Counting the Cost of Soil Compaction

2007 Compaction Trials in South West Victoria Jeremy White DPI Ballarat

ISBN 978-1-74217-091-6 (Print) ISBN 978-1-74217-092-3 (PDF)

It is often argued compaction of cropping land does not occur on some soil types, or in some regions, but most soil scientists dispute this view. It is difficult to determine exactly how much this form of soil degradation is costing conventional farmers each year, as past research has shown many cropping soils across Australia are already compacted. To show farmers the effects of soil compaction, three trials were set up in 2007 on a range of soil types in South Western Victoria. The trials were created by selecting six 10m by 10m areas in a paddock and compacting three of them using one pass back and forth with the grower/co-operator’s tractor. Sites were chosen on paddocks which were largely uncompacted as they had been cropped using Raised Beds or Controlled Traffic systems for a number of years, allowing the compaction treatment to affect soil structure. Where degradation was caused by tractor compaction, between 15 and 30 per cent yield losses were recorded- costing between $300 and $450 per hectare in the 2007 season. Both clays and sandy-loams were shown to compact enough from one pass of a tractor to cause these yield losses, so continued wheelings by these and larger machines (particularly harvesters in moist soil) over many years could be costing growers many hundreds of thousands of dollars annually. Dry soil conditions appear to reduce the impact of tractor wheelings on soil structure. But as soon as soil becomes moist, the evidence is that machinery wheels damage structure, reducing both water infiltration and the water holding capacity of the soil. All grain growers should be asking themselves the question, “how much is soil compaction costing my farm business?” Controlled Traffic Farming is the only farming system that can eliminate random wheelings across cropping soils and eliminate the yield and environmental costs of compaction. Funding: Federal Department of Agriculture, Fisheries and Forestry- National Landcare Program Department of Primary Industries Victoria Researcher: Jeremy White (Crop Agronomist, DPI Ballarat) Acknowledgements: – George Burdett, David and Peter Bufton and Stewart and John Hamilton for provision of land and cooperation throughout the year – Martin and Jo Peters of Farmworks for their technical support – Steven Holden and Kathryn Robertson (DPI Hamilton) Wickliffe Site Figure 1- Soil bulk densities from Wickliffe taken in June, after compaction treatment is applied

Wickliffe Bulk-Densities 35 30 Soil Depth (cm)

25 20

Uncompacted

15

Compacted

10 5 0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

The Wickliffe data shows a common increase then decrease with depth between both plot’s bulk-densities, due to the buckshot layer present in that soil at around 15cm in depth. It is also clear that just one pass of a relatively small tractor has caused significant compaction of this soil, with a constant 0.10 to 0.15 g/cm3 difference between treatments down the soil profile.

3

Soil Bulk Density (g/cm )

Note that for a clay loam such as this a bulk density of greater than 1.6 g/c m3 would prohibit root growth through the soil profile, with roots instead following preferential pathways such as cracks and between large aggregates (Hazelton and Murphy, 2007). This slows root development, whilst restricting moisture and nutrient uptake to these preferred paths.

Figure 2- Photograph of crop growth differences between compacted (left) and uncompacted (right) plots at Wickliffe The deleterious effects of this can clearly be seen in the photo, showing a sample of six plants taken from the same seeder row and same bed in midAugust. A clear difference in plant height and root development can be seen between the plots.

Wickliffe Final Yield

Yield (t/ha)

Figure 3- Final Yield at Wickliffe The above figure shows a significant yield difference between the treatments at Wickliffe. In this instance, the compacted plots yielded 15% less, with only 5.1 t/ha versus 6.1 t/ha for the uncompacted. In a season like this, with feed wheat prices at $300/tonne, this would represent a $300/ha loss of income per hectare!

6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Uncompacted

Compacted Treatment

Staughton Vale Site Figure 4- Soil bulk densities from Staughton Vale taken in June, after compaction treatment is applied

Staughton Vale Bulk-Densities 40 35

Soil Depth (cm)

30 25 Uncompacted

20

Compacted

15 10 5 0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

The above data from Staughton Vale shows that there is no difference in bulk density recorded between compacted and uncompacted plots. Therefore, in this instance, compaction was not caused by the single pass of the tractor. A number of factors may have caused this to occur.

1.8

Soil Bulk Density (g/cm3)

The compaction treatment had to be applied before sowing so as not to confound compaction with crop trampling affects. At this time in the season, Staughton Vale had received little rainfall, and so soil moisture was perhaps not enough to cause significant damage to the soil. Another cause could be the possible presence of random wheelings already at the site. Further analysis will be carried out of the wheelings caused by the Bufton’s machinery, as the absence of the header as well as any other implements from a fully aligned CTF system may mean these results were actually due to the compaction treatment already being present across the paddock (i.e. bulk densities could not increase as the entire paddock is already compacted). As a consequence there was no crop differences recorded at the Staughton Vale site, so the recordings from throughout the year will not be discussed further here.

Inverleigh Site Inverleigh Bulk-Densities 40 35

Soil Depth (cm)

30

Figure 5- Soil bulk-densities taken from Inverleigh in June, after the compaction treatment is applied

25 Uncompacted

20

Compacted

15 10 5 0 1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Soil Bulk Density (g/cm3)

The graph above indicates the difficulty in sampling for bulk-densities in a highly variable soil such as that around Inverleigh. Although there is a sandy loam surface at the site, this gives way to a rolling heavy clay below which varies greatly in its depth from the surface. The graph shows a criss-cross of bulk-densities between treatments, and these subsurface gilgai seem to have had more effect on the bulk density sample results than the compaction treatment below 20cm in depth. Inverleigh Soil Strength

Figure 6- Soil strength at Inverleigh, using soil penetrometer. Taken at the same time as bulk density samples

35 30 Soil Depth (cm)

This data more accurately depicts the effect of the compaction treatment, as it is derived from a greater number of samples using a simple handheld penetrometer. Visually, the 10 m2 plots were clearly defined, with greatly reduced crop growth throughout the season.

40

25 20

Uncompacted

15

Compacted

10 5 0 0

1000

2000

3000

4000

5000

6000

Pressure Applie d with Pe netrometer (kPa)

Once again, at least in the top 20cm, the compaction treatment has caused bulk density to increase to above 1.7 g/cm3 which on a light sandy topsoil like that at Inverleigh would limit root penetration into the soil (Hazelton and Murphy, 2007). The photograph below gives a visual representation of the difference in plant and root development between treatments that was clear even at mid-tillering.

Figure 8- Photograph of crop growth differences between compacted (left) and uncompacted (right) plots at Inverleigh

Inverleigh Final Yield

4.5

Figure 6- Final yield at Inverleigh

4.0

Yield (t/ha)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Uncompacted Site

Compacted Site Treatment

This data illustrates a huge difference in yield at Inverleigh, with compaction dropping yield from 4.4 to 3.1 t/ha, which is around a 29% reduction. At hard wheat prices of $350/tonne (conservatively at the moment!), this represents a loss of $446/ha, obviously a huge loss when the same rates of inputs were used on each treatment!

Kathryn Robertson (DPI, Hamilton) and Andrew Whitlock (DPI, Ballarat) will continue these compaction trials to see if the same effects occur in season 2008/09.

For more information contact Andrew Whitlock Precision Agriculture Agronomist DPI Ballarat Phone: 03 5336 6721 Mobile: 0458 312 589 Email: [email protected]

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ISBN 978-1-74217-091-6 (Print) ISBN 978-1-74217-092-3 (PDF)