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20MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEthat of soils containing only bacteria and fungi(Kautz and Topp 2000).EarthwormsEarthworms are generally considered positive for thehealth of broadacre agricultural soils, but quantifyingtheir impact on soil function has not been extensivelystudied in Australia. Since earthworms do not haveteeth they ingest both organic matter and soil, usingthe soil to help grind up organic residues internally.Their waste (worm casts) is a resultant mix ofstrongly aggregated soil and organic residues thatare rich in plant available nitrogen (0.6 per cent) andphosphorus (2.8 mg/100 g). The carbon content ofthese casts is on average 1.5 times that of the bulksoil (Bhadauria and Saxena 2010).Significant numbers of earthworms are required tostimulate an improved soil structure. For example,Fonte et al. (2012) determined the equivalent of 144worms per m 2 in the top 10 cm of a soil that alsohad high fungi and bacteria numbers and activelygrowing roots, was required before a six per centincrease in aggregate stability was measured. Thishas led to estimates of drainage rates up to 10times faster and infiltration rates six times faster insoil with earthworms compared to those withoutearthworms.Significant numbers ofearthworms are required tostimulate an improved soilstructure.In Australian cropping soils, only a few speciesof earthworms are associated with improved plantgrowth (Blackmore 1997). Current estimates ofearthworm populations in Australian agricultureare between 4/m 2 and 430/m 2 , although higherpopulations have been observed in some pasturesystems (Buckerfield 1992; Mele and Carter 1999;Chan and Heenan 2005). In north-eastern Victoriaand southern New South Wales, the density ofearthworms across 84 crop and pasture sitesaveraged 89 earthworms/m 2 , with a low speciesrichness of 1-2 per site (Mele and Carter 1999). Thisis similar to the average abundance (75 earthworms/m²) reported by Chan and Heenan (2006).Earthworm populations generally increased inwetter environments above 600 mm annual rainfalland were three times higher in pasture sites thancropping paddocks (Mele and Carter 1999). Zerotillagecombined with stubble retention was alsoshown to support earthworm populations up toseven times higher than in disturbed systems whereresidues were burnt (Chan and Heenan 2006).Variability in earthworm numbers has also beennoted within seasons, increasing during the wintermonths to between 160/m 2 and 501/m 2 for cropand pasture systems respectively and betweenseasons (Chan and Heenan 2006).Earthworms are not readily supported in somesoils, including very coarse sands or acidic soils(pH Ca less than 4.5), and are often absent regardlessof management. In part this is due to low levels ofcalcium in these soils, which is required continuouslyby earthworms.Soil-borne plant pathogensOrganisms that attack living plant tissue andcause plant diseases are called pathogens. In soil,undesirable organisms include a range of insects,parasitic nematodes, protozoa, viruses, bacteriaand fungi, which may be present even where thereare no visible symptoms. For a disease to developseveral criteria must be met. There must be asuitable host plant, a pathogen and an environmentsuited to its growth. Climatic patterns also affectthe types of fungal pathogens that are dominant ina region. Disease outbreaks can be caused by anincrease in the population of the pathogen, or byan increase in susceptibility of the plant, which isaffected by factors such as its age and nutritionalstatus, environmental stress, crop type or variety.The degree of root damage will generally relate tothe number and type of disease pathogens present.Undesirable organisms influence soil health andproduction through their influence on root and plantvigour and changes in the soil food web. A highincidence of pathogens can slow root growth anddecrease the ability of roots to acquire water andnutrients, decreasing grain and pasture yields, andconstraining organic matter inputs into soil. In doingthis, the addition of fresh organic inputs that favoursthe growth of a diverse range of beneficial organisms(as compared to pathogens) is constrained.Soils with high levels of soil organic matter andbiological activity seem to prevent pathogensfrom taking hold due to increased competition forresources, which constrains pathogen activity, or
21MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEby sheltering antagonistic or predatory microbes.For example, a decline in plant-parasitic nematodesand an increase in saprophytic nematodeswere observed with the use of diverse rotationalsequences, addition of organic matter, covercrops, green manures, composts and other soilamendments (Widmer et al. 2002).Suppressive soilsSuppressive soils are those that naturally suppressthe incidence or impact of soil-borne pathogens.Disease suppression can develop over 5-10 yearsand is a function of the population, activity anddiversity of the microbial biomass (Roget 1995,2006). In southern Australia, sites with high levelsof disease suppression to rhizoctonia (Rhizoctoniasolani) and take-all (Gaeumannomyces graminis var.tritici) have been associated with higher inputs ofbiologically available carbon and greater competitionfor food resources (Roget 1995, 2006).The increase in available carbon at these siteswas associated with intensive cropping, full stubbleretention, limited grazing, no cultivation and highyielding crops. However, the adoption of thesepractices does not always result in suppression ofsoil pathogens as evidenced by the many instancesin which disease continues to be prevalent. To datethere are no indicator species associated with thedevelopment of suppression in agricultural soils,though research in this area is progressing.ORGANIC MATTER TURNOVER TIMESThe ‘turnover’ or decomposition rate of organicmatter refers to the time taken for organic matter tomove into and through the various organic matterpools, including living, actively decomposing andstable. This movement is a continual processand is vital to the functioning of all ecosystems.As new organic matter enters the soil it supportsbiological processes, releases nutrients throughdecomposition and contributes to soil resilience.The difference between organic matter inputs andoutputs and the rate at which they are transformeddetermines the size and stability of the organic matterpools (see Chapter 1). Soil biological function is lesssensitive to the total amount of organic matter thanthe rate at which organic matter turns over, whichis related to the size and nature of the soil organicmatter pools and soil depth (Dalal et al. 2011).Understanding and quantifying the mechanismsdriving turnover of organic matter between the poolsis critical to the capacity to increase and maintainsoil organic carbon in different soils and climates.The resistant soil organic matter fraction can takeseveral thousand years to turn over and is relativelyinert, while the stable humus pool generally takesdecades or centuries. In contrast, the activelydecomposing pool, which includes the particulateand dissolved organic fractions, has a turnover timeof less than a few hours through to a few decades.As organic matter is decomposed and moves fromthe rapidly decomposing fraction through to the
Page 1 and 2: MANAGING SOILORGANIC MATTERA PRACTI
Page 3 and 4: FOREWORD1The one constant in agricu
Page 6 and 7: TABLES, FIGURES AND PLATES4MANAGING
Page 8 and 9: 6INTRODUCTIONMANAGING SOIL ORGANIC
Page 10 and 11: 018SOIL ORGANIC MATTERMANAGING SOIL
Page 12 and 13: Table 1.1 Size, composition, turnov
Page 14 and 15: Table 1.2 Functional role of soil o
Page 16 and 17: Figure 1.7 The influence of soil ty
Page 18 and 19: 16HOW MUCH ORGANIC CARBONREMAINS IN
Page 20 and 21: 0218BIOLOGICAL TURNOVEROF ORGANIC M
Page 24 and 25: 22MANAGING SOIL ORGANIC MATTER: A P
Page 26 and 27: 2403MANAGING SOIL ORGANIC MATTER: A
Page 29: less than 15 per cent of carbon inp
Page 36 and 37: Figure 4.1 Nitrogen cycling in soil
Page 39: mycorrhizal associations have been
Page 42 and 43: Table 5.1 Influence of soil charact
Page 44 and 45: Figure 5.2 Adjustment of organic ca
Page 48 and 49: 0646ORGANIC MATTER LOSSESFROM SOILM

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