Future Directions for Dryland Soil Management Under Direct Seeding Techniques - an Australian Perspective: Part 2

In parts of this paper, data is presented from in-paddock observations, or extrapolated from other regions, with some notable exceptions. Although this is in part due to evolution in our understanding of the factors at play, and limitations in the resources available to perform wide-ranging studies, caution must be exercised in the interpretation of some of this data, particularly in regard to:

paddock history; conversion from traditional tillage to direct drilling may not overcome the limitations of, say, the presence of a tillage induced hardpan;

lack of time for conservation treatments to demonstrate measurable improvement, particularly on paddocks with a long history of traditional practices;

lack of comparative analysis from replicated experiments;

pooled data sets from sites that are not directly comparable;

lack of conformity in defining soil treatments such as `direct drilling'; for example, `direct drilling' may or may not include the burning of crop residue, may or may not include the occasional presence of livestock during certain times of the season, and although usually limited to one pass at sowing, the amount of soil disturbance can vary substantially;

lack of standardisation in the depth of sampling and time of sampling within a crop cycle.

Despite these limitations, the following case is argued:

Although the benefits of conservation tillage methods have been proven in many circumstances and the rate of soil degradation has slowed, an improvement in soil quality has not always been demonstrated. The reasons for this are examined, based on the available evidence, leading to the conclusion that all facets of the cropping system should be designed to lead toward improved soil quality. Barriers to adoption need continued exploration.

The impacts of tillage on soil physical quality - an overview

The impacts of tillage on soil physical quality* at the paddock scale have been well documented (Harte 1984, Packer et al. 1992, Murphy et al. 1993, Geeves et al. 1995, Macks et al. 1996, Edwards & Zierholz 2000, Lawrie et al. 2000, Chan et al. 2003). Most studies conclude that agricultural activity has accelerated soil and landscape degradation. Specifically, tillage has been shown to increase soil erosion and bulk density*, and reduce hydraulic conductivity*, water holding capacity and organic carbon content. The rate of soil degradation can be rapid, sometimes resulting in major impact after only a few years of tillage (Harte 1984, Chan & Mead 1988, Murphy et al. 2003). At the catchment scale, crop and pasture establishment in dryland cropping areas of Australia using frequent tillage and stubble burning has caused significant landscape degradation, with subsequent impacts on catchment water quality and visual amenity (for example, see Pratley & Robertson 1998).

Since the 1940's, earthen soil conservation banks were often built to minimize erosion, designed to intercept runoff before it reached erosive velocities on bare soil. However, due to the presence of bare soil between these banks, erosion still occurred (Murphy & Flewin 1993), including catastrophic erosion events when runoff volumes exceeded the design capacity of the banks. With the introduction of knockdown herbicides to Australia in the 1960's, alternative cropping systems now broadly known as `conservation tillage' became possible. These systems involved fewer tillage operations to control weeds, and sometimes involved direct seeding and stubble retention, to address agronomic, environmental and economic issues such as optimum sowing time, increased farm management flexibility, reduced wind and water erosion, reduced labour costs and increased machinery life. In a recent survey (D'Emden & Llewellyn 2004), farmers nominated `soil conservation' as the most common reason to adopt conservation tillage practices. Erosion rates have declined significantly as a result of the adoption of conservation tillage practices (Packer et al. 1992, Hairsine et al. 1993, Lawrie et al. 2000).

More recently, there has been a change of focus from erosion control to a focus on broader environmental and economic benefits of improved soil quality using additional or more specialised conservation farming practices in crop and pasture enterprises. In the survey by D'Emden & Llewellyn (2004), 26% of farmers nominate soil quality factors as reason to adopt conservation tillage. This has been possible with the implementation of cropping systems such as zero tillage and controlled traffic, and planned grazing systems such as high intensity - short duration grazing management. These practices are designed to maximise plant biomass production and retention, as a means of constructing biopores and maximising soil biological activity, as well as maximising harvestable product.

One of the challenges is to scientifically quantify the soil improvements being observed, and to identify agronomic and economic advantages and limitations of these systems, a difficult task when considering soil biological criteria (Bell et al. 2003). There is, however, data from Australian and other research to support the conclusion that enhanced soil physical quality will increase soil biological activity, and this benefits crop performance. For example, in a replicated study of soil fauna under alternative tillage treatments on two sites in central NSW, Longstaff (et al. 1999) found that species diversity and abundance of springtails and mites, selected as indicator species in this study, responded to tillage treatment, with increased numbers in direct drilled and stubble incorporated plots. To some degree, this would be expected, but a shift in species composition was also noted, implying that this shift in trophic groups is likely to influence energy and nutrient dynamics. In comparing direct drilling with conventional cultivation at two sites in central west NSW, Pankhurst (et al. 1997) measured significant increases in soil microbial biomass and activity with direct drilling, and also noted the more beneficial fungal domination of the soil biomass* under direct drilling.

Is `direct drilling' sufficient to improve soil physical quality?

In the previous section, it is claimed that numerous studies have shown that conventional tillage practices, involving multiple passes of tillage equipment prior to seeding, has a detrimental impact on soil quality. It can be concluded that direct drilling (i.e. soil disturbance only at the time of planting) is likely to result in a less detrimental impact on soil quality. Direct drilling is frequently practised by farmers in the region, often as a step toward a no tillage or zero tillage regime. Many farmers perceive that soil disturbance, even if limited to the seeding zone, is necessary to optimise the microenvironment surrounding the seed. Baker (2003) suggests this should not be the case, and untilled (but loose) soil combined with adequate residue cover provides the better microenvironment potential, on condition that the characteristics of the seeding slot, fertiliser placement and opener design are optimal.

Some studies have shown that direct drilling, although less detrimental, may not result in significant soil improvement. In most cases, direct drilling results in less soil erosion than conventional tillage, as described earlier, but the contribution to other measures of soil quality is not always as consistent. Lawrie (et al. 2000), averaging the data of Geeves (et al. 1995) from 75 sites in southern NSW and northern Victoria, showed that on average, the direct drilled sites with stubble retention had higher soil carbon levels in the 0 to 20 mm layer and higher surface hydraulic conductivity than traditionally tilled sites. However a more careful examination of the data showed that in some cases the direct drilled sites were not in better soil physical condition. It seems that the adoption of "direct drilling" alone is not sufficient to ensure an improvement in soil physical properties. Given that direct drilling means that crops are sown into an untilled seedbed, and if the site being direct drilled has had little plant growth and biomass production in the previous season or is already in a degraded condition, then the expected benefits of this `better' cropping practice will not necessarily be apparent. A major finding was that the `best' cropping treatment remained significantly lower in soil carbon content and surface hydraulic conductivity than adjacent woodland soils and lightly grazed soils, and heavily grazed pastures fared equally poorly.

In a review of many field trials of conservation tillage on light textured soils in southern Australia, Chan (et al. 2003) observed that although storage of soil organic carbon is greater in sites that are direct drilled, compared to conventional cultivation, the differences are only significant in higher rainfall areas (>500 mm), and that soil quality is likely to remain fragile in lower rainfall zones. A number of qualifications and observations are discussed by these authors. These levels are often lower than reported overseas, indicating local climatic or soil effects are relevant, and long term data is not readily available. Increases in soil carbon content are usually limited to the topmost (0 _ 50 mm) soil layer. Although differences in soil carbon content have only been modest, the frequent reporting of improvements to soil quality under direct drilling are consistent with those increases being of high quality (i.e. more labile*) carbon fractions. Importantly, many trials have not been designed to separate stubble effects from tillage effects.

In a paired paddock comparison of crop establishment practices on 5 soil type classes from 22 sites throughout southern NSW, Packer (et al. 1998) found no significant improvement in infiltration rate at _10 kPa potential, organic carbon content, or soil bulk density after three years of `conservative' practice, recognising that more time may have been necessary to develop improvement. In another set of experiments, Packer (et al. 1984) found no significant differences in soil bulk density, soil organic matter or sediment loss between direct drilled and tilled treatments of a fine sandy loam surface textured soil in southern NSW, after 4 years. The authors believed the lack of difference related to the following: the degree of soil disturbance with direct drill seeding; variation in biomass production between sites; the amount of grazing pressure; the duration of the experiment; the number of samples; the use of `hybrid' seeding systems by some landholders (combinations of direct drilling and conventional tillage).

A pasture phase is often recommended as a means of restoring soil physical quality and managing soil water. Lucerne (Medicago sativa) is often recommended as the preferred pasture species due its extensive root system as well as its fodder value (for example, Hirth et al. 2001, Ridley et al. 2001). However, in a comparison between lucerne pasture and direct drill treatments (with and without stubble retention) on a Red Alfisol in southern NSW, Roberts & Packer (2000) found that full stubble retention was the only treatment to record a significantly higher infiltration rate. The relatively poorer soil structure under lucerne pasture and direct drilled treatments without stubble retention appears due in part to a compaction layer (as measured by increased soil bulk density) at a depth of 8 _ 12 cm. Lawrie (et al. 2000) describe grasses as better contributors to aggregate stability than legumes. It is also clear that the intensity of grazing of pastures is a key factor, with heavy grazing resulting in significantly poorer soil physical quality than even some cropping treatments (Lawrie et al. 2000). These are important considerations where livestock and crop production are used together in farming systems.

It may take many years to detect a measurable improvement in soil quality. Packer (et al. 1992), in a 7 year trial, conclude that 5 years of `conservation tillage' practice is required on loam and sandy loam surface textured soils before differences in runoff volume, hydraulic conductivity, soil loss and soil bulk density (at 100 mm) become significant and consistent. Roberts & Packer (2000) report that four years were required to create improved surface soil physical quality under direct drilling with stubble retention in a Red Alfisol in southern NSW. These reports are consistent with other workers, with Hamblin (1980) suggesting the relatively dry climate a contributing factor.

It appears that many factors contribute to improved soil physical quality, including retention or burning of residue, the presence and grazing intensity of livestock, and the history of the paddock. Consequently, if soil improvement is the goal, then direct drilling alone may not be sufficient.

The importance of other components of the farming system.

There is general agreement that improvement in soil physical quality is dependant on development, stability and maintenance of soil macropores. Packer (et al. 1992) used soil hydraulic measurements to show that formation of soil macropores greater than 0.75 mm diameter was necessary to create significantly greater porosity and reduced runoff in a sandy loam surface textured soil, observing that the mechanisms for macropore development related to soil fauna activity and plant root development. Although the contribution of soil organic matter is acknowledged, they conclude that macroporosity itself is of greater importance. Murphy (et al. 1993) showed that changes in hydraulic properties associated with tillage systems was largely associated with changes in pores > 0.75 mm in size. Activity which disrupts macropores and their development and stability will therefore reduce soil physical quality directly, as well as indirectly via unfavourable alteration to the habitat of soil fauna. These activities not only include tillage, whereby a single tillage pass may be sufficient to cause substantial disruption to soil macroporosity and soil biomass activity (Pankhurst et al. 1997), but also compaction from machinery wheels and livestock.

Rummery & Coleman (2000) compared three years data of yield, gross margin and water use efficiency for wheat, chickpea and sorghum crops on conventionally established crops and those established by no tillage plus controlled traffic. Their data relates to northern NSW, but encompasses an area of some 60,000 ha. In all crops in all years, the performance of conservation farming systems exceeds that of conventional crop establishment. In a study of the effects of wheel compaction on a vertisol in Queensland, McHugh (et al. 2003) measured a 4-fold increase in hydraulic conductivity at 100 mm below the seeding depth, and a 43% increase in plant available water holding capacity after 22 months following the removal of wheel traffic, principally due to the prevalence of larger transmission pores. Blackwell (et al. 2003) report a 13% increase in yield with 10% reduction in input costs with controlled traffic systems in Western Australia. Boydell & Boydell (2003) report a 53% increase in infiltration rate and a 6% increase in water holding capacity after 4 years of controlled traffic in northern NSW, together with reduced soil penetration resistance. Although these results cannot be translated directly to other soil types and regions, it is clear that soil compaction due to wheel traffic has a significant impact on soil physical quality.

The detrimental impacts of livestock on soil physical quality are well documented (Packer 1988, Greenwood & McKenzie 2001, Southorn 2002). When livestock are included as part of a crop management strategy (for example, in grazing of stubble or for pre-sowing weed management), soil physical quality is likely to be compromised in the short term due to compaction pressure exerted by hooves, particularly when the soil water content is high. For example, in a study comparing tillage treatment to permanent pasture on sandy loam surface texture soil in southern NSW, Packer (et al. 1984) found significantly lower soil bulk density (to 100 mm) with direct drilled soil compared to permanent pasture (and compared to other tillage treatments). If pastures are included as part of a crop rotation, some soil benefit may accrue, but it will depend on the botanical composition of the pasture, its ability to construct biopores, and the grazing tactics used during the term of the pasture phase.

Retention of crop residues can contribute to soil physical quality directly (in the case of residue providing protection from raindrop impact) and indirectly (by providing additional soil organic matter, and helping control soil evaporation) (Charman & Roper 2000). Traditionally, stubble has been burnt to remove it from the field, to enable conventional tillage and seeding machinery to work unimpeded, and to assist with control of soil borne diseases, but these practices generally result in a reduction of soil organic carbon (and nitrogen) compared to direct drilling and stubble retention (Heenan et al. 1995). Now, machines are available that can work in large quantities of stubble, with crop rotation playing an important part in disease management. A critical factor must be the timing of stubble burning as this will determine the length of time and amount of rainfall energy to which the soil surface is exposed. A late burn just prior to sowing will clearly reduce the amount of rainfall energy to which the soil surface is exposed. On the other hand an early burn soon after harvest will expose the soil surface to the maximum amount of rainfall energy. It is therefore not surprising that studies where a late burn was used did not clearly demonstrate significant differences in soil properties between burn and no-burn direct drill treatments. (Heenan et al. n.d.). Retaining stubble can also result in improved economic performance of a cropping system, compared to stubble burning (Brennan et al. 2001).

Crop rotation may also have a direct influence over soil physical quality. Packer (et al. 1994) report significantly higher infiltration rate and aggregate stability, and significantly lower runoff and soil loss, from bare soil with canola (Brassica napus L.) compared to bare soil with wheat, assumed to be as a result of greater ground cover and surface root density from canola during the growing season. No difference was observed when both treatments carried stubble, further highlighting the potential of crop residue in protecting the soil surface. Blair (2003) suggests that crop type, specifically the proportions of residue straw and roots and their rate of breakdown, are likely to influence the supply of labile soil carbon.

The beneficial contribution of soil biota needs to be considered, given their role in soil aggregation, aggregate stability, and construction of macropores (for example, Lee & Foster 1991, Pankhurst et al. 1994). The diversity, abundance and activity of soil biota will be enhanced by cropping systems that cycle a larger quantity of substrate for soil biota, sustain their physical habitat, avoid biotic toxins, and provide adequate soil water for long periods of time. These criteria are best met by controlled traffic, zero till, stubble retention, cropping systems with flexible rotations and livestock excluded.

Some farmers in central west NSW practice such systems. Increased crop water use efficiency, reduced tractor power requirement, reduced fuel and labour costs, and increased (but often more variable) crop gross margins are frequently reported.

Discussion

There appears to be sufficient evidence to indicate that soil physical quality will be optimised if soil disturbance is minimised, plant biomass maximised, and compaction stresses removed. At the same time, increased soil water use resulting from increased water holding capacity and increased rooting depth can help alleviate the risk of dryland salinity. More importantly in this region, where rainfall represents a common limitation to crop yield, increased water use efficiency can be achieved.

In commercial agricultural systems, conservation farming must be translated into business profit, whilst managing risk. In a summary of wheat, chickpea and sorghum gross margins over three years in northern NSW, Rummery & Coleman (2000) show that although the cost per hectare is greater with `conservation farming', in this case no tillage plus controlled traffic, yields have always been sufficiently greater to generate better gross margins in all years for all crops investigated. In addition to direct benefits associated with crop establishment method, they list a number of secondary effects: conservation farmers generally have better crop rotations and management of inputs; conservation farming creates better alignment with variable rainfall patterns due to improved timeliness and higher water use efficiency; reduced capital associated with machines for conservation farming. However, they also cite potential difficulties with herbicide resistance, and maintaining control of chemical costs, as management risks.

Herbicide resistance has emerged as a major challenge to conservation farming practice in Australia and other locations where conservation farming methods have been used for some time. In a survey of 384 grain farmers across 4 States in Australia, D'Emden & Llewellyn (2004) report that more than half the farmers surveyed in NSW believe herbicide resistance is present on their farms, with a higher percentage in some other States. In the same survey, herbicide resistance is cited as the major reason why 18% of `no-till' farmers surveyed have discontinued the use of this practice, and that more are likely to use tillage again in the future. Despite this, most farmers are preparing to adopt conservation tillage practices in the future, an indication they believe the potential benefits exceed herbicide resistance risk. Whilst D'Emden & Llewellyn (2004) conclude that more research is necessary to manage resistance in conservation tillage systems, existing risk management strategies include more appropriate use of chemical application rate, better timing of herbicide application, appropriate rotation of chemicals, precision weed targeting, crop rotation tactics, management of weed seed during harvest, and non-chemical methods for weed control.

Stratification of soil quality factors is also emerging as a potential issue; i.e. in the absence of soil inversion due to tillage, improvements in soil quality can be confined to the topmost soil layer, with soil quality limitations remaining at depth. In addition to stratification of indicators such as soil organic carbon content, incorporation of soil ameliorants such as lime becomes problematic. For example, Heenan (2002) reports strongly stratified soil pH two years after lime application with direct drilling compared to tillage, but that after four years, there was little difference. Selective tillage has been advocated as a means to overcome stratification (Bell et al. 2003). Selective tillage may also be necessary to ameliorate pre-existing induced compaction layers prior to the introduction of conservation tillage practices. Some farmers may not feel comfortable with immediate adoption of zero tillage, preferring instead to `experiment' with conservation tillage as part of a learning phase.

Transmission of soil borne root diseases has been nominated as another area of concern with zero tillage systems, and a reason cited for continuation of tillage and stubble burning. In addition to the use of crop rotations as a means of control, there is evidence that increasing soil biomass and activity, and increasing the proportion of fungi compared to bacteria in the biomass, outcomes associated with high input zero tillage systems, can help suppress certain root disease organisms (Pankhurst et al. 1997, Drew et al. 2004).

Conclusion

Much has been learned over the last two decades about the importance of reducing tillage during dryland crop establishment. Significant reductions in soil erosion have occurred during this time. Reduced tillage and stubble retention will reduce erosion risk, increase soil carbon content, and assist with macropore development and retention, key factors in soil physical quality. The potential of these mechanisms will be maximised with zero tillage, with purpose-built seeding tools now commercially available. Maximising plant biomass is a parallel objective, requiring a high level of management of other inputs and the retention of all crop residue. Removing the causes of soil compaction (machinery wheels and livestock) is necessary to ensure soil quality improvement. Whilst these principles can be stated clearly, their implementation is not so easy, because the operational interdependence (Roberts & Packer 2000) of these factors on any farm is complex, and the mix of factors on each farm is unique. Engineering challenges remain, including optimum wheeltrack placement, wheeltrack standardisation, and controlled traffic at harvest.

A shift from traditional tillage to reduced tillage has occurred to a large degree, with a further shift to zero tillage much more gradual. Leading farmers have demonstrated that such a shift is possible within the normal constraints of farming in central west NSW. It is suggested that this has been accompanied by a shift in attitude toward soil management and a recognition of the role of soil quality in sustainable production. In addition, new management systems require continuous evaluation of crop rotation options, input strategies and crop budgets, a preparedness to invest in information and new methods, and a realignment of cropping strategies to match the variability of rainfall patterns. A different approach to risk management emerges.

Improvement in soil physical quality alone does not guarantee increased yields, despite obvious potential for increased soil water use, and this may act as a disincentive to some farmers. Farmers may need to be patient whilst benefits accumulate, a difficult request of any businessperson. It is necessary to manage risks associated with disease management and dependence on chemical weed control. Whilst farmers tend to focus on yield, one of the primary determinants of farm profit, the environmental benefits of improved soil quality should also be considered, including carbon sequestration. Farmers should evaluate all determinants of farm profit, which include reduced operating costs and improved timeliness of farming operations, both of which are dependant on crop establishment methods.

Although rates of adoption of conservation tillage and zero tillage are likely to increase, the impacts of the remaining non-adopters on catchment health need consideration. Continued and additional incentives and programs need to be implemented as part of the on-going reform process of Catchment Management in NSW, together with effective research and extension action.

Acknolwedgements

Our sincere thanks to Ian Packer for providing this article and the pictures. This paper was written by:
Neil J. Southorn, Faculty of Rural Management, The University of Sydney, Orange, NSW, Australia. neil.southorn@orange.usyd.edu.au
Ian J. Packer, NSW Department of Infrastructure, Planning & Natural Resources, Cowra, NSW, Australia.
Brian W. Murphy, NSW Department of Infrastructure, Planning & Natural Resources, Cowra, NSW, Australia.
John W. Lawrie, NSW Department of Infrastructure, Planning & Natural Resources, Wellington, NSW, Australia.

References used in the glossaries:

Hunt, N. and Gilkes, B. (1992) Farm Monitoring Handbook. The University of Western Australia.

Whitbread A.M., Lefroy R.D.B., and Blair G.J., 1996. Changes In Soil Physical Properties and Soil Organic Carbon Fractions With Cropping On a Red Brown Earth Soil. In Proceedings of the 8th Australian Agronomy Conference, Toowoomba.

Daybreak disc seeding units attached to a farmer constructed air seeding system. Wass Brothers, Nyngan.

Field peas sown into stubble using GPS controlled traffic. Wass Brothers, Nyngan.

Soil Physical Quality - relates to soil structure, aggregate stability, porosity, infiltration

Bulk density - mass or weight of a soil per unit volume. The greater the pore space for a given weight of soil (ie the greater the volume), then the less will be the bulk density. When a soil is compacted, the volume is reduced and the bulk density increases. It is generally desirable to have a soil with a low bulk density (Hunt and Gilkes, 1992).

Hydraulic conductivity - ability of the soil to drain water and it depends on soil properties, especially texture and structure.

Soil Biomass - the mass of living organisms in th soil (bacteria, fungi, soil animals etc)

Erosion rates have declined significantly as a result of the adoption of conservation tillage practices....

......It seems that the adoption of "direct drilling" alone is not sufficient to ensure an improvement in soil physical properties......

Labile carbon fraction

-soil carbon can be divided into active (labile) and resistant fractions. The active carbon consists of water soluble carbon such as simple sugars, organic acids and proteins. This carbon is held in the mass of living organisms in the soil and can be readily metabolised during the initial stages of decomposition.

- may be associated with soil nutrient dynamics and have a role in the stabilisation of soil structure (Whitbread et al, 1996)

....In all crops in all years, the performance of conservation farming systems exceeds that of conventional crop establishment......

......Even with direct drilling, removal of stubble by burning has been shown to reduce earthworm numbers, soil respiration rate and soil fungal mass......

Small air seeder unit with parallelogram sowing units sowing into wheat stubble. R. Chewings, Central West CMA.

In commercial agricultural systems, conservation farming must be translated into business profit, whilst managing risk.

......herbicide resistance is cited as the major reason why 18% of `no-till' farmers surveyed have discontinued the use of this practice, and that more are likely to use tillage again in the future......

Soil Health