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Research unravelling complex soil pH interaction

A trial in NSW is optimising ways to ameliorate soil acidity and free up phosphorus.
Photo: Ag Grow Agronomy Research

Lime-responsive and non-responsive soils have been targeted by NGN-driven research that is developing best-practice amelioration protocols.

Non-responsive soils

By Barry Haskins, Ag Grow Agronomy Research and Jason Condon, Charles Sturt University

In the lower central west of New South Wales, soil testing has revealed a gradual but persistent decline in pH. Historically, however, some of these soils have proven unresponsive to lime application and incorporation. GRDC investment has made it possible to ask why and already new knowledge is being generated on how best to manage soil-related yield constraints in this region.

The three-year project started in 2022 and is led by Ag Grow Agronomy Research, with Jason Condon at Charles Sturt University. The project also involves grower groups FarmLink, Holbrook Landcare Network and Central Farming Systems.

The trial is based at the Ag Grow Agronomy Research station near Griffith in paddocks with a wheat (2022), canola (2023), wheat (2024) and pulse (2025) rotation.

Central to advances being made is the observation that elevated phosphorus in these soils is not available for crop growth. The availability of phosphorus fertiliser is set by a pH-dependent process, with acidic soils known to lock up this important and expensive nutrient.

Trials have shown that liming the acidic soils of south-western NSW produces an increase in soil pH that increases phosphorus availability. Responses around seven to eight milligrams per kilogram Colwell-P (a measure of available phosphorus) have been observed in limed versus unlimed plots.

These trials involve liming at three different rates using four incorporation methods – shallow, deep once, deep twice and rotary hoe – to mix lime into the top 25 centimetres of soil.

The magnitude of the response from these early findings has the potential to reduce fertiliser phosphorus costs by 20 to 30 per cent – a significant saving given an average cost of $100 per hectare for starter fertiliser. There are additional economic benefits given the treatment also produces yield gains irrespective of phosphorus. Additionally, the researchers became aware that growers were not seeing a yield response to micronutrient fertiliser applied as a foliar spray despite crops being deficient in zinc, copper and molybdenum.

A preliminary analysis shows that liming also promotes uptake of these micronutrients, resulting in yield gains in this trial worth an extra 0.5 tonne/ha.

Ultimately, the goal is to provide growers in south-western NSW with clear soil amelioration guidelines that result in economic benefits by reducing input costs and closing yield gap potential.

Ongoing progress will be reported at fields days, industry events and online.

Removing the acid handbrake on crop rotations

Many growers in southern NSW are applying lime every eight to 10 years at a rule-of-thumb rate of 2.5 tonnes per hectare to raise the pH enough to stop aluminium and manganese toxicity affecting crops. However, local research found that while these practices have often been beneficial, they have not prevented the formation of acid layers from about seven centimetres to 15cm in some of the best cropping country. Despite applying lime, acidity continues to affect the growth of susceptible plants, especially pulses.

A change in sampling practices is making tactical lime application more effective. This involves sampling in 5cm depth intervals, to 20cm, to find the depth and extent of acidity. This then informs the rate and application method.

If acid layers exist in the top 20cm, they can be remedied by adding enough lime to remove the acidity by incorporating the lime to the depth of acidity. If such incorporation is not practical, then maintaining soil pH near 5.8 pHCa above the acidity allows the liming effect to move down the profile to the site of the acid layer.

Over the past four years, Associate Professor Jason Condon (Charles Sturt University) and Helen Burns (NSW Department of Primary Industries) have been working with collaborators from FarmLink, Holbrook Landcare Network and Central West Farming Systems to establish a network of trial sites testing the new pH targets (5.8 pHCa) and methods of lime application.

A series of workshops that demonstrate how to develop better liming strategies is planned for 2023 through each grower group.

Often the benefits of liming are not seen in the first few years after liming. It takes time for the lime to dissolve and correct acidity. Once that handbrake is removed, growers are no longer constrained to grow only acid-tolerant crops and can make the most of pulse options.

The GRDC-invested cropping sequence program demonstrated the financial and system benefit of pulses in cropping rotations. This is only possible if acidity is not constraining pulse nitrogen fixation.

More information:  Barry Haskins,; Jason Condon,

Responsive soils

By Sara Hely, Riverine Plains

Subsoil acidity is a prevalent geographical feature of soils in the Riverine Plains of the southern region. Most growers need to ameliorate soil pH through liming. However, many have reported mixed results and, due to the costs involved, are keen for trials that help optimise practices and maximise benefits.

Those trials, led by Riverine Plains, are now underway in a two-year project. It was due for completion in 2024 but was extended to 2025 due to wet conditions in 2021 that caused waterlogging, slug and disease issues.

One replicated field trial has been established to demonstrate best-practice liming strategies using commercial-grade farming equipment on soils that have been fully characterised.

There is also a field demonstration site that highlights the impact of using different types and sources of lime. Other variables being investigated include:

  • different incorporation methods – top-dressing, spading, discing and ripping;
  • the interaction of lime and moisture;
  • the depth of lime incorporation; and
  • the timing of the lime application.

Analysis from this field work will include comparisons of both agronomic and economic returns using Acid Soils SA calculator tools. Impacts are being assessed by monitoring yields and soil chemistry.

Growers have been consulted early in the design of the trial and, as a result, a broad range of well-targeted extension activities are included in this project, such as:

  • field days;
  • an annual trial book; and
  • a website of liming case studies.

More information: Dr Sara Hely,

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