Long Term Field Trials at Avon, Kapunda and Waikerie in South
Australia to Study the Effects of Rotation and Tillage on Soil and Root
Health and Profitability.
By
Albert Rovira*, Gupta, V.V.S.R.** and David Roget***
*Formerly CSIRO and CRC for Soil & Land Management
**CSIRO Division of Entomology, Glen Osmond, SA
***Honorary Research Fellow, CSIRO Division of Entomology; Formerly CSIRO Sustainable Ecosystems, Glen
Osmond, SA
Abstract
The Avon field trial was started in 1978 in a typical Mallee environment to study the effects of
rotation and tillage on the fungal disease Take-all which attacks wheat roots. In the third year
another root disease of wheat caused by the fungus Rhizoctonia solani became a serious problem
in direct drilled wheat. Rhizoctonia root rot was reduced by soil disturbance so the study was
extended to develop narrow sowing points for direct drilling. A decline in root disease caused by
the Take-all fungus and Rhizoctonia in all rotations after three years was due to the development
of biological suppression of the diseases. Measurement of changes in total soil nitrogen and N
input and output budgets at Avon indicated that up to 18 to 20 kg of nitrogen per hectare was
biologically fixed each year; this was linked to the retention of crop residues which provided the
energy for the free living nitrogen fixing micro-organisms.
In 1983 a similar trial was started at Kapunda on a typical red brown earth with a higher rainfall
than Avon. The initial emphasis at Kapunda was also on the effect of rotation and tillage on
Take-all but the work was extended to study the effects of tillage and rotation on soil structure,
organic matter level and earthworms.
A third long term field trial was established in 1998 at Waikerie in the drier part of the South
Australian Mallee to demonstrate the impact of tillage, rotation and adequate fertilizer inputs on
production, profitability and beneficial biological functions.
Key Words
Soil borne root diseases of wheat, rotation, tillage, sowing points, non-symbiotic nitrogen fixation,
water use efficiency, productivity, profitability.
Introduction
Fourteen soil fumigation field trials conducted in Victoria and South Australia between 1971 and
1976 demonstrated that one of the major constraints to production by cereals was root disease
(Rovira, 1988). Soil fumigation gave yield increases ranging from 0.3 to 2.65 tonnes per hectare –
the highest responses were obtained in calcareous sandy soils of the Mallee in Victoria and
Streaky Bay in South Australia. Control of soil borne root diseases such as Take-all
(Gaeumannomyces graminis var.tritici), cereal cyst nematode (Heterodera avenae) and
rhizoctonia root rot ( Rhizoctonia solani) was mainly responsible for the yield increases but the
release of mineral nitrogen from the soil biomass also contributed.
At this stage no rotation trials had been conducted in Australia to study the effects of rotation on
these three diseases. In the early 1970’s a knock down herbicide, “Sprayseed” (ICI, paraquat +
diquat) was being tested as a tool to enable the direct drilling of crops which offered a promising
strategy to protect fragile and erosion prone soils. At this stage we decided to set up trials at Avon
and Kapunda in South Australia to study the impacts of rotation and tillage on root diseases of
wheat in two environments representing the major wheat growing regions of southern Australia.
When these two trials were completed, another was set up at Waikerie where the rainfall and soil
type is typical of the Mallee region in South Australia, Victoria and New South Wales.
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
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Trials Reviewed
Avon, South Australia.
Experimental Design
A rotation x tillage trial was established in1978 at Avon, 100 km north of Adelaide, which has an
average annual rainfall of 280mm (range 200 to 510mm) and typical Mallee soils ranging from
sand over limestone (Dy3) on the sand hill, loamy sand over limestone (Gc1) on the slope and
clay loam (Gc2) on the flat swale (Classifications according to Northcote et al. 1975).
Avon was selected over several other sites with similar soils and rainfall because the Take-all
fungus occurred over the whole site and cereal cyst nematode occurred at very low levels so that it
was possible to initially concentrate on Take-all. Later Rhizoctonia root rot was shown to be a
yield limiting factor throughout the site when wheat was sown by direct drilling so we extended
our work to consider this disease.
Initially the plots were 300m long over the three soil types but after two years the plots were
reduced to 100m on the slope. There were five two year rotations, viz. continuous wheat, wheatgrass medic pasture, wheat-sown medic (grass free) pasture, wheat-peas, and wheat-oats. There
were two tillage treatments viz. conventional cultivation with three cultivations before sowing and
direct drilling after spraying the pasture with ICI Sprayseed (paraquat/diquat) one to three days
before sowing. Initially the direct drilled plots were sown with the 2m wide experimental drill
(SIRODRILL) designed to give minimum soil disturbance through having a fluted coulter in front
to cut through stubble and pasture residues followed by a narrow Janke point followed by a press
wheel. After four years when Rhizoctonia root rot became a serious problem in the direct drilled
plots the SIRODRILL was replaced by a seed drill with narrow modified McKay lucerne points
(Figure 1) which gave more soil disturbance.
Figure 1, Sowing points tested at the Avon field site including modified McKay Lucerne
Point (MP)
The trial was duplicated side by side but one year apart so that each year one phase was sown to
wheat while the other phase was in the alternative rotation phase. This design gave root disease
data and a wheat harvest each year which helped to compensate for the high variability in rainfall
(Figure 2).
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
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Figure 2. Annual and April-October Rainfall at Avon.
Avon Rainfall 1976 - 2005
Year Total
Apr-Oct Total
600
500
400
mm 300
200
100
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
1980
1979
1978
1977
1976
0
Year
Key Findings and Principles
Effect of Rotations and Tillage on Take-all and Wheat Yields.
In the second year of the trial (1979) the effect of rotation on the level of disease and on grain
yields was clearly established. Figure 3 shows that the lowest levels of Take-all and highest
yields were obtained when wheat was sown with cultivation following sown medic (grass free)
pasture and peas. Wheat after oats (a non-host for G. graminis var. tritici) also had low Take-all
but yields were lower than after peas or medic because the legumes fixed nitrogen. Wheat in the
rotations wheat after wheat and wheat after grass-medic pasture had high levels of Take-all on
their roots and lower yields. The lower yields in direct drilled wheat was probably due to two
factors, firstly, crowns of wheat and grasses from previous years remain intact with direct drilling
enabling the Take-all fungus to survive, whereas cultivation breaks up crowns and residues, and
secondly, Rhizoctonia root rot was already having an effect on root health and hence yield.
Figure 3. Effect of Rotation and Tillage on Take-all and Wheat Yields.
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
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Effect of Rotation and Tillage on Root Disease, Available N and Water Use Efficiency
Following the publications by French and Schultz (1984a, 1984b) on the use of Water Use
Efficiency (WUE) as an alternative to grain yield as the best indicator of the efficiency of
cropping systems we decided to use WUE as well as yield to indicate the best cropping system.
Data in Figure 4 shows that when root diseases are low and available nitrogen is high both high
yields and maximum WUE are obtained.
Figure 4. Effect of Rotation and Tillage on Root Disease, Available Nitrogen and Water Use
Efficiency.
Link between Rainfall and Incidence of Take-all on Wheat
During the first eight years of the Avon trial there was a high correlation between growing season
rainfall and the incidence of Take-all on the roots of wheat (Figure 5). Ninety percent of the yield
loss from Take-all was accounted for by the early incidence of the disease on the wheat roots and
the September rainfall (Roget & Rovira, 1991).
There was also a correlation between the previous Spring rainfall and the incidence of the disease
and we concluded that wet conditions favour the build up of the Take-all fungus on roots and
crowns of hosts such as wheat and grasses which then provide a high inoculum level the following
year.
Photo: Rainout shelter facility to manipulate seasonal rainfall in field situations
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
-4-
Figure 5. The Link between Rainfall and Take-all.
Relationship between rainfall and the incidence of take-all
Avon S.A
Rainfall
Take-all predicted
400
350
90
300
Apr - Oct 250
Rainfall 200
(mm) 150
70
100
50
20
80
Wheat
plants
50
with
40
take-all
30
(%)
60
10
94
92
90
88
86
84
82
0
80
78
0
Year
After a number of field observations that in years when summer rains occurred there was less
Take-all than expected, experiments were conducted in undisturbed soil cores in rain shelters at
Avon where summer rainfall events could be simulated. The results showed that for each rainfall
event of 25mm or more in January, February or March the level of Take-all fungus fell by 30%.
Figure 6. The Effect of Summer Rainfall on the Level of Take-all Inoculum in Soil (source:
SARDI Resource manual on root disease management, p 20).
Rise and Decline of Rhizoctonia Root Damage over 15 Years.
The build up of Rhizoctonia damage to roots from 1979 to 1982 was much greater in direct drilled
wheat than wheat sown after cultivation, but in all treatments the level of disease declined to
virtually zero in the next eight years (Figure 6). This decline was shown to be due to the build up
of microbes in the soil which suppressed Rhizoctonia; this was associated with increased carbon
inputs from roots and stubble. The process could be reversed if mineral N or plant material with
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
-5-
a narrow C:N ratio was added to the soil which is illustrated in Figure 7 when wheat residues high
in N (sprayed at anthesis) were added to the soil.
Figure 7. Rise and Decline of Rhizoctonia Root Damage in Direct Drilled Wheat.
Rhizoctonia Levels after Opening Rains.
When the Avon experiment commenced in 1974 the most common rotation in South Australia
was grass/medic pasture for sheep followed by wheat. The grass component of the pasture was
barley grass, brome grass and rye grass, the first two of which are excellent hosts for the Take-all
fungus. Barley grass is the first grass to germinate after the opening rains with up to 5000
seedlings per square metre, each seedling has several roots and assays showed that most of these
roots carried both Take-all and Rhizoctonia (Rovira, unpubl.). Thus, the “graze, spray, sow”
technique, which permitted farmers to make maximum use of the early barley grass growth for
sheep after the Autumn break, advocated as an advantage for direct drilling was considered by us
as setting the scene for high root disease. An experiment was conducted to determine whether the
early removal of grasses 1 -2 weeks after the opening rains and then a period of three weeks
before sowing would have an effect on the Rhizoctonia inoculum in the soil. Figure 8 shows that
Rhizoctonia inoculum builds up if the volunteer pasture is permitted to grow until sowing where
as with a period of “chemical fallowing” the level of Rhizoctonia in the soil falls.
Figure 8. Rhizoctonia Levels after Opening Rains.
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
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Effect of Sowing Point on Rhizoctonia
An advantage of having a site such as Avon was that supplementary trials could be conducted
adjacent to the plots to investigate interesting leads arising from the long-term trial. One of these
was a study we undertook to investigate a variety of sowing points for direct drilling.
There was a big economic advantage to farmers who wanted to move from conventional
cultivation to direct drilling if they could simply change the points on their seed drills rather than
buying expensive machinery designed for no-till farming. For several years we studied the effects
of five narrow sowing points and the conventional wide point on Rhizoctonia disease on wheat
roots (Figure1). This study by Roget et al. (1996) showed that all the narrow points were
effective in reducing disease provided the points disturbed the soil down to 10cm and the seed was
sown shallow at around 5cm. The highest disease occurred with the wide conventional point
sowing at 5cm with no disturbance below 5cm. Neate (1987, 1988) had shown that Rhizoctonia
propagules were associated with particulate organic matter concentrated in the top 10cm of soil
and that the fungus grew from these propagules to attack roots. Hence, by disturbing the top
10cm of soil the fungal strands in the soil were broken and by placing the seed at 10cm the roots
grew in a zone of soil low in pathogen level.
Nitrogen Fixation associated with Stubble Retention
Contributions from biological nitrogen fixation processes, both symbiotic (SNF) and nonsymbiotic nitrogen fixation (NSNF), are highly desirable for both the economic and
environmental sustainability of crop production especially in low input dryland farming systems
in Australia. Until recently the contribution of free-living nitrogen fixing microorganisms to the N
inputs into the dryland cereal cropping systems has received little attention. One of the important
and unexpected results to come out of the Avon long term trial was the finding that under
intensive cereal systems with stubble retention there was an average contribution of nitrogen into
the soil of 20kg N/ha/yr (Table 1). Gupta et al. (2006) were able to allocate this “free” N input to
free living nitrogen fixing bacteria building up on the decomposing plant residues. This is
another plus for conservation farming.
Table 1. N budget for 17 years of continuous wheat at Avon, South Australia, 1979-1996.
(Source: Gupta et al. 2006)
N source
or sink
Soil N
Fertiliser N
Calculation details
Total N (0-10cm)
1979 = 0.140% (+/- 0.005)
1996 = 0.135% (+/- 0.005)
N from soil OM (Soil bulk density 1.2g/cc)
No fertiliser added
Net change in N
(kg/ha)
+ 60
0
Grain N
Total grain harvested = 19.7 t/ha
Average grain N = 0.020%
- 394
_____________________________________________________________________
Total N deficit
334
Annual N deficit
19.7
_____________________________________________________________________
Available carbon, soil moisture and temperature are the major regulating factors for NSNF in
Australian soils. In the low organic matter Australian soils, crop residues are the major source of
C for total microbial activity and for the (NSNF) during the months of January to June. Cereal
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
-7-
crop residues with their wide C:N ratios (100:1 or wider) provide ideal conditions for the
expression of nitrogenase activity because of the large amount of available C coupled with very
low levels of available inorganic N. If all the C in the straw is utilized to support N fixation,
contributions of up to 75kg N per tonne straw is possible (Kennedy and Islam 2001). Halsall and
Gibson (1985) have measured 72kg N/t straw under laboratory conditions with co-inoculation of
cellulolytic and diazotrophic microorganisms. The presence of high levels of inorganic N has been
shown to have a negative effect on the expression of nitrogenase activity and N2 fixation.
Rhizosphere is another area where carbon availability would meet the energy requirement by the
NSNF microorganisms. However, due to low temperatures during the early phase of crop growth,
the contribution of rhizosphere associated N2 fixation to the N status in agricultural systems in
southern Australia is uncertain.
Currently our knowledge about the effects of soil type and environment on this biological process,
in particular in southern Australian region, is limited. Integration of the contribution from NSNF
should be useful for agronomists and extension officers to help explain changes in N status within
paddocks or within specific farming systems and assist in providing more accurate advice on N
fertilizer requirements, particularly in low-input farming systems. This should result in more
efficient use of N inputs, greater benefits from a soil biological function and increased grower
returns.
Profitability of the Different Farming Systems
Although the trials showed conclusively that the rotations which reduced root disease gave the
highest yields and maximum WUE the true value to farmers is to do an economic analysis of the
different rotation and tillage treatments. Roget and Krause (Pers. Communication) undertook an
economic analysis of all the treatments over the period 1979 to1996 including the total cost of
each treatment (fertilizer, chemicals, labour etc) and the sale of produce at market rates,
extrapolated this to a whole farm basis and then calculated the gain or loss in equity for a typical
Mallee farm. Figure 9 demonstrates that a rotation Peas-Wheat planted by direct drilling gave a
225% gain in equity. Next came Oats-Wheat sown with conventional cultivation – oats being a
non-host for G.graminis var.tritici lowers the disease level but does not fix nitrogen like peas.
Figure 9. Change in Farm Equity of Selected Rotation and Tillage Treatments
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
-8-
Kapunda, South Australia.
Experimental Design
This was a red-brown earth (Northcote et al., 1975) representative of many of the cereal growing
regions in southern and south-eastern Australia with a winter dominant average annual rainfall of
496mm. The surface 10cm is composed of 11% clay, 19% silt, 60% fine sand and 9% coarse sand
with 1.5% organic carbon. This rotation x tillage trial commenced in 1984 after 8 years of
naturally regenerated clover-grass pastures. The three tillage treatments were: (a) conventional
cultivation (CC) comprising three cultivations to 70mm using 150mm wide sweep shares and
sown with the same implement (b) reduced tillage (RT-one cultivation before seeding) and (c)
direct drilling (DD) where the regenerated pasture was sprayed with ICI “Sprayseed”
(paraquat/diquat) one day before sowing with the SIRODRILL. Both treatments were sown on
the same day in early June.
There were three rotations: wheat-wheat, lupins-wheat and pasture-wheat with six replicates.
There were two phases of the trial: one started in 1983 and a second started in 1984 so that each
year there was an assessment of Take-all on the roots and also a wheat harvest. Plots were 60m
long x 1.5m wide. Root samples were taken along each plot in the wheat phase at anthesis, washed
and examined for the incidence of Take-all lesions. Rhizoctonia root rot did not occur at a
significant level at Kapunda so no assays for this pathogen were necessary. In August, 1986,
replicate blocks of soil (20 cm x 10cm x 10cm (deep)) were taken from each plot in the wheat
phase for earthworm assays.
In early July, 1987, ten soil samples to a depth of 5cm, each of approximately 1kg, were taken
from each plot between the rows of wheat which had been sown 3 weeks previously. These
samples were used for the measurement of water stable aggregates and organic matter.
A supplementary experiment was undertaken in an adjacent field to study the optimum time of
spraying the grasses from the annual grass-subterranean clover pasture to reduce the carryover of
the Take-all fungus into the following wheat crop.
Key Findings and Principles
Effect of Rotation and Tillage on Take-all
In the first year moving from long-term pasture to cropping there was a clear effect of tillage on
the level of soilborne diseases such as Take-all on the roots of wheat plants (Table 2). In general
very little rhizoctonia bare patch disease was observed at this trial.
Table 2. Effect of Tillage on Levels of Take-all on Roots of Wheat.
__________________________________________________________________
Treatment
Take-all incidence (%)
__________________________________________________________________
Conventional cultivation
27
Reduced tillage
41
Direct Drill (with SIRO drill)
45
Level of significance
** (LSD P<0.05 = 10)
__________________________________________________________________
Yields of wheat were highest in the wheat following lupins in both CC and DD tillage treatments
which is a reflection of lower Take-all disease and the higher nitrogen levels from the lupin
residues and the nitrogen saving during the life of the lupins as they fix all the nitrogen they need.
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
-9-
Effect of Grass Control on the Level of Take-all Fungus in Soil.
Experiments were conducted using selective herbicides to remove grasses at different times from
July to September from subterranean clover/grass pastures to assess the best time for the removal
of grasses to reduce the level of Take-all fungus in the soil.
Herbicide applications in July and August reduced grass density to 5% while application in
September reduced it to 15 – 25%; clover population increased as the grass population decreased.
Removing the grass in late July and early August reduced the incidence of the Take-all fungus to
30% of the unsprayed pasture, while removal in September only reduced the incidence to 83%
compared with the unsprayed pasture. These results indicate the need to remove the grasses from
the pasture as early as possible in order to reduce the carryover of the Take-all fungus into the
following year. However, removal of grasses even early in the season is still not as effective as
growing a grass free break crop such as lupins, peas, oats or canola which reduces incidence to 5 –
10% (Inward et al,. 1992).
Effect of Rotation and Tillage on Earthworm Populations and Biomass
Table 3 shows that tillage has a marked effect on total earthworm populations and biomass while
rotation has a small but not significant effect on numbers but a large effect on biomass because the
worms were larger in the pasture-wheat rotation than in the lupins-wheat rotation. These results
are reported in more detail in a paper by Rovira et al. (1987).
Table 3. Effect of Tillage and Rotation on Earthworm Population and Worm Biomass
___________________________________________________________________________
Tillage
Rotation
Direct Drill
Conventional
Wheat after Pasture Wheat after lupins
Cultivation
___________________________________________________________________________
Total worms
6.57
3.64***
5.47
4.75 ns
Worm biomass 1.52
0.72***
1.51
0.73***
___________________________________________________________________________
Effect of Rotation and Tillage on Soil Organic Matter and Soil Physical Properties.
Organic matter level was higher with direct drilling compared with conventional cultivation but
was not affected by crop rotation (Table 4). The study also showed that the stability of the soil
aggregates was greater in the direct drilled plots than in those sown by conventional cultivation.
These results are reported in Smettem et al. 1992. This benefit in terms of organic matter content
with direct drilling disappeared when we changed from the SIRODRILL with minimum soil
disturbance to the use of narrow sowing points which caused much more soil disturbance. This
result confirms the fragile nature of the organic matter in red brown earths where the clay content
is not sufficient to protect the organic matter from disturbance.
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
- 10 -
Table 4. Effect of Tillage and Rotation on the Stability of Soil Aggregates and the Organic
Carbon in Aggregates
__________________________________________________________________________
Percent of Water Stable Aggregates Percent Organic Matter in Aggregates
__________________________________________________________________________
1 to 2 mm aggregate
CC wheat/wheat
5.3
1.40
CC wheat/pasture
4.8
1.60
CC wheat/lupin
4.3
1.49
DD wheat/wheat
14.6
1.78
DD wheat/pasture
15.8
1.78
DD wheat/lupin
7.5
1.81
4 to 5.13 mm aggregate
CC wheat/wheat
3.0
1.29
CC wheat/pasture
4.9
1.47
CC wheat/lupin
5.4
1.34
DD wheat/wheat
14.1
1.70
DD wheat/pasture
23.8
1.87
DD wheat/lupin
20.4
1.91
_________________________________________________________________________
Note: No significant effect of rotation; highly significant effect of tillage for both small & large
aggregates.
These results clearly demonstrate the beneficial effect of zero tillage on the stability of this redbrown earth.
Waikerie, South Australia.
The Mallee region covers an area of some 5m ha of SA, Vic and NSW. It is characterised by dune
/ swale landforms that contain soils that are generally coarse in texture (5-15% clay). The soils are
inherently of low fertility and low in organic matter (total C = 0.5%). Rainfall is low (250350mm / year) and variable and is winter dominant (Roget and Gupta, 2004).
The marginal production characteristics of the Mallee have led to the development of low risk
farming systems based on cereal / pasture or cereal / fallow rotations with low fertiliser inputs
which have consisted largely of additions of P. This has resulted in low productivity both in terms
of crops and pastures with cereal yields achieving around 50% of the water limited potential
(average wheat yield 1.2t/ha). Productivity can be limited by numerous factors including cereal
root diseases and chemical subsoil constraints but overall it is limited in dry years by lack of
rainfall and in better rainfall years by lack of fertility. The low productivity combined with other
factors including heavy grazing, fallowing and endemic wind erosion have resulted in low returns
of organic matter to the soils with subsequent limitations to microbial activity and microbial
functions.
A project was initiated in 1997 (Mallee Sustainable Farming Project) to investigate the potential to
significantly improve Mallee farming systems. The project was based on the hypothesis that
productivity gains of up to 100% could be made by more efficient utilization of the available
rainfall with more intensive cropping and improved tillage and fertiliser strategies. This increase
in cropping intensity with increased productivity in association with limited grazing and zero
tillage would result in a substantial increase in the return of organic matter to the soil with an
associated increase in microbial activity and function, which are essential for the long-term
sustainability of any farming system. In these coarse textured Mallee soils with low levels of
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
- 11 -
organic C and limited opportunity for protection of organic matter it was hypothesised that an
increased flow of microbially available C would quickly provide a significant improvement in
microbial biomass, microbial activity and populations of microbial functional groups involved in
nutrient cycling and other microbial functions.
A brief summary of results on the impact of agronomic management on soil microbial functions
from the Mallee Sustainable Farming Project at Waikerie, SA is given below.
Experimental Site
A field trial was established at Waikerie, South Australia, (34O 17’ S, 140O 02’ E) in 1998. The
climate is Mediterranean, characterized by hot dry summers and a winter-dominant, average
annual rainfall of 260mm. The soil is an alkaline calcareous loamy sand, classification Um5
(Northcote et al., 1975), or gray-brown or red calcareous soil (Stace et al., 1968), or alfisol (Dudal
1968). Soil chemical properties at the start of the trial were pH(water) 8.6, organic C 0.68%, total N
0.05%, Colwell bicarbonate-extractable P 12 mg/kg, and CaCO3 0.4%. Particle size distribution
(%) is clay 6, silt 1, fine sand 43 and coarse sand 47.
Key Findings and Principles
Agronomic and economic performance
Data on yield and economic performance of various farming system treatments over the last 9
years of the trial clearly indicated that intensive cropping combined with the management of
inputs to meet the predicted yield estimates are better than the conventional low-input pasturecrop system. During the entire experimental period, the performance of the opportunity cropping
system has been very good, especially considering the average rainfall decile has been 4.1 over
the last 9 seasons compared to the long term average of 5.5 which is a reduction from the average
of 25%. Despite the expected benefits from N fixation through legume-rhizobium symbiosis, low
grain yields from the grain legumes resulted in lower economic returns from the wheat-pulse
system.
1400
W
1200
Cumulative GM ($/ha).
W
C
W
Wheat / Pasture DP
W
1000
Wheat / Pulse
W
Opportunity Cropping
800
C
600
W = wheat
C = canola
W
400
W
200
0
1998
1999
2000
2001
2002
2003
2004
2005
2006
Soil biological functions:
Microbial biomass (MB) is a storehouse for plant-essential nutrients and it plays an important role
in the availability of soil organic nutrients and fertilizers to plants. Microbial biomass carbon
levels in mallee soils ranged from 165 to 700 mg per kg soil and this accounted for 2-5% of soil
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
- 12 -
organic matter. The size of MB and the amounts of nutrients in MB are influenced by soil type
(physical and chemical properties), plant type, the quality of soil organic matter including crop
residues and soil moisture availability. Nutrients such as nitrogen in MB are less prone to losses
through leaching than freely available N including fertilizer nitrogen.
Data shown in Table 5 indicate changes in microbial biomass C and nutrients (N and P) in the
different farming system treatments that differ in carbon inputs to the soil through improved
production, increased cropping intensity and stubble retention. Soils from intensive cropping
systems contained higher levels of MB-C and MB-N compared to the traditional low-input
pasture-crop (DP) system. In this experimental trial the pastures were not grazed and the pasturewheat rotation receiving high levels of fertilizer inputs also resulted in increased MB levels
probably a benefit from the C inputs from pasture phase. Intensive cropping systems also
exihibted higher levels of microbial activity during the off-season and at the time of sowing. In
general, a higher level of microbial activity reflects a greater opportunity for nutrients (such as
nitrogen and phosphorus) to be made available to plants.
Table 5: Microbial biomass carbon and nutrient levels in the surface soils (0-10 cm) of
selected treatments at Waikerie core site (after 4 years) (Roget and Gupta, 2004).
__________________________________________________________________________________________
Cropping system
MB-C
(kg C/ha)
MB-N
(kg N/ha)
MB-P
(kg P/ha)
Microbial activity§
(g CO2 /m2/hour)
__________________________________________________________________________________________
Pasture-Wheat (DP)
Pasture-Wheat (HI)
Legume-Wheat (HI)
Canola-Wheat (HI)
265a#
370b
370b
357b
26 a
43cd
46 d
36bc
16.0a
21.0b
13.0a
16.5a
0.105
0.185
0.210
0.175
____________________________________________________________________________________________
#
§
values in each row followed by the same letter are not significant at P<0.05.
average values from 6 in situ respiration measurements made in each experimental plot.
MB levels change during the season and the level of nutrients in MB at the beginning of the
season would reflect the nutrient supplying capacity of a soil during that season (Gupta and Roget,
2004; Roget and Gupta, 2004). Cropping systems that support higher levels of MB and maintain
high levels of microbial activity will have greater amounts of nutrient mineralization. This
improvement in function was due to an increase in microbial activity as a result of an increase in
microbially available C.
Table 6. Amount of N mineralised (kg N / ha) during the off-season (summer and autumn)
as influenced by cropping system type at Waikerie, SA (Roget and Gupta, 2004)
_________________________________________________________________________
Treatment
Off-season mineralization
Treatment
Off-season mineralization
2000
(Nov 2000-May 2001)
2001
(Nov 2001-June 2002)
__________________________________________________________________________
Wheat-DP# (1)
10.3
Pasture-DP
18.5
Wheat-HI§ (3)
14.0
Pasture-HI
37.0
Wheat-HI (8)
36.1
Peas-HI
35.0
Wheat-HI (9)
24.7
Canola-HI
23.1
Wheat-HI (10)
34.0
Wheat-HI
23.0
Canola-HI (11)
41.5
-__________________________________________________________________________
#
DP – district practice fertiliser rate comprising 10 kg P/ ha; 5 kg N/ha
§
HI – high fertiliser rate comprising 15 kg P/ha; 27 kg N/ha (N excluded for pulse crops)
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
- 13 -
The level of nitrogen mineralization was generally greater following legume (pasture and grain
legumes) and canola crops compared to wheat. Data in Figure 10 on the mineral N levels in the
soil profile at the time of sowing shows considerable seasonal variability. Since 2002, mineral N
levels were generally higher under the high input pulse-wheat rotation compared to the continuous
wheat system. Improvements in the quantity and quality of particulate organic matter (from crop
residues) coupled with better contribution from legume-rhizobium symbiosis are two of the main
reasons for this trend. Results from other work at this site indicated that care must be taken in the
type of ‘in crop’ herbicide used in order to gain maximum N inputs from legume-rhizobium
symbiosis.
Figure 10. Effect of crop intensification on mineral N levels in the soil profile (1M) at the
time of sowing in the Waikerie core trial on a Mallee soil.
M ineral N at Sowing - Waikerie Core Trial
160
Cont Wheat
P ast-Wheat
P ulse-Wheat
Mineral N (kg/ha)
140
120
100
80
60
40
20
0
1998
1999
2000
2001
2002
2003
2004
2005
2006
Overall the results from MSFP on light textured Mallee soils in SA, Vic and NSW indicated that
management changes that increased productivity and resulted in the return to the soil of higher
levels of microbially available C were the driver for greater soil microbial activity and improved
microbial functions. Soil microbial functions, including non-symbiotic N-fixation, N and P
availability and disease suppression can be significantly improved through changes in on-farm
management in the Mallee. The course textured Mallee soils are likely to be particularly
responsive to increased C inputs due to the low soil C levels and rapid turnover of added C as a
result of the limited protection offered by these soils. Therefore benefits from intensive cropping
and stubble retention may only be realised in no-till systems as disturbance accelerates
decomposition of crop residues and lead to leaching of mineralized N.
Conclusions
Before these trials soil borne root diseases of wheat were a major constraint on grain production in
southern Australia, especially on light calcareous soils. These trials are unique anywhere in the
world in that they were set up to study biological constraints on wheat yields sown by either
conventional cultivation or direct drilling with different rotations. This presents a far more
complex scenario than long term field trials on fertilizers and plant nutrition.
One lesson from the long term trial at Avon was "always expect the unexpected" and follow-up on
leads from the trials by either changing some of the treatments, e.g. our change from the
SIRODRILL to narrow points or conduct supplementary trials around the long term trial to answer
questions thrown up from the main trial. Examples of this at Avon were: short term chemical
Paper presented at the Long-term fertilizer workshop held during June 2007, WestBeach, Adelaide, SA, 6(1-16). 2007.
- 14 -
fallow before direct drilling to control Rhizoctonia root rot and the impact of summer rainfall on
the level of Take-all fungus in soil. Other new and unexpected findings from the main trial were
(a) Significant non-symbiotic fixation associated with stubble retention, (b) Development of
biological suppression of both Rhizoctonia root rot and Take-all fungus with stubble retention (d)
The link between spring rainfall and Take-all. Results from the farming system trail at Waikerie
indicated that soil microbial functions, including N mineralization, non-symbiotic N-fixation,
availability can be significantly improved through changes in on-farm management in the Mallee.
Management changes that increased productivity and resulted in the return to the soil of higher
levels of microbially available C were the driver for greater soil microbial activity and improved
microbial functions. These improvements in microbial function have contributed significantly to
the economic and environmental outcomes achieved from the modified management strategies.
Regular farmer field days were held at Avon, Kapunda and Waikerie. These trials in South
Australia provided farmers with information which they could then adapt to their own farms and,
together with the Tarlee Trial conducted by Jeff Schultz and co-workers (Schultz, 1995) of the
South Australian Department of Agriculture, were in large part responsible for the widespread
adoption of No-till and minimum tillage on most cereal farms in the state.
The involvement of Mike Krause, agricultural economist, in the final analysis of the full cost input
and the dollar value of the yields for each treatment and expressing this in terms of farm equity at
Avon and Waikerie is of far more value to farmers than simply grain yields or water use
efficiency. This approach was also used by Schultz (1995) on the Tarlee long term rotation x
tillage x fertilizer trial. In our opinion the team made up of an agronomist and an economist to
conduct the financial analysis of different treatments at the completion of the trials represents a
very important innovation which could be adopted in other long term trials across Australia.
Acknowledgement
Financial support for Gupta V.V.S.R. and David Roget was provided by the CSIRO and Mallee
Sustainable Farming Inc. Authors wish to thank all the technical officers involved in the three
long-term experiments for their efforts with field work and laboratory analysis.
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