Nanneella broadacre farmer Alistair Gibson stands in a sinkhole in his ryegrass crop to indicate how far it sank during the growing season.
Farmers around Nanneella continue to be vigilant with looking for sinkholes in their paddocks as they begin to harvest lucerne and other crops.
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Sinkholes are generally rare in Australia but can occur frequently in the middle of crops in areas of the western Goulburn Valley dominated by sandy loam soils and deep-rooted plants.
Peter Gibson grows lucerne near Nanneella and believes no-one ‘really understands’ sinkholes.
“Well, I certainly don't anyway,” Mr Gibson said.
“The water just disappears into the ground — megalitres of water — you can pour it in and it just takes it.
“Where the hell is that water going?”
Mr Gibson said some sinkholes were deep enough to fit a tractor into.
“If you hit one — you just can’t see them — you’d just break your neck.”
Other nearby farmers also harvest with caution.
AgVic soil scientist Rebecca Mitchell said the phenomenon was usually related to activities very deep in the subsoil.
“Consider clay soils — when they get wet they can really change their structure, they can then become quite soupy and the top soil can heave down on it,” Ms Mitchell said.
“Or the clay soils when they dry, it can crack and come in on itself and then the sinkhole can be formed.”
Ms Mitchell said research by Grains Research and Development Corporation (GRDC) had been only anecdotal and had not found a consistent trend of soil type, land form or area where it occurs.
Some farmers have speculated an association between sinkholes and deep-rooted lucerne crops which can remove water from deep subsoil.
Agronomist Dale Boyd said historical research done by noted soil scientist Bernard Noonan in western Victoria looked for consistencies in the occurrence of sinkholes.
“He was just collating farmers' experiences and trying to see if there was consistencies, and then more recently (others) have been working with growers around Lake Charm. It probably has been associated with lucerne production as well.
“I guess there's a correlation between the deep-rooted heavy moisture extraction of lucerne and interactions with soil and creating voids.
“But its research is sort of anecdotal and environmental observation; it probably needs to have more time and money put towards it.
“It mostly creates issues when you’ve got a big stand and you’ve got to cut and you can’t see the surface of the soil.”
Ms Mitchell said sinkholes could also result from subsoil sitting on a free-draining limestone layer which could allow subsoil to slip down into it.
“We know it’s related to clay — we know it’s related to deep down,” she said.
Stories from local farmers describe abandoning dam constructions because of ‘bubbling’ in the soil and excavators being forced to then pat down and repair the soil surface.
Mr Gibson said sinkholes were ‘definitely’ an irrigation phenomenon and on a resulting canyon face he once saw the depth of lucerne roots.
“The lucerne roots go all the way down,” Mr Gibson said.
“We were irrigating some beautiful Hunter River lucerne, and as the water went out across it, you could see almost like a foamy sort of water coming up and bubbles coming up with the soil and then the next thing the soil would go — phoom! — down.”
“They seem to come along in lines, with a bit of a pattern along a ridge.
“We have a paddock we actually call the the sinkhole paddock, and it was bloody hard to fix them.
“If only we could dig really, really, really deep to just see what it's doing.”
A water-filled sinkhole drops down in a pasture paddock on Peter Gibson's farm at Nanneella.
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