Agronomic consequences of tractor wheel compaction on a clay soil
11.01.07
| Title | Agronomic consequences of tractor wheel compaction on a clay soil |
|---|---|
| Description | Research Update for Growers - Southern Region - August 2005 |
| Authors |
Chan KY Oates AA Hayes RC Sandral GA Dear BS Wagga Wagga Agricultural
Institute NSW Agriculture PMB Wagga Wagga NSW |
| Presented | Condobolin NSW |
Take-home message
Wheeled traffic causes compaction in clay (vertisol) soils that can reduce the yield of sensitive crops such as canola.
This is expected to be a common occurrence under conventional farming systems involving random traffic.
Alternative systems such as controlled traffic may have to be adopted for long-term productivity and more sustainable management of these soils.
Abstract
In southern NSW farming operations using tractors often occur when the soils are moist and prone to soil compaction. However the extent of soil compaction and its relative impact on crop yield in this region have not been quantified.
In this experiment re-compaction due to tractor wheel traffic on a sodic brown clay (vertisol) was monitored under simulated controlled traffic conditions after deep tillage to remove a pre-existing subsoil pan.
Soil physical properties under wheel tracks were compared to those between wheel tracks. Differences in the growth and yield of canola and wheat in the two areas were also measured.
A compaction pan re-formed under the wheel tracks after sowing in the first season of cropping. In the second season soil in the 5-10 cm layer under wheel tracks had significantly higher penetrometer resistance and bulk density than that between wheel tracks. The 'least limiting water range' of the compacted soil was reduced to zero.
Conditions under wheel tracks were unfavourable to plant roots but favourable conditions were maintained in the area between wheel tracks throughout the whole available water range.
This finding was supported by a significant reduction in canola and wheat root growth in the wheel tracks.
Whilst there was no yield penalty for wheat grown on wheel tracks the canola yield on wheel tracks was only 34% of that between wheel tracks (1.1 vs 3.2 t/ha).
These results highlight the potential loss in crop yield due to compaction by tractor wheels and indicate the potential benefits of adopting controlled traffic in farming systems for these soils in southern NSW.
Introduction
Soil compaction by machinery traffic is a well recognised problem in many parts of the world.
The severity of soil compaction is a function of soil machinery management
environmental factors and their interactions. Compaction can reduce crop
yield and adversely affect soil hydrology and biodiversity.
The extent of soil compaction problems due to wheel traffic and their
agronomic consequences have not been investigated but are believed to
be very widespread.
In the cropping belt of southern NSW tillage and sowing operations often have to be carried out when soil is moist and vulnerable to compaction. This is especially the case for the heavy soils with poor internal drainage commonly found in the lower part of the landscape.
Serious crop losses due to waterlogging on clays (vertisols) and red brown earths (alfisols) are common in wet seasons but the extent to which this has been exacerbated by soil compaction due to wheel traffic is not known.
This study explored the formation of a subsoil compaction pan due to wheel traffic in a clay soil characterised the physical properties of the compacted soil and related the degree of compaction to the growth and yield of wheat and canola the two most common crops grown in southern NSW.
Materials and methods
The trial was conducted on a sodic brown clay (vertisol) at Grogan NSW where the average annual rainfall of around 500 mm is fairly well distributed throughout the year.
The trial soil had a clay content of 32% at 0-10 cm and 60% at 150 cm depth. The top 10 cm of the soil was sodic (12% exchangeable sodium) and sodicity increased down the profile (Fig. 1). The soil was also saline beyond about 40 cm.
The site had been cropped and preliminary penetrometer measurements indicated the presence of a cultivation pan at 10-20 cm depth (Fig. 2).
Prior to the experiment the site was sown to lucerne to dry out the profile then deep-tilled to 20 cm to remove the compacted layer. Care was taken to ensure soil moisture at the time of ripping would promote shattering and not smearing. Gypsum applied at 5 t/ha in autumn of 1999 and 2.5 t/ha in 2000 was incorporated to 10 cm depth. Randomised plots were then sown to a range of pastures and crops.
All tractor traffic was restricted to the same path so wheel-track and non-wheel track areas were maintained throughout the trial.
Results and discussion
Penetration resistance readings taken before and after ripping showed that the pre-existing compaction layer had been removed (Fig. 2) but a compaction zone in the 0-10 cm layer was evident directly under wheel tracks in ripped soil after sowing in the first season.
In the second season penetrometer results showed the compaction layer had re-formed at a depth of 5-10 cm under the wheel tracks (Fig. 3).
Penetrometer resistance was higher in soil under wheel tracks than in areas between the wheel tracks at all soil water contents measured (Fig. 4) and increased markedly under the wheel tracks as the soil dried out.
However penetration resistance remained relatively unchanged in the soil between the wheel tracks whether it was wet or dry.
Penetrometer resistance of the soil under wheel tracks was greater than two megapascals (2 MPa) - considered to be the critical limit for root growth - at all moisture levels tested.
The bulk density of soil under the wheel tracks was also significantly higher than in the area between the wheel tracks. Moreover even at "field capacity" the air-filled porosity of soil under the wheel tracks was only 0.07 m3/m3; lower than the 0.1 value considered to be the critical lower limit for adequate soil aeration (Table 1).
These readings indicate conditions in the soil under the wheel track were limiting root growth throughout the available water range. At the wet end of the range the soil under wheel tracks is likely to be anaerobic (waterlogged) and have higher than desirable soil strength. As the soil dries its strength rapidly increases to levels providing a barrier to root growth.
In contrast aeration and physical conditions in soil between the wheel tracks were consistently favourable for plant growth.
For both crops root mass density in the 5-10 cm (Table 1) and 10-20 cm layers under wheel tracks was significantly lower than between wheel tracks.
Canola dry matter production and grain yield were significantly lower on wheel tracks than in the area between wheel tracks and canola yield on wheel tracks was only 34% of that in non-compacted soil. However there were no such differences in wheat performance (Table 1).
This may be related to the sensitivity of the two different crops to soil compaction. Canola plants with their tap root systems tend to be more sensitive to soil compaction than wheat plants which have a more fibrous root system.
In addition the compacted soil also had poor aeration under wet conditions making it less favourable for crops such as canola that are sensitive to waterlogged conditions.
| Canola | Wheat | |||
|---|---|---|---|---|
| Wheel | Non-wheel | Wheel | Non-wheel | |
| Bulk density (Mg/m3 5-10 cm layer) | 1.58* | 1.29 | 1.50* | 1.25 |
| Air-filled porosity at FC* | 0.07* | 0.187 | 0.09* | 0.226 |
| Root mass density (g x 106 /cm3 soil) | 9.2* | 27.5 | 75* | 118 |
| Dry matter (t/ha) | 4.7* | 11.8 | 12.0 | 12.6 |
| Grain yield (t/ha) | 1.1* | 3.2 | 5.5 | 5.3 |
(FC = field capacity; Values under wheel tracks followed by * are significantly different (P<0.05) from the corresponding values under non-wheel tracks for the same crop).
Figure 1. Sodicity and salt profiles of the brown clay
Figure 2. Penetration resistance profile of un-ripped wheel tracks and non- wheel track areas measured in the first season
Figure 3. Contour plot showing distribution of penetration resistance of soil profile; cross section perpendicular to direction of tractor traffic
Figure 4. Changes in penetration resistance as a function of soil water content at 7.5 cm depth under wheel track and between wheel tracks (FC = field capacity; PWP = permanent wilting point)
Acknowledgements
Financial support from GRDC is acknowledged. Cooperation provided by the
local Grogan and Morangarell Landcare Groups was appreciated.
