Identifying the Causes of Unreliable Nitrogen Fixation in Medic Based Pastures


Research organisaton
Funding source

Trial details


The broad aim of this 3 year SAGIT funded project was to investigate if current management tools for medic based pastures, such as herbicides, fertilisers and rhizobial inoculants, are affecting N fixation by medic pastures under field conditions typical of the upper Eyre Peninsula. These results should also be relevant to other low rainfall Mallee systems where medics are used.

Annual medics (Medicago spp.) are self-regenerating legumes that are well suited to crop rotations on neutral to alkaline soils in the low to medium rainfall areas of southern Australia. They provide highly nutritious feed for livestock, act as a disease break for many cereal root pathogens, and improve soil fertility through N fixation.  However, it appears that some of these pastures are not providing sufficient N reserves (as mentioned at local farmer meetings) for the following cereal crops, even where the medic has been quite productive. The longer term decline of protein levels in cereal crops are of concern. Medic pastures are now often sprayed with a range of herbicides and pesticides, both to ensure its productivity as pasture for livestock, and to ensure that minimal weed seeds are carried into the following cereal crop. This project examined if commonly used management strategies reduced N fixation by the medic pasture, and consequently mineral N supply to the following crop.

Key messages
  • Applying phosphorus to a soil with low P reserves when establishing a medic pasture boosts shoot and root dry matter, improves root health and improves N fixation.
  • Residues of the herbicide Logran can severely stunt medic growth.
  • Applying a full label rate of Agritone 750 (late) decreases pasture production and N fixation in actively growing pastures.
  • Any management practice that reduces medic biomass will reduce N fixation, such as the late application of certain herbicides, eg Agritone 750.
  • In a dry growing season when medic plants are already moisture stressed, herbicides will have little impact on medic production and N fixation.
  • Regardless of seasonal conditions, the management of newly established medic pastures with regard to herbicides and nutrition, appears to have no impact on the yield and protein levels of the wheat crop in the following year.
Lead research organisation SARDI Minnipa Agricultural Centre
Host research organisation N/A
Trial funding source SAGIT 515
Related program N/A

Thank you to Greg Scholz, Brent Cronin and their families, for allowing us to conduct the trials on their properties and their support of research; and Andy Bates (Bates Ag Consulting) for his valuable input.

Other trial partners Not specified
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Crop type Medic
Treatment type(s)
  • Fertiliser: Type
  • Herbicide: Type
  • Herbicide: Timing
Trial type Experimental
Trial design Replicated

Pingbong 2017

Sow rate or Target density 60 kg/ha wheat
Sowing machinery

In the 2017 season, the 2016 medic trial sites at Pinbong and Piednippie, were sown with Scepter wheat.

Sow date 18 May 2017
Harvest date 13 November 2017
Plot size 6m X 1.5m
Plot replication 3
Paddock history 2016: Medic, 2015: Barley, 2014: Mace wheat.
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Trial source data and summary not available
Check the trial report PDF for trial results.
Observed trial site soil information
Trial site soil testing
Not specified
Soil conditions
Trial site Soil texture
Pingbong, SA Red sandy loam
Derived trial site soil information
Australian Soil Classification Source
Trial site Soil order
Pingbong, SA Calcarosol
National soil grid Source
NOTE: National Soil Grid data is aggregated information for background information on the wider area
Actual soil values can vary significantly in a small area and the trial soil tests are the most relevant data where available

Soil properties



Pingbong SA 2017

Observed climate information

Rainfall avg ann (mm) 321mm
Rainfall avg gsr (mm) 227mm
Rainfall trial total (mm) 307mm
Rainfall trial gsr (mm) 150mm

Derived climate information

Pingbong SA


Some data on this site is sourced from the Bureau of Meteorology

SILO weather estimates sourced from
Jeffrey, S.J., Carter, J.O., Moodie, K.B. and Beswick, A.R. (2001). Using spatial interpolation to construct a comprehensive archive of Australian climate data , Environmental Modelling and Software, Vol 16/4, pp 309-330. DOI: 10.1016/S1364-8152(01)00008-1.

Trial report and links

2017 trial report

2017 trial report

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Trial last modified: 24-04-2019 12:32pm AEST