Read part 1-2, Curtain call on soils.

Part 3: Acidity

The challenge

With soil acidity one of the major yield constraints for WA growers, solutions to address this ever-increasing challenge are critical for WA’s grain industry, particularly in the central and eastern wheatbelt.

DPIRD research officer Dr Gaus Azam and his team have been working to understand soil acidity, its link to aluminium toxicity and the combined effect on plant root growth.

He says soil acidity increases when land is cleared; when paddocks are continuously cropped, particularly with legumes in the system; and through the regular use of ammonium-based fertilisers.

“When topsoil pH levels are below 5.5, we don’t see any improvement in the soil pH in deeper levels and subsurface acidity becomes an issue,” he says.

When subsoil pH levels are below 4.8, aluminium becomes toxic to plant roots.

Aluminium becomes toxic to more sensitive canola and barley at levels of 2-5mg/kg, and to most tolerant wheat above 5mg/kg. Excess toxic aluminium affects root cell division and deforms the root tips, arresting root elongation. This makes them unable to extract deep soil water or nutrients.

Typically, high aluminium levels in the subsurface cause the plant to shut down as roots reach 8-15cm below the surface.

Dr Azam says many WA growers continued to apply lime, without incorporating it, for many years but have not seen yield responses immediately, as they hoped for.

“Growers have hoped rain events would move lime through the soil profile, but we now know lime can sit on the surface for many years without moving,” he says.

“In fact, with the acidic soil pH levels which growers are now faced with, it is now our understanding that it will take decades for lime to move through the profile without intervention.”

The research

Eight trials in the central and eastern wheatbelt looked at lime applications on sandy soils, with and without incorporation.

Other research investigated various amelioration methods, in different soil types, to move the lime through to the subsoil to reduce acidity and remove the toxic aluminium.

Trials compared lime application without incorporation and with deep incorporation, one-way ploughing, and spading. Separate trials also investigated the interaction between lime and gypsum in these acidic soils.

Dr Azam says some growers are not comfortable with any cultivation method and believe adding gypsum to the lime application will move the product trough the soil profile much quicker.

“We wanted to test that idea,” he says.

“Given gypsum does not increase soil pH (it might slightly acidify some sandy soils if applied at a higher rate), this was an interesting trial and something that needed close monitoring.”

The outcomes

A trial in Wongan Hills, where a total of 8.5t/ha of lime was applied at the surface in three separate applications over 23 years showed 40 per cent  of the lime was still sitting on the topsoil at the end of this 23-year period.

The unincorporated lime allowed for a slight improvement in the subsoil pH levels, which translated to a wheat yield improvement in 2018of 750kg/ha after the 23 years. But, a 250kg/ha additional yield increase was achieved in just one season simply by incorporating the lime that had previously been applied.

Separate trials in Kalannie and Merredin, comparing different incorporation techniques, highlighted shallow ploughing did not create any significant immediate yield response.

Dr Azam says best results in these incorporation trials were from deep incorporation – at 250mm or deeper.

Results from the lime and gypsum combination trial at Kalannie showed up to a 30 per cent yield increase when both compounds were applied and incorporated.

Dr Azam says while gypsum can only be used on suitable soils, the trials showed decreased effects of aluminium toxicity in the soil and an increase in plant ability to take-up water and nutrients.

Take-home messages

• If you farm in a low-rainfall area with sandy soils, it is likely your soil will have low pH and high aluminium levels.
• Soil testing is critical to determine soil buffering rates, which will determine lime application rates.
• Invest in lime applications with incorporation, but be conscious of wind erosion at critical times of the year.
• Consider deep tillage incorporation methods over shallow tillage.

Part 4: Sodicity

The challenge

Found predominately in heavy soils in lower-rainfall regions, sodicity restricts a plant’s ability to access water, thereby reducing crop yield potential. According to DPIRD senior research scientist David Hall, sodicity is the accumulation of sodium and salts in the soil profile.

Excessive sodium causes clay to disperse, clogging soil pores and restricting water infiltration, drainage and root growth. The salts reduce water availability to crops.

While rising water tables as a result of land clearing have long been considered the main cause of salinity in WA soils, scientists are now also acknowledging the widespread impact of transient salinity, or cyclical salt in the soil, caused predominately by the salts contained in normal rainfall events accumulating in the soil over many decades.

While sodicity can also be a problem in higher-rainfall areas, the biggest yield impacts are felt in heavy clay soils in lower-rainfall regions, particularly during low decile rainfall years.

“Sodicity causes soil structure to decline and leads to the development of transient salinity over many years,” Mr Hall says.”

“Innovative solutions that stabilise these soils are required to ensure plant health in the long-term.”

The research

During the past five years, through Soil Constraints West, many research trials across the eastern and southern wheatbelt have investigated the impact of innovative solutions to the challenge of sodic soils.

Trials considering the impacts of soil amelioration strategies, such as delving and deep-ripping, focused on breaking up the soil profile to create water and root pathways in these clay soils.

“Heavy clay soils respond very differently to certain soil amelioration techniques when compared to sandy soils,” Mr Hall says.

The trials assessed effects of breaking through the heavy clay dome and bringing that clay up to the top layer of the soil.

Other trials considered the application of gypsum, mulch and the injection of manures into the soils.

While adding organic matter to a broadacre business is an expensive solution, Mr Hall says the project left ‘no stone unturned’ in attempts to find ways to stabilise sodic soils.

Other trials assessed water harvesting by creating mounds and furrows during the sowing process. Further research compared saline soils due to rising water tables to sodic soils due to transient salinity.

The outcomes

Mr Hall says the research demonstrated that gypsum is a useful resource for growers with sodic soils that have an exchangeable sodium percentage of 10 or above.

“The simple solution to improve sodic soils is to replace the sodium with calcium,” he says.

“Gypsum is predominately calcium and is very mobile through the profile, which move the sodium off the exchange sites and improves soil structure.”

But not all soils are responsive to gypsum, and growers must test to ascertain their exchangeable sodium percentage in the top 30cm of the soil.

This research also considered the impact of acidifying alkaline soils to combat the effects of the sodium through improved soil structure.

The application of elemental sulfur, dissolved at a rate of between 30 and 60 per cent, effectively converted the calcium carbonate contained in these soils to gypsum. But Mr Hall says understanding the chemistry involved in this strategy is critical.

“In some of the trials, we did see the pH levels change from 8.5 down to 3.9, which will obviously cause different soil constraint problems, but the concept of acidifying these soils, even slightly, to reduce their sodicity was promising,” he says.

Take-home messages

• Soil testing is imperative to determine your soil type and structure.
• Understanding the results is important to ensure the best strategies are employed to re-engineer your soils.
• When considering gypsum application, you must know the percentage of exchangeable sodium in the top 30cm of your soils.
• Optimal gypsum application rates are 3-5t/ha for clay soils.
• High yield increases from mounds and furrows usually only occur in lower-rainfall years.
• Mulches may see big yield increases but are potentially uneconomical.

More information: Chris Gazey, chris.gazey@dpird.wa.gov.au