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Root growth of canola and wheat in the deep yellow sands of the northern sandplain of the Western Australian wheat belt.

Stephen P. Milroy, Senthold Asseng and Michael L. Poole

CSIRO, Centre for Environment and Life Sciences, Private Bag 5 Wembley, WA 6913. Email:


A field study has been initiated to examine the potential yield of wheat on the deep sands of the northern sandplain landscape of the Western Australian wheat belt and what constraints are limiting growers’ capacity to achieve such yields. In the first year’s experiment, there was no difference in the root distribution of canola and wheat and no indication that the contrasting root systems responded differently either in terms of rooting or water extraction. Simulation analysis suggested that high nitrogen application rates could overcome the impact of compaction in this system. The relative importance of restriction to water and nitrogen uptake by compaction in these soil types needs to be explored.

Media summary

We need to explore the relative importance of restriction to water and nitrogen uptake by compaction in the northern sandplain soils of the Western Australian wheat belt.

Key words

Root distribution, compaction, water, water extraction


The sandplain soils of the northern wheat belt of Western Australia produce some 15% of Australia’s grain production. Production in the area is subject to a range of biophysical constraints.

The sandplain landscape in the north of the wheat belt is characterised by deep sandy soils. Some of the soils are acidic with subsurface acidity in particular being an issue (Tang et al. 2002). In addition, the sand can compact either through natural settling or through vehicle movement (Jarvis 1982). Good yield responses can be gained through the amelioration of compaction through deep ripping (Delroy and Bowden 1986). This has been attributed to more rapid rooting and hence better access to soil water and mobile nutrients which leach readily due to the combination of winter rainfall exceeding evaporation and low soil retention ability (Tennant and Hall 2001). Rainfall is low and variable. For example, Dalwallinu, in the centre of the region, receives an annual average of 360 mm (Decile 1 = 230.1mm, Decile 9 = 472.4mm) with approximately 75% falling in the growing season of May to October.

In this paper we report results from our first experiment in a study of yield constraints in the northern sandplain cropping systems. The results compare the root growth and water extraction of wheat and canola.

Materials and Methods

Field experiment

The experiment was conducted on a commercial grain-growing property near Buntine (29.99S, 116.57E) in Western Australia. The soil was a deep yellow sand to at least 3.0 m with a small percentage of gravel found below 1.5 m. The previous crop in the field was wheat. A survey conducted prior to establishing the experiment (D. Murphy, The University of Western Australia, pers. comm.) found little evidence of salinity or low soil pH. There was however, a compaction layer at around 20-30 cm depth.

A randomised complete block trial was established incorporating a range of species as initial phases of crop rotations typical of the region. Plots were 10 m by 40 m long. Within these main plots canola and wheat were split into four nitrogen rates of nil, 40, 80 and 120 kg/ha of applied N. The local average is around 40 kg/ha. The nitrogen was applied as urea and broadcast on 5 June. Wheat was sown at 70 kg/ha and canola at 6 kg/ha on 22 May 2003, at the break of season.

Roots were sampled three times during the growth of the crop: 31 July, 2 September and 1 October. At each time four locations were sampled per plot, two from between the plant rows and two on the plant rows. The soil was sampled in 200 mm depth intervals with the two samples nearest the surface being 100 mm. The data for a given layer from each of the four locations in a plot were pooled prior to analysis. The roots were washed from the soil samples, stained and root length measured using image analysis. The samples were then dried and weighed. Soil water content was measured regularly using a neutron moisture meter calibrated for the soil type. At maturity, the crops were harvested using a plot harvester.

Simulation study

APSIM (Keating et al. 2003) was configured with the Nwheat crop module (version 1.55s), SOILN2, SOILWAT2 and RESIDUE2 soil and residue modules ( This model configuration simulates carbon, water and nitrogen dynamics and their interactions within a wheat crop/soil system that is driven by daily weather information (rainfall, maximum and minimum temperature and solar radiation). It calculates the potential yield, that is, the yield not limited by pests and diseases, but limited only by temperature, solar radiation, water and N supply. The model has been successfully tested against data from field experiments in Western Australia and elsewhere (e.g. Asseng et al. 1998). Simulation experiments were carried out for un-ripped and a ripped sandy soil with long-term daily weather records (1954-2003) from Buntine, Western Australia with an average annual rainfall of 341 mm. Each simulation run commenced on 1 May and was re-set each year at lower limit (LL) and with 50 kg mineral N/ha in the top 90 cm. Wheat was sown between 5 May and 31 July after rainfall of >25 mm in May or >10 mm thereafter. Nitrogen fertiliser treatments of nil, 30, 60, 90, 120 and 150 kg N/ha were simulated.


Field experiment

The root length density of wheat and canola grown under high nitrogen application are shown in Figure 1. Canola produced a significantly lower root length density (RLD cm/cm3) than wheat in the upper layers (Fig. 1a). At greater depths, the difference was not significant. There appeared to be a restriction of root growth of both species at the 20-40 cm depth sample. This corresponds to the first sampling depth below the suspected compaction zone detected in the pre-season survey. Consistent with this, the specific root weight (SRW, mg/m), a surrogate for root thickness, showed a marked increase (Fig. 1b). The SRW of wheat roots increased two-fold and that of canola by a factor of three. There was still some indication that root length densities below 30cm were low at later samplings in September and October (data not shown).



Figure 1. (a) Root length density and (b) specific root weight for canola and wheat at the first sampling in July 2004.

Although root distribution appeared to be restricted in the 20-40 cm depth interval, this did not halt water extraction below this depth. Both wheat and canola were extracting water to approximately 70cm in June (Figure 2). At this time, both species were extracting water to the depth of the wetted portion of the soil. At later samplings the depth of extraction could not be determined due to continued rainfall and downward movement of the wetting front.

Simulation study

The simulation analysis showed that wheat is likely to have yielded more if the rooting restriction was removed but not when the applied nitrogen rate was high (Figure 3). The simulated effect of deep ripping was to increase yields at low nitrogen and to have no effect at the highest N rates.



Figure 2. Soil water extraction by (a) wheat and (b) canola between 22 May and 24 June 2004.

Figure 3. Simulated response of wheat yield to applied nitrogen for Buntine, Western Australia, with and without deep ripping. Results are means for simulated yield across 50 years of climatic data.


The results showed that while canola had a lower RLD than wheat, there was little difference between the species in terms of their root distribution down the profile. Thus there was no indication that the different structures of the dicotyledonous and monocotyledonous root systems rendered them more or less likely to be impeded by the compaction zone. While we did not see any difference in root distribution, it is not possible to say how the two species might compare in their effect on the compaction zone for subsequent crops. This will be examined in the 2004 season.

The observed root distributions were similar to those measured for wheat on compacted earthy sands at East Chapman (28.50S; 115.15E) (Schmidt et al. 1994). However, the depth of extraction compares well to an expected depth of extraction based on a potential extraction front velocity of 2.2mm per degree day (Keating et al. 2001), suggesting that in this case the compaction layer did not greatly restrict the advance of the extraction front. As the roots had reached the depth of the wetting front, the small discrepancy between the expected and observed depth of extraction, could not be definitely attributed to compaction.

The simulation results suggest that compaction may restrict yields but that the addition of nitrogen could overcome the yield limitation due to compaction: at the highest N rates, yield of non-ripped crops was similar to yield of ripped treatments. The pattern observed in these simulations was similar to the N responses of wheat in ripped and non-ripped treatments on deep loamy sand at Wongan Hills (30.90S; 116.72E) (Delroy and Bowden 1986).

In light of the compaction zone found in the pre-season soil assessment and the root restriction observed, experiments in the following year will incorporate a deep ripping treatment in factorial combination with nitrogen rates. This will permit the relative importance of limitations to nitrogen and water uptake by compaction in this soil type to be explored. The impact of canola and wheat crops grown in 2003 on soil properties and root growth in 2004 will also be assessed.


Thanks to Kelley Whisson and Ben Pace (CSIRO) and Lester Snooke (Agritech Services) for their great help in conducting the experiment. We thank Stuart and Leanne McAlpine for access to the site and members of the Liebe Growers’ Group for their collaboration. The project is partially funded by the Grains Research and Development Corporation of Australia.


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