managing soil organic matter - Grains Research & Development ...

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