Australian Agriculture Assessment 2001 - Ways forward for agriculture

Australian agriculture has well demonstrated its capacity to adapt and innovate in response to environmental challenges. Australian farmers are conscious of the need to manage natural resources sustainably and to deliver a 'clean and green' product. They are progressively improving their activities within the broader contexts of increased profitability and community demands for improved catchment management.

The resource assessment components of Australian Agriculture Assessment 2001 focused on soils and nutrients - both on- and off-farm.

Australian Agriculture Assessment 2001 improves our understanding of natural resource processes active in agricultural landscapes-providing pointers to priorities for management action and further investigation.

Quantifying key management issues

Nutrients have increased to five times their natural levels in agricultural landscapes - increasing the potential for leakage from land to rivers and estuaries and therefore closer attention to nutrient management on farm.
Biomass productivity from agricultural landscapes has doubled from natural levels - reflecting the role of fertilisers and farm management systems in delivering profitable agriculture and buoyant rural communities.
Almost 1200 million tonnes of soil potentially moves on agricultural hillslopes each year - demonstrating the need for close attention to in situ soil management.
About 50 million tonnes of sediment from hillslopes each year enters the rivers with about 50,000 tonnes of phosphorus attached to the sediment - indicating the need for closer attention to sheet and rill erosion, especially maintenance of grass cover on our grazed landscapes.
About 44 million tonnes of sediment each year enters the rivers from gullies, including about 11,000 tonnes of attached phosphorus - demonstrating that even following half a century of government and industry attention to this most obvious of soil erosion activities, much still remains to be done in improving land use practice.
About 33 million tonnes of sediment each year enters the rivers from river banks, including about 9000 tonnes of attached phosphorus - clearly identifying that continued attention to riparian area management is essential.
Nearly 19,000 tonnes of total phosphorus and 141,000 tonnes of total nitrogen are predicted to be exported down rivers to the coast each year - demonstrating the importance of minimising soil erosion and quantifying the extent of enrichment of estuarine and coastal waters and the likelihood of long term algal blooms affecting our fisheries and recreational areas.
44 million hectares or 50% of Australia's agricultural soils have soil pH values below optimal levels (< 5.5) for acid sensitive agricultural production systems-foreshadowing major reductions in the productivity of our soils and underlining the need for good fertiliser management.
Amelioration of soil acidification by the current use of lime at a regional scale is less than adequate; projections suggest that liming needs to increase significantly-possibly up to 80 times present levels of use at appropriate rates of application.

Soil and nutrient management involves recognising inherent soil properties and maximising productivity with minimal degradation. Other soil properties that need to be part of management include soil organic matter, soil biota, soil compaction and structure, contaminants, salinity, waterlogging and soil sodicity. These were not able to be addressed by Australian Agriculture Assessment 2001 within the time frame and resources available but nonetheless are important both on- and off-farm and as part of farm management planning.

Soil management and regional climates

Agriculture productivity and maintenance of the natural resource base needs to be managed as a 'package' - understanding cause and consequence on farm and delivering to off-farm objectives set in a catchment context. Australian Agriculture Assessment 2001 provides useful insights into natural resource processes and the 'footprint' of agriculture. Natural resource issues coincide and interact in the landscape-regional differences need to be understood and will help set priorities and shape decision making both on and off farm

Five associations between climatic regimesand natural resource attributes were observed:

Organic matter levels in surface soils (estimated by soil organic carbon) were broadly related to regional rainfall and temperature regimes (Figures 3.6, 2.4). Thus, levels were usually higher in cooler and wetter environments and in irrigation regions than in more arid, dryland agricultural regions (e.g. the arid mallee soils of Victoria and South Australia and the northern wheat belt of Western Australia had very low levels). This pattern can be associated with substantial regional differences in the quantity of plant biomass generated annually (and hence gains in photosynthetic carbon), which in turn, affects carbon inputs into soils. Regional temperature mainly exerts its effect on the rate at which organic matter is decomposed in cultivated soils - rates being higher in tropical regions and warmer inland areas. Soil organic matter plays an essential role in nutrient supply, water holding capacity and structural stability. Practices to maintain soil organic matter are important for all agriculture and imperative in the more arid regions. Examples of key practices include maintaining adequate plant residue cover; adopting minimum/zero tillage, stubble retention systems; avoiding cultivation in high erosion risk periods; no stubble burning or over grazing.
Soil pH values in some agricultural regions tended to be lower as annual rainfall increased-therefore lower near the coast than further inland (Figure 4.5). This broad observation, particularly noticeable in transects inland from the eastern and southern coastlines, may be associated with soils in higher rainfall areas being naturally more acidic or, as in most cases, associated with the rate of induced acidification being more rapid in these higher rainfall environments. An integrated approach to fertiliser management will involve assessment of nutrient requirements and soil acidity hazard. While important for all of Australian agriculture, fertiliser management (Figure 3.3) is an imperative for the higher rainfall regions - from a soil acidity perspective and also recognising leakage of nutrients to groundwater and transport of nutrients by overland flow to waterways.
Soil erosion and tropical Australian grazing systems provide a particular management challenge. Losses from hillslope erosion from grazing lands might be comparatively low per hectare when compared to cropping - of the order of 1-2 t/ha/yr for grazing compared to measured figures for the cane industry of up to 200 t/ha/yr before the implementation of green cane harvesting. The total volume of sediment from the larger catchments and extensive grazing lands such as the Queensland catchments of the Fitzroy and Burdekin is very substantial with impact in-river, through estuary to near shore marine zone of the Great Barrier Reef lagoon. Much of this is a feature of the interaction of land use and climate. Many tropical environments experience an 'annual drought' with reduced grass cover before the onset of the high intensity storms and then monsoonal rains from November onwards each year. Practices to manage grazing pressure and retain pasture cover, crash graze and spell, minimise impact on river frontage country and trap sediment leaving the paddock are essential.
Water use efficiency, important for all Australian agriculture from a perspective of maximising productivity, becomes doubly important in those landscapes with a propensity to dryland salinity, particularly much of temperate Australia. Trends in wheat crop and other cereals productivity (Figure 7.7) were variable across the southern cropping regions of Australia, being generally greater and more consistent in the more reliable rainfall areas, where more intense and higher-input farming systems are practised and are more profitable. Broadly based, higher yield performances were particularly evident in Western Australia. Areas with consistently higher wheat yield gains, regardless of variation in rainfall, demonstrate the successful application of farming systems working with climate variability, are essential for maximising productivity in Australian agriculture.

Nevertheless, the water use efficiency index, defined for wheat-growing shires across southern Australia between 1983 and 1997, indicates that the level of water use by dryland wheat rarely exceeded 70%, with many shires below 50%, and some below 20% (Figure 7.3). Modelled estimates of deep drainage of soil water beyond the rooting zone (Figure 2.13) generally support low water use efficiency in many southern regions. This regional information can be associated partly with forecast risks of dryland salinity (NLWRA 2001) and secondly with soil acidification, where the leaching of soil nitrate is a major contributor to acidification (see Soil acidification section). Major risks exist on the sandier soils of Western Australia (Anderson et al. 1998) and in parts of New South Wales and Victoria.

In some regions, more diversified and intense systems of cropping with appropriate crop management will need to be adopted to minimise future risks of deep soil drainage of water and nitrate leaching. In others, the replacement of annual pastures with perennials maybe a more viable option to address acidity, water use efficiency and salinity hazard issues (Ridley et al. 2001, Heng et al. 2001).
Managing nutrients on farm and in landscape has climate and soil type components that needs to be recognised in best practise. The distribution of acidic land (pH_Ca 4.3 - 5.5) extending from central New South Wales through Victoria into the south-eastern region of South Australia (Figures 4.5, 4.6) closely resembles the distribution of soils with marginal soil phosphorus status (20 - 30 mg P/kg; Figure 3.13) - with the exception of the irrigated areas in north-central Victoria. This association suggests that soil phosphorus availability in these regions may be limited by acidic soil conditions. Dryland salinity risks are also predicted to increase in these regions of southern Australia (NLWRA 2001) and this may be linked to higher deep drainage losses of soil water and phosphorus beyond the rooting zone in these winter dominated rainfall areas (Figure 2.13).

The largely negative nutrient balances (signifying nutrient depletion) estimated for major regions of Queensland and the Wimmera in Victoria are associated with relatively low applications of fertiliser on soils of naturally high soil fertility status. Soil fertility decline will need to be closely monitored in these regions so that fertiliser use can be increased as soil fertility decline starts to impact on productivity.

By contrast, the highly positive nutrient balances recorded for higher rainfall regions where dairy and horticultural industries often co-exist relate to the regular use of fertilisers and generally higher soil fertility status (see Nutrient management section). Attention to nutrient balance, minimising applications of fertiliser surplus to plant needs is essential in these regions and will contribute to minimising any off farm impact from these industries.

Soil management-essential for integrated agricultural land and landscape management

Soil management decisions must also lead to:

optimising agricultural productivity by identifying and alleviating soil constraints to yield;
countering longer-term degradation to soils through current soil management practices that overcome insidious soil processes, including impacts on the physical, chemical and biological properties of the soil root zone; and
minimising off-site impacts in catchments and downstream.

A systems approach. Agricultural research has repeatedly demonstrated that changes in soil use induce a myriad of complex changes to soil processes that affect soil health in both beneficial and detrimental ways.

A change in tillage systems to minimise soil erosion can affect availability of soil nutrients (Robson & Taylor 1987).
Applications of lime to arrest soil acidification increases availability of some soil nutrients (e.g. phosphorus and molybdenum) by altering soil chemical reactions, and may change the risk of disease or induce imbalances in the copper status of livestock (Brennan & Bruce 1999).

These 'cause and effect' relationships need to be carefully assessed in formulating 'site-specific' management practices. For example, the strongly positive yield trends observed recently in the Western Australian wheat belt have been attributed to farmers adopting an integrated package of new crop and soil management practices, which together produced synergistic impacts to increase crop yields.

Recognising on farm variability. Agricultural systems place varying demands on soil resources. Conversely, soils with widely varying properties need to be managed differently, irrespective of the land use practised, since some are more fragile, while others are more resilient.

We need to manage agricultural landscapes according to the known distribution and characteristics of soil types. For example, in the mallee regions of southern Australia, sandy dunes are often managed differently (including not being cleared for agriculture) to the loamy flats (e.g. cereal rye is grown on the top of dunes, barley on the sides and wheat in the flats). Experience has shown that these particular land uses, with different soil and crop husbandry practices, better match land capability within the landscape and deliver higher productivity.

Building off-farm needs into soil management. Many of the problems encountered in achieving or maintaining sustainability in farming systems arise because agricultural management systems are not well matched to the landscape and its needs. Excessive leakage of nutrients and water is a widespread problem associated with many of the annual crops and pastures in southern Australia. Solutions include increased adoption of perennials - located to buffer rivers and watercourses - to maximise environmental returns, while minimising economic losses (perennials are normally less profitably than annuals). Placement of crops and pastures requires a good understanding of their environmental requirements, as well as, a good knowledge of how these requirements vary across a landscape. Regional soil information and an appreciation of interactions with the local landscape are critical.

Australian agriculture is progressively adopting and developing precision agriculture and site-specific management to meet the challenge of variable soils and landscapes.

Underpinning soil management-building better knowledge and information to support integrated resource management

Australian Agriculture Assessment 2001 has brought together the best available data and information on the condition of Australian landscapes used for agriculture. It has relied on major data collections by public agencies and private industry and interpreted these to provide management orientated information. The development of Australian Soil Resources Information System is a good example. This initiative has delivered soil properties information from national to regional scales. It is based on a diverse range of soil mapping activities across Australia over the past 20 years - many previously inaccessible to farmers and poorly presenting management orientated information.

Better data to reduce risks in decision making

Information needs to be closely linked and driven by the decision making process whether at a paddock, enterprise, small catchment, region or nation scale.

Agricultural industries need better information to:

match land use and practice with land suitability;
gain market advantage by demonstrating the sustainable nature of production systems; and most importantly
maximise productivity.

Regional communities need better information to:

prepare or review regional development management plans and set realistic natural resources targets;
prioritise works and then assess the efficiency of works in meeting their regional targets; and
improve awareness among all land users of landscape processes and priorities for management.

Government agencies need better information to:

develop policy frameworks to encourage sustainable and productive use of our natural resources;
identify priority initiatives and then assess the effectiveness of natural resource management programs;
implement trading schemes (e.g. for salt, water or carbon) to achieve better natural resource management outcomes; and
set baselines and to monitor trends.

Commodity research and development groups need better information to:

create improved management tools such as simulation models to assess the production opportunities of farming systems; and
develop improved understanding of soil and landscape processes.

Trade-offs between the desired and practically feasible level of data and information provision are inevitable. The greater the detail, accuracy and precision, the greater the costs of gathering, interpreting and reporting.

Information provision-mapping, monitoring and modelling: the tools

Mapping, monitoring and modelling land condition are complementary activities. In isolation, each fails to provide appropriate information for soil management and planning. Combined, they provide a powerful means for improving the quality of agricultural land management in Australia.

A major challenge facing those supporting agriculture - the public agencies, commodity research groups and industry bodies - is to achieve better integration and application of these activities.

Land resource mapping-establishing a baseline

Land resource mapping provides a structured description of landscape attributes. Land resource mapping delineates repeating patterns of landscapes and associated soils. Key parameters of the landscape and soils that influence soil health and productivity are recorded including:

terrain attributes of slope and relief;
vegetation and land use descriptions of the major soil types and associated soil properties such as soil texture, structure, water-holding characteristics;
pressure or absence of root limiting layers; and
fundamental soil chemistry-pH interpretations are often linked to land resource mapping-suitability and versatility assessment or susceptibility to landscape issues such as waterlogging or erosion.

Land resource mapping provides a framework for extending our detailed knowledge of one location to other locations with similar characteristics. This is essential for planning and managing land at all scales. It provides the baseline for determining resource condition and input data required by models that predict likely response to changes in the landscape.

Good progress has been made over the past decade to improve the land resource information base - particularly through the National Landcare Program and Natural Heritage Trust. Australia is vast and a great deal remains to be done to meet the growing demand for high quality and resolution information. Resource constraints inevitably mean that information collected must be prioritised and targeted to areas of most significant need.

Figure 9.1 Mapping, monitoring and modelling are complementary activities for natural resource management.

In the process of building the Audit's Australian Soil Resources Information System a number of significant deficiencies in the current land resource mapping coverage were identified:

The coverage of land resource map in agricultural areas is incomplete and in most areas the scale is too broad to be useful for decisions at the farm level.
Agencies have used incompatible methods for surveying land resources. This made compilation of an Australia-wide overview of land resource condition and provision of soil property information very difficult.
Many key soil and land attributes controlling land degradation or productivity have not been measured in a rigorous manner, seriously limiting our capacity to make assessments linking land resource condition with practice.
Lack of adequately geo-referenced, time-series data on critical soil properties.
Soils have not been representatively sampled on the landscape nor on a statistical basis.

To support the information requirements of regional planning and evaluation - in the mid-term (10 to 15 years) - Australia should aim to have a land resource survey at nominal scales of 1:50 000 for intensive agricultural lands (irrigation, horticulture), 1:100 000 for dryland agricultural areas, both cropping and pasture, and 1:250 000 for the extensive pastoral regions.

Achieving these scale targets, even in priority areas, will require long-term investment in survey activities. The commitment requires permanent resource assessment groups in State and Territory agencies to ensure continual improvements to natural resource databases and better links with modelling and monitoring groups.

Simulation modelling-building understanding and developing scenarios

The projects of Australian Agriculture Assessment 2001 have demonstrated that computer simulation modelling of farming systems and landscape processes (e.g. erosion, soil acidification) can be used to improve understanding, set targets and prioritise management of Australia's land and water resources. To fully realise the potential benefit of simulation models we need to:

ensure that survey and monitoring programs make data accessible through data libraries (e.g. Australian Natural Resources Data Library: adl.brs.gov.au) for running and validating models (with statements on accuracy and precision); and
have an active research program to develop integrated simulation models useful for guiding land management decisions - at a range of scales, both on and off farm.

Land condition monitoring-measuring progress

Many programs for land condition monitoring have been implemented during the last decade, generating significant benefits (e.g. community-based monitoring programs have provided basic data relating to weather and bird populations). Most programs for monitoring land condition have focused on improving land literacy rather than generating a technically sound monitoring network and accompanying database.

The focus on land literacy is most commendable, but there are few regions in Australia where comprehensive trends in land condition, and soil properties in particular, can be deduced from reliable time-series data.

Land condition monitoring programs must:

have a clear purpose and be closely linked to a decision-making process at farm, catchment, State or national level. This link may be direct (e.g. a farmer monitoring nutrient levels and planning fertiliser applications accordingly) or more general (e.g. Birds Australia documenting the decline of particular bird species in agricultural areas). Such programs lead to increase community awareness, attract publicly funded programs and encourage landholder action.
include monitoring sites - to establish reference point or reference condition. These sites should be located after land resource or ecological surveys have been undertaken so that sites represent well-defined landscape units and land use systems. Results can then be extrapolated with confidence.
have monitoring activities closely aligned with modelling activities to assess whether change in land condition can be detected in a reasonable time and cost-effectively. Modelling can also be used to predict trends at locations beyond those used for direct monitoring by capitalising on the understanding gained from the field measurement program-the monitoring sites can be used to validate model predictions.
focus monitoring in areas where early change in land condition is likely. This avoids wasting resources on measurement programs and ensures that it provides an early-warning system.

The proposed strategy for land condition monitoring for sustainable agriculture recognises that monitoring requires significant resources, cannot be undertaken everywhere and therefore must be clearly focused.

Land condition monitoring must provide multiple benefits by:

assisting in site-specific decisions;
being applicable and aggregated to regional scales; and
providing inputs to predictive models-based on sound understanding of natural resource processes and interrelationships within the landscape and over time.

Key elements are:

Community programs providing support for community land condition monitoring programs, where motivation is strong and technical capacity or support is sufficient. This should include the provision of protocols for measurement, training, database maintenance and feedback on trends and utility of the data.
Industry programs providing support for soil monitoring (e.g. by fertiliser companies) by providing protocols to ensure data compatibility in sampling, site and profile characterisation, geo-referencing, laboratory measurement and database maintenance.
Setting a baseline providing support for establishing a distributed set of reference sites within land resource survey programs. The properties of these sites would be thoroughly characterised to establish a baseline upon which future changes in properties can be assessed. Provisional protocols for these sites are being developed by the Audit and a scheme for soil carbon has been published (McKenzie et al. 2000).
Long-term sustainability issues providing support for a number of substantial, long-term scientific studies on ecosystem and landscape processes in catchments that represent Australia's main agricultural regions. These long-term studies would measure and model water, sediment, nutrients, biological production and related processes and would be essential for developing an improved understanding of processes controlling agricultural sustainability.

Active partnerships between industry, government, research and community groups will be a key ingredient for the success of monitoring activities across Australia. It will be important to build on the achievements of, and draw support from, technical coordination activities such as the Australian Collaborative Land Evaluation Program. Participative structures that ensure the collaboration of farmers, community groups, policy makers and researchers are essential if the agricultural landscapes of Australia are to be well understood and managed in a sustainable manner. Photo 9.4

Conclusions

Australian Agriculture Assessment 2001 has highlighted opportunities for continuous improvement in the information base to support Australian agriculture:

Progress made by Australian Agriculture Assessment 2001

Agribusiness, industry and government partnerships. Building and improving agribusiness knowledge and information bases to support agricultural development, investment decisions and establishing environmental credentials to enhance market access. The Sustaining Our Natural Resources - Dairying for Tomorrow project provides an excellent example of an industry-led initiative where information was gathered to support sustainable development and to improve practice at regional and national planning scales.

Best available information. Information bases developed by the Audit such as the Australian Soil Resources Information System will need to be updated and information products developed, as new information and better understanding of soil processes becomes available. The full value of Australian Soil Resources Information System will only be realised if it is maintained and efforts coordinated Australia wide, probably through the Australian Collaborative Land Evaluation Program.

Setting the context for soil management works and activities. For the first time, Australia has a comprehensive assessment on the transport and fate of sediments and nutrients in agricultural landscapes. Sources and sinks for sediments and nutrients have been identified and the findings can be used to set priorities and to target actions.

Defining monitoring needs. Australia needs to establish a monitoring framework upon which progress and change in resource condition and the effectiveness of private and public investments can be assessed. Adoption of an integrated 'map-monitor-model' framework for land condition assessment will provide a basis for reporting and predicting change.

Areas for improvement-filling the gaps

Better links to on-ground activities. A key input to Australian Agriculture Assessment 2001 assessments was land use data. Improved geo-referencing of land use, productivity and practice information will provide the next upgrade path for these assessments (e.g. input of nutrients from point sources such as piggeries and feedlots were beyond the scope of this assessment because of a lack of geo-referenced data).

Enhanced assessment. Improved data on riparian vegetation (extent, type, condition and effectiveness in terms of buffering function) and river hydrology (degree and type of change from a reference condition) would increase accuracy of Audit assessments of sediment and nutrient transport from land to the river.

Industry leadership in information provision. Data and information are the currency of most organisations and industries. Structured approaches for their collection from industry sources would significantly enhance the capacity of industry and government to work in partnership towards sustainable resource and industry development. This could include improved understanding on the distribution and application of fertiliser and lime on agricultural landscapes, supported by routine soil, plant and water testing - as a budget approach to nutrient management. Partnerships between companies, industry peak bodies and government as demonstrated by the Audit nutrient management assessment through the member bodies of the Fertilizer Industry Federation of Australia are critical to achieving a coordinated approach.

The dairy industry has also clearly demonstrated (through Sustaining Our Natural Resources - Dairying For Tomorrow) that access to detailed regional data and information on the operational, socioeconomic and environmental
activities and adoption of best management practice are fundamental to planning and implementing a sustainable future for the industry.

Research and development. The impact of agricultural land management and climate variations on soil and landscape processes and the inducement of changed soil properties requires continuing and accelerated research effort so that land, water and vegetation management targets can be realistically set and achieved. Off-site effects of soil acidification are at best surmised conceptually and based on anecdotal information.

Where to from here?

Increased emphasis on integrated land management in Australia can deliver both productivity outcomes on-farm and natural resource benefits off-farm. Success of such initiatives needs:

leadership in monitoring and reporting from within agricultural industries and their research and development corporations;
tracking progress on the adoption of best management practices, and working to targets while continuing research, development and extension to refine management practices;
soil management including soil erosion control and revegetation of riparian lands;
nutrient management, increasing attention to soil fertility and nutrient balance on farm, based on a partnership with Australia's fertiliser industry, their extensive activities in soil testing, extension and farmer support;
green credentials, developing consistent and Australia-wide accreditation systems, positioning Australian agriculture as the global leader in 'clean and green' commodities;
comparable data, ensuring implementation of standards and protocols for the collection of land condition data and information - to support regional action planning, evaluation and monitoring activities-particularly for initiatives such as the National Action Plan on Salinity and Water Quality and Natural Heritage Trust programs; and
improved access to data and information to and from community groups and land managers.

References

Anderson G.C., Fillery I.R.P., Dunin F.X., Dolling P.J. & Asseng S. 1998, 'Nitrogen and water flows under pasture-wheat and lupin-wheat rotations in deep sands in Western Australia 2. Drainage and nitrate leaching', Australian Journal of Agricultural Research vol. 49, pp. 345-61.

Brennan R.F. & Bruce R.C. 1999, 'Molybdenum', in K.I. Peverill, L.A. Sparrow & D.J. Reuter (eds), Soil Analysis: an interpretation manual, CSIRO Publishing, Melbourne.

Dalal R.C., Lawrence P., Walker J., Shaw R.J., Lawrence G., Yule D., Doughton J.A., Bourne A., Duivenvoorden L., Choy S., Moloney D., Turner L., King C. & Dale A. 1999, 'A framework to monitor sustainability in the grains industry', Australian Journal of Experimental Agriculture, vol. 39. pp. 605-20.

Heng L.K., White R.E., Helyar K.R., Fisher R. & Chen D. 2001, 'Seasonal differences in the soil water balance under perennial and annual pastures on an acid Sodosol in southeastern Australia', European Journal of Soil Science vol. 52, pp. 227-36.

McKenzie N.J., Ryan P. J., Fogarty P., Wood J. 2000, Sampling, measurement and analytical protocols for carbon estimation in soil, litter and coarse woody debris, National Carbon Accounting System Technical Report No. 14, Australian Greenhouse Office, Canberra.

National Land and Water Resources Audit 2001, Australian Dryland Salinity Assessment 2000. Extent impacts, processes, monitoring and management options, Commonwealth of Australia.

Ridley A.M., White R.E., Helyar K.R., Morrison G.R., Heng L.K. & Fisher R. 2001, 'Nitrate leaching loss under annual and perennial pastures with and without lime on a duplex (texture contrast) soil in humid southeastern Australia', European Journal of Soil Science vol. 52, pp. 237-52.

Robson A.D. & Taylor A.C. 1987, 'The effect of tillage on the chemical fertility of soil', in P.S. Cornish & J.E. Pratley (eds), Tillage: New directions in Australian agriculture, Inkata Press, Melbourne.

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