1. Introduction
Soil structure degradation,
often termed soil compaction, is one of a recognised suite of land degradation
types, commonly considered ubiquitous to the world’s cropping and grazing
lands. Other types of land degradation include salinity, erosion, invasion of
weeds and fertility decline. Structure degradation has been ranked as the
greatest problem in terms of damage to Australia’s soil resource (Williams,
1998). Soil erosion and salinity produce more immediately visible effects but
human-made structure degradation has the greatest impact on agricultural
profitability. As a cost example, Fray (1991) states that soil structure
degradation alone has caused $144 million worth of damage in the Murray –
Darling basin, Australia. High costs have also been associated with programs
aimed at repairing structure degradation, employing deep cultivation.
At a most general level, it
is a “change for the worse” of a soil’s inherent structure that describes soil
structure degradation. Soil structure refers to the size, shape and degree of
development of soil units that are composed of primary soil particles (sand,
silt and clay), and the arrangement of these units with the spaces (pores)
within and between them. Good soil structure, typified by many interconnected
spaces, is important for the movement of water and gases in the soil system,
the proliferation of plant roots, and is the prime regulator of water and
nutrient supply to plants. It is the loss of this pore space, especially the
interconnected pores, through compression and shear that best defines soil
structure degradation.
Identification, diagnosis
and rationalisation of structure degradation are firmly based on the
description and measurement of soil structure. This causes the recognition and
measurement of structure degradation to more difficult than recognising and
measuring other forms of land degradation (Bie, 1990; Biot et al., 1995). Not
only can the degradation be located, characterised, reported and statements
made on degree but the same set of descriptive and/or quantitative assessments
can be employed to monitor and define subsequent improvements in soil structure
state with altered, better management systems.
In this report the sole
emphasis will be the qualitative appraisal of structure through soil profile
description. The field description of soil structure uses five defined
features: type, size, grade, fabric and orientation of structure units,
together with size and number per unit area of associated inter- and intra-
unit pores (McGarry, 1996). These are common to most systems of soil profile
description, globally. They have also become the basis for teaching soil
structure degradation recognition and rationalisation in farmer and agronomist
participatory workshops, like SOILpak (McKenzie, 1998). Observations of soil
structure are made in soil pits, either small spade-dug holes or large
backhoe-dug pits. McKenzie (1998) provides methods for pit digging. The use of
soil profile descriptions answers the common demand for “robust and cheap”
methods of assessing land degradation (Biot et al., 1995) to facilitate
initial, broadscale, rapid but repeatable assessment. Abundant, quantitative
measures for both field and laboratory assessment of structure degradation
exist in the scientific literature (eg Coughlan et al., 1991; McGarry, 1993)
but tend to be single trial specific and more costly and time consuming than
profile description, requiring specific skills for correct use and
interpretation. Studies that link change in soil profile attributes with
measured soil physical degradation and yield reduction (eg McGarry, 1987) are
vital to legitimise the interpretation of soil profile features as physical
degradation that has yield implications.
Whether qualitative or
quantitative information is being recorded, both paired sites and on-site
farmer discussions are vital tools in developing regional and National
perspective on the nature, location, degree and effect of soil structure
degradation. Paired site comparisons are opportunistic and may include
cultivated versus non-cultivated (a treeline and an immediately adjoining
field), wheel track versus non-wheel track, different types of cultivation on
one soil, before and after a repair strategy, etc. Farmer input is vital. Only
they can truly describe previous management that has resulted in the current,
recordable soil structure state. Matching visible signs of structure
degradation with the farmer’s tractors and tillage equipment is a most telling
and constructive form of rationalising the visible, degraded state.
This report emphasises
human-made structure degradation. This is caused by farm machinery and farm
animals and has the greatest, potential (if not actual) negative effect on
productivity in soils with no or minimal inherent structure degradation, ie a
country's prime cropping and grazing lands. Some soils do naturally
self-compact with no additional human input, ie hardsetting and crusting soils.
Because of the inherent, negative physical properties of these soils, the
agricultural/grazing potential, the intensity of their use, and the
expectations of their cropping potential is relatively low (McGarry, 1993;
1998). Far more is expected from a nation’s principal cropping soils, so the
potential for degradation from intense usage is high. The inherently best soils
are being degraded.
The aim of this report is to
present a map of the location and current status of soil structure degradation
in Queensland cropping soils. Toward this, a review of previous work that has
mapped soil structure degradation will be presented. Then, four topics on
structure degradation will be covered: extent, location and nature;
contributing factors; impact; and repair and control programs. It is the
integration of these four factors that leads to the rationalisation of the
location and current status of structure degradation in Queensland cropping
soils. Globally, investigation and rationalisation of the same four factors
will provide strong, basic information on the status of this form of land
degradation whether in a selected cropping industry or region.
The “snap–shot” approach of
this report needs to be emphasised. The determinants of the degradation and
hence the content of the map classes are active. Change is anticipated and
certainly encouraged as knowledge grows. Structure degradation is human-induced
so strategies of repair, prevention and control can be integrated with farm
practices to address the problem. As will be discussed, this has already
eventuated in some cropping industries. Hopefully this report will be a
catalyst for change in those cropping industries ranked poorly in terms of
prevention and control of structure degradation. Experience from industries
with better prevention and control practices can be transferred via training and support programs.
Several of these have already occurred (McGarry, 1992a; 1992b; 1999a) based on
the SOILpak manual (McKenzie, 1998). It is beyond the scope of this report to
suggest strategies for wider adoption and support of structure degradation
awareness and prevention programs. McGarry (1999b) details the example of
Australian cotton. To date, the general trend has been for trial-based science
to be extended to farmer groups through farm meetings and participatory
workshops. Farmers make the science practicable and economic - adapting
equipment and altering management systems. In time, farm equipment
manufacturers respond to meet new design requirements to match the new system.
A mix of R&D Corporation and Federal/State project grants has funded all
levels of this work. It is to be emphasised that the prevention and control of
structure degradation is a long term goal to be achieved over decades, rather
than single cropping seasons.
2. Previous work and the background of this current report
Queensland - wide and
national statements on soil structure degradation have tended to be brief,
general in content and sometimes accompanied by a highly generalised map of
location.
The Department of
Environment and Heritage, Queensland (DEH, 1990) presented three paragraphs on
soil structure decline in a report on the state of the Queensland environment.
They commented that reliable information on its occurrence in Queensland is not
available, but that work in cotton areas shows it to be a more serious and
extensive problem than previously supposed. The Australian Bureau of Statistics
(Castles, 1992) presented a brief report on soil structure decline, with
emphasis on the effects of cattle grazing. They made the important observation
that the effects of soil structure degradation can be addressed and even
reversed. They presented an Australia-wide map delineating areas where there
had been significant yield loss under certain seasonal conditions.
The most detailed statement
on structure degradation in Queensland cropping lands was by the Department of
Primary Industries in their Decade of Land Care Plan (DPI, 1992). This
presented the current situation under the headings of: definition,
location/extent, contributing factors, impacts and control measures. No map was
presented.
The current report aims to
provide the present status of soil structure degradation in Queensland cropping
soils. The source material comes from a decade of projects, funded by Federal
National Landcare and National Heritage Trust sources, and Cotton, Grains and
Sugar R&D Corporations, covering the years 1989 – 1999. These commenced
with farm consultations in three major agricultural areas of Queensland to
ascertain farmers’ perceptions of the problem of structure degradation (Mortiss
and McGarry, 1993). Specifically, a three-step model was employed. Farmers
first participated in a structured meeting to collect their perception of the
symptoms and causes of structure degradation. These were followed by visits to
individual farms to gauge the reality of the problem, before the workshop
participants were led on a group tour of each district to inspect locally
pertinent areas of structure degradation. A major gap identified was the lack
of locally trained staff, able to lead and support farmers in degradation identification,
repair and control. For eight years, local training workshops for farmers and
their agronomists have been conducted in many of the cropping lands of New
South Wales and Queensland ((McGarry, 1991; 1992a; 1992b; 1999a; McKenzie and
McGarry, 1999). Recently, emphasis has shifted to predicting the risk of soil
structure degradation based on the inter-relation of farmer-available soil
chemistry attributes, climate (to model soil wetness) and farm system analysis
(to pinpoint traffic and cultivation pressure times) (McGarry and Sharp, 1997).
Throughout, there have been many field and laboratory experiments, specifically
measuring soil structure degradation and amelioration in field and glasshouse
studies and when possible linking measurements with visible soil attributes and
images of soil structure from image analysis (eg, Conway et al., 1996; McGarry,
1987; McGarry et al., 1996;1997;1998;1999; Pillai and McGarry, 1999). Over the
course of this work, thousands of field sites have been visited, and inspection
holes in the form of spade and backhoe pits excavated to inspect soil structure
degradation. All visits included on-site discussions with farmers to address
cause, effect and repair strategies. The studies have covered a wide variety of
soils in each of the cotton, grain, horticulture, small crops and cane lands of
Queensland and New South Wales. Many ideas and issues from the projects were
reported in McGarry (1993; 1995; 1998) which provide background reading for
this current report.
3. Extent, location and nature
The extent and location of
structure degradation are of interest to both the farming community who are
trying to prevent and control the phenomenon at the farm level, and to land use
planners and managers who are tackling regional and national issues. Extent,
therefore, can be considered from the farm to regional levels, covering
individual fields, to catchments, to Government regions. Personnel at each
level require different information, reflecting different interests and
priorities. Farmers, and the cropping industries that represent them, require
practicable solutions to the problem. They pose such questions as: is structure
degradation restricted to specific areas of a field or farm, or is it related
to a specific crop or type of cultivation, or does it occur in one field but
not another? Catchment or regional issues include questions on the effect of
structure degradation on the short-term viabilities of specific cropping
industries in unseasonably wet or dry years and whether structure degradation
exacerbates other environmental problems such as soil erosion.
Difficulties surround the
precise definition of the extent and location of structure degradation. The
reason is that structure degradation principally occurs in the upper subsoil
and is hidden from view. As a result, structure degradation is blamed for many
soil and crop problems that have no immediately obvious cause. Conversely, many
crop failures due to structure degradation are wrongly blamed on other factors,
eg. root disease and soil pathogens. With structure degradation, most crops
fail as their roots are unable to penetrate a physical barrier. There may well
be root disease, but it is exacerbated because of waterlogging and poor root
performance from structure degradation.
In terms of data collection
toward mapping the extent, location and nature of structure degradation there
is currently no better method than visual recognition of the phenomenon in a
spade or backhoe pit. Such observations are the basis of SOILpak which is a
soil management decision support system (Daniells et al., 1996; McKenzie,
1998). The system was originated for irrigated cotton but there are now other
manuals either completed (Daniells et al., 1994) or in production for other
cropping areas.
There are two important
issues in the consideration of locating soil structure degradation.
-
Crop
growth and yield are poor indicators for assessing the extent and location of
structure degradation. Many examples have been observed where severe
degradation did not affect crop growth and yield. One explanation is that crop
growth is highly related to the amount and periodicity of during-season rain or
irrigation. Generally, small but regular applications of water ensure good crop
growth, irrespective of the presence of soil structure degradation. Conversely,
wheat crops planted into full moisture profiles have wilted and died when there
was no follow-up rain. Upper subsoil structure degradation prevented the roots
accessing subsoil moisture. Also, farmers have reported that additional
fertiliser applications help mask the effect on yield of structure degradation.
-
The
extent and location of structure degradation needs to be addressed both in the
horizontal (across the field) and vertical (soil profile) planes. The degree
(magnitude) of the degradation in each plane must also be assessed.
4. Contributing factors
Soil water content at the
time of traversing or cultivating a soil is the principal determinant of the
severity and extent of soil structure degradation. Tractor loads, implement
design, speed and tyre size, type and inflation are all important but soil
water content is the prime determinant (Kirby and Blunden, 1992). Soil water
content at key times is particularly important, for example during primary
cultivation and at harvest, and the water content at those times depends on
climate and current weather patterns, as well as irrigation scheduling.
Soil type is important in
determining and rationalising the severity of structure degradation. Of
greatest relevance is that different soils hold water for varying lengths of
time. Some soils remain more plastic, hence more degradable, than others at
similar times after similar amounts of rain or irrigation. Clay soils tend to
stay wetter for longer as their fine particles hold more water, more tightly
than a sand or loam. Critical to the inter-relation of soil type and the
potential for structure degradation is a soil's Plastic Limit water content (PL). PL is the water
content of a soil above which it will compress and shear when loaded, ie the
soil is in a “plastic” state and is prone to structure degradation. Soil
cultivated drier than PL will fracture rather than smear so structure
degradation will not occur.
The level of management
awareness is potentially a major contributor to the occurrence of structure
degradation. Imperfect understanding is the key and occurs at many different
levels. Especially up to the late 1970’s primary producers lacked an
understanding and awareness of the physical frailty of soil structure. There
had been a European paradigm for cultivation - repeated, deep, cultivation of
soil close to field capacity. Early farmers, unknowingly, had assumed
Australian soils were as physically robust as European soils. Adding to the
problem, Australian farms are large. This necessitates large machinery, gives
inflexibility in timing of cultivation, sowing and harvesting, generally on a
wide range of soils with different levels of robustness. The problem is not
that farmers knowingly have over-used the soil, rather they were unaware of the
high level of care needed to maintain the resource.
Though best for the soil,
there are several considerations that preclude cultivating and trafficking at
optimal (ie sub – PL) soil water contents. Planting operations are commonly
done when the soil is wetter than PL, particularly in clay soils. Farmers wait
for rain or pre-irrigate to attain soil water contents for planting. Generally,
seed requires water contents much greater than PL to ensure germination and early
growth. Additionally, many clay soils have a wilting point water content (ie
the water content below which plants cannot extract more soil water) that is
greater than PL. As a result, even following a dry season and dry harvest, soil
water contents may still be greater than PL. Much pre-season cultivation,
therefore, is done in soil that can compress and shear. The soil may be “dry”
to farmer observation but retains sufficient water to be degradable. For a full
discussion of the effect and the measurement of a soil's PL, see McKenzie
(1998). Video footage of the method of determining PL in the field is given in
Bray et al. (1997).
Cropping industries vary
markedly in terms of the incidence and degree of soil structure degradation. At
the better end, systems such as minimum till wheat with controlled traffic or
irrigated cotton in retained beds have the best potential to ensure that
degradation is minimised in the crop/root growing areas. At the other end of
the spectrum, crops growing in environments where there is periodic, intense
cultivation and/or potential for wet harvesting - have high potential for
producing structure degradation. Such scenarios will be addressed in section 7
below.
5. The impact of structure degradation
The
impact of soil structure degradation is felt at several different levels. This
section will first present the broader cost of structure degradation – beyond
yield loss. Then examples of structure degradation will be presented to show a
variety of common effects, types and causes.
Primary
producers tend to solely consider the impact of structure degradation on crop
productivity. If the structure degradation does not effect yield, they consider
that there is no problem. For at least two reasons, this is an unrealistic
appreciation of the problem.
First, soil structure
degradation only increases the potential for yield reduction. The word
potential is stressed as crops can grow well in structurally degraded soil, if
there is frequent irrigation or rainfall. The crop grows almost hydroponically.
However, when irrigation water or rain is scarce, crops in structurally
degraded soil will fail long before crops in well-structured soil. McGarry
(1990a) presents an example of adjoining cotton crops, one with structure
degradation from wet harvesting. Irrigation water was scarce, so was withdrawn
from the degraded field, resulting in a 70% yield reduction in the degraded
field relative to the non degraded and irrigated field. The structure
degradation itself did not cause the yield loss, rather the presence of the
degradation gave poor, early crop growth and the farmer withdrew irrigation
water from that crop. McGarry (1993) presents another example where the
presence of strong soil structure degradation (from wet harvesting) did not lead
to yield loss in the following cotton crop, solely because the farmer tripled
the number of during-season irrigations. If irrigation water had been scarce,
the crop would have yielded most poorly.
Second, there are
potentially high costs involved in both forming soil structure degradation and
then initiating repair and control strategies. Cultivation and traffic cause
structure degradation. Yet, on many occasions the aim of the cultivation was to
alleviate soil structure degradation. However, if the soil was too moist or wet
at the time of the cultivation, structure degradation ensues. So, the farmer is
paying threefold – the cost of the cultivation and traffic, the cost of
negative responses (yield loss, increased irrigations, poor seedbeds, etc) that
then require more cultivation (with traffic) to repair – again running the risk
of producing more structure degradation. This is a typical “downward spiral”
associated with structure degradation.
Six examples of soil
structure degradation follow. All are from Queensland and have been recorded
and photographed by the authors to represent structure degradation of cropping
soils. They are, however, descriptive of structure degradation in many national
and global situations. Each will be
described, together with the causal factors. The potential for repair and
future prevention of structure degradation at these sites will be discussed in
Section 6, below. One vital point is that for all the examples presented, the
farmer at the site had no prior knowledge that structure degradation underlay
the field and was a major contributor to the agronomic problems that they were
experiencing.
Structure degradation
commonly appears as platy structure that forms horizontal pans in the upper
subsoil. This loam soil just north of Dalby, cropped to rain-fed grains, shows
3 or 4 distinct degraded pans to 0.25 m, each associated with a different
cultivation (Figure 1). Three cultivations (scarifyings) preceded sowing the
field for barley. The crop did not reach a harvestable state as it was
incapable of penetrating the pans and accessing subsoil water below the
compacted layers. Additionally, there is a deeper platy layer at 0.35-0.4 m,
related to a deep working some years previously.
Fig. 1
Soil structure degradation in a loam soil growing rain-fed grains, near Dalby.
Degradation is evident as several layers of strong platy structure. Five or six
separate layers are evident, attributable to different types and times of
cultivation.
A second example of
structure degradation caused by cultivation is a most severe compacted layer
located beneath rain-fed wheat, north of Goondiwindi (Figure 2). The layer
occurred over 100% of the field and that season’s wheat crop died in the
ground, again because it could not access subsoil moisture. No roots penetrated
the compacted layer even though the soil beneath was quite moist. Repeated
workings in wet soil with blade ploughs and chisels with wide sweeps formed the
layer. In the photograph the 0.1 m of loose topsoil has been removed to expose
the compacted layer.
Animal hooves on moist/wet
soil can also cause soil structure degradation. Severe degradation of the soil
surface is evident in this sown pasture on a shallow black earth, west of Moura
in Central Queensland (Figure 3). The grazier reported poor rain infiltration
and most poor pasture growth at this site despite the deep (and expensive)
chiselling operation that preceded sowing of the pasture. The cattle had
degraded the soil surface through trampling moist/wet soil that had only
recently been made soft (hence prone to degradation) by the chiselling
operation. The degradation led to poor infiltration and water ponding. In turn,
this caused the soil surface to remain wet for long periods of time, so
worsening the surface degradation through puddling under cattle hooves.
Fig. 2
A most severe, structurally degraded layer beneath rain-fed wheat, near
Goondiwindi. The layer covered 100% of the field and was attributable to blade
ploughs and chisels with sweeps, used for primary cultivation and weed control.
Fig. 3
Severe compaction from cattle hooves of the immediate soil surface, to 0.2 m,
of a shallow black earth soil, sown to pasture, Moura, Central Queensland.
Controlled traffic, where
hills and furrows (inter-rows) are retained, is widely used in the cotton
industry to reduce the negative impacts of structure degradation in the crop
zone and improve traction in the traffic zones (McGarry, 1995). The system
emerged after early research demonstrated that harvesting and pre-season
workings in wet soil had strong, negative impact on cotton yield (McKenzie et
al., 1992). The structure degradation in the 0.2-0.35 m zone of this cotton bed
(Figure 4) gave a measured 50% reduction in cotton yield (McGarry, 1990b).
Almost all the roots in these beds had an acute “L” shape as they were unable
to penetrate the degradation. The degraded zone is an old wheel furrow. The
farm owner had removed the previous season’s hill and furrow system, as the
rows were mis-aligned. However, wet soil at the time prevented deep
cross-chiselling to completely remove the compacted furrows. Unknowingly, in preparation for the next
cotton crop the farmer located the new hills over the old, degraded furrows.
Fig. 4
Upper subsoil compaction in a grey clay under irrigated cotton. The compacted
zone, from 0.2-0.35 m under the cotton hill, is a buried wheel track giving
serious root growth restrictions and yield loss.
A single pass of a tractor,
on soil wetter than the PL, has the potential to cause structure degradation.
The strong degradation to 0.4 m in this brown clay soil, north of Goondiwindi
was formed at the time of sowing this zero till wheat crop (Figure 5). The
degraded zone pictured here precisely matched the tyre footprint of the sowing
tractor (that had dual tyres front and back), so the degraded areas recurred
cyclically across the field. The degradation reduced the wheat growth by 0.2 m
in the row located on the centre of the tyre.
Combinations of traffic and
tillage are the most common cause of soil structure degradation. Repeated
discing and trafficking in moist/wet soil caused the severe degradation from
the soil surface to 0.25 m in this red clay under sorghum, near Bauhinia in
Central Queensland (Figure 6). The structure degradation covered 100% of the
field. The farmer experienced difficulties with germination each season,
principally as there was such a shallow seedbed of loose soil.
Fig. 5
Structure degradation from one pass of a tractor tyre, sowing zero till wheat
on a brown clay soil, north of Goondiwindi. The degradation with associated
plant height reduction recurred cyclically across the field, uniquely matching each
of the dual wheels of the tractor.
Fig. 6
Structure degradation of the soil surface to 0.25 m from repeated traffic and
tillage of a silty topsoil for sorghum production, Bauhinia, Central
Queensland.
6. Repair and control
programs to address structure degradation
Once structure degradation
is located, then repair and control measures can commence. It is imperative
that soil management/crop problems are correctly linked to the recorded
presence of structure degradation before repair and control practices are
begun. The high failure rate of many repair practices can be directly related
to the mis-interpretation of crop failure. Quite simply, structure degradation
was not the problem. Location, with spade-dug holes or soil pits, needs to be
at different scales; parts of fields, across fields, across farms, etc.
Location in the soil profile is also important, to correctly choose the best
repair strategy. If the problem lies in the top 0.1 m, there is no need to cultivate
to 0.4 m. Deep cultivation is expensive and has strong potential to produce
adverse effects by inducing deep soil smearing and compaction, and bringing
subsoil with poor chemical properties to the soil surface.
Repair can either be
biological or mechanical, or a combination of both. Biological methods are
preferable as they not only remove the possibility of further damaging the soil
by mechanically-removing the structure degradation but also are more
sustainable and have minimum costs. Current biological options include rotation
crops, pasture phases, earthworms and green manures. These activate natural
soil processes of swelling and shrinking, the production of natural soil pores,
and organic matter improvement.
Different soils as well as
different degrees of structure degradation vary in their response to repair
practices. Generally, cracking clays respond well to repeated wet/dry cycles
under rotation crops (Pillai and McGarry, 1999), whereas non-swelling soils
react better to increases
in earthworm activity and
root hole formation, and additions of organic matter. Mechanical options of
ripping and cultivating must only be done after digging pits to ensure the soil
(to a depth below the intended cultivation zone) is drier than PL. This will ensure
brittle failure of the soil rather than plastic flow (which would give further
structure degradation).
Before initiating prevention
practices careful consideration should be given to repairing any inherent soil
structure degradation. The initial removal of degradation is particularly
important if the degradation is severe or the soil has little, inherent
self-repair ability (it is not a cracking soil) or if zero till will be
practised in the new system. Under zero till, even a strongly cracking soil will
take several seasons to repair degradation through biological means. Initial
improvement of structure degradation “kick-starts” the new prevention system in
which all-future traffic is controlled and the need for future deep cultivation
is removed. Subsoiling, deep ripping or square ploughing are potential devices
for the initial degradation repair but must only be used in soil below PL,
where a problem has been identified (spade holes or backhoe pit) and its
location in the profile noted.
Currently, prevention
measures include controlled traffic, minimum tillage and flotation tyres. Each
will be addressed in turn.
The essence of controlled
traffic is to remove the randomness of trafficking and cultivating fields. With
controlled traffic in its purest form, all wheels always use the same path
(Murray, 1994). The current best example is the irrigated cotton industry where
the retained bed system ensures all wheels are kept out of the plant growth
zones. In that system every effort is made to concentrate the traffic zones of
tractors and harvesters, so structure degradation is only in known, narrow
strips. In this system, cotton beds have been retained for up to six or seven
years, during which time the combination of rotation crops, minimum till (only in
dry soil) and controlled traffic ensure excellent bed conditions (McGarry,
1995). A most profitable offshoot of the retained bed system is that large,
ripping tractors and heavy, tillage equipment are redundant, giving cost
savings in servicing, fuel, garaging and operator wages. In the grains
industry, controlled traffic is practised but only the tractor and harvester
wheels are consistently kept in traffic zones, separate from the plant lines.
Press wheels, coulters and depth wheels (on planters) all potentially traffic
the plant line, so have potential to compact the soil. Considering that all
planting is done in moist to wet soil, the potential for degradation from these
sources is strong. In broadacre cropping, a marker system is needed to
permanently mark wheel paths. Removing or blocking the sowing hose immediately
behind the tractor/seeder tyre may be a simple method of marking tyre location
for subsequent passes/seasons. Cultivating tines are also removed from behind
tyres to maximise compaction in the traffic zone. Global positioning system
(GPS) technology is another important tool to guide tractors and harvesters to
eliminate gaps/overlays in “guess–rows” and keep equipment on pre-selected
lines. Integrating controlled traffic systems with contour banks has been
achieved in Central Queensland, where traffic runs up and over the banks (D.
Yule, pers. comm.)
Minimum till is already
widely practised in cotton and grains. Practices vary markedly between
industries and regions but in essence minimum tillage aims to reduce
cultivation, retain plant stubble and commonly sow back into (close to)
previous plant lines. Many grain growers aim for zero till where sowing is the
only implement pass, and weeds are controlled chemically. Minimum tillage in
cotton tends to be shallow cultivation, only of beds for pre-season bed
preparation and early season weed control. With minimum tillage, it is vital
that all tillage is
conducted in soil drier than
PL or structure degradation will occur. This is particularly true when
cultivating the beds in a controlled traffic situation as the beds are most
soft and friable, hence susceptible to compactive forces. As an example of this
latter point, in cotton two inter-crop tillage practices have strong potential
to degrade soil structure of beds: the middle-busting of cotton hills to
0.25-0.3 m and the cultivation of hills to 0.1 m for Heliothis pupae control.
Not only does each have strong potential to degrade soil structure if done in
moist/wet soil (McGarry, 1989; 1995) but the effectiveness of each cultivation
practice is questionable as soil disturbance will be less than if done in dry
soil.
The place of flotation tyres
in minimising and preventing soil structure degradation in Queensland cropping
lands has not been fully investigated. However, pertinent are the results of
Kirby and Blunden (1992) who compared the ground pressures of a range of
tractors and tyre combinations, though not flotation tyres. They reported that
whereas soil strength varied one hundred-fold with change in soil water content
(from field capacity to PL), vehicle ground pressure varied by only five to
seven fold. It is much more important, therefore, to get the soil dry than it
is to choose another vehicle or wider tyres. Lower ground pressures, for example
in flotation tyres, merely increase the moisture content range that may be
trafficked without damage. This could permit a farmer or a harvesting
contractor to traffic a few days earlier with no increased damage, if time is
vital. Flotation tyres may also be advantageous on farms with contrasting soil
types in single fields. One part of the field is ready for harvest or sowing
before the other, as one soil dries more rapidly than the other. Flotation
tyres would permit access with less damage to the whole field, particularly to
the wetter soil area.
Referring to the examples of
soil structure degradation in section 5, above, statements can be made on the
potential for and the most effective type of repair and prevention practices
Repair strategies for the
soil in Figure 1 are most limited. Loams have minimal capacity to self-repair
as they do not shrink and swell with wetting and drying cycles. With this soil,
the farmer’s best option is to first shallow rip (with points) the soil to
approximately 0.2 m (after checking that the soil is below PL to 0.3 m at that
time), before enacting minimum tillage with controlled traffic. Subsequent
crops will begin to create air spaces in the soil from their roots, and the
stubble retained after harvest should begin to attract earthworms, also capable
of giving soil aeration (McGarry et al., 1999; Robertson et al., 1994). Roots
and soil fauna are the only mechanisms for giving continuous airways in a soil
such as this. Controlled traffic will ensure all future structure degradation
is in permanent, known areas.
Repair potential for the
soil in Figure 2 is better than for the previous example as the soil has
moderate swell/shrink potential. However, the degraded layer is so severe and
thick that a preliminary soil ripping (to approximately 0.2 m) is required to
give the soil a “kick start” by providing a thicker seedbed over what remains
of the compacted layer. Again, the soil must be below PL to 0.3 m when
cultivated. This could prove difficult, as crop roots cannot grow in the
current degraded layer. The thicker seedbed should provide a better root medium
to ensure better early crop growth and subsequently these larger, maturer
plants can penetrate and begin to ameliorate the degraded layer. Again,
controlled traffic and minimum till must be enacted after this ripping, or the
problem will simply return and the cost of ripping lost.
The soil in Figure 3 has
potential for moderate swell/shrink with wet/dry cycles under a crop. However,
the immediate problem is the lack of a seedbed as the degradation occurs from
the immediate soil surface. Shallow cultivation in dry soil is needed to
prepare a seedbed, then subsequent crops should initiate cracking of the
compacted subsoil. Critical, however, is that the soil must not be trampled
while moist/wet or the problem will return. Fencing is required to better
control stock movement. Thought should be given to the creation of a
“sacrifice” paddock where stock are kept and hand-fed in wet times, to save the
soil structure on the remainder of the farm. Dependent on stock numbers and
economics, this area could be concreted and used as a wet-weather, stock feed
area.
Repair potential of the soil
in Figure 4 is high as it has strong swell/shrink characteristics that can be
fully utilised with irrigated crops. One problem is that plant roots are unable
to penetrate the current degraded layer. However in time, particularly with
fibrous rooted crops, the layer will begin to self-repair. Mechanical
cultivation is a poor option on this soil as the high clay content causes the
soil to remain wet for long periods of time, hence wetter than PL.
The degraded wheel track in
Figure 5 may not require amelioration if it is treated as a form of controlled
traffic. If the farmer can always traffic the same zones (some marker system
will be required) then they have achieved a controlled traffic/zero till
system. The growth/yield reduction in the wheel row is inevitable but
preferable to random, across-field degradation.
In contrast to the previous
rainfed situation, the structure degradation in Figure 6 arises from both
tillage and traffic. Similar to the soil in Figure 3, the most immediate
problem with this soil is the lack of a seedbed. Priority should be given to
shallow cultivation (at water contents below PL) to achieve a seedbed. Better
early crop growth should follow and those crops should begin to ameliorate the
remaining degraded zone. The soil has low to moderate shrink-swell potential,
so controlled traffic is vital on this type of soil to prevent re-occurrence of
the degraded layer.
7. The current situation - towards a map of location and status
It
is not the intention of this section to map the type and location of observed
or measured soil structure degradation. As stated above, it is known that
structure degradation is ubiquitous to Australian cropping land. This section
has the more important and constructive aim of assessing current farming
practices of the major cropping industries in terms of their potential to lead
to structure degradation, and their current implementation of repair and
control practices. Such an assessment will identify industries where current
management is sub-optimal, hence highlighting the need for better management
practices to optimise soil structure for plant growth.
The definition and mapping
of the current status of structure degradation needs to be considered both in
terms of location and extent, as well as the current usage of repair and
control practices. This follows the logic that locating, describing and
explaining structure degradation is only a first step. Initiating repair and
control practices are vital to ensure that not only does the soil become
rejuvenated but will remain so.
Table 1 presents the major
cropping industries of Queensland against four determinants of the status of
soil structure degradation. It is the interaction and sum of these four factors
that cause soils to be both at risk of structure degradation and present the
potential/current practice of repair or control practices. The results of Table
1 are integrated to give a map of the current status of structure degradation
in Queensland cropping soils (Figure 7).
The current situation in Queensland is that all cropping soils have soil
structure degradation (Figure 7). Recent National Land Care funded research has
shown that no one farm visited or soil type examined was free of structure
degradation and that the nature of structure degradation was quite varied, with
the variability almost wholly being determined by current management techniques
(McGarry, 1991; 1992a; 1992b; 1999a).
At present, in terms of the
prevention and control of structure degradation the most sustainable systems
combine controlled traffic and minimum till with cultivation only in soil drier
than PL. Such systems account for 90% of irrigated cotton (McGarry, 1995) and
large areas of rain-fed grain land. For example, Blacket (1992) stated that 50%
of Waggamba Shire (near Goondiwindi, Queensland) practised zero or minimum
till. In such systems most of the structure degradation is restricted to
defined wheel tracks, leaving inter-wheel areas free of compaction (see Figure
5). These wheel tracks can be quite massive to 0.3 or 0.4 m, but the
degradation is not in the major growth zone. A less sustainable system is associated
with large areas of rain-fed grain and oil seeds where conventional cultivation
(commonly cross - cultivation with discs and tines) gives a shallow (often very
rough) seedbed over a relatively thin (0.1-0.2 m thick) degraded layer, that
overlays natural structure below 0.25-0.35 m.
Table 1: Determinants of the status of soil structure
degradation in Queensland cropping soils
for
each of the four principal cropping industries.
Potential for wet
Minimum till Controlled
traffic/ Rotation crops
sowing/working/harvesting practices retained beds for structure repair
Grains/oil seeds Wet
sowing, common; wet Widespread
in grains; Need lessened
by Not practical as
harvesting possible but two
passes per year minimum
till; restricted by low
impact lessened if header (sowing and harvesting) increasingly soil water
fits with controlled traffic. and weeds sprayed-out. common in grains availability
Harvest
potentially
harmful
due to
various
axle widths.
Irrigated cotton All impacts lessened
by Widespread; promotion 90% of industry. Widespread; also
widespread adoption of of natural structure green manuring
minimum till and retained production accepted. and nitrogenous
beds. break
crops.
Sugarcane Very high,
especially None;
routine removal None;
experimental None.
harvesting (no in-field of beds and "plough- work in infancy.
shelf-life after harvest, and out” to 0.4 m depth.
mill requirements).
Horticulture High, especially at None. Rare. Rare
(esp. vegetables) harvesting as
based on
market demand
and crop-readiness.
Currently, the least
sustainable systems in terms of preventing and controlling structure
degradation are of two types. The practice of using cattle or sheep to forage
weeds and stubble on moist/wet soil leads to severe, across-paddock degradation
of the immediate topsoil. This topsoil becomes the following season's seedbed,
which in a minimum till situation will receive no cultivation prior to sowing.
The second is in sugarcane, peanuts and horticultural land where intense
cultivation, harvesting and hauling-out (in sugarcane) has strong potential to
give structure degradation of the 0- 0.45 m soil layer, with associated poor
root penetration and rough seedbeds (Boyd-Law and Deuter, 1994; Bridge and
Bell, 1994; McGarry, 1999a). Exacerbating the problem in sugarcane and
horticulture is a scarcity of break crops that have soil structure regeneration
capacity. Paradoxically, it has been demonstrated that sugarcane has strong
capacity to create good soil structure in the rows ie around and beneath the
stool. This is associated with the ratoon nature of sugarcane, where rows
(plant lines) and inter-rows (traffic zones) remain in place for 6- 7 years
(commonly four ratoons), giving good soil structure in the rows and high
density and strong soil in the traffic zone (McGarry et al., 1996; 1997). This
attractive form of controlled traffic is lost after the last ratoon when,
currently, the whole field is cross-cultivated to remove the old stool. Trials
are currently investigating the potential of retaining the old row and
inter-row configuration through removing (mechanically or chemically) only the
old stool with no yield or soil pathogen penalties (McGarry et al., 1998).
Peanut production, almost exclusively on red and yellow clays (kraznosems and
euchrozems, ie Ferrosols) has led to surface sealing under rain and structure degradation
to 0.6 m (Bridge and Bell, 1994; Loch et al., 1987). Wet soil conditions at harvest are seen as a major contributor to
this structure degradation. Rehabilitation of krasnozems, degraded under peanut
production, has been recorded (Bell et al., 1997) In particular, pasture
(kikuyu) leys and introduced earthworms improved porosity and through drainage,
and reduced surface crusting under rain.
8. Conclusions
Intensive,
machinery-based cropping systems, like almost all cropping in Queensland,
cannot avoid soil structure degradation. It is possible, however, to control
the degradation by restricting it to known, designated parts of the field, ie
the wheel tracks. All efforts are then focussed on the zone between the wheel
tracks, the plant growth zone, to optimise soil structure for plant
growth. The aim is minimum tillage,
only in dry soil, and the retention of organic matter to promote natural
structure aggregation for optimum germination, root growth, water entry and
fertiliser utilisation. Beneficial soil fauna such as earthworms flourish in
these bed conditions – further enhancing soil porosity and natural aggregation.
There
are difficulties and challenges to the true enactment of the above management
system across all cropping systems. All wheels must strictly follow the
designated traffic areas, particularly as with time the beds between the wheel
tracks become soft and friable, and most susceptible to structure degradation.
Cultivation of beds should use only “light” equipment and be done in dry soil
or the beds will rapidly become degraded. Certain soils, especially the
hardsetting and crusting soils, will respond slowly to this new type of system.
The build-up of organic matter is vital to structure optimisation in such soils
and this can be a most slow process. Difficulties arise where several soil
types occur in one field, as one part of the field is sufficiently dry to
traffic without damage but the other soil remains too wet. In such a case,
flotation tyres on tractors, headers and harvesting bins may be beneficial,
rather than controlled traffic, to minimise structure degradation.
Break
crops are a most important element in the control and reduction of structure
degradation. They enhance “biological ripping” in cracking soils; activating wet/dry
cycles that swell/shrink the soil to provide natural cracking. In non-cracking
soils they assist by raising organic matter levels (green manures are one
possibility) and by providing increased soil porosity from root holes and
subsequent earthworm activity.
Enactment
of the above management systems is current in cotton. Grains are well advanced.
Horticulture, small crops, peanuts and sugar are behind but learning fast.
Achieving controlled traffic and finding break crops to fit specific cropping
systems and climates takes time; cotton required up to 10 years to get the
system working. In all cropping systems, vigilance is needed to ensure the
controlled traffic/minimum till/break crop system is properly adhered to or
degradation can begin to invade beds, observed as hard shoulders along side
wheel tracks, and compacted and smeared layers in the beds. In such a case,
changing to the new system was time and effort wasted.
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