New Mallee trial investigates the relationship between Nitrogen and Soil Carbon

A new research project, being undertaken by Birchip Cropping Group (BCG), is investigating ways to improve crop biomass production and yield on the poor performing sandy soils common across the Mallee.

The project, supported by the Mallee CMA, through funding from the Australian Government’s National Landcare program, is investigating the relationship between increased biomass production through targeted nitrogen (N) applications and whether these practices can improve soil carbon levels under local conditions, and if so, the degree to which this can be achieved. The project is being run in parallel with a secondary project with support from the Wimmera CMA which is investigating the impacts of a variety of soil amelioration techniques including deep ripping, claying and manure spreading on crop biomass and yield.

“Every farm has patches that for some reason, year after year, fail to perform as well as the rest of the paddock. Even with adequate nutrition, disease and weed control” BCG Research Agronomist Kate Maddern explains “crops grown in these patches often don’t produce as much biomass during the season, which can carry through to lower yields and/or poor quality at harvest.”

Poor performing patches can be due to underlying soil constraints, such as differences in water-holding capacity, a compaction layer, a difference in soil type, or other issues such as sodicity, nutrient deficiencies, salinity or acidity/alkalinity and nitrogen deficiency. Research also suggests that depleted levels of soil organic carbon can also be a contributing factor in crop performance.

“Soil organic carbon is an important contributor to the chemical, physical and biological fertility of soils. Increasing soil organic carbon can help to increase nutrient availability, help to improve soil structure and water-holding capacity and stimulate the growth of beneficial soil micro-organisms” Ms Maddern continued “However, when lower plant production is combined with consistent organic matter removal, through hay cuts, stubble burning or harvesting, soil organic matter and soil organic carbon, the little bits of plant matter in the soil and the carbon derived from that plant matter, can be depleted”

Applicable Learnings from the Wimmera CMA Soil Amelioration Trials

In 2019, BCG, supported by the Mallee and Wimmera CMAs, through funding from the Australian Government’s National Landcare program, developed a trial to investigate whether soil constraints could be addressed, and biomass improved, through a combination of addressing sub-soil constraints and whether these practices could increase levels of soil organic carbon. A range of commercial practices were investigated, including clay spreading, deep ripping, fertiliser and manure spreading.

The early results of a trial conducted on a poor section of an otherwise productive paddock, near Lubeck in the Wimmera, shows the potential of soil amelioration for Mallee farmers. The trial was located on a sandy gravel rise typical of a Wal Wal Sand, whereas the rest of the paddock was a higher performing black/grey clay loam (Figure 1).

Figure 1: Location of trial (black outline) on 2018 yield map generated on the header, where green is higher yielding and red is lower yielding.

The trial showed promising early results, with clear differences in biomass and yield being seen between the different treatments throughout the season.

Plant establishment was significantly impacted in those plots that received the clay spreading and deep ripping treatments – unripped treatments showed higher plant establishment than the ripped treatments. Those plots spread with clay also showed lower plant establishment than those that were not (Figure 2).

Figure 2: Plant establishment (p<0.001, LSD=34.25, CV=26.7% (95% confidence)) across treatments, displayed with standard error bars. Different letters indicate significant differences.

Applying 20 t/ha of manure significantly increased biomass (confirmed by NDVI data) and yield when compared to the control, ripped and clayed treatments. However, manuring also increased protein and screenings in the wheat quality, indicating that the bulkier biomass had ‘hayed off’ in the dry spring, as the water holding capacity of the sandy soil proved too low for the crop to access adequate moisture to finish (Figure 3)(Figure 4).

Figure 3: Visual differences in biomass between treatments at 2/9/19.
Figure 4: NDVI (greenness) response between treatments at 7/8/2019 (p<0.001, LSD=0.040, CV=8.1% (95% confidence)), displayed with standard error bars. Different letters indicate significant differences.

Ripping resulted in lower yield than the unripped treatments, with some differences between treatments being statistically significant (Figure 5). This could potentially be due to the reduced plant establishment seen in the ripped treatments. However, there was no yield penalty associated with the reduced plant establishment and the clay treatments.

Figure 5: Yield (t/ha) across treatments (p=0.008, LSD=0.944, CV=28.2% (95% confidence)), displayed with standard error bars. Different letters indicate significant differences.

A more detailed report on the results of the trial can be found here.

Overall, the trial demonstrated that the increase in biomass and yield from different soil amelioration techniques varies and does not provide farmers with a one-size-fits-all approach. It confirms that determining which soil amelioration technique best fits your farm and farming system requires an in-depth understanding of the local soil types, the soil constraints present and their impacts on production.

The trial also identified several considerations that farmers exploring deep ripping, claying or manuring should factor into their decision making:

  1. When undertaking amelioration that alters the seed bed or sowing depth (claying, ripping, delving, spading etc.), there is a risk of poor plant establishment the following season. To minimise the risk of poor establishment, consider cultivating, rolling or incorporating prior to sowing. Choosing a crop type and/or variety with longer coleoptile length may also help to mitigate the risks associated with variable seeding depth.
  2. When ripping or claying, the removal of organic matter from the surface may make herbicides ‘hotter’, increasing the risk of crop phytotoxicity. Pay careful attention to labels to prevent this from occurring.
  3. Alternatively, depending on the mode of action, placing large amounts of organic matter on the soil surface, such as chicken litter, may lead to herbicides binding the organic matter rather than reaching the target, potentially reducing herbicide efficacy.
  4. Take steps to ensure that the amelioration technique being utilised doesn’t cause a larger problem than the one you are trying to resolve. Undertake testing to ensure that ripping or claying do not bring sub-soil acidity, salinity or toxicities such as boron to the surface, where they can have a bigger impact than if they were at depth. Likewise, chicken manure varies from shed to shed. Testing the clay, manure or sub-soil through a laboratory before ameliorating ensures that you are aware of any potential issues – for example, the sample tested for this trial contained salt and boron.
  5. If the laboratory results come back clear, consider ameliorating a section or strip in a paddock to ensure that this is the best technique for your soil type, and that you are made aware of any potential issues before conducting the amelioration on a larger scale.
  6. While this trial saw a negative response to deep ripping in the first year, other trials have seen a positive yield response to deep ripping. When considering whether or not to deep rip, consider where you are most likely to see a positive response: responsive soil type (sands are more responsive than loams and clays), compaction is an issue at a depth that can be ripped, no other major soil constraints present, having adequate stored soil water and that the machinery is physically able to rip to the required depth.

Establishing a Nitrogen trial in the Mallee

BCG, with the support of the Mallee CMA and LaTrobe University Associate Professor James Hunt, have finalised the design of a research trial in the Mallee to investigate the application of nitrogen (N) as it relates to soil carbon. The trial has been running for two years and is beginning to show interesting results, and the support from Mallee CMA will allow the trial to be continued for another four years.

“Research undertaken in 2018 determined that nitrogen underapplication is the biggest contributor to Australia’s wheat yield gap. It is estimated that Australia currently produces 55% less wheat than it could with our current rainfall due to poor agronomic practices, with 40% of this yield gap being due to underapplication of nitrogen fertilisers.” Ms Maddern explained when outlining the need for increased research “Due to the low amounts of nitrogen in the soil, if not enough nitrogen fertiliser is applied, crop yields will be lower than they should have been. Applying too much nitrogen can also cause problems, due to the increase in the cost of production and the increased risk of ‘haying off’ the crop in a drier season.”

“This trial will investigate different nitrogen application rates and strategies that could be used as a tool to increase soil organic carbon, plant biomass and crop yield in Mallee farming systems.”

Trial Objectives

Over the next four years, this trial aims to determine:

  • Can nitrogen application rates and strategies increase soil carbon?
  • Can nitrogen application rates and strategies increase crop biomass? 
  • Can nitrogen application rates and strategies increase crop yields?
  • Can nitrogen application rates and strategies increase farmer profitability? 
  • Can a ‘nitrogen bank’ strategy provide farmers with a tool to decrease the complexity of making nitrogen decisions and better manage nitrogen in their farming system?

“Results from the research will be extended through a series of workshops and events will be held over the life of the project to share the results with Mallee farmers and answer any questions they may have – these will be promoted on the events page of the BCG website, through member communication and local and regional media” explained Tom Draffen, BCG Senior Manager Extension and Communication “however due to the restrictions on gatherings implemented during the COVID-19 outbreak in Australia, BCG have enacted a continuity plan in early March to ensure the success of our trials program – due to this the extension events planned for BCG Research Trials will be delivered as Webinars and Digital Workshops until restrictions are eased.”

Information regarding upcoming events can be found here.


This article is supported by Mallee Catchment Management Authority, through funding from the Australian Government’s National Landcare Program.

The trial discussed in this article is supported by Wimmera Catchment Management Authority, through funding from the Australian Government’s National Landcare Program.  

Thank you to Mayo Park Farms for hosting the Wimmera trial and for providing machinery and time to implement the treatments.

Thank you to Paul Barclay for hosting the Mallee trial.

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