Nitrogen management in a stubble retained system in the Wimmera and Mallee

  • Wheat stubble contributes a small percentage (2-3%) of nitrogen (N) requirement to the following cereal crop.
  • N applied to crops that have hayed off can contribute to crop biomass and yield in the subsequent season.
  • If soil tests are below 40kg N/ha in the top 60cm then you should apply 20kg N/ha at sowing (to make sure the crops get through to the beginning of stem elongation).

BCG logo with white background

Stubble retention can modify the nitrogen (N) requirements of crops compared to removed stubble or burning. During wheat stubble decomposition, immobilisation of N is common, reducing the immediate availability of N to crops. Subsequent remineralisation of N derived from the stubble may augment N supply later in crop development, or in a subsequent crop.

Microbial immobilisation of N during stubble decomposition competes with N demand by a crop, especially with cereal crop stubble. Often, additional N fertiliser is applied, both to meet the requirements of the growing crop and, to hasten stubble decomposition.

Micro-organisms are responsible for crop residue breakdown and the release of nutrients. The higher the soil organic matter (represented by carbon content in soil tests), the quicker the soil can break the residue down due to an increased level of nutrients and micro-organisms. Microbial Biomass (MB) in soil is a store-house for nutrients and acts as an engine for biological processes involved in carbon and nutrient transformations. Soil organic carbon level, soil texture and management practices such as stubble retention, tillage and crop rotation can influence the amount of MB and thus affect nutrient (N, P and S) supply capacity in a soil.

Soils low in organic matter will take much longer to break residue down and are more likely to need additional nutrition with incorporation (Kirkby, 2011). Decomposition is influenced by soil type, soil texture and incorporation of residues.  It is not unusual for farmers to see immediate structural improvements and greater water holding capacity from incorporation, however, nutritional benefits can take as long as three to five years. This explains the slow yield gains when switching from conventional tillage to minimum till operations. The use of break crops with higher N content in residue can speed up this process.

Nitrogen Budgeting

Creating a nitrogen budget can help you track and estimate how much N is needed for a growing crop. To make this simpler there are eight steps which analyse soil test data to interpret current soil N status and how much N is required at sowing and in-season. The decision to retain stubbles needs to take into account consideration of other management issues such as: soil borne disease, herbicide interception by stubble, difficulty at sowing with high stubble loads etc.

Key steps:

  1. Soil testing: Measure the N status of your soil as close as practical to the time of sowing so that the N budget calculated is most effective. Take six to eight soil cores per paddock with cores taken off row with the increments 0-10 and 10-60cm. For Yield Prophet® the depths need to be 0-10, 10-40, 40-70 and 70-100cm).
  2. Soil test results: by summing soil ammonium and nitrate, total mineral N can be calculated out. Soil bulk density is used to calculate kg N/ha from ppm.
  3. Applying N at sowing: If soil tests are below 40kg N/ha in the top 60cm then you should apply 20kg N/ha at sowing to make sure the crops get through to the beginning of stem elongation. If soil test results indicate that you have more than 40kg N/ha in the soil then do not apply additional N as there is adequate N in the soil for the crop to reach stem elongation in the Wimmera and Mallee.
  4. Top-dressing: the most reliable time to top-dress crops is in early stem elongation, this is the time when you will know most about yield potential and the crop is still responsive to additional inputs.  N can be applied up until flowering provided there is rain and/or soil moisture. Ideally N is applied prior to flag leaf emergence (GS49). Aligning urea applications to rainfall and seasonal forecasting is vital.
  5. Nitrogen budget – nitrogen requirement: the N requirement of a crop is calculated per tonne of grain produced. As a rule of thumb a wheat crop will need 40kg N/ha applied to produce 1 tonne of grain, barley requires 35kg N/ha to produce 1 tonne of grain and canola needs 80kg/ha of N to produce 1 tonne of grain.
  6. N budgeting – N supply: the N available to a crop comes from the N within the soil at sowing, mineralisation of N during the growing season and additional N applications. There is no set method for farmers to correctly measure their in-crop mineralisation. An estimated method for mineralisation includes GSR (mm) x organic carbon % x 0.15 = kg N/ha (a rule of thumb).
  7. Estimate the yield: Yield Prophet® can predict yield potential based on different seasonal conditions and historic climate data (disease and weed pressure needs to also be taken into consideration). The steps for yield estimation are at calculating yield in paddock
  8. Putting everything together: After going through the above steps and a nitrogen budget is calculated, the final value indicates your total N requirement

For a guide to yield potential calculations and more information

The amount of soil N that might be immobilised at sowing (not available to the crop in the short-term) is dependent upon the quantity and N concentration of stubble (C:N ratio) and soil fertility. It is also seasonally dependant as the decomposition of stubble from summer rainfall can reduce the amount of N immobilised at sowing. To estimate the amount of N immobilised you will need to know your stubble load, harvest index and N concentration (or C: N ratio) of the stubble (or use the estimate of 120:1). An immobilisation calculation can be done by here.

Tracing crop nitrogen from previous stubble

Crop residues are one of the major sources of carbon for soil biota especially in the low fertility soils of the Southern region of Australia. Therefore, retaining stubble can provide numerous benefits by changing the soil physical, chemical and biological properties. A trial conducted by BCG and CSIRO at Horsham, has investigated N cycling in cereal stubble retained systems by labelling wheat stubble (in 2014) with a 15N-isotope so that it can be tracked through the cropping system.

Treatments: surface stubble, incorporated, no stubble, standing stubble

Data: stubble retention significantly increased the amount of microbial biomass carbon, N supply potential and soil water content in the soil at 0-10cm depth. Stubble retention increased dissolved organic carbon levels but there was no difference between stubble treatments in total soil organic carbon. N-isotope analysis results showed that ~3% of 2014 stubble N was found in the 2015 wheat crop (first crop) and 11% in the 2016 wheat crop (second crop). A dry finish to the 2015 crop resulted in the low amount of previous season’s stubble N in the crop and the unused 2014 stubble N was taken up by the 2016 crop. These findings suggest that N from previous cereal stubble is transferred through a microbial biomass pool before being taken up by the following cereal crop.

Key outcomes: the management of wheat stubble on N nutrition of the following cereal crop is more through its influence on the microbial activity and N cycling processes. Its direct contribution to the N requirement of the following cereal crop is generally small.   

For more information see Nitrogen cycling in cereal retained systems.

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Figure 1. Amount of nitrogen (kg N/ha) in the different soil mineral and organic nitrogen and microbial nitrogen pools in the surface 0-10cm depth soil at 2016 sowing, fertiliser N applied and N taken-up by the 2016 wheat crop in the stubble N-management experiment at Horsham, Victoria. NSP=N supply potential, IMP= N immobilization potential, MB=microbial biomass, SOM=soil organic matter, POM=particulate organic matter – Source: Gupta Vadakattu.

Key points

  1. Wheat stubble contributes to a small percentage (2-3%) of N requirement of the following cereal crop.
  2. Cereal stubble retention significantly increases microbial activity and significantly influences all the processes involved in N availability including N mineralisation.
  3. Stubble retention increases the N tie-up potential of the soil suggesting that stubble load from cereal crops should be considered when deciding fertiliser management strategies in the following cereal crop.

Stubble nutrients into soil nutrients

Stubble on the soil surface is a vital source of carbon and therefore stubble retention practices affect soil organic matter levels (SOM) levels. Soil organic carbon has many influences on soil characteristics, many of which can influence a crops performance. The concentration of SOM within soils will vary with soil texture (clay content) and stubble levels, sandy soils may require a larger amount of stubble to increase SOM in comparison to clay soils.

Stubble type will influence the amount that will be converted into soil organic matter. Stubbles with narrow carbon to nitrogen ratio (C:N), such as pulse residues, generally have higher rates of conversion to SOM, whereas stubbles with wider C:N ratio (ie N poor cereal stubbles) will require additional nutrients (N, P and S) to be added to achieve efficient conversion. Dr Clive Kirkby has found that under ideal conditions up to 30% of carbon from stubble residues can be converted to soil humus, if other inorganic nutrients are not limiting.

Table 1: Stubble nutrient humification calculator indicated which stubble nutrient components are broken down with humification rate of 20% with a stubble load of 10t/ha.

Stubble nutrient concentration (%) C

45.0%

N

0.70%

P

0.10%

S

0.10%

Nutrients in the stubble (kg/ha) 4500 70 10 10
Carbon to be humified and nutrients required (kg) 900 77.0 9.2 11.7
Carbon remaining (kg) 3600

Deep banding: Choosing between deep banding and surface urea applications at sowing can influence how your cereal stubble affects yields. It was found that large stubble amounts can reduce yields in subsequent crops due to a large rate of N immobilisation. In 2016 a trial set up in Temora (58kg N/ha soil/mineral N at sowing) with 5.1t/ha of retained wheat stubble where 122kg N/ha was either banded beside and below the seed or spread before sowing. Anthesis dry matter indicated that deep banding urea had an increase of 1.3t/ha in anthesis dry matter and a yield increase of 1.1t/ha showing that where stubble loads are high (~>5t/ha) deep banding urea increased yield (Table 2).

Agronomy

Table 2: Wheat performance sown into 5.1t/ha wheat stubble at Temora with 122kg/ha N as urea applied either by deep banding or spread at sowing.

Growth stage GS30 GS30 GS30 Anthesis (GS65) Anthesis (GS65) Harvest GS99
Assessment Emergence count Plant dry matter Plant nitrogen Nitrogen uptake Plant dry matter Nitrogen uptake Yield
Units Plants/m2 t/ha %N kg N/ha t/ha kg N/ha t/ha
Deep 132 1.4 4.6 60.0 9.2 136.4 5.2
Surface 137 1.4 3.8 51.6 7.9 102.5 4.1
P value NS NS NS NS (<0.001) 0.007 0.001

For further information CSIRO researcher Tony Swan has described a C:N ratio calculator in a paper (February 2017) summarising the key findings from the GRDC stubble initiative.

The reality of receiving enough rain to justify applying 122kg N/ha at sowing is not realistic in the Mallee region. Instead growers in the Mallee region match paddock yield potential and rainfall in terms of nitrogen applications. High rainfall years in the Mallee region is predominately winter rainfall, allowing for in-season application of nitrogen to supplement growth. When dry sowing there must be care taken to avoid seed burn from placing the seed too close to fertiliser. In dry years urea applications in-season can hinder a crops performance, haying off can occur due to the crop running out of soil moisture, but how much of this N can be recycled for the subsequent crop?

Practical Application

Nitrogen management after a dry season – how much residual n can be utilised?

In 2015 BCG carried out a nutrition trial, to illustrate the response of barley varieties to varied nitrogen rates at Kalkee in the Victorian Wimmera region. The trial was severely affected by dry conditions which caused plants to hay off. However, this failed nutrition trial provided BCG with an opportunity to examine the effect of residual or unused nitrogen inputs on a subsequent crop and to determine effective nitrogen management in paddocks where crops ‘hayed off’ in the previous season.

Treatments:

2015 – Six barley varieties, with five different N rates were sown 22 May 2015

2016 – Grenade CL Plus wheat was sown over the top of the 2015 trial stubble on 19 May 2016. Each plot was then segmented into half. Additional N was applied at GS15/22 at 41.4kg N/ha to one half of the plots. While the remaining half had no additional N applied in–season.

Results:

NDVI conducted early in the season showed a significant response towards increased vigor with more residual N and additional N applied early in season. This indicates that the size of the response to added N in 2016 depended primarily on the 2015 N treatment, and consequently upon the 2015 yield. High N treatment plots that had hayed off in 2015 had significantly higher NDVI measurements, but less of a response to added N in 2016.

A wet 2016 spring delivered notable yield results that reflected the same patterns of response shown earlier in the season. The wheat sown over plots where higher N treatments had been applied in 2015 produced higher yields, demonstrating a significant interaction between 2016 and 2015 N treatments (Figure 2).

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Figure 2: Average yields in 2016 by 2015 N treatment. Error bars indicate least significant difference (LSD) between 0N and +N treatments in 2016, but differences in average yield between 2015 N treatment were also all significant. Stats: P=0.001, LSD=0.16, CV = 5.5%.

For further in depth analysis of results see the article published in the 2016 BCG Season Results Manual.

Key findings:

  • N applied to crops that have hayed off can contribute to crop biomass and yield in the subsequent season.
  • There is always going to be a trade off with applying more N in a dry or wet year. Will the N go towards grain yield or will it be made to be residual N for utilisation? Knowing your soil N status is key.
  • Progressing from a dry year to a wet year, conservative N management deliver the best economic returns over 2015 and 2016.

Reference List

Ie, Sebastian,. 2016, 2016 BCG Season Research Results, “Saving N for a rainy day” pp. 126-131.

Kirkby C A. Richardson A E, Wade L J, Conyers M, Kirkegaard J A (2016). Inorganic nutrient increase humification efficiency and C-sequestration in an annually cropped soil.
Kirkby CA, Kirkegaard JA, Richardson AE, Wade LJ, Blanchard C, Batten G (2011) Stable soil organic matter: A comparison of C:N:P:S ratios in Australian and other world soils.

By Jessica Lemon                                                                                                                     Research Officer (BCG)                                                                                                     jessica@bcg.org.au

GRDCLogoStacked_TM_CMYKThis research is being conducted by BCG as part of the GRDC Maintaining profitable farming systems with retained stubble initiative (project BWD00024 Maintaining profitable farming systems with retained stubble in Victoria and Tasmania).

 

Disclaimer: Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of the Birchip Cropping Group (BCG) or the Grains Research and Development Corporation (GRDC). No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. BCG and GRDC and contributors to these guidelines may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. BCG and GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication. 

Stubble project overview: This guideline has been developed for BCG Farming Systems Group as part of the Maintaining Profitable Farming Systems with Retained Stubble initiative, funded by the Grains Research and Development Corporation (GRDC). The initiative involves farming systems groups in Victoria, South Australia, southern and central New South Wales and Tasmania collaborating with research organisation’s and agribusiness to explore and address issues for growers that impact the profitability of cropping systems with stubble, including pests, diseases, weeds, nutrition and the physical aspects of sowing and establishing crops in heavy residues.

 

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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.
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