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The carbon hidden away in soil originatesfrom plants and is the unglamorous andoften forgotten part of the cycle of carbonbetween the land, oceans and atmosphere.Yet it is a fundamental component of soil’sorganic matter, integral to healthy soils andthe productivity of agricultural and widerecological systems, and an importantsequester of the carbon in CO2 thatcontributes to global warming.

Soil organic matter comprises all theliving, dead and decomposing plants,animals and microbes in the soil alongwith the organic residues and humicsubstances they release.

‘It includes carbon, hydrogen, oxygen,

nitrogen, phosphorus and sulphur and it isa small but vital part of all soils,’ says MrJan Skjemstad, formerly of CSIRO Landand Water. ‘We now recognise four differ-ent types – crop residues, particulateorganic matter, humus (usually the largestpool) and recalcitrant organic matter likecharcoal – so it is a complex mixture ofmaterials that vary in size, chemistry,degree of decomposition and interactionwith soil minerals.’

Dr Ram Dalal, of the CRC forGreenhouse Accounting and theQueensland Department of NaturalResources, has also studied soil organicmatter in croplands for many years and has

reviewed its role.1 He describes soil organicmatter as containing both living and non-living organic components.

‘Soil microbes, especially fungi andbacteria, and larger soil fauna are the livingcomponent while plant residues andvarious other organic materials representthe non-living portion,’ says Dalal, ‘and it isthe microbes that are the main drivingagent for organic matter turnover.’

The amount of organic matter in soilresults from the balance of carbon inputs(vegetation and roots) and outputs(decomposition, leaching and erosion).This offers the prospect of fiddling withthe rates of these inputs and outputs to

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Aware of a remorseless decline in the vital organic matter of Australian farmland soils because of landclearing and agricultural production, researchers, land managers and now, commercial players, areseeking ways to reverse the trend. It’s an important quest: without organic matter, soil is essentiallysterile, reducing the viability of both agricultural and surrounding natural systems.

A dust storm rolls across central NSW farmland. As our soils become more deficient in organic matter, they lose their structural stability and aremore vulnerable to weather extremes. CSIRO Land & Water

1 Dalal RC and Chan KY (2001). Soil organic matter in rainfed cropping systems of the Australian cereal belt. Australian Journal of Soil Research 39, 435–464.

Saving the life of

farmland soils

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manage or manipulate soil organic matterlevels.

While this sounds promising, thebiological, physical and chemical processesinvolved are complex, so boosting organicmatter by, say, altering cropping systemsisn’t always that simple. For example, thereare thresholds of soil organic mattercontent above which the various benefitsfor productivity or soil function level off,and clay soils respond differently to moresandy soils.

Soil organic matter is now universallyregarded as important stuff. In a GRDCreport,2 Dr Evelyn Krull, of CSIRO Landand Water, and her colleagues, said thatmany farmers see increases in soil organicmatter as desirable because this is usuallydirectly related to better plant nutrition,ease of cultivation and seedbed prepara-tion, greater aggregate stability, reducedbulk density, improved water holdingcapacity, enhanced porosity and earlierwarming in spring. At a fundamental level,soil organic matter provides a reservoir ofmetabolic energy that enables a range ofbiological processes to occur in the soil.

A disappearing actMany studies have documented the seem-ingly inevitable decline of soil organic

matter following land clearing and use ofconventional cropping systems.

To give an idea of this, researchers3

found that percentage losses of organiccarbon from Australian soils, already lowin carbon compared to European soils forexample, varied from 10–60 per cent over10–80 years of cultivation, and otherstudies have recorded similar losses.4

Why, then, are land clearing and crop-ping so detrimental to soil organic matter?

‘The declining organic matter levels incroplands,’ says Dalal, ‘are mostly due tochanges in temperature, moisture, soilaeration, exposure of new soil surfaces asaggregates are disrupted, reduced input oforganic materials relative to grassland orforest returns, increased erosion, and theexport of carbon and nutrients, such asnitrogen, phosphorus and potassium, fromfarms in produce.’

Large losses of soil carbon can lead toland degradation while restoration of soilorganic matter improves soil quality andenhances ecological sustainability. Indeed astudy5 by Dalal, and several colleagues, inthe Brigalow region of the Murray–DarlingBasin, found that soil organic matter is auseful indicator of both ecological andeconomic sustainability. They confirmed

that organic matter declined exponentiallyas the cultivation period for cereal crop-ping increased. Consequently, nutrientssuch as soil nitrogen were in shorter supplyand grain protein and grain yields fell, withclear economic implications.

One of the key reasons for the decliningfertility in agricultural soils is the removal –some would say ‘strip mining’ – ofnutrients in crops and other farm produce,and the degradation of organic matterfollowing soil cultivation. Instead of being

Land clearing, cropping and the export of agricultural produce from farmlands reduce soil organic matter. Composts made from reclaimed andrecycled organic wastes from urban refuse could help restore essential nutrients to farmland soils. Suprijono Sujarjoto/ CSIRO Land & Water

2 Krull ES, Skjemstad JO and Baldock JA (2004). Functions of soil organic matter and the effect on soil properties. GRDC, www.grdc.com.au/growers/res_summ/cso00029/summary.htm3 Russell JS and Williams CH (1982). Biogeochemical interactions of carbon, nitrogen, sulphur and phosphorus in Australian agro-ecosystems. In The Cycling of Carbon, Nitrogen, Sulphur and Phosphorus in Terrestrial

and Aquatic Systems. (Eds LE Galbally and JR Freney) pp. 61–75 (Australian Academy of Science: Canberra).4 To estimate organic matter, scientists usually multiply the (more easily determined) soil organic carbon value by a conversion factor of 1.72. If a ‘Loss on Ignition’ method is used, the relationship is the other way

around.5 Dalal RC, Eberhard R, Grantham T and Mayer DG (2003). Application of sustainability indicators, soil organic matter and electrical conductivity to resource management in the northern grains region. Australian

Journal of Experimental Agriculture 43(3), 253–259.

One of the key reasons for the declining fertility in agricultural soils is the removal – some would say ‘strip mining’ – of nutrients in crops and other farm produce, and the degradation of organicmatter following soil cultivation.

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recycled into the soil, as they would in anatural system, plant and animal material(such as grains and meat) are sent tofaraway markets and end up beingconsumed by people in towns and cities ormanufactured into other foods or goods,or end up, eventually, as waste in garbagebins or as human waste in sewage. Urbancentres, in fact, have become huge nutrientand carbon sinks. Over time, soils losingtheir carbon and nutrients to this processbreak down.

Declining soil fertility isthe reason why, asQueenland EPA researchindicates, average grainyields over the last 30 yearshave remained static,despite improved cropmanagement practices(including the use ofimproved plant breeds),higher fertiliser applicationrates (e.g. nitrogen 300%)and better weed control.6

Sending organics homeAustralia’s organics recycling industry,represented by CompostAustralia, is on a missionto reverse this trend.Dr John White, ManagingDirector of GlobalRenewables Limited(GRL), one of 60 organisa-tions represented underthe peak national body, ispassionate about the needfor sustainable agriculturein this country andbelieves that the industry can play animportant part by collecting and process-ing organic residues, and returning themto farm soils as high quality mulch orcompost products that will boost soilorganic matter, improve water use efficiency and increase productivity.

He has a vested interest too, as GRLcollects urban waste, usually sent straightto landfill, and recycles the various compo-nents, including organic wastes, using aprocess known as UR-3R. The systemstabilises the organic component, produc-ing a compost that meets the stringentAustralian standard for application to soilsfor farming.

‘I believe Australian farmers are losingtheir soil organic matter and, encouraged by

fertiliser companies, are increasinglyaddicted to synthetic fertilisers and pesti-cides,’ Dr White told Ecos. ‘And it is a viciouscycle because as soil organic matter declines,soils degrade, microbial activity declinesand farmers must use more and morechemicals to maintain fertility, controlweeds and pests and fight plant diseases.

‘This is all detrimental to the environ-ment, in particular as nutrients enter ourrivers, lakes, groundwater and even marinewaters, causing algal blooms, damage to

coral reefs and so on. Also, loss of soilorganic matter means soils retain lesswater, so farms are more vulnerable to theeffects of drought,’ White said.

Peter Wadewitz, Chair of CompostAustralia, knows first-hand of the moistureboosting potential of organic compostsand the promise they hold. ‘We have seendramatic water savings effects through theuse of organic mulches in the wine indus-try and in tree crops,’ he says.

‘However,’ he goes on, ‘we cannot expectthe farmers to fix Australia’s problems ontheir own. We all have to help, and there isan easy way we can do that. In 2004/05, theorganics recycling industry convertedapproximately 3.5 million tonnes oforganic residues into valuable mulch and

compost products. However, there are stillmillions of tonnes of useful organic mate-rials wasted in Australian landfill sites eachyear. Much of this material could be sepa-rated, preferably at source, adequatelyprocessed and used to improve farm soilsand productivity.’

Dr John White at GRL concurs. ‘Some40 to 60 per cent of the waste in garbagebins is organic – food scraps, garden clip-pings, grass – and this amounts to about11–12 million tonnes going to Australian

landfill sites each year. AtGRL, we separate this out,clean and deodorise it,and use microbes toprocess it into a compostthat is suitable for agricul-tural soils. So we close theloop, sending organicmatter back to farms, inmuch the same way thatsmall-scale traditionalvillage systems returnedwastes to the soil.’

Dr White has beenurging the AustralianGovernment to follow thelead of the UnitedKingdom, which infollowing the EuropeanUnion’s 1999 LandfillDirective,7 passed laws lastyear to progressively banorganics at landfills overthe next decade, tominimise pollution ofwater tables throughleaching and reducegreenhouse gas emissions.He argues that govern-

ment incentives are needed to encouragefarmers to manage their soils sustainablyand suggests that ‘organic’ farming, alreadybooming, is the best way to achieve this.

‘Our government is beginning to getinterested,’ said White, ‘perhaps motivatedby greenhouse pollution problems, farmersgoing bankrupt and the likelihood thatEuropean countries, especially, will soondemand truly clean and green produce ininternational trading.’

Whilst, according to Compost Australia,strict organic farmers usually aim atminimising external inputs, and thatincludes carbon (in composts), the argu-ment for recycling urban organics throughagriculture is slowly gathering momentumacross the wider farming community as the

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The soil carbon cycle. Soil carbon originates from plants, while the amount ofsoil organic matter in the soil results from the balance between inputs (plantresidues) and outputs (mostly microbial decomposition). JO Skjemstad

Substrate Clitter and roots

Photosynthesis

Assimilation

Death

CO2

Mineralisation

Humifiedorganic carbon

Microbialbiomass C

6 Queensland EPA (2004). State of the Environment Queensland 2003, Brisbane.7 The 1999 EU Landfill Directive demands from member countries that they progressively reduce the amount of organic matter (or unstabilised waste) that goes to landfill.

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importance of maintaining soil organicmatter levels is underscored, and the bene-fits of organic composts become moregenerally accepted. There are also mecha-nisms being worked on to reduce thesignificant cost commitments required forcompost applications.

Some scientists, however, urge cautionon soil amendments. Professor KeithCameron and his colleagues at LincolnUniversity, New Zealand, argue8 that whilewastes may contain nutrients and, usedproperly, can serve as valuable fertilisers foragricultural production, they should notjust be ‘dumped’ on soils.

‘There needs to be good management ofwaste composition and application rate

and appropriate guidelines to minimisedamage and maximise benefits,’ saidCameron. ‘Recycling nutrients can improvesustainability, but excessive rates of appli-cation have been shown to cause nitrogenand phosphorus pollution of surface orgroundwater and some wastes containheavy metals, salts and pathogens or mayhave extremely low or high pH.

‘Land application of wastes can have avariety of beneficial or detrimental effectson soil conditions, depending on thenature of the waste and the soil. Researchand monitoring programs are thereforenecessary to ensure that waste applicationsystems are sustainable and do not damagesoil quality.’

This sentiment is fully supported byCompost Australia. ‘If we as an industryare not able to produce high quality, recy-cled organic products that are environ-mentally benign, that are fit for purposeand that deliver tangible benefits to theuser, we will not last,’ says Peter Wadewitz.

According to him, many farmers wouldlike to use good quality compost on theirland to improve soil properties, but areunsure whether this long-term investmentin their soil ‘infrastructure’ pays off.Research around the country is currentlyfocused on assessing the agronomic andeconomic as well as the environmentalbenefits of using recycled organic productsin agricultural production systems, sofarmers can make a well-informed deci-sion about whether to use recycled organ-ics or not.

In Queensland, the Brisbane CityCouncil funded ‘Returning the Favour’Project still has to run a year before yieldand economic benefits are determined forvegetable growers in the Lockyer Valley.Meanwhile, Bob Paulin from the WesternAustralian Department of Agriculture andFood has identified some of the benefits ofcomposted soil amendments to vegetableproduction in a four-year study.9 Hisresearch found significant improvements,particularly on the sandy soils, in all soilcharacteristics measured, includingincreased soil organic matter, organicnitrogen, biological activity and diversity,cation (positively charged ions) exchangecapacity, volumetric soil moisture alongwith improved soil pH and reduced bulkdensity. Economic analysis also indicatedthat the use of compost in vegetableproduction increases returns.

Bob Paulin’s aim, like others, is not toreplace the use of synthetic fertilisers, butrather to develop and establish trulysustainable agricultural productionsystems that are able to maintain andenhance soil fertility, supply healthy food-stuff and deliver an adequate income tofarmers.

Long-term farming system trials, suchas the famous Rothamstad trial atBroadbalk in England (since 1843),10 haveshown that use of mineral fertilisers ontheir own were not able to increase soilcarbon levels. At best they maintainedthem. In many cases, combined use oforganic and inorganic fertilisers providedthe best yield responses.

Soil organic matter (SOM) performs three functions – biological, physical and chemical –which are each interrelated. From Krull ES, Skjemstad JO and Baldock JA (2004)

Fungi, bacteria and larger soil fauna such as Collembola and earthworms (above) are theliving component of soils. Microbes decompose organic materials to produce detritus whilelarger fauna produce castings and aerate the soil. CSIRO Land & Water

8 Cameron KC, Di HJ and McLaren RG (1997). Is soil an appropriate dumping ground for our wastes? Australian Journal of Soil Research 35, 995–1035.9 Paulin B, Wilkinson K, O’Malley P and Flavel T (2005). Identifying the benefits of composted soil amendments to vegetable production. Horticulture Australia (HAL Report VG 990016).10 See http://bretonplots.rr.ualberta.ca/quicktime/files/index.asp?page=Powerpoint. Look at slide 52 of the Powerpoint presentation.

Biological functions

• source of energy for biological processes

• a large reservoir of potentially available nutrients

• contributes to the resilience of the soil/plant system

Physical functions

• improves structural stability

• influences water retention and water holding capacity

• alters soil thermal properties

Chemical functions

• increases cation exchange capacity andholds nutrients (macro and micro)

• buffers changes in pH

• complexes cations

Functions of SOM

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Managing soil carbonAnother way to reverse declining soilorganic matter is by managing the soildifferently. We can’t detail here the variousland and crop management options andsystems for boosting soil carbon levels, butsuffice to say that the following aspects andpractices feature prominently:

• use of pasture leys (where land is left infallow to recharge);

• crop rotations, legumes and fertiliserapplications;

• ‘no till’ or ‘minimum till’ systems andcrop residue retention (conservationfarming);

• no burning of stubble (as this destroyspotential organic input and releasescarbon);

• application of manures or recycledorganics;

• lower stocking rates on grazing lands;• retention of grasslands and trees or

conversion of marginal lands to nativevegetation;

• forestry plantations; and• soil erosion control.

Pastures are an important tool becauseorganic matter concentrations tend to bemuch higher under grass. Observations11

of the world’s ecosystems show thatorganic carbon concentrations in soils (toa depth of one metre) under various landuses were: 122.7 tonnes per hectare fortropical forests, 117.3 for tropical savannas,96.2 for temperate forests, just 80 for crop-lands, but 236 for temperate grasslands.

Similarly, the importance of treesemerged in a recent study conducted by Dr Rick Young, of the NSW Department ofPrimary Industries, and several colleagues.12

‘We found that continuous cultivationand cropping over 20 years or more signifi-cantly depleted carbon concentrationscompared with grassy woodlands to adepth of 0.2 metres at all sites and to 0.6metres at three sites in north-westernNSW,’ says Dr Young. ‘The outstandingfinding was that comparisons between landuses and the total amount of carbon storedat the seven sites we studied were domi-nated by the number of trees per hectareand the size of the trees.’

The researchers concluded that in mostcases, maximum carbon – both soil carbonand total carbon on site (includingbiomass carbon) – will be maintainedwhere some tree cover is retained.

Dr Brian Tunstall of the EnvironmentalResearch and Information Consortium,and formerly with CSIRO, brings anecological perspective to the question ofrestoring soil organic matter.

‘Unfortunately, more than 75 per cent ofAustralian farming soils have organiccarbon contents less than 1.75 per cent,’ hesays, ‘indicating a widespread need toimprove soil organic matter concentrations.’

He too is critical of the current broad-scale farming approach with its high inputsof mineral fertilisers, herbicides and insec-ticides, which he regards as symptomatictreatments that are unsustainable in thelong term.

‘We need to increase levels of soil nutri-tion to increase the amount of organicmatter being recycled,’ argues Dr Tunstall.

‘But, to be sustainable, the nutritionshould be increased by developing thenatural processes, a functional soil, ratherthan treating soils as a hydroponic-likemedium that simply acts as a reservoir forwater and applied nutrients.

‘The idea is to build up biological popu-lations in the soil, not least the micro-organisms in organic matter whichmaintain a supply of essential elements toplants, rather than relying only on mineralfertiliser additions which negate the signif-icance of microbes. A good practicalexample of this is sugar cane farmers inQueensland who have been using microbesto incorporate organic matter into the soilafter harvesting.’

Overall, however, there are differingviewpoints on the benefits or otherwise of

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Organics from municipal waste are cleaned and processed into organic compost at GlobalRenewables Limited’s recycling facilities. GRL

On cultivated land, soil carbon gradually declines in the top 10 cm of various soils.From Dalal RC and Chan KY (2001)

11 Watson RT, Noble IR, Bolin B, Ravindranath NH, Verardo DJ and Dokken DJ (2000). Land Use, Land-use Change, and Forestry: A Special Report of the IPCC. Cambridge University Press, Cambridge.12 Young R, Wilson BR, McLeod M and Alston C (2005). Carbon storage in the soils and vegetation of contrasting land uses in northern New South Wales, Australia. Australian Journal of Soil Research 43, 21–31.

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mineral fertiliser additions. On the onehand, some scientists argue that reliance onmineral fertilisers contributes to theproblem because these do not return carbonto the soil, while other scientists recom-mend use of mineral fertilisers to promoteplant growth and hence increase the returnof crop residues to the soil. What farmerschoose to do probably depends on theirpreferred farming system and philosophy.

Soil carbon, sinks and global warmingIncreasing concern about greenhouseemissions and climate change is renewinginterest in soil’s capacity to hold carbon.Perhaps surprisingly, there is more carbonsequestered in the world’s soils (2500billion tonnes, or 2500 Gigatonnes (Gt))than in the atmosphere (807 Gt) or above-ground vegetation (560 Gt). Some 1550 Gtof this soil carbon, more than half, is in soilorganic matter.

So the historical losses of soil organicmatter represent a considerable contribu-tion to greenhouse emissions. In Australia,more than half the CO2-e (carbon dioxideequivalent) emissions from our agriculturesector are due to loss of organic carbonfrom the cereal belt. The good news is thatthis is a reversible process and there arebonus benefits.

‘If we can reverse these cereal belt lossesby restoring soil organic matter, there is thepotential to lock up about 50 Megatonnes(Mt) of CO2-e per year in the soil profileover a 20-year period,’ said Dalal. ‘That’salmost 10 per cent mitigation of thecurrent annual CO2 emissions in Australia.’

Brian Tunstall calculates that an averageorganic matter increase of 2 per cent to adepth of 30 cm, achievable in many claysoils, represents a sequestration of 35tonnes of carbon or 128 tonnes of CO2 perhectare … and there are many hectares offarm land.

Jan Skjemstad is more cautious on soilsequestration. ‘The problem is that theonly way to increase soil carbon is toincrease the amount of carbon going inthrough plant residues,’ he said. ‘This islimited by rainfall so even if stubble isburnt and all residues returned to the soil,there is a definite limit which is deter-mined by climate, soil type and manage-ment.

‘We estimate that if 16 million hectaresof cropping land were completely put tofull stubble retention and with morepasture included in rotations and withcrop yields increased, 10 Mt of carbonsequestration per year (or 35 Mt CO2-e) isabout as much as you would get over a30–40 year period. This assumes nodroughts.’

Mr Skjemstad also points out thathistorically our lands have not beencropped for that long. ‘As a result, not all ofour soils are that “flogged” yet and so donot have the potential to recover thatmuch. It really depends on when the soilswere originally cleared,’ he said.

He also highlights that there is greatvariation to take into account with the soiltypes and make-ups across Australia, andtherefore their treatment and potential.

‘Western Australia’s soils are very sandy

with relatively little organic matter, whileacross on the plains of Toowoomba inQueensland, farmers enjoy a very rich soilwith high organic matter and nutrientlevels – it can endure more intensive agri-culture.’

Evelyn Krull, meanwhile, points out thatadopting sensible land management prac-tices to boost soil carbon has benefits thatextend beyond the much-discussed green-house gas sequestration. She says directbenefits to the landowner includeincreased productivity, sustainability and,above all, better soil quality.

Besides the greenhouse benefits, there isa view that the next dramatic increase incrop production could come about by adeliberate increase in the biological activityof soil and by accessing nutrients throughmicrobe-driven mechanisms.

One way of doing this is by addition oforganic fertilisers on some farms, whichcould build soil carbon levels and, as alikely consequence, increase yields.James Porteous and Steve Davidson

Declaration: Dr John White is an invitedmember of the Ecos Editorial AdvisoryCommittee, made up of representatives fromareas of the national sustainability agenda.

Tomato yields in fields with poor soils (left) were improved (right) after the addition of 10 tonnes per hectare of carbon- and nutrient-richcompost (seen as the dark top covering) from recycled municipal waste. GRL/Katie Webster EcoResearch

More information:Soil organic matter overview:www.dpiw.tas.gov.au/inter.nsf/WebPages/TPRY-5YW6YZ?open

Compost Australia:www.compostaustralia.com

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