Contributions and constraints of stubble to nutrient supply in relation to stubble and soil analysis

Key messages:TrialsBookCover

  • Trial was infected by high numbers of ARG and was sprayed out prior to harvest to prevent weed seed set in the farmers paddock
  • There were no significant differences in dry matter production for any of the treatments applied
  • Soil analysis is currently being conducted to determine soil moisture, nutrient and carbon levels in 2015 following treatments in 2014.

Stubble retention can have serious impacts on nutrient supply. High percentages of nutrients contained in stubble are lost when stubble is burnt or removed. Stubble breakdown is slow and can delay nutrient availability to subsequent crops. Guptar, VSR has conducted trials across Victoria, South Australia and NSW to determine nutrient levels of various crop residue and related biological activity.

Table 1: Typical amounts of nutrients contained in various stubbles. Sourced from GRDC web site, GRDC Update papers, Biological and nutritional value of stubble. VSR Gupta.

Stubble type C N P S K C:N N:P N:S
  Kg / one tonne of stubble
Wheat 400 5 0.4 1 13 87 12 4
Canola 400 6 3 3.2 30 91 3 2
Lucerne 400 26 2.1 2.2 28 15 13 12

From Table 1 a wheat stubble of 5t/ha can potentially provide 25 units of N. Time taken to release these nutrients will vary depending on a range of factors including environmental factors, soil type and residue quality all of which have strong influence on biological activity.

Clive Kirkby, CSIRO has proven in several studies that it is possible to increase humic organic carbon fractions of a soil, (this is where a high percentage of mineralisation of N is sourced) by providing balanced quantities of N, P & S in the ratio of 6N:2P:1S for every ton of stubble retained. Subsequent research and modelling has determined that a 1% increase in humic C can provide an additional 33kg/ha of N at Temora in NSW. This additional N gave modelled yield increases of 1.8t/ha at the 50% level of probability. Surprisingly there was only a very small rise in PAW capacity in this soil as a result of this humic C increase.


Figure 1: Modelled yield increases related to soil organic carbon levels at Temora NSW. Sourced from GRDC web site, GRDC Update papers, Soil organic matter, what does it mean to you? Van Rees et al.

SFS in consultation with Clive Kirkby and John Kirkegaard, CSIRO Canberra, designed and implemented a trial testing the impacts of balanced nutrients N, P & S, applied according to stubble dry matter levels, compared to the impacts of balanced nutrients applied in relation to stubble levels and stubble nutrient content analysis results. The aim was to test the potential of soils in this region to increase organic carbon levels and subsequently N mineralisation and yield with the addition of nutrients determined by stubble level, stubble nutrient analysis and soil analysis.

Additionally SFS used 2 different compounds as sources of applied nutrients:

  1. Liquid
  2. Granular

The HRZ environmental conditions of the western districts of Geelong are considered very favourable for humic C increases. The 2014 season however was well below average for rainfall and this may have had a detrimental effect on biological activity and stubble break down.

SFS gratefully appreciate and acknowledge the contributions of Myles Read and family and Swayne and McCabe in the running of this trial.

Seasonal conditions

2014 was a Decile 1 year. Average annual rainfall in Westmere is 530mm but the trial site received only 370mm. Growing season rainfall (GSR) was a Decile 2, with Westmere receiving only 300mm from April to November compared to a long term average GSR of 400mm. A large portion of this rain fell in winter with spring rainfall well below average. Below average rainfall was recorded in every month except April and June.

Agronomic information

Table 2: Agronomy for 14 GRDC STUB 16 WHT MR in 2014
Operation Date Product rates
N, P, S applied 23/4/14 Granular mix applied as per schedule
N, P, S applied 24/4/14 Liquid mix applied as per schedule
Incorporation of stubble 25/4/14 Cultivated whole site with Vaderstaad topdown
Crop 5/6/14 Beaufort wheat
Seed treatment 4/6/14 Jockey
Sowing rate kg/ha 5/6/14 70kg/ha
Target plants/m2 5/6/14 160/m2
Fertiliser 5/6/14 MAP 100kg/ha + 4ml Impact
Fertiliser 13/7/14 Urea 70kg/ha
Fertiliser 20/7/14 UAN 139/ha
Herbicide 3/6/14 Gramoxone 2l/ha PE
Herbicide 5/6/14 Sakura 118g/ha PE
Herbicide 5/6/14 Avadex 1.6l/ha PE
Herbicide 5/9/14 Axial 0.6l/ha Post E
Herbicide 5/9/14 Velocity 0.65l/ha
Surfactant 5/9/14 Adigor 1%
Molluscicide 25/6/14 Meta 7kg/ha
Molluscicide 7/7/14 Meterex 10kg/ha


Single block, 3 replicates of 4 treatments. Plots 12m wide x 25m long.

UAN applied with 12m boom with streaming nozzles, granular fertiliser applied by hand spinners. All fertiliser treatments were incorporated following after application using Vaderstaad Topdown donated by Swayne & McCabe.

report 1 graph 2


Table 3: Stubble and soil analysis (0-80cm) results pre sowing 2014.
Soil & Stubble analysis 2014 (3.8t/ha stubble) %C %N %P %S
Nutrient in stubble 41.6 0.411 0.038 0.071
Nutrients in soil (kg/ha) 15.3 22.8 22.8

Stubble was analysed and found to contain 41% C. Of this 0.411% was N, 0.038% was P and 0.071% was S. Soil analysis determined there was 15kg/ha N, 22.8kg/ha P and 22.8kg/ha S.

Table 4: Nutrients added to plots for individual treatments pre sowing 2014
  %N %P %S TRT
Nutrients required for stubble load present (kg/plot) 0.74 0.099 0.104 1 & 3
Nutrients available in stubble 0.459 0.684 0.684
Nutrients required for stubble load minus available nutrients (kg/plot) 0.281 -0.585 -0.580 2 & 4

A complicated calculation was run to determine how much fertilizer for each compound would be required to satisfy the treatment parameters. Fertiliser blends in liquid and granular form were used for each balanced nutrient mix.

Table 5: Quantities of applied compounds according to stubble & analysis.
TRT TRT description Compound Kg or L/plot
1 Applied as per stubble load Liquid NPS – N = 22.8% w/v 3.2
2 Applied as per stubble + analysis Liquid NPS – N = 22.8% w/v 1.2
3 Applied as per stubble load Granular NPS – N = 46% 1.6
4 Applied as per stubble + analysis Granular NPS – N = 46% 0.6


As the crop was sprayed out to prevent weed seed set no grain yield results are available for the 2014 report. We collected replicated dry matter cuts from each plot and analysed them for differences. There were no significant differences in dry matter between treatments in 2014.

report 1 graph 3

Figure 3. Fertiliser treatment according to analysis


The potential of crop production systems to increase the humus content of soils is being tested widely in several projects throughout Australia. Kirkby has developed and tested methods of increasing humus content in soils by applying NP&S in ratios that will maximise this potential. Modelling has identified significant increases “potentially” available if soil humus can be increased from 0.8% to 1.8%.

Growers considering adopting this system will need additional information and to consider several factors from a whole of system point of view. I have outlined several below:

  • Applying balanced nutrients is costly and returns from actual grain yield increases, from five years of trials, have not provided returns on the initial investment.
  • Over 3 years at Harden NSW applying additional NPS resulted in 7.5t/ha of extra C at an estimated cost of $1612 ($215/t)
  • These increases required incorporation of stubble residue early in the season as possible. This may have detrimental impacts on soil health.
  • Research by Kirkegaard and Kirkby estimates it cost $215 in NP&S to sequester 1t/ha of C.
  • Dry matter increases have been evidenced but this increase has not been transferred to the higher grain yields determined in crop modelling outputs comparing 0.8% C to 1.8% C.
  • Pasture increases C by 0.15%/an
  • Additional dry matter could be used as a forage source for livestock but there is no information on returns from this system at present.
  • Research by Kirkegaard and Kirkby estimates it cost $215 in NP&S to sequester 1t/ha of C.
  • C is currently valued at $6-$10/t on the world market.
  • Testing for soil C fractions is time consuming and expensive.

The SFS trial is aiming to test how applying the additional nutrients, in liquid & granular form, will impact on crop production in the high rainfall zone. If stubble nutrient content is considered when calculating nutrient application rates can initial costs be reduced without a subsequent drop in soil C. Stay tuned.GRDCLogoStacked_TM_CMYK

The stubble project – maintaining profitable farming systems in Victoria and Tasmania with retained stubble (project number BWD00024) is funded by the GRDC.

About BCG

Birchip Cropping Group Inc. (BCG) is a not-for-profit agricultural research and extension organisation led by farmers in the Victorian Wimmera and Mallee.
This entry was posted in Crop nutrition, Research results and tagged , , , , . Bookmark the permalink.

1 Response to Contributions and constraints of stubble to nutrient supply in relation to stubble and soil analysis

  1. Pingback: 2014 field trials | The stubble project: Victoria and Tasmania

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