As part of a wider study at a number of sites in the UK and over several years, the effects of harvest compaction, subsoiling and post subsoiling traffic were assessed. One site, classified as a sandy loam, had been cropped with winter wheat for the previous two years. Post-harvest, penetration resistance (soil strength) was measured, and then additional compaction applied “track by track” using a tracked and a wheeled combine harvester and a tractor and trailer, with loads as shown in the table. 

MachinesMaximum wheel or track load, Mg
Tracked combine9.5
Wheeled combine7.5
Tractor and trailer3.5/3.75

As one can see from the illustration below, there was a significant increase in strength of the soil due to the machines running over it, particularly in the top 15 centimetres compared with the “as found” condition.

Key: TComb – tracked combine; WComb – wheeled combine; WT&T – wheeled tractor and trailer

The site was then subsoiled to around 30 cm depth and a week or so later, compacted as above with wheeled and tracked tractors typically associated with cultivation and drilling machinery, as detailed in the following table.

MachineryWheel or trackLoads, Mg
CTF0
Wheeled drill tractor (WDT)3.5
Wheeled cultivation tractor (WCT)5
Tracked cultivation tractor (TCT)7

The next illustration shows what happened to soil strength in terms of penetration resistance. 

On the left of the graph, the non-trafficked subsoiled condition (CTF) can be compared with the “as found”, showing a very significant reduction in soil strength as a result of subsoiling. However, it is very obvious from the other traces, that any traffic on the soil after subsoiling increased the strength beyond the “as found” condition to 10 cm depth, while two passes mostly increased the strength beyond the “as found” to around 25 cm depth. Additionally, all the compaction treatments increased the strength of the soil beyond the “as found” condition below 25 cm depth.

So, what are the take-home messages from these results?

  1. The rubber tracks on the combine were not able to compensate for the 2 tonne heavier load of this machine and soil strength was increased beyond that of the wheeled combine.
  2. A tractor and trailer combination caused just as much compaction as a wheeled combine to 20 cm depth.
  3. Single passes of wheeled and tracked vehicles soon after subsoiling compacted the top 10 cm to a greater extent than its pre-loosened condition. Two passes similarly compacted it to around 20 cm depth. All vehicle passes compacted the soil to greater than its pre-loosened condition below 25 cm depth.
  4. What one does after subsoiling is just as important as getting the subsoiling operation right because just one pass of a modest-sized tractor can cause more compaction than was there in the first place!
  5. Post subsoiling is a good time to introduce a controlled traffic system or to tread extremely lightly.

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