Managing non-wetting soils in South West WA

 

With an estimated ten million hectares of sandy soil, Western Australian agriculture is at particular risk from water-repellent or non-wetting soils.

Sandy soils are more prone to this significant production constraint because the larger sand particles gives them a smaller total surface area compared to clay particles. This means they are more easily coated by waxy compounds, left behind after the breakdown of organic matter, which repels water from entering the soil profile. The outcome is uneven wetting, patchy germination and more weed pressure from delayed germination.

Already costing industry an estimated $251 million annually, the problem appears to be getting worse since the adoption of no-till farming, which concentrates organic matter at the soil surface. Smaller and less frequent rainfall events at the break of season are also likely to have an effect along with earlier sowing than in the past.

Tincurrin agronomist Clinton Mullan agrees that less autumn rain has probably contributed to the growing problem, but he sees the complete absence of tillage as another problem.

“I think it’s slowly getting worse, particularly on our sandy gravels, with our use of knife points and press wheels,” Mr Mullan said. “We’re not stirring the soil back up. I’m a believer in resetting the topsoil on certain soil types when you get the opportunity.”

Strategic tillage

Mr Mullan isn’t the only advisor promoting the use of strategic tillage to invert the soil and bury the water repellent layer below the soil surface. With trials running in the Kojonup area, Department of Primary Industries and Regional Development (DPIRD) research scientist Glenn McDonald believes that strategic tillage, whether with a Plozza plough, mouldboard plough (MBP), one-way plough or spader, is the most effective and profitable technique to deal with the issue on many soil types.

“The advantage of these operations is you do them once and get a long-term affect,” Mr McDonald said. “In long term trials we’re still seeing effects after eight years and counting.”

While tillage is often seen as detrimental to soil structure and soil carbon, evidence suggests impacts from a single event may not persist, and loss of carbon may be mitigated if the area has been previously limed. It is possible that in the long-term, soil inversion could increase soil carbon if it removes constraints and results in greater plant biomass production. However, given losses are often greater on sandy soils, which are most prone to repellence, and also given the effect of the same practice can vary from site to site, assessing the benefits with trials on different soils may be warranted.

One of Mr McDonald’s key principles for improving benefits of strategic tillage is to treat as many other soil constraints at the same time, such as subsoil acidity and compaction.

“The thing about lime is it doesn’t move very fast and can potentially take decades to get to the depth of a MBP (30-40 cm). So, if you’ve soil tested and have a lower pH at depth, applying a good chunk of lime up front before any strategic tillage means the lime will be transferred into the subsoil to treat acidity.”

Mr Mullan agrees, saying lime can have a significant impact.

“It helps with crop health, nutrient availability and root disease. Once you’ve got your pH right and nutrition balance better, you’re exposed to less frost risk. You won’t stop a big frost, but you can withstand the smaller events better if your nutrient balance is right.”

The choice of machine for soil amelioration can come down to cost and suitability for specific soil types. Mr Mullan has looked at several machines, but doesn’t think many are suited to our rocky, gravelly soils.

“For our sandy gravels, I think an old plough with Plozza discs gives us the best result long term. Hence why I’ve got a Shearer 5GP plough that’s halfway through getting converted. It’s slow and a bit more work compared to what a $180,000 machine will do for you, but for a $15,000 investment, I’m happy to slowly go around and fix our sandy gravels for a more long-term effect.”

Mr McDonald agrees, saying that while a MBP can handle roots and rocks, it tends to jump over them and lift them out of the soil whereas the Plozza tends to roll over rocks.

“The Plozza is more favoured in the forest country where there’s more roots and rocks because it leaves more of them below the surface.”

Nevertheless, according to Ben Webb, who farms west of Kojonup, dealing with those that do come to the surface and levelling out paddocks after tilling can be a challenge, and they can be an art to drive. Mr Webb started using a Plozza in 2019 and has ploughed 250 hectares of mainly gravelly duplex loam over clay soils. He thinks he got his money back on the investment, but would also like to compare it with claying.

“In the worst of the non-wetting [area], there is a very small percentage that is still a bit non-wetting. I think it needs some actual clay mixed in with it,” Mr Webb said.

Claying and clay delving are more costly techniques that some farmers have taken on, especially on the south coast, because it’s more of a permanent change to the soil.

However, Mr Mullan believes other constraints should be considered first.

“You need to know the constraints before you start spending money on claying. That goes for things like pH and potassium as well. Fix everything else first because that’s where most of your production is going to come from.”

The downside of the Plozza is that while the amount of soil inversion can be varied depending on the Plozza disc shape and plough angle, Mr McDonald says it can’t fully invert the profile like a MBP, so won’t completely bury herbicide-resistant weed seeds if that is an issue that also needs to be addressed.

“There will still be a thin strip of weeds if you maximise the inversion, so you need to think about what outcome you want.”

Another consideration is what might be bought up from the subsoil in the tillage operation.

“If the subsoil’s a bit toxic (e.g. sodic, high in boron), you don’t really want to bring that to the surface with any machine,” Mr McDonald said. “There’s been numerous examples where people have done that and the paddock’s gone backwards permanently.”

Mr McDonald also says the Plozza is generally cheaper to set up by modifying an old one-way plough and adding the Plozza discs, compared to purchasing a MBP. It also means you have your own machine to use when your conditions are ideal, rather than relying on a MBP contractor to be available at the right time. Given that water repellent soils are usually light, sandy and at a relatively high risk of wind erosion, getting the timing right can be important.

“None of the strategic tillage options are without environmental risk,” Mr McDonald said. “A lot of people try to do their amelioration before seeding. That has some big risks when it comes to things like erosion because you expose the soil to autumn winds.

“For those with livestock, they might be able to graze the paddock until late winter – early spring, ameliorate and then follow it up with a cover crop of some sort or a cash summer crop.  This later timing will be most relevant where wind erosion is almost certain. Sowing with a winter or early spring timing has a real place in the southern half of the state.”

While tillage with these machines can address compaction to 30-40 cm, deeper compaction is addressed with deep ripping. Mr McDonald generally recommends that any deep ripping is done after strategic tillage.

“If you go back over a deep ripped area with a spader or plough you’re just re-compacting the deeper area.  Also, a lot of strategic tillage works better if the soil hasn’t been pre-loosened. There are exceptions but that’s the general rule. The soil folds better with inversion ploughs if the soil hasn’t been ripped first.”

Soil Wetters

Where resetting the soil isn’t an option, Mr Mullan is using soil wetters to get the crop established as best he can.

“Wetters have gained a lot of traction in the past two years with the dry starts. Initially, SACOA brought out Lure H₂0^TM, and I think that still has a fit for farmers with pasture.”

One farmer who agrees is Simon Williamson from Kukerin who first used Lure H₂0^TM in Williams ten years ago and found it worked better when applied immediately prior to rain. In 2020, he used it on a paddock that normally doesn’t get going until August.

“I applied 10 litres per hectare with 190 litres of water in late May when rain was forecast. The paddock is 64 hectares and I applied it to 18 hectares. That year it was the first patch on the farm to get green and the sheep in that paddock ran on that 18 hectares most of the year.

“It was between rainfall events that you definitely saw a difference. It stayed greener and recovered better after rain. Our neighbour noticed it from a kilometre away. That was enough evidence for me.”

However, for crops, spraying the entire paddock including inter-rows is less economical, so SACOA developed a furrow wetter, as Mr Mullan explains.

“They looked at an on-furrow treatment, but that didn’t work if a wind event blew repellent soil on top. So, they came up with an in-furrow product called SE14^TM.”

Mr Mullan believes it’s an easy decision to use wetters on paddocks where 60-70% of the soil is non-wetting, more commonly found west of Narrogin and Williams.

“It’s a no-brainer, you just do the whole paddock.”

However, soil wetters aren’t effective on all soils. On Mr Mullan’s farm, only 20-30% of the soil has an economic response to the product.

“We ran trials with it here, measuring yield with the header. We saw some really good gains with the non-wetting soils, but as soon as we went off those soil types there was no economic gain. So, we were spending $22 per hectare for no benefit on those soils.”

To reduce costs, Mr Mullan realised he needed to get better at putting the product in the right place. So, he mapped out his non-wetting soils.

“If you are only treating part of a paddock, the investment is pretty good. I think out this way, it’s probably [the way] to go. For us, that brought the cost down to $4-5 per hectare.

Mr Mullan said that while some farmers might not think they know their soil types, he’s found that where they’ve had five- or ten-years’ experience with the farm, they get most of them right.

“I’ve sat down with a lot of clients over the years trying to map out soils and found they know them pretty well.”

Mr Mullan also suggested the thorium layer that comes with electromagnetic [EM] and radiometric mapping provides a pretty good representation of gravelly soils (non-wetting in most sandy soils), and this can help managers that are less experienced with a property. The potential of gamma radiometrics to map soil water repellency is the focus of a new project being undertaken by Murdoch University with funding from Soils Cooperative Research Centre.

Mr Mullan is also a strong believer in doing soil cores and soil tests to 30cm across his farm to get a better idea of what the constraints are.

“Some of our soils are sand to 50cm, so you’ve just got to accept that it’s a poor performing soil, or you’ve got to clay it. The problem is these soils without a clay base are limited with water holding capacity and yield anyway. So, you get the crop up, but they tend not to be the best performing soils.”

Mr Mullan thinks using SE14^TM to get the crop established could help to understand a particular soil’s productive capacity by effectively stripping away the non-wetting constraint to better understand its fertility and water holding capacity.

In-furrow wetters tend to be most responsive when sowing with marginal moisture, so another option to reduce cost is to not use it in wetter years. This is an approach used by Ben Webb who has been using SE14 for two years.

“If we’ve got a wet year we wouldn’t use it,” Mr Webb said. “Or we might start using it but once you get to the cereals towards the end of seeding, turn it off.”

Mr Webb also agrees that SE14 works better in gravels.

“The loamier the soil the less effective it is. Those soils generally hold the moisture a bit better.”

Like many farmers, Mr Webb tries to maximise moisture by retaining stubble and spraying summer weeds.

“Anything to keep moisture in the soil.”

The other consideration with using in-furrow wetters is the need to put another liquid system on the bar so the wetter can be placed close to the seed, separate from the UAN system which is banded a few centimetres below the seed to avoid toxicity.

“It costs roughly $10,000 on a 40-foot bar for plumbing,” Mr Mullan said. “Then you’ve got the tank and pumps as well as a truck and a tank to get it to the paddock. So, it’s not a cheap exercise, and running two tanks adds another level of complication.”

However, having a second liquid system can provide other benefits as Mr Mullan has found out.

“We’ve gone to a second liquid system anyway to place trace elements and fungicides with the seed. So it wasn’t that much of a step to start adding wetters.”

Another option which some growers appear to be getting away with is using the wetter with UAN and placing them both with the seed. However, Mr Mullan won’t be advising his clients to do that until more replicated trial work has been conducted.

“I can’t say it won’t work, but it’s a risk and there’s no backing for you if you smoke 2,000 hectares of crop. There will be certain conditions and soil types where you could get some toxicity.”

Near-row sowing

Another option to mitigate water repellency is on- or near-row sowing, placing the seed close to existing infiltration pathways made by roots from the previous crop. These pathways are maintained with a no-till system.

Soil moisture in these old rows also increases microbial activity in that part of the soil, including a large group of bacteria that degrade waxes and reduce repellency. These microbes can be present in far greater numbers under old rows compared to the dry inter-row.  Promoting these wax-degrading microbes may be assisted by liming, which is a source of nutrition and can create a more favourable soil pH for microbial activity if soils are acidic. Inoculation with the microbes in the field has been less successful, unlike pot-trials, while some other groups of microbes can induce repellency.

Mr Mullan says he has a few clients that have applied near-row sowing successfully using ProTrakker.

“Being able to sow two centimetres alongside last year’s row where rain will channel down last year’s root pores does give you a good crop establishment.”

Mr Mullan agreed that there is the possibility of human error with a ProTrakker and ending up with the crop too far from last year’s rows.

“You don’t want 100% of your crop 4cm away! You’ve got to have your RTK system in place and be happy with your run lines and what fences to work off before you can get the ProTrakker. Again, that’s a big investment to have all that done, and then it’s a $30,000 investment in a ProTrakker.”

An easier though less effective alternative to this system is cross-seeding, where run lines are angled across the previous season’s crop so some seeds will be placed on preferred pathways. However, a proportion of seeds will not be on preferred pathways, and increased traffic lanes will increase compaction risk.

A range of soil management options for cropping situations can be compared with the ROSA (Ranking options for soil amelioration) tool, developed by DPIRD.

In summary, farmers looking for cost effective solutions should consider testing proposed treatments on different soil types to see which ones respond, and then map these responsive soils to minimise costs. Also, consider whether other constraints such as subsoil acidity can also be addressed with strategic tillage. Given non-wetting soils are usually light and at risk of wind erosion, it’s also important to consider the timing of any strategic tillage.

This article was produced by South West Catchments Council’s Regional Agriculture Landcare Facilitator, through funding from the Australian Government’s National Landcare Program.

South West Catchments Council has no affiliations with any commercial services or products mentioned in this article.

Tags: Plozza plough SE14 water repellency