soil organic matter - agric.gov.ab.cafile/organicmatter.pdf · soil organic matter is primarily...

5

Click here to load reader

Upload: truongtram

Post on 04-Sep-2018

212 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Soil Organic Matter - agric.gov.ab.caFILE/organicmatter.pdf · Soil organic matter is primarily plant residues, in different stages of decomposition. ... 17% in mineral soils), though

Soil Organic MatterOrganic Carbon and NitrogenAnalysis of soil organic matter (SOM), conducted on the AESA Soil Quality Benchmark sites, has revealedsome significant differences in the per cent of organic carbon and total nitrogen in Alberta�s agriculturalecoregions, depending on soil horizon and slope positions. In ecoregions located further north in theprovince, the per cent of organic carbon in topsoil generally increased. The per cent of organic carbon insubsoil was higher in ecoregions that had greater below-ground plant productivity. The per cent of nitrogenin both topsoil and subsoil were higher in ecoregions that had higher levels of soil organic matter. Organiccarbon and nitrogen levels increased at downslope locations in all ecoregions. Because SOM varies betweenecoregions, and within a single field based on slope position, the assessment of SOM is very site-specific. Thestudy findings highlight the importance of conserving high SOM levels in soils and identify some waysfarmers can do this.

What is Soil OrganicMatter?Soil organic matter is primarily plant residues, indifferent stages of decomposition. The accumulationof SOM within soil is a balance between the returnor addition of plant residues and their subsequentloss due to the decay of these residues by micro-organisms. SOM is found naturally in all Albertasoils, in small quantities (0 - 17% in mineral soils),though the amount of natural SOM variesconsiderably from place to place.

Why is SOM Important?SOM is important because it improves both thephysical and chemical properties of soil and hasseveral beneficial effects on agricultural soil quality.SOM helps stabilize soil particles, thus decreasingerosion. It also improves soil structure andworkability, enhances aeration and waterpenetration, increases water-holding capacity, andstores and supplies nutrients for growth of bothplants and soil micro-organisms.

What Factors Influencethe Amount of SOM?Climatic conditions, such as temperature andrainfall, exert a major influence on the amount oforganic matter found in soil. Typically, accumulationof organic matter in soil is greater where there ismore precipitation and cooler temperatures.Decomposition of organic matter is greater inwarmer, drier climates. Other factors that affect therate of organic matter decomposition include soilaeration, pH level, and the microbial population ofthe soil.

Agricultural management practices can alsoinfluence the amount of SOM. Increased tillage ofthe soil decreases organic matter. Tillage increasesaeration, which leads to drier soils and greater ratesof decomposition. Increased summerfallow in croprotation also decreases SOM, because fewer planttissue residues are being returned to the soil.

Fertilization increases SOM because it increasesproductivity and therefore adds plant residues.Increased use of manure and other soil amendmentshas a similar affect. Increased use of forages in croprotations increases SOM because forages leavemore residues than other crops. Irrigation canincrease SOM because it increases productivity and

Page 2: Soil Organic Matter - agric.gov.ab.caFILE/organicmatter.pdf · Soil organic matter is primarily plant residues, in different stages of decomposition. ... 17% in mineral soils), though

leads to greater soil moisture. Increased erosion,caused by runoff and other factors, decreases SOMbecause the soil particles richest in organic matterare usually transported most readily.

Alberta Agriculture Food and Rural Development(AAFRD) has published an AGRI-FACTS handouton Soil Organic Matter (Agdex 536-1) that describesin greater detail the importance and management ofSOM.

Soil Quality BenchmarkProjectIn 1998, Alberta Environmentally SustainableAgriculture (AESA), in conjunction with theprovince�s agri-food industry, initiated a program toidentify and monitor the impacts of agriculture onsoil resources and to collect soil information to helpdevelop environmentally sustainable agriculturalpractices. The program established 43 soil qualitybenchmark sites, located on typical farm fieldsthroughout the province�s agricultural areas(Figure 1). One phase of the program was todetermine and compare soil organic carbon (OC)and total nitrogen (N) levels among Alberta�secoregions.

Sampling Methods atthe Benchmark SitesSamples were collected at three slope positions �upper, mid and lower � to determine differenceswithin a given field. Samples were collected fromtopsoil (the A horizon), subsoil (top of theB horizon) and from the parent geological material(the C horizon). Samples collected from the topsoiland subsoil were analysed to determine the percentof organic carbon and nitrogen and the ratiobetween the two. Total organic carbon values fortopsoil and the soil profile to one metre depth werecalculated from organic carbon percentages, horizondepths and bulk densities.

Figure 1.Locations of Soil Quality benchmarks, Ecoregions andEcodistricts in Alberta

Results of the StudyTTTTTopsoilsopsoilsopsoilsopsoilsopsoilsThe soil benchmark study showed the percent of OCin topsoils was different among the six ecoregions(Figure 2).

About Organic Carbon

Organic carbon (OC) is an essential part of allorganic compounds. Soil organic mattergenerally contains approximately 56% OC.The following equation is used to estimate thetotal organic matter content of soil from OCmeasurements:

% Organic Matter = % Organic Carbon x 1.78

Figure 2.

Topsoils in the Mixed Grassland Ecoregion had thelowest percent OC. The further north thebenchmark sites were located, the greater was thepercent of OC in topsoil. The higher soil moisture

Page 3: Soil Organic Matter - agric.gov.ab.caFILE/organicmatter.pdf · Soil organic matter is primarily plant residues, in different stages of decomposition. ... 17% in mineral soils), though

levels and cooler temperatures in the northern partsof the province resulted in greater accumulationsand slower decomposition of SOM. The greater useof forages in crop rotations and the lower use ofsummerfallow in the north may also be contributingfactors in the pattern of OC percentages in the soil.

Past agricultural practices typical of the MixedGrassland Ecoregion may have affected the levels ofSOM in this area. Fertilizing at lower rates andusing crop/fallow rotations to conserve soil moisturemay have resulted in less crop residue beingreturned to the soil and less SOM beingaccumulated.

The benchmark study also showed the percent ofOC in topsoil was equal to or slightly higher thanpreviously reported averages for cultivated topsoilswithin each ecoregion (Table 1).

Table 1.Typical SOM levels in cultivated soils

the Peace Lowland, Aspen Parkland, Moist MixedGrassland and Fescue Grassland ecoregions(Figure 4).

These ecoregions are located in optimal climaticzones for production and crops have deeper rootpenetration and greater root biomass than cropsgrown in the Mixed Grassland ecoregion. Theseroots eventually decompose and add organic matterto the subsoil. Soils in the Boreal Transitionecoregion also have few moisture restrictions forcrop growth, but historically, these soils were treedand more organic matter accumulated on thesurface than in the subsoil.

Soil profileSoil profileSoil profileSoil profileSoil profileThe pattern of OC percentages in the overall soilprofile to one metre depth are similar to the patternof OC in topsoils, with the exception of the PeaceLowland Ecoregion (Figure 5). The percent of OCin the soil profile was highest in the Peace Lowland,whereas the percent of OC in topsoil was highest inthe Aspen Parkland ecoregion. Ecoregions in thenorth have more OC, deeper in the profile, due tomore extensive root systems, greater cropproductivity, less decomposition, and moremoisture, compared to southern ecoregions.

Source: AAFRD Agdex 536-1, 1985

The study also showed the percent of OC in topsoilsin all the ecoregions was influenced by slopeposition. On lower parts of slopes, the percent ofOC in topsoils was greater than at the upper andmid slope positions. The increases in organic matterat the lower slope positions were primarily due totheir higher crop productivity and slowerdecomposition. To a lesser extent, the increase inthe percent of OC at lower slope positions may bedue to topsoil accumulation caused by wind andwater erosion and soil displacement frommechanical tillage. These processes result in theaccumulation of organic carbon-rich topsoil in low-lying areas (Figure 3) across the province.

SubsoilsSubsoilsSubsoilsSubsoilsSubsoilsIn subsoil, the percent of OC was also differentamong the ecoregions. Instead of progressivelyincreasing in ecoregions located further north,OC percentages in subsoil appear to be higher in

noigerocE enoZlioS MO% CO%

LP yargkrad 3-2 7.1-1.1

TB yarg 2-1 1.1-6.0

PA,GF kcalb 6-4 4.3-2.2

GMM nworbkrad 4-3 2.2-7.1

GM nworb 3-2 7.1-1.1Figure 4.

Figure 3.

Page 4: Soil Organic Matter - agric.gov.ab.caFILE/organicmatter.pdf · Soil organic matter is primarily plant residues, in different stages of decomposition. ... 17% in mineral soils), though

About Nitrogen

Nitrogen (N) is absorbed from soil and usedby plants in its inorganic form, as eithernitrate (NO3

-) or ammonium (NH4+). Both

forms occur naturally in soils and are commoncomponents of inorganic fertilizers. Nitrate isthe principal form in which nitrogen is takenup by plants, due to its mobile nature andgreater abundance than ammonium.

However, inorganic N represents only 2 to 5%of the total nitrogen in the soil. Most soil N isbound to organic matter and not readilyavailable to plants. Total nitrogen (TN) is ameasure of both inorganic and organic formsof nitrogen and is expressed as a percentage.

Levels of N vary with temperature andmoisture in the same way as levels of SOM,that is, N increases with cooler temperaturesand more moisture.

Figure 7.

Figure 8.

TTTTTopsoilsopsoilsopsoilsopsoilsopsoilsThe study showed the percent of total N in topsoil inAlberta�s ecoregions reflected the amount of SOMwithin each ecoregion. As the percent of OCincreased, so did the percent of N (Figure 6). Thisconfirms that the majority of N in soils is closelyassociated with SOM and emphasizes theimportance of conserving and maintaining thisorganic matter.

Figure 5.

As in other aspects of the study, organic carbon inthe soil profile was greatest at the lower slopepositions and decreased upslope.

The percent N in topsoil increased downward alongslopes in all ecoregions (Figure 7). The increasereflected higher OC percentages in topsoil at thelower slope positions, due to the accumulation ofricher organic matter in low-lying areas.

SubsoilsSubsoilsSubsoilsSubsoilsSubsoilsEcoregions with higher OC levels in subsoil, hadhigher levels of N in the subsoil (Figure 8).

Figure 6.

Page 5: Soil Organic Matter - agric.gov.ab.caFILE/organicmatter.pdf · Soil organic matter is primarily plant residues, in different stages of decomposition. ... 17% in mineral soils), though

FS2002-1SQ

Carbon � Nitrogen RatiosA close relationship exists between carbon andnitrogen (C:N) levels of soils. The C:N ratio ofarable topsoil ranges from 8:1 to 15:1, though itusually falls between 10:1 and 12:1. The variationsthat occur in these ratios are generally related toclimatic conditions. Soil C:N ratios tend to be lowerin arid regions than in humid regions, when annualtemperatures are about the same. Soil C:N ratiosare also lower in warmer regions than in coolerones, if rainfall is equal. The C:N ratio is generallylower in subsoil than in topsoil.

Carbon to nitrogen ratios of different plants andfarm materials vary greatly, ranging from 15 to 30:1for legumes and farm manures, to a high of 100:1for straw residues. If plant residues with high C:Nratios are added to the soil, decaying micro-organisms will immobilize the ammonium andnitrate and help decompose the SOM. However, ifmaterial added to soil contains more N inproportion to C, (lower C:N ratios), there isnormally no decrease in mineral N. Carbon tonitrogen ratios of 20 or 30:1 are the dividing linebetween immobilization and mineralization.

The study results showed C:N ratios for topsoil wereslightly lower in the Mixed Grassland Ecoregion,since it is slightly warmer and more arid than theother ecoregions and decomposition is greater(Figure 9).

Implications for FieldManagementCreating and maintaining soil organic matter is vitalfor improving the physical and chemical propertiesof agricultural soils. Though SOM is not increasedquickly, studies have shown that certain agriculturalpractices can help farmers increase SOM or reducethe loss of SOM from their fields. The followingpractices have been shown to be beneficial:

� Use of permanent cover crops and continuouscropping; less use of summerfallow.

� Use of legumes and forages in crop rotations toincrease plant residues and SOM.

� The addition of manures to increase SOM.� Use of fertilizers to increase crop productivity,

add plant residues and increase SOM.� Use of conservation tillage or reduced tillage.� Better crop residue management.� Improved erosion control.� Use of site specific soil testing to determine

organic carbon and nitrogen levels. Soil samplesshould be collected from several differentlocations on a field to obtain composite andrepresentative samples for analysis.

For More InformationFor further information on soil organic matterconservation and management, contact your localprovincial government crop specialist. You may alsowish to read The Nature and Properties of Soils(10th edition), by N.C. Brady, published byMacMillan Books in 1990, or Soil Fertility andFertilizers (5th edition), by Tisdale, Werner, Beatonand Havlin, published by MacMillan in 1993.

For additional copies or more information on thisFact Sheet, refer to FS2001-1SQ. If you have furtherquestions regarding the Soil Quality BenchmarkStudy, contact:

Karen CannonAESA Soil Quality ProgramConservation and Development BranchAlberta Agriculture, Food and Rural DevelopmentPhone: (780) 422-4385E-mail: [email protected]

Figure 9.

As expected, subsoil C:N ratios are slightly lowerthan topsoil C:N ratios for each ecoregion, becausethere are lower amounts of OC in that horizon.