South West

Better Soils And Potatoes – Whole System Approach To Potato Quality

This trial is now complete

Key Messages

  • Managing cover crops to increase soil organic matter is fundamentally different to cover crops for weed, disease or erosion control. The focus must be on maximizing biomass production. So growers need to consider seeding rates, fertiliser rates and even partial irrigation.
  • Cultivation caused an overall decline in soil carbon at some sites. While cultivation is an integral part of the industry, it can get over-used and must be rationalised in order to build soil carbon.
  • Considering all parts of the system, such as using brown manuring instead of bare fallow, will also help to conserve soil carbon.
  • Trial Rationale
  • Steps Taken
  • Lessons Learnt
  • Looking Forward

Trial Rationale

The Busselton-Marybrook Grower Group formed in 2014 with a shared vision to improve the quality of their potatoes and improve profitability. Group members believe potato quality is linked to soil health, with poor health causing plant stress and exposure to disease.

From 2014 to 2017 the group partnered with Murdoch University and the South West Catchments Council (SWCC), through funding from the Australian Government’s National Landcare Program to trial high-biomass cover crops aimed at increasing soil carbon, the measurable component of soil organic matter and a key indicator of soil health.

Busselton potato growers on sandy soils typically have soil carbon levels of 1.5-2%. The same soil types under nearby pasture typically have levels around 3.5%, suggesting an increase is attainable.

Soil organic matter (SOM) is beneficial to crop growth in several ways. Like clay, SOM is negatively charged, so can hold cations like potassium, reducing loss to leaching. It can also hold several times its own weight in water, improving resilience in times of moisture stress.

The benefits of SOM to soil structure are linked to its capacity to support soil organisms.

Crop residues and exudates from living plant roots are decomposed by bacteria and fungi (microbes), providing them with energy to grow and multiply. A side-effect of this activity is a sticky substance (glomalin) produced by fungi that glues soil particles into aggregates, the building blocks of soil structure. An increase in crop residue and associated microbial activity provides a food source for larger soil organisms that tunnel through the soil, improving drainage as they go. Soil organisms also store nutrients which are mineralised when they die.

Steps Taken

An oat cover crop was trialled on two farms and a caliente cover crop on a third farm, with the aim of increasing soil carbon compared to the standard rotation into ryegrass and clover.

Lessons Learnt

One of the early lessons learned from their trial was that cover crops grown to produce high levels of biomass need to be treated like a cash crop, as project coordinator Steve Milroy from Potato Research WA (Murdoch University) explained.

“Managing cover crops to increase soil organic matter is fundamentally different to cover crops for weed, disease or erosion control. The focus must be on maximizing biomass production. So growers need to consider seeding rates, fertiliser rates and even partial irrigation.”

Daniel Taylor and his father Keith grow sorghum, lupins, caliente and oat cover crops and use irrigation to establish them where they can. This practice is now catching on with others.

“We irrigate for a month to get the sorghum going before we need to move the irrigation,” Daniel says. “Sorghum is probably our biggest gain and we should put more effort into it with irrigation.”

The grower that instigated the project, Vaughan Carter, says it’s not enough to rely on left over nutrients to fertilise cover crops.

“You’ve got to maintain a good fertiliser regime to get the bulk.”

He also says he’s realised the importance of choosing the best cover crop.

“If you’re going to use an oat, choose a good one, not just anything. Find one that works in your country and doesn’t host disease.”

Other multi-benefits of cover crops were discussed at the group’s 2017 field walk by Murdoch University Professor Richard Bell.

“Species like sorghum, that produce deeper roots, will help to keep better structure deeper in the profile, reducing the risk of subsoil compaction and waterlogging,” Professor Bell says.

“Growers shouldn’t underestimate the importance of the root systems for building soil structure. Roots have more lignin so more is converted into humus and preserved.”

Cover crops typically don’t need to be incorporated into the soil. Professor Bell believes it’s probably better to mulch or a knock-down the crop with crimping and leave it on the soil surface.

“You’re keeping the surface cool, it’s in contact with microbes in the soil that help decomposition, and you’re not causing massive soil disturbance which runs against the goal of developing soil structure.”

Keeping the soil covered and protected from raindrop impact may also reduce surface crusting.

Unfortunately, soil disturbance is unavoidable in cropping. This means that gains in organic matter from cover cropping, along with any aggregate formation, can be easily lost.

“Basically, every time you disturb the soil you speed up the breakdown of organic matter,” Professor Bell says. “The more disturbance, the harder it is to maintain carbon levels.”

Steve Milroy said that cultivation caused an overall decline in soil carbon at some sites.

“Cultivation is an integral part of the industry, but it can get over-used. Rationalizing cultivation may have a greater impact in managing soil organic matter than efforts to increase biomass inputs. So we need to pay more attention to cultivation.”

While Mr Carter says his heavy clays were more friable after an oat cover crop, he agrees that cover cropping is just one part of the solution.

“I think the project has shown that we can’t achieve the desired outcome by changing just one aspect of our cropping system. We need to look for improvements across the whole system, and that includes the number of cultivations and also the methods we currently use.”

Considering all parts of the system, such as using brown manuring instead of bare fallow, will also help to conserve soil carbon. Other practices, such as traffic control, improved nitrogen use, integrated pest management, optimised water management and a diverse rotation sequence will also improve general soil health.

But cover cropping with minimum tillage is perhaps the first step for these potato growers. The Taylors have been experimenting for many years and say that where they have been cover cropping the longest, their soils are in the best shape they’ve been for a long time.

“All we’ve seen in soil tests is a very slight upward trend in organic matter, but it’s had an exponential effect on the plants and their ability to pick up nutrients,” Daniel says.

“There’s no downside except a small overhead cost. As far as we know, it’s paying us back 5, 10 or 20 times.”

Looking Forward

Grower Vaughan Carter said that he will adopt cover cropping.

“If I am going to be planting 20 hectares, I’ll be doing 20 ha of cover crop.”

Busselton

Project Snapshot

Land Manager
Vaughan Carter (Busselton-Marybrook Grower Group)
Shire:
Busselton
NRM Region:
South West
Property Size
40 ha
Average Rainfall:
Very High - More than 750mm
Enterprise Mix:
Potatoes and Grazing
System Constraints:
Soil Organic Carbon, Soil Structure Decline
Partners:
Murdoch University, NLP, SWCC
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