Productivity may be reduced on sodic soils. Management to alleviate this constraint should be considered. Opportunities to refine nutrient inputs may also be available.

  • High soil sodium levels cause the breakdown of soil structure, which affects both aeration and water penetration, making these soils difficult to manage.
  • Sodium-dominated soils with an exchangeable sodium percentage (ESP) greater than 6% are unstable when wet and will disperse.
  • A soil ESP in excess of 15% indicates severe structural deterioration in most soils.
  • Small areas with high levels of sodium within a block are often called soda patches.
  • In sodic soils, sodium displaces many of the other cations (calcium, magnesium and potassium), on cation-exchange sites.
  • Clay particles disperse when high levels of sodium are present on the cation- exchange sites of the clay particles.
  • Clays that disperse reduce soil pore spaces and permeability to both air and water.
  • Electro-Magnetic (EM) mapping can help identify zones within blocks that have different levels of ESP.
  • A soil sample taken from the sub-soil (40-60 cm) will assist in the diagnosis of sodicity and assessing whether it may be restricting rooting depth.
  • Excessive sodium in the soil reduces cane yield. In the Burdekin, for every 1% increase in sub-soil ESP (25-50 cm) there was a 2.4 t/ha reduction in yield (Nelson and Ham, 1998). Similarly, in the Central region there was a 1.5 t/ha reduction in cane yield for every 1% increase in sub-soil ESP.
  • Sodic soils may be acid or alkaline and may also be associated with salinity.

Managing sodic soils

  • A key step to improving productivity of sodic soils is to reduce the amount of sodium on the cation exchange sites of the soil.
  • This reduction of soil sodium levels requires an integrated approach that involves amelioration, irrigation/rainfall and drainage.
  • Sodium can be removed by displacing the sodium ions on the cation-exchange sites with calcium ions. The most practical way to do this is by applying gypsum or lime. Lime is used when the sodic soil pH(water) is less than 5.5 and gypsum is used when sodic soil pH(water) is greater 6.0.  These products should be thoroughly mixed into the soil through cultivation to speed up the amelioration process. This is most effectively done well before planting.
  • When gypsum or lime is applied, the displaced soil sodium is leached deeper into the soil profile, thereby providing a more favourable root zone for crop growth.
  • Variable application of ameliorants to sodic zones identified using EM mapping is likely to be more cost effective than broadcast applications of these products across the whole block.

 

GYPSUM APPLICATION RATES FOR SODIC SOILS
Exchangeable sodium percentage (ESP%) Gypsum rate (t/ha) for all regions except Burdekin Gypsum rate (t/ha) for the Burdekin
<5 0 0
5-10 2 5
10-15 4 7.5
>15 6 10

Gypsum application rates are affected by product quality (concentrations of sulfur and calcium and particle size can differ).

 

  • Sufficient irrigation/rainfall is needed to leach the displaced sodium through the soil profile and out of the root zone.
  • For this to occur, it is important that surface and subsurface drainage is improved before ameliorant application.
  • The addition of organic matter on sodic soils is also recommended (green cane trash blanketing, mill by-products, crop residues/composts), as these help to counter the dispersing effect of the sodium and improve soil structure.
  • Mill ash and mud-ash mixtures will increase the productivity of sodic soils, primarily by improving soil porosity and hence increasing plant available water.
  • Mill by-products also supply several essential nutrients to the cane crop and these need to be taken into account in any fertiliser program (see Accounting for nutrients contained in mill by-products tool).
  • Management of saline sodic soils differs from the guidance above. If unsure about methods to ameliorate saline sodic soils, seek advice from a trusted advisor.

Fine tuning nutrient rates on sodic soils

  • If high ESP levels are present, measures to manage sodicity such as gypsum, lime or mill by-product application should be undertaken as the first priority.
  • If these measures have not been undertaken or haven’t been effective in improving soil condition and productivity remains restricted, a reduction in nutrient rates could be considered.
  • In some cases, yields have been maintained on sodic soils at reduced nitrogen rates (SRA projects 2015007* and 2015065*). However, these results may not apply to all situations.
  • It is recommended that any change in management is 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.
  • Initially small reductions (~10%) to nutrient rates should be tested on-farm in consultation with a trusted advisor.
  • In the Burdekin region, it may be more appropriate to evaluate a nitrogen rate based on the district yield potential of 150 t/ha (DYP 150) on sodic soils.
  • The use of enhanced efficiency fertiliser (EEF) products may reduce productivity risk associated with lowering nitrogen rate as these products potentially reduce nitrogen losses to the environment.
  • Where variable-rate technology is available, nutrient application rates should only be adjusted on sodic zones.

 

*Trial results from SRA projects 2015007 and 2015065, both being conducted in the Central region, will be included as case studies once data is available following the completion of these projects.

 

RP20 project sites 6 and 8

These trials were conducted in the Burdekin region on sodic duplex soils as part of the RP20 project. Sub-soil (40-60 cm) ESP % was 9.7-15% at site 6 and 8.9 – 9.8 % at site 8. Results show similar grower economic returns between the DYP 150 and DYP 180 SIX EASY STEPS nitrogen guidelines. At the lower nitrogen rate (DYP 150), there is a trend for slightly lower cane yield but slightly higher CCS in some seasons.

  • RP20 Trial Results – Site 6
  • RP20 Trial Results – Site 8

 

MOSES project

This case study from the Central region reports on a trial looking at variable nitrogen rate application in a block with zones containing high sodicity. The zone with high sodicity and low cane yield did not show a response to nitrogen above 90 kg N/ha.

  • Markley and Hughes – A two zone paddock with productivity influenced by elevated sodium levels

Disclaimer:

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

 

Banner photograph kindly supplied by Pontarelli Farms and Robert Stockham and Sons Contracting.

 

Version: April 2020.

 

Disclaimer: In this disclaimer a reference to ‘SRA’, ‘we’, ‘us’ or ‘our’ means Sugar Research Australia Limited and our directors, officers, agents and employees. Although we do our very best to present information that is correct and accurate, we make no warranties, guarantees or representations about the suitability, reliability, currency or accuracy of the information we present in this publication, for any purposes. Subject to any terms implied by law and which cannot be excluded, we accept no responsibility for any loss, damage, cost or expense incurred by you as a result of the use of, or reliance on, any materials and information appearing in this publication. You, the user, accept sole responsibility and risk associated with the use and results of the information appearing in this publication, and you agree that we will not be liable for any loss or damage whatsoever (including through negligence) arising out of, or in connection with the use of this publication. We recommend that you contact our staff before acting on any information provided in this publication. Warning: Our tests, inspections and recommendations should not be relied on without further, independent inquiries. They may not be accurate, complete or applicable for your particular needs for many reasons, including (for example) SRA being unaware of other matters relevant to individual crops, the analysis of unrepresentative samples or the influence of environmental, managerial or other factors on production.