Legume break crops may contain significant amounts of nitrogen. Nitrogen fertiliser rates in the plant cane crop should be adjusted to account for available nitrogen from the legume crop.

  • Legumes break the sugarcane monoculture between crop cycles.
  • In general, legumes improve soil condition, add organic matter to the soil, protect soil from erosion during the wet season and help reduce pest, weed and disease pressures.
  • Some legumes are affected by specific nematodes hosted by sugarcane.  Legume crop selection should take this into account. An information sheet can be found here.
  • Nodulation of the legume crop is susceptible to low soil pH, resulting in reduced nitrogen fixation. Consequently, fields selected for legume break crops should have a soil pH (water) above 5.5 or be limed to reach a soil pH (water) above 5.5.
  • Legume crops should be inoculated at planting with the appropriate group of Rhizobium bacteria to stimulate nodulation.
  • Excessive nitrogen fertiliser should not be applied to a legume crop as this will inhibit nodulation and the ability/necessity for the crop to fix atmospheric nitrogen.
  • Well-grown legume crops can contribute significant amounts of nitrogen to the soil. The amount available to the plant cane crop will depend on legume biomass and nitrogen content, legume termination method, tillage and rainfall.
  • Non-legume and mixed species cover crops are also grown in sugarcane farming systems. Further work is required on these systems to understand how they affect nutrient availability.

Calculating legume N

  • The nitrogen contributed by legume crops should be accounted for when fertilising a plant cane crop.
  • SIX EASY STEPS provides guidelines to estimate the amount of N contained in legume break crops (Table 1).
  • The estimate is based on crop size (dry biomass) and likely nitrogen content (N%) for different legume species.
  • The estimate is adjusted for crops that are harvested as removal of grain exports approximately 2/3 of the legume nitrogen from the field.

Table 1. Estimating legume fallow crop N content

*N contribution includes estimate of N contained in legume roots

**Non-inoculated or poorly nodulated legume crops may have lower N% than indicated in this table

Refining biomass and N concentration (%) calculations

While the guidelines in Table 1 are based on previous industry research, legume crops should be sampled to estimate crop size and actual nitrogen concentration (%). This provides more precise information for decisions. Guidelines for this process are contained in the links below.

 

  • Initially, the nitrogen in a legume crop is in organic form within the roots, stems, leaves and grain.
  • These plant materials need to be broken down by soil organisms to release the nitrogen contained within it.  This process is called mineralisation and results in ammonium and then nitrate being released into the soil.
  • The mineralisation process is faster when soils are warm and moist, and is even more rapid when the legume material has been broken up and incorporated into the soil by tillage. This is due to greater access to legume residues by the soil organisms.
  • Both ammonium and nitrate can be taken up by a sugarcane crop, although the crop shows a preference for ammonium.
  • Nitrate can be easily lost from the soil through leaching, run-off or denitrification.
  • On high pH soils [pH(water) > 8.0], ammonium can also be lost via ammonia volatilisation, especially if it is at the soil surface.

 

  • The rate of decomposition and resulting nitrogen mineralisation from legume crops is greatest when nitrogen concentration in the legume residue is highest (e.g. in green-manured crops). Harvesting legume grain reduces the amount of residue available for mineralisation.
  • The rate of mineralisation is increased by cultivation and incorporation of the legume crop into the soil.
  • The rate of mineralisation can be slowed if the legume is sprayed out or matures and is left standing in the field with residues retained as standing stubble. Harvesting grain reduces these benefits, as standing stubble is broken into small pieces during harvest and distributed on the soil surface.
  • Residues retained on the soil surface for as long as possible prior to planting cane will ensure the greatest synchrony between N release and N demand by the emerging cane crop.
  • Rapid mineralisation of nitrogen to nitrate from legume residues incorporated by tillage can lead to substantial nitrogen losses through either denitrification, runoff or leaching.
  • Do not plant cane immediately after the incorporation of green legume residues as the decomposing organic matter can affect germination and establishment of the plant cane crop.

 

  • High rainfall and soil temperature influence the availability of nitrogen contributed by a legume crop as it speeds up the mineralisation process and increases the risk of nitrogen losses.
  • If high rainfall occurs between legume crop incorporation and sugarcane planting or when the plant cane crop is small and still establishing, there is a risk that some or all of the nitrogen contributed by the legume crop may be lost.
  • Depending on timing of incorporation, high rainfall from April – August could pose a risk to legume nitrogen availability.
  • To get an indication of the remaining nitrogen under these circumstances, a nitrate test strip can be used to assess nitrate present in the root zone.

 

  • Nitrate test strips can be a good indicator of whether the nitrogen from a legume crop is still present in the soil in adequate amounts for the following cane crop.
  • Nitrate test strip assessments are best carried out immediately prior to top-dressing.
  • If the nitrate test strip indicates a high reading, top-dressing with fertiliser nitrogen may not be required.
  • If the nitrate reading is low, fertiliser nitrogen may be required at top-dressing.
  • Analysis of soil mineral nitrogen provides similar information, but the time required for laboratory analysis may be a limiting factor.
  • It should be noted that nitrate test strips do not detect all forms of nitrogen. If the legume nitrogen is still in organic (yet to mineralise) or ammonium forms it will not be detected on a nitrate test strip. Under very dry conditions, a nitrate test strip may indicate low nitrate levels as the legume nitrogen is yet to mineralise. Given this, nitrate test strips need to be used together with an understanding of what conditions have been like in the field and how these conditions are likely to have affected the mineralisation of nitrogen from the legume residue. It is recommended that growers seek advice about the use of and interpretation of nitrate test strips.

Determining nitrogen rate after a legume crop

Nitrogen rates at top-dressing following a legume break crop should be guided by:

  • Legume crop nitrogen content (Table 1)
  • Management of the legume residue
  • Weather conditions, particularly rainfall
  • Nitrate test strip (if available).

Each season will be different. Some likely scenarios are outlined below. In each case, it is assumed that a low nitrogen rate (10-30 kg/ha) was applied at planting.

 

Legume N contribution: High
Management: Delayed incorporation
Rainfall since incorporation: Minimal
Nitrate strip test result: High
  • In the above situation, no fertiliser nitrogen would be recommended at the top dress stage

 

Legume N contribution: High Small-moderate High but grain harvested
Management: Early incorporation Delayed incorporation Delayed incorporation of remaining stubble
Rainfall since incorporation: Moderate Minimal Minimal
Nitrate strip test result: Medium Medium Not available
  • In the above situations, some fertiliser nitrogen would be recommended at the top dress stage.

 

Legume N contribution: Any
Management: Any
Rainfall since incorporation: High
Nitrate strip test result: Low
  • In the above situation, a top-dress application to meet the SIX EASY STEPS recommended nitrogen fertiliser rate for plant cane may be required. The combination of high rainfall after crop incorporation and a low nitrate test strip result indicate that the legume N may not be available to the following cane crop. The top-dress application must take the nitrogen applied at planting into account.

 

  • Any change in nutrient management should be tested on-farm. This will build confidence in both the new nutrient rates but also the process of fine tuning a nutrient management program as part of steps 5 & 6 in the SIX EASY STEPS.
  • A guideline for conducting on-farm trials is included in the SIX EASY STEPS toolbox.
  • Leaf testing also provides a valuable method for checking on the adequacy of nutrient inputs.
  • If required, growers should seek assistance from an experienced advisor when fine tuning nitrogen rates following legume fallows.

Disclaimer:

Papers published from the Australian Society of Sugar Cane Technologists annual conference are also available at www.assct.com.au

 

Version: April 2020

 

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