Soil compaction in WA is estimated to cost $330 million in lost production annually.
But that loss is mostly associated with cropping regions. In contrast, there has been little research into the impacts of soil compaction in the high rainfall zone (>600 mm annual rainfall).
It’s an issue worth investigating since higher rainfall often equals shallow root growth and that could have implications for nutrient use efficiency, soil carbon sequestration and water logging.
With this in mind, we went back and reviewed our previous investigations into compaction in the high rainfall zone to see what learnings could be gained.
Our 2019 compaction survey
South West NRM surveyed 21 high rainfall livestock farms west of Boyup Brook for soil compaction in 2019.
We hired a digital penetrometer that recorded penetration resistance down the soil profile every 25 mm to a depth of 300 mm, taking an average of 12 insertions per site over 39 sites.
We did the same under adjacent fencelines that had not been exposed to machinery, vehicles or livestock.
Our results suggested that almost 50% of sites were compacted enough to restrict root growth below a depth of 100 mm. Compaction increased slightly with depth for sandier soils, whereas compaction declined below 100 mm depth in clayey soils.
Fencelines tended to be significantly less compacted for both soil types and most depths, except in clayey soils below a depth of 150 mm. This suggests that compaction in clayey soils is on the surface and probably caused by livestock, whereas compaction is deeper in sandy soils and more likely caused by machinery movement.
There was no obvious correlation between compacted soil and lower productivity from our observations.
UWA research
Our observations mirrored earlier research from UWA’s Dr Mattias Leopold.
He investigated a Busselton property where testing had indicated increased pasture productivity following deep ripping. He found a compacted layer between 150 mm and 300 mm, and concluded that machinery and frequent grazing by cattle had compacted the shallow subsurface.
He suggested caution should be taken using heavy machinery on these sites and strongly recommended reducing tyre pressure. He suggested that while deep ripping appeared to help, current practices were likely to cause a return of the problem.
Treating compaction with aeration
Due to concerns about bringing up rocks, roots or acidic soils to the surface, we found most farmers in the high rainfall region don’t treat compacted soils.
Some operating on sandy coastal soils were using aerators to treat compaction to a depth of 200 mm, but drone imagery failed to detect an obvious response to aeration compared to non-aerated areas in the same paddock.
We sought expert advice on the matter and the suggestion was that the tractor pulling the aerator may have caused more compaction than the aerator was fixing!
At a separate site, aeration appeared to produce an increase in germination due to increased infiltration on water repellent soils.
Treating compaction with deeper rooted perennials
One impact of soil compaction is of course restricted water infiltration and water logging.
We’ve received anecdotal evidence from farmers in the region that deeper rooted perennial species such as chicory reduced flooding – potentially due to better infiltration caused by roots. Others have also seen improved drainage from deep ripping, especially on clayey flats.
One prominent researcher has suggested that combining deep ripping with the establishment of deeper perennials may help reduce the rate of compaction recurrence.
Treating compaction with deep ripping
In 2023, we were given the opportunity to investigate the issue further by incorporating deep ripping into a trial run in Busselton.
The trial site, very close to the UWA research site, had deep sandy soil with significant compaction, and the farmer had observed improved production from ripping elsewhere on the farm in the past, with the effect lasting for up to five years.
Our trial aimed to address several soil constraints at the same time, so while replicated plots were used, the effect of deep ripping was not observed in isolation to other treatments like liming for soil acidity.
However, results showed a significant reduction in compaction in areas ripped to 400 mm. Other treatments used perennials and other deep-rooted species to treat compaction, but no change in compaction was recorded (although this was a single year trial).
Unfortunately, the site didn’t lend itself to observing the effects of ripping on waterlogging. Also there was no evidence that deep ripping increased access to the water table and extended the growing season, but this may have been due to the early finish to the season.
Similar research in the cropping region in the same year found that in dry seasons, deep ripping could actually be detrimental, because increased root growth could dry out the soil profile quicker. This was seen anecdotally on a nearby property as well.
An assessment of root mass in the 10-20 cm zone of trial plots suggested that root mass was higher in the ripped plots (not statistically tested).
If true, this suggests that deep ripping may help to increase root growth and therefore soil carbon at this depth.
Concluding thoughts
Safe to say, soil compaction has a big impact on productivity, and this seems to extend into high rainfall regions.
There is some mostly anecdotal evidence that treating compaction provides an economic return for high-rainfall livestock producers, but other potential benefits also exist, including improved nutrient use efficiency, increased soil carbon (which could buffer compaction) and reduced waterlogging.
Farmers looking to trial deep ripping and/or use of perennials to treat compaction should leave at least three areas in the paddock untreated to increase confidence that any noticeable affect is the result of the treatment and not another factor.
Another option is to control traffic movement around paddocks especially with heavy machinery, and reduce tyre pressure in wet conditions.
And if deep ripping, try to rip behind the wheels.
