Jim Laycock, IPF Technical Agronomist

By August 2020, after a number of below average seasons and in many areas no crop at all, the winter crop yield potential in many areas was one of the best seen. On the east coast, a good autumn break meant crops were planted on time; soil nitrogen levels were above average as a result of carry-over nitrogen and good late summer-autumn mineralisation; and rainfall was above average. By harvest, many areas had achieved above average yields. The only dark spot was the frost damage, quality downgrades due to weather damage and yield reductions due to a dry September experienced in some areas.

As we plan for the 2021 winter season it is likely that those areas that achieved above average yields last year will see crop nutrition issues that have not been seen for a number of seasons. The key questions for 2021 are:

• Is your current soil-testing program adequate?
• What amount of nutrients were removal last year?
• Have you mined your phosphorus?
• How much nitrogen will you require and what is the best time of application?
• Is stubble your friend or foe?

Soil testing program

Our experience and data indicate that, for winter cereal cropping, random sampling at 0-10 cm depth for macro (except nitrogen) and micro nutrients is still the recommendation. In northern vertosols, include BSES P, Colwell P and PBI at 10-30 cm depth as part of the soil sampling/nutrition management plan if it has not been done previously.

It is important to sample to consistent depth, avoid any non-representative areas of the paddock (such as headlands, residual fertiliser). Take a minimum of 30 cores and mix thoroughly before despatching the representative sample to Nutrient Advantage Laboratory Services for analysis.

For nitrogen, deep N testing can commence as soon as practical. In cropping paddocks that have not had a deep N for 3 or 4 years, split the profile into layers to assist in identifying any sub-soil constraints and also to identify at what depth the nitrogen is located in the profile after above average crop yields.

What has changed is the ability to geo-locate soil samples and use the data as a layer in regional and farm mapping programs. The Incitec Pivot LabSTREAM soil sampling app allows you to log and record sampling co-ordinates in the paddock.

We also continue our work to assess the correlation between sampling depth, location and access to residual fertiliser bands under control traffic minimum tillage sub two-centimetre RTK guidance farming systems.

Phosphorus in 2021

 Planting is the only opportunity to apply plant available water-soluble phosphorus in annual cropping systems to ensure it does not limit yield potential. If phosphorus availability in winter cereals is restricted in the first six to eight weeks of growth, full yield potential may not be realised.

The build-up of phosphorus over several dry, low-yielding years presented the opportunity to reduce rates in 2020 without major economic impact. However, as the grain nutrient removal data in Table 1 shows, removal rates from the high-yielding 2020 harvest are likely to be very high. Therefore, at a minimum, the strategy should be to replace all macro nutrients to avoid soil nutrient mining.

Table 1

Source: Incitec Pivot Fertilisers Grenfell long-term trial site – wheat 2016 and canola 2020.

Nitrogen requirement and timing

Deep N soil testing is a useful tool for assessing the potential for crop nitrogen availability, but it is not the only tool. Crop modelling, nitrogen budgeting, decision support systems, tiller densities, tiller nitrogen, NDVI’s can all be used at various stages of crop growth.

For those paddocks in 2020 that returned high yields and proteins less than 11%, nitrogen may be limiting. So, consider an early application of pre-plant nitrogen in 2021 especially in grazing cereals.

 Stubble – friend or foe?

Definitely friend! Retention of stubble over summer provides ground cover, assists in moisture retention, maintains cooler soil temperatures and improves soil structure. These soil conditions will maximise the mineralisation of organic nitrogen into plant-available N.

Stubble retention can create a temporary nitrogen lag during early crop growth stages, as microbes ‘borrow’ soil nitrogen to break down the stubble from the previous crop. This is known as nitrogen tie-up or immobilisation.

Nitrogen tie-up can leave the current crop short on available nitrogen during the initial stages of plant growth if adequate fertiliser nitrogen is not supplied. A rough rule of thumb is that every tonne per hectare of cereal or canola stubble will tie-up 5 kg/ha of nitrogen.

The type of stubble, in addition to the amount of stubble, also influences the extent and duration of nitrogen tie-up. Nitrogen tied-up in microbes will be freed up later in the season (4-8 weeks) and could contribute to later season growth depending on seasonal conditions.

Additional nitrogen at sowing helps to provide the nutrition required to break down the previous stubble and also makes sure the germinating crop has sufficient nitrogen to establish, which ultimately helps maintain yield potential (Kirkegaard, 2018).

Remember that any successful farming system relies on a multitude of factors. Soil testing and the resultant fertiliser recommendation is only one facet of that system.

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