Diagnosing seedling fertiliser damage in canola

Root damage from contact with drilled fertiliser can delay or reduce germination and reduce plant growth.

Variable emergence between and within seeding rows
Plants fail to germinate, and survivors are stunted
Seedling recovering from fertiliser-burnt taproot

Contributing factors

Plant species

  • Tap rooted species are more severely affected.

Fertiliser type:

  • Fertilisers with a high salt index (particularly muriate of potash).
  • Urea has a low salt index but produces ammonium carbonate when it breaks down, which causes alkaline conditions around the fertiliser granules. This may cause the production of ammonia gas which is highly toxic to germinating seeds.

Fertiliser rate:

  • Higher rates increase toxicity risk.
  • Seeding using wide rows increases the concentration of fertiliser near the seed.

Proximity to the seed:

  • With placement closer to the seed, contact between high concentration solutions and the seedling is more likely
  • With no-till practices, seed and fertiliser are often placed close together, which increases the chance of germination being reduced by toxicity. Possibly counteracting this is less disturbance and generally more compact, better moisture-holding soil near the seed, particularly if press wheels are used.

Soil type and moisture content:

  • Toxicity levels are highest when seeding into moist, light sands and when seeding in any soil type is followed by a warm dry period.
  • Toxic effects will be reduced or eliminated when effective rains occur immediately after seeding. Rain dilutes the salt solution, dissolving ammonia and removing fertiliser from around the seed by leaching
  • Greater toxicity has been seen where the seedbed has been left loose and dry and thus holds less moisture to dilute the concentrated salt solution from the dissolving fertiliser.

What to look for


  • Bare areas or poorer plant growth that is more common in sandy soils or specific drill rows.


    • Bare areas or poorer plant growth that is more common in sandy soils or specific drill rows.
  • Plants fail to emerge, have delayed emergence, and early growth stunting.
  • Cereals may have older leaf death but recover completely.
  • Canola and legume tap roots growth can be stunted reducing their capacity to use deep subsoil water.

What else could it be

Condition Similarities Differences
Deep seeding in canola Reduced germination and weak poor seedlings Distinguished by coleoptile/hypocotyl length. Emerged plants that were sown deeply have a longer hypocotyl, and smaller cotyledons with pale margins
Diagnosing poor seedbed soil structure Reduced germination and weak poor seedlings Can be distinguished by crusted or dispersed soil

Management strategies

  • Toxic effects of fertilser on crop germination can be avoided by banding fertilisers away from the seed or by top-dressing.
  • Banding about 5 cm below or to the side of the seed rows is usually sufficient to avoid germination problems.
  • Because nitrogen fertilisers can leach, banding nitrogen fertiliser above the seed rows was tested in trials, but reduced establishment and early vigour.
  • A 'safe level' cannot be stated because many contributing factors influence fertiliser toxicity on germinating seed.
  • Fertiliser may greatly reduce germination, but the remaining plants may grow luxuriantly in a high-fertiliser-responsive soil and produce a higher yield than would have occurred had a lower, non-toxic fertiliser rate been used.

Where to go for expert help

Mark Seymour
+61 (0)8 9083 1143
Page last updated: Friday, 17 April 2015 - 2:09pm