomposition. The half-life of SOMcan range from weeks to centuries, depending on the types of compoundspresent and how strongly they are adsorbed onto soil particles.Soil organic matter is an integral part of many processes that arecritical to soil health and nutrient supply to crops, including:¾¾ Response to tillage (e.g., ease of seedbed preparation).¾¾ Resistance to wind and water erosion.¾¾ Water-holding capacity, water infiltration, and water movementin soil.¾¾ Capacity to retain and release nutrients for plant growth.¾¾ Population and diversity of soil biota.There is no laboratory test to directly quantify SOM. Estimation ofSOM is usually accomplished through a measure of soil organic carbon(SOC), total soil carbon, or weight loss on ignition (Tabatabai, 1996). Alllaboratory methods have limitations and inherent sources of error.Photo: Dan Sullivan, Oregon State UniversitySummaryThis publication provides guidance on soil organicmatter (SOM) testing for the purpose of monitoringchanges in soil health over time. Before using SOMtest results for this purpose, it is important tounderstand that measurable improvements in SOMin response to soil management practices often takeat least 3 to 5 years. Growers should not be overlyconcerned if SOM changes are not immediate.Practices that increase SOM include increasingcrop biomass production, retaining crop residues inthe field, reducing or eliminating tillage and applyingorganic matter (OM) amendments.Improper soil sampling methods can produceinconsistent and misleading SOM test results. Thisproblem can be avoided by following consistent soilsampling practices. We recommend removing mulchfrom the soil surface prior to sampling. Soil sampledepth must be consistent because SOM typicallydeclines with depth.SOM testing methods are based on themeasurement of soil organic carbon (SOC), total soilcarbon or weight loss upon ignition. These methodsdiffer in cost, accuracy and reproducibility. Choose asoil testing laboratory and SOM testing method thatcan provide consistent data quality.Dan M. Sullivan, professor and Extension soil scientist; Amber D. Moore, Extension soil fertility specialist; and Linda J. Brewer, seniorfaculty research assistant; all of Oregon State UniversityE M 9251August 2019
Soil organic matter and nutrientsAlthough SOM is not itself a plant nutrient, itis an important source of nutrient supply to crops.Macronutrients such as nitrogen (N) and sulfur (S), alongwith micronutrients such as boron (B), are stored withinOM and are released as OM decomposes. Soil organicmatter is negatively charged and attracts and retainspotassium (K), magnesium (Mg) and calcium (Ca), whichare positively charged.A soil with more OM contains more total N.However, total SOM percentage is not a quantitativeindicator of the capacity of soil to supply plantavailable nitrogen (PAN) for plant growth. The timingand amount of PAN released from SOM depend on itscomposition, soil temperature, soil moisture and othersoil management factors.Soil organic matter equilibriumSoil properties affect SOM retentionFigure 1. Conceptual example of expected soil organic matter(SOM) increase over time in response to a managementregime under which OM inputs exceed SOM loss. Improvedmanagement was implemented in Year 1 and continuedthereafter. The rate of OM accumulation over time depends onhow much the new management regime shifts the balance infavor of OM accumulation.Soil texture and drainage affect the capacity ofsoil to retain OM. Heavier-textured soils (e.g., claysand clay loams) are better able to protect OM fromdecomposition than are lighter-textured soils (e.g.,sandy loams, loamy sands). SOM decomposition is lesswhen soil pores are filled with water than when poresare filled with air. Therefore, under similar climate andmanagement, poorly drained soils usually have greaterSOM than well-drained soils.Soil pH and other chemical properties also play arole in regulating the rate of OM decomposition. Forexample, decomposition of SOM via microbial activity isfaster when soil pH is near neutral and slower when soilis strongly acidic (pH below 5).Soil parent material also affects SOM retention insoils. For example, soils derived from volcanic ash bindvery strongly with SOM, thereby protecting it fromdecomposition.of SOM in equilibrium with consistent crop rotation,tillage, and erosion control practices. In the example,we assume that sufficient time has occurred for SOM tocome to equilibrium (OM inputs outputs).The upper line, “SOM with improved management,”shows the expected response in SOM percentage whena major change is made to alter the OM input/outputbalance in favor of OM accumulation. The rate of OMaccumulation over time depends on how much the newmanagement regime shifts the balance in favor of OMaccumulation. In most cases, the maximum expectedchange in SOM is less than 2 percentage points (forexample, from 2 percent to 4 percent SOM or from4 percent to 6 percent SOM). Larger changes in SOMare possible over time with repeated application ofmanure or compost (see “Management actions toincrease SOM,” page 3).Changes in SOM usually occur slowly and requireat least three to five years to become measurable. Thetime required to achieve a measurable change in SOMdepends on many site-specific factors. Soil organicmatter near the soil surface changes most rapidly inresponse to typical soil management practices (Figure 2,page 4). High-rate applications of organic materialsbrought in from off-farm (e.g., compost or manure) canincrease SOM more rapidly (see Appendix A, page 7).SOM will slowly reach equilibriumChanges in SOM reflect the long-term balancebetween OM inputs and outputs (Figure 1).Management practices that increase OM inputs tothe soil or reduce the rate of SOM decomposition canincrease SOM concentration. Figure 1 shows the generalpattern expected when improved soil managementpractices are implemented to increase OM inputs ordecrease OM loss, resulting in net accumulation ofSOM.The lower line in Figure 1, “SOM with currentmanagement (equilibrium),” shows a low concentration2
Management actions to increase SOMresidues are retained, the SOM input/output balanceshifts in a positive direction.Reducing tillage is also an important strategy toretain SOM, as tillage exposes OM to decomposition.When SOM is inside a soil aggregate, few soil organismscan access and consume it. When the soil aggregate iscrushed by tillage, the OM becomes a readily accessiblefood source for soil organisms. As decompositionproceeds, soil carbon (C) is lost as CO2 gas. The morefrequent and intensive the tillage, the faster SOMdecomposes and is lost.Off-farm sources of OM (e.g., compost, animalmanure, or municipal biosolids) can increase SOM,although the use of these materials is limited by locationand cost. Some sources are more effective than othersin increasing SOM. For example, composts are generallymore effective than raw organic materials. Research indryland wheat-fallow cropping systems in Oregon andWashington demonstrated that municipal biosolidswere highly effective in increasing SOM (Cogger, et al.,2013; Wuest and Reardon, 2016; Sullivan, et al., 2018).However, repeated applications of manure, compost, orbiosolids can lead to excessive nutrient accumulations insoil.Forecasting changes in SOM over time is aninexact science because the interactions among soilmanagement practices and soil processes are complex.A single soil management practice can alter the rate ofboth OM accumulation and loss. For example, a changefrom dryland to irrigated farming alters both OM inputs(irrigated crops produce more biomass) and the rate ofOM decomposition (OM decomposition speeds up whensoil is moist).Table 1 shows categories of management actions thatfavor OM retention in soil. The overall balance betweenOM inputs and outputs over the years determines thenet accumulation or loss of OM.Soil erosion must be controlled as a prerequisite toincreasing SOM. Soil organic matter concentration ishighest at the soil surface (the source of eroded topsoil),and SOM is lighter than mineral soil particles, so soileroded from fields is usually rich in OM. For example,when a topsoil contains 2 percent SOM, the watereroded soil from the field may contain at least 3 percentOM (Sharpley, 1985; Schiettecatte, et al., 2008).Often, the most effective ways to increase SOMare to reduce fallow periods and increase biomassproduction. For example, switching from wheat-fallowto annual direct seeding has been demonstrated toreverse declining SOM in the inland Pacific Northwest(Brown and Huggins, 2012). Winter annual cover cropshave been shown to increase SOM in irrigated croppingsystems (Mitchell, et al., 2015).Crop residues are a critical contributor to SOM.When crop residues are routinely burned or baled,SOM often declines over time. Conversely, when cropTable 1.—Management actions that can maintain orincrease soil organic matter.PracticeExamples ofmanagement actionsReduce soil erosion bywind and waterMaintain vegetative cover. Retaincrop residue on the soil surface.Increase crop biomassApply nutrients at agronomic rates.Maintain soil pH in the optimumrange. Grow crops that producegreater amounts of above-groundand below-ground crop residue.Increase duration ofcrop growthWhen feasible, plant a cover cropbetween cash crops. Rotate toperennial grass. Consider relay(interseeded) crops.1Retain crop residuesLeave crop residue in the fieldinstead of burning or baling it.Reduce tillage intensity,frequency, or depthUse a less-intensive tillageimplement. Substitute no-till or striptill for more intensive tillage (e.g.,discing, plowing, rototilling). Useherbicides to replace tillage for weedcontrol.Add organic matterfrom off-farm sourcesApply compost, manure, ormunicipal biosolids.Monitoring changes in SOM in responseto managementPeriodic SOM testing, when properly done, canassist in understanding the cumulative impact of factorsthat can be controlled (soil management practices,Table 1), as well as natural factors (e.g., precipitation,temperature, soil texture). Figure 2 (page 4) illustratesmeasured SOM changes.Both soil sampling methods and laboratory analysismethods must be consistent when comparing SOMpercentages from the same field location over time.Large variations in reported SOM percentage can occurwhen soil sampling depth is inconsistent, when mulchfrom the soil surface is mixed with the soil sample, orwhen different laboratory methods are used. See thesidebar “Changes in SOM in a soil profile in response tomanagement” (page 4) for more information about theimportance of sampling at a consistent depth.Even with optimal procedures, SOM test valuesare not perfectly reproducible. A SOM test resultreflects variation in sample collection and variationRelay cropping, or interseeding, is the practice of plantinga cover crop or successive crop before the current crop isharvested.13
associated with the laboratory test. In most cases,soil test SOM values within 0.5 percentage point (e.g.,2.0 and 2.5 percent) from the same location in a fieldare not different. Figure 7 (page 10) shows variationencountered when a well-mixed composite soil samplewas analyzed by different testing laboratories.¾¾ Always remove thatch or mulch from the soilsurface prior to sampling.¾¾ Inspect composite soil samples after collectionand remove any obvious pieces of crop residue.¾¾ Georeference soil sampling locations within thefield when possible.Frequency of sampling. Testing SOM levels onceper crop rotation or every five years can be useful inmeasuring the impact of long-term soil managementpractices. Except for intensive long-term monitoringprojects, testing soil for SOM more than once everythree to five years is usually not useful, as randomerrors in sample collection and errors in laboratorymeasurement usually obscure small changes in SOM.More frequent soil sampling can be useful when manureor other organic amendments are applied.Soil sampling and testing methodsfor OM determinationSoil sampling methodsThe same general principles that apply to soilsampling for nutrient evaluation (Staben, et al., 2003)apply to soil sampling for determination of OM.A consistent soil sampling protocol is essential.¾¾ Collect soil samples to consistent soil depth(s).¾¾ Maintain consistency in the number of soil corescollected per composite sample.Changes in SOM in a soil profile in response to managementSituation. The Crop Residue Long-Term Experiment wasinitiated in a winter wheat-summer fallow cropping system in1931 at Pendleton, OR (Ghimire, et al., 2015).Objective. This trial evaluates the effect of soilmanagement practices on grain yield and soil properties,including SOM. We focus here on the effect of OM inputhistory on SOM present in the top 24 inches of soil.Methods. Table 2 shows annualized C and N input fromthree treatments as an average across alternating cropand fallow years. Each treatment included crop residues(retained straw and roots) plus a specific source of C and N,as indicated.In 2010, soil samples were collected by depth (0–4, 4–8,8 –12, and 12–24 inches) from treatment plots. Soil C wasdetermined by the combustion method (see Table 3, page5). We estimated SOM using the reported soil C and anapproximate conversion factor: SOM soil C x 1.72.Results. Treatment differences in SOM were moreapparent near the soil surface than at greater depth(Figure 2). After 80 years of different management, SOMconcentration at the 0- to 4-inch depth was about twice asmuch with the manure treatment (3.4 percent) than with theinorganic N fertilizer treatment (1.5 percent).Conclusion. Most of the change in SOM in this exampleoccurred in the top eightinches of soil. Maintaining aconsistent soil sampling depth is essential when trackingchanges in SOM over time.Table 2.—Annualized C and N input tosoil, Pendleton Crop Residue Long-TermExperiment.Treatment ID1CN(lb/a)(lb/a)N fertilizer (NB45)1,89035Manure (MN)3,29075Pea vine (PV)2,27032Treatment IDs (NB45, MN, and PV) are those used inthe published research. Source: Ghimire, et al., 2015.1Figure 2. Soil organic matter with depth in a Walla Wallasilt loam soil in a winter wheat-summer fallow croppingsystem under three OM input regimes initiated in 1931 andmaintained thereafter (Crop Residue Long-Term Experiment,Pendleton, OR). Soil samples were collected in 2010.Adapted from Table 3 in Ghimire, et al. (2015).4
Timing of soil sample collection. Plan to sample ata consistent time each year. Collect samples when thesoil has not recently been disturbed by tillage and at leastfour to six months after a manure or compost application.Equipment. The goal is to obtain a sample thatcontains an equal volume throughout the sampleddepth. A shovel will not achieve this goal. Standardsoil probes ensure uniform soil volume throughout thesampled depth.Area sampled (sampling unit). For long-termmonitoring of trends in SOM, we recommend using asampling approach that will reduce variability due tosoil type, landscape position, crop residue management,and organic inputs (e.g., manure). Rather than collectinga “whole field” sample, consider monitoring SOM ina reference area (a few acres) that represents a singleNRCS soil mapping unit and a uniform cropping history.In fields that have received manure inputs, choose areference area that has uniform soil test P and K levels,as levels of these nutrients are indicators of historicalmanure applications. Consider collecting replicate soilsamples at the first sampling date to assess inherentvariability in SOM.Walkley-Black. Soil C is estimated via a wet chemicalmethod, using potassium dichromate as an oxidizingagent. The dichromate (Cr2O7) ion remaining afterreaction with the soil sample is quantified by titrationwith FeSO4. Alternatively, the Cr3 generated by thereaction of dichromate with organic C can be quantifiedby colorimetry (Sims and Haby, 1971).Combustion. Soil C is determined after combustionof the sample at temperatures above 1,000 C. Aninfrared or conductivity detector is used to determinethe amount of C present in the exhaust gas (CO2).These testing methods differ in cost, accuracy andreproducibility, as well as in suitability for alkaline soilsthat contain carbonates (Table 3). Appendices B, C andD discuss additional aspects of these testing methods.Photo: Dan Sullivan, Oregon State UniversityFigure 3. Organicmulch present onthe soil surfacecan contaminate asoil sample. Scrapeaway organic mulchfrom the soil surfacebefore collecting asoil core. Also removeany organic matteradhering to the outsideof the soil core.Laboratory testing methods for SOM and CThe following three methods are commonly used toestimate SOM or soil C:Loss on Ignition (LOI). Soil organic matter isestimated indirectly by measuring sample weight loss onignition at 360 C. At this temperature, OM compoundsin the soil are converted to carbon dioxide gas, thuslowering the weight of the soil sample.Table 3.—Advantages and limitations of test methods for determining SOM or soil C.MethodTest costand availabilitySuitability for soilwith carbonateCommentsLoss-on-Ignition(LOI)Low cost; often included inroutine soil test “package”Carbonates can be lost fromthe sample at very high oventemperatures, causing OM tobe over-estimated. To avoidcarbonate volatilization, use anoven temperature of 360 C.SOM values determined by LOI are usuallyhigher than those obtained with the WalkleyBlack method. The LOI method is mostinaccurate when SOM is low (less than2 percent). The oven temperature and durationof the test may vary among laboratories.Walkley-Black1Moderate to high cost;some labs are phasingout this test because itgenerates toxic chromiumwaste.SuitableSome laboratories use a modified Walkley-Blackmethod (Sims and Haby, 1971).Combustion2Moderate to high cost;available as an additionaltest upon request at manylabsNot suitable for soils withcarbonates, unless inorganic Cconcentration is also measuredor carbonates are removed. SeeAppendix C.Sample size (grams of soil analyzed) variesamong combustion instruments. To improvetest reproducibility, an instrument using a largesample size is preferred.The traditional Walkley-Black method measures C concentration by redox titration of the remaining dichromate, while themodified method (Sims and Haby, 1971) determines the chromate ion (Cr3 ) in extract by colorimetry. See Appendix B for additionalinformation.2SOM is estimated as C multiplied by a conversion factor. The most common conversion equation is SOM C x 1.72. Somelaboratories employ other C-to-OM conversion factors based on proprietary data.15
Organic matter as a soil health indicator: Total SOM vs. “active” SOMSoil health is defined as “the capacity of a soil tofunction within ecosystem boundaries to sustain biologicalproductivity, maintain environmental quality, and promoteplant and animal health” (Doran and Parkin, 1994). Soilorganic matter is considered a critical Tier 1 indicator of soilhealth (Soil Health Institute, 2018).This publication addresses total SOM, which respondsslowly to soil management changes.Conceptually, total SOM can be divided into passiveand active pools based on its stability, or resistance todecomposition (Weil and Magdoff, 2004). The half-life of thepassive pool ranges from decades to centuries. This portionof OM is responsible for soil characteristics such as colorand cation exchange capacity. The active pool has a half-lifeof a few years and therefore is more sensitive to short-termchanges in soil management practices.Ongoing research is evaluating the sensitivity andapplicability of direct and indirect indicators of active OM(Soil Health Institute, 2018), including the following: Short-term C and N mineralization rate Permanganate oxidizable carbon Soil Protein Index Soil enzymes: B-glucosidase, B-glucosaminidase, phosphatase, arylsulfatase Microbial community characterization: phospholipidfatty acid (PLFA), ester-linked fatty acid methyl ester(EL-FAME) Genomics Diffuse reflectance spectroscopyPhoto: Ed PeacheyFigure 4. Adding a winter cover crop to a rotation won’timmediately produce a measurable increase in total SOM,but it likely will increase the active SOM fraction.6
Appendix A. Soil organic matter response to repeated dairy manure applicationsSituationConclusionDairy manure is a common organic matteramendment for cropping systems in the Snake RiverValley of southern Idaho. A long-term study is beingconducted to evaluate the impact of repeated dairymanure applications on soil properties and cropproduction.This trial demonstrates that SOM tends towardequilibrium in response to OM inputs and outputs.In this example, SOM concentration stabilized afterfive years of manure application, despite additionalannual applications. In the fertilizer treatment plots,SOM percentage remained constant over the entirestudy period, as would be expected when OM inputs(crop residues) and outputs (CO2 lost via respiration)remain relatively constant over time.This study had very high OM inputs (7.4 ton OM/acre/year as dairy manure) compared to most croppingsystems that do not include manure or compost input.Therefore, the change in SOM over time in this exampleis much greater than that expected for systems thatmodify tillage practices or increase crop biomass inputs.MethodStockpiled dairy manure was applied each fall from2012 to 2017 to a Portneuf silt loam in two adjacentirrigated fields near Kimberly, Idaho. Prior to initiationof the study, chemical fertilizers were routinely appliedto these fields to supply nutrients for crop production.Beginning in 2013, a wheat-potato-barley-sugar beetcrop rotation was established. The experimental designwas a randomized complete block design with fourreplications.Each fall, manure was applied at 15.5 ton/acre(dry weight basis). Manure contained 48 percent OMand 52 percent ash on a dry matter basis, so 7.4 ton/acre of OM was applied each year. Manure was tilledinto the soil within one to two days after application.The control nutrient application was a commercialinorganic fertilizer treatment, based on University ofIdaho recommendations. Aside from manure or fertilizerapplication, all plots in the experiment were managedwith the same tillage, irrigation and other managementpractices.Soils were sampled (0- to 12-inch depth) in Marchor early April from 2013 to 2018. Composite sampleswere created from 10 soil cores collected from eachtreatment plot. Soil organic matter was determined bythe Sims and Haby (1971) method, a variation of theWalkley-Black method (Table 3 and Appendix B). SoilOM percentage presented in Figure 5 is averaged acrosssoil samples collected from each treatment.Figure 5. Soil organic matter concentration in response tomanure or inorganic fertilizer application, Kimberly, Idaho.Manure was applied annually (15.5 ton/acre/year on a dry weightbasis). Inorganic fertilizer was applied at agronomic rates, basedon university recommendations. Manure was applied in the fall,and soil samples were collected in the spring. Source: AmberMoore and April Leytem, unpublished data.ResultsWhile soil organic matter increased with the manuretreatment, it remained relatively constant (1.3 percentto 1.5 percent) with the chemical fertilizer treatment(Figure 5). With manure application, SOM increasedfrom 1.6 percent at the start of the trial (2012) to2.6 percent in 2016, an average increase of 0.25 percentper year. Soil organic matter did not increase in 2017and 2018, despite continued annual manure applicationsat the same rates.Photo: Amber Moore, Oregon State UniversityFigure 6. Field application of stockpiled dairy manure to manuretreatment plots, Kimberly, Idaho.7
Appendix B. Laboratory methods for SOM determinationLoss on Ignition (Western Region MethodS 9.20)Except as noted, this Appendix is derived from Soil,Plant, and Water Reference Methods for the WesternRegion (Gavlak, et al., 2005).This method estimates SOM by measuring sampleweight loss following burning in a high-temperaturemuffle furnace (360oC). Organic matter burns at thistemperature, and mineral soil particles do not. LOIorganic matter is estimated by the difference in sampleweight at 360 and 105oC. Some laboratories furtheradjust raw LOI data via a regression equation betweenLOI and another established method (e.g., WalkleyBlack). The method is generally reproducible within 20 percent (Gavlak, et al., 2005).Walkley-Black (Western Region Method S 9.10)This method quantifies oxidizable soil C byreacting soil with dichromate and sulfuric acid. Thedichromate remaining after reaction with the soilsample is quantified by titration with FeSO4. Becausedichromate does not oxidize all soil C, raw lab data isadjusted with a correction factor to estimate soil C. Themethod detection limit is approximately 0.1 percentC and is generally reproducible to within 8 percent(Gavlak, et al., 2005). Soil organic matter is estimatedby multiplying soil C by 1.72 (Walkley-Black C x 1.72 OM).Large amounts of manganese or carbonates insoils may interfere with C results from this method.Pretreatment of soil with 0.1N HCl can removecarbonates and manganese.Dichromate is a hazardous waste and must bedisposed of properly by the laboratory. For this reason,the Walkley-Black method is being phased out ofgeneral use.The Sims and Haby (1971) method is the same asthe Walkley-Black method except that it quantifiessoil organic C via colorimetry and measures the Cr3 generated by the reaction of organic C with dichromate.Combustion (Western Region Method S 9.30)This method determines total C released from a soilsample by combustion in a furnace at a temperature ofat least 1,000oC using a proprietary instrument. Carbonpresent as carbon dioxide (CO2) gas after combustionis determined by an infrared or conductivity detector.Prior to CO2 determination, combustion gas is treatedto remove moisture and to ensure complete conversionof soil C to CO2. For acid soils, total soil C as determinedby the instrument equals soil organic C because acidsoils do not contain significant quantities of carbonates(inorganic C).When soil pH is greater than 7.4, carbonates typicallyare present. When a soil sample contains carbonates,two approaches can be used to estimate soil organicC. See Appendix C (page 9), “Using the CombustionMethod for SOM Determination in Soils ContainingCarbonates,” for details.Soil sample weight and fineness required for testreproducibility vary among combustion instrumentmanufacturers and models. Some instruments requirethat soils be pulverized to pass 60-mesh sieve.The detection limit for this method is instrumentdependent, and is approximately 0.02 percent C. It isgenerally reproducible to within 5 percent (Gavlak, etal., 2005).8
Appendix C. Using the combustion method for soil organic C determinationin soils containing carbonatesDisadvantages:¾¾ Two separate analyses are required(combustion method for total C and inorganic Cdetermination). Each analysis can have associatederror.¾¾ This method assumes that the distributionof carbonates is identical in the two samples.Identical distribution may be difficult to achievein some situations, especially when the soilcontains carbonate “chunks.” Grinding soil tofine mesh size is required to achieve samplehomogeneity.Analytical problemSoils with pH above 7.4 may contain “free lime” orcarbonates (XCO3, where X Ca, Mg, or Na). For thesesoils, test results for soil C via the combustion methodare unreliable because measured C is the sum of organicC in SOM plus inorganic C in carbonates.Soils that contain carbonates can be identified by aqualitative test (the “fizz test” with acid).SolutionsHere we describe two approaches to avoidinterference by carbonates:2. Pretreat the soil with acid to remove carbonates1. Estimate organic C by differenceDescription:¾¾ Measure soil C via combustion after carbonateremoval with acid.Advantages:¾¾ Only one analysis is required. The combustionmethod directly measures soil organic Cremaining after carbonate removal.¾¾ Acid pretreatment can be done on a relativelylarge soil sample, reducing error due tononuniform distribution of carbonate in thesample.Disadvantages:¾¾ The acid pretreatment may also remove some ofthe organic C present in the soil.¾¾ The acid pretreatment procedure is often notstandardized for time and acid molarity, makingthe results less reproducible.¾¾ When carbonate is removed, soil sample weightdecreases. Results for C via the combustionmethod must be corrected for this weight loss.Description:¾¾ Determine soil total C (organic inorganic) in asubsample via combustion.¾¾ Quantitatively determine inorganic C in anothersubsample.¾¾ Estimate organic C as the difference: soil organicC (total C – inorganic C).Advantages:¾¾ Precise and accurate methods for quantitativedetermination of carbonates exist.¾¾ A soil analysis for carbonate percentage hasother useful applications. For example, it can bean indicator of exposed subsoil and may provideuseful information to guide P fertilization rates.9
Appendix D. Comparison of methods for organic matter testingSituationMore than 100 soil testing laboratories participatein the North American Proficiency Testing (NAPT)program. Participating laboratories receive a commonset of soil samples each quarter and report analyses to acentralized database (NAPT, 2019).MethodWe used the NAPT sample analysis database for2018 from Quarters 1, 3, and 4 to compare test resultsfor the combustion method, the Walkley-Black method,and the LOI method. For the combustion method data,we used reported values for total C (n 30 l
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Thomas Talarico, Nicole Inan . Pennsylvania Policy Forum, from Solicitor, Richard Perhacs, in which he stated "Empower Erie" and the "Western Pennsylvania Policy Forum" are private entities separate and distinct from the County of Erie." Mr. Davis's question to Council regarding this is that, if Empower Erie is separate from the County, why did Tim McNair current Chair of Empower Erie send a .
