25 May 2025

Author: Bede O’Mara – IPF Technical Agronomist

Make the most of the rotation window

Last season delivered strong outcomes for growers, and with an early picking season in many areas now completed ahead of traditional schedules, now is an ideal time to be considering the rotation and be reconfiguring fields for the next crop.

For those who experienced late season rainfall that continued late stubborn regrowth, the uptake of soil N may have persisted later into the season than usual. This may result in increased immobilisation from those residues to accumulate nitrogen later. This may mean soil N may be more depleted than usual leading into the next crop.

Start with yield, then replace what’s removed

Quantifying crop yield and nutrient removal is essential. Once ginning and actual yields are known, refer to reliable agronomic estimates of nutrient removal per hectare to determine what has been drawn from the soil. Table 1 below shows the average nutrient removal rates in cotton – based on established industry benchmarks.

Depending on the grower’s approach and soil test and yield results, one of three strategies applies:

  • Mine:
    • Where soil nutrient reserves exceed known optimum ranges, removal can be sustained for many seasons to come without much negative impact.
  • Maintain or replace:
    • Where soil nutrient levels need to be maintained for system resilience and supply for the next or proceeding few seasons. This means simply replacing what’s been removed.
  • Build capital:
    • Where soil nutrient levels are below optimum and require a deliberate lift over one or more seasons. This approach is used alongside a maintenance program. Depending on the quantity required, a capital approach can be deployed/split over future seasons and is in addition to maintenance or replacement requirements.

Table 1: Average cotton nutrient removal (kg/ha).

Source: Adapted from Rochester 2014, Bell & Griffiths, in CRDC Cotton Production Manual, 2014.

Soil testing: The foundation of smart planning

Regular soil testing will help  gauge the amount and distribution of residual nitrogen in the profile. . This is an important step to planning the amount of fertiliser nitrogen required.

The Fertcare Soil Sampling Guide (Gourlay & Weaver, 2019), summarises the appropriate sampling depths for topsoil:

  • For comprehensive analyses where cotton is irrigated and grown on hills or beds in Qld, or all cotton enterprises in NSW, looking at most nutrients, salts, exchangeable cations, and soil health indicators:
    • 0-30cm for topsoils should be selected.
  • For irrigated cotton grown on the flat in Qld, or rain-grown cotton in Qld and NSW:
    • 0-10cm for the topsoil depths should be selected,
      • along with considering subsoil sampling of comprehensive nutrients etc in the 10-30cm layer also.

Measure more with the Soil Health Test Package

It’s also important to measure some key soil traits that influence nutrient access and uptake. Our Soil Health Test Package helps track biological, chemical, and physical indicators in the topsoil. Key analytes include:

  • Chemical:
    • Labile (active) Carbon
    • Total Carbon
    • Total Nitrogen
    • C:N Ratio
  • Physical:
    • Loveday & Pyle Slaking & Dispersion
  • Biological:
    • Microbial Respiration

This is a test designed to provide a reference point that subsequent testing (every 4-5 years or at key times in a crop rotation) can be compared to. If you do set up a soil health benchmarking program it is important the testing is done in the same location, and at a consistent depth and soil moisture.

This is also a good test to help develop a sound amelioration program.

Deep sampling for more informed nitrogen planning

Deep soil samples (segmented at 30-60cm and 60-90cm) should be considered to better understand the quantity and position of mobile nutrients (N, Sulphur and Boron) in the profile. It will also highlight the location and possible impacts on any subsoil constraints present where salinity and cations are analysed.

This will mean you can better plan your nitrogen application rates and placement position early in the fallow.

To be effective nitrogen needs to be available and well distributed in the soil profile. However, nitrogen does not become plant-available until after the applied fertiliser has undergone physical, chemical, and microbial breakdown. These processes depend on soil temperature, moisture, pH, and soil microbes, and they take time.

A range of factors affects cotton’s ability to take up nitrogen, including:

  • Application timing: lower soil temperatures in winter can result in less soil nitrate being available and reduced soil microbial activity.
  • Weather conditions: periods of high rainfall can lead to waterlogging, denitrification and leaching losses.
  • Form of nitrogen fertiliser: Urea can be slower taken up by the plant than other fertilisers due to the chemical breakdown of ammonium nitrogen to nitrate nitrogen.
  • Placement of fertiliser in the soil: placing fertiliser at the correct depth can minimise volatilisation losses and boost plant availability.


Reduce losses with Enhanced Efficiency Fertilisers

Nitrogen losses are costly to a farm business. IPF has two Enhanced Efficiency Fertilisers (EEFs) to help reduce these losses:

  • Green Urea NV – to reduce volatilisation losses when broadcasting urea.
  • eNpower Urea – to reduce dentrification (nitrous oxide) and leaching losses from pre-plant incorporated urea.

These are products that have been developed over many years by IPF right here in Australia, with trials and proven results to give growers and advisors confidence when using them. They have been tailored and formulated for Australian conditions, not simply brought in from overseas and sold without backing and support. So, you can have confidence in using Australian developed products when using an IPF EEF!

Australian-developed and trialled, these EEFs offer growers confidence backed by results – not just imported formulations.

Image: Cotton crop in-field ready for harvest

Source: Image stock / Generic

Further Information

For more information about cotton nitrogen management, see your local Incitec Pivot or Nutrient Advantage accredited agronomist.

References

Gourley CJP and Weaver DM (2019), A guide for fit for purpose soil sampling, Fertiliser Australia, Canberra, Australia.

CRDC, (2014), Australian Cotton Production Manual, Greenmount Press, Toowoomba.