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Fertiliser Toxicity

The increasing number of paddocks with a low soil Nitrogen status has led to a significant increase in the amount of Urea being applied at sowing.  Whilst this is an efficient way to apply Urea, there is a risk under certain conditions that fertiliser burn can reduce crop germination and establishment.

In chemical terms, fertilisers are a salt.  Fertiliser burn occurs when fertilisers (salts) affect the ability of the seed to absorb water by osmosis.  If there is too much fertiliser near the seed and it is unable to absorb moisture, a desiccation or ‘burn’ may occur.  The risk of fertiliser burn occurring depends on the type of fertiliser, the sowing system, soil type, soil moisture and soil temperature, plus crop type.

All fertiliser can cause fertiliser burn at high concentrations.  However as a general rule, Nitrogen and Potassium fertilisers have the highest salt index and hence the most likely to cause problems.  In the context of broad acre farming in southern NSW, very few issues are seen with starter fertilisers such as MAP or DAP.  However Urea has the potential to cause problems if adequate separation from the seed is not achieved.

The sowing system affects the risk of fertiliser burn, through the separation (or lack of separation) that it provides the seed from the fertiliser.  Placing seed and fertiliser in close proximity increases the risk of burn, while separation decreases the risk.  A separation distance of 25 mm is usually enough to minimise or eliminate the risk of fertiliser burn.  This can easily be achieved by most double boot sowing systems.

Sowing systems with wide row spacing, narrow points and no defined seed and fertiliser separation have the highest risk of fertiliser burn occuring.  Sowing systems with narrow row spacing, wider points or with defined separation of seed and fertiliser have the lowest risk of fertiliser burn occuring.

Germinating crops are most susceptible to fertiliser burn in light soils with cool, dry soil conditions.  Heavier soil types with good soil moisture and warm soil conditions have a lower risk.

Canola is the most sensitive crop to fertiliser burn, followed by wheat, peas, barley and oats.

Due to the complexity in the events which cause fertiliser burn, it is difficult to provide a number or rate which is safe.  Published figures suggest that the safe limits for wheat sown with narrow point sowing systems on medium to heavy soil types with no seed separation and good soil moisture, are as low as 43 kg/ha Urea for a 225 mm (9”) sowing row spacing and 32kg/ha for a 300 mm (12”) sowing row spacing.  In practice, these rates have been exceeded with no apparent problems, due to adequate separation between seed and Urea.

Individual experience with sowing plant, soil type and environment, provides the most valuable information, but in the absence of this, it is wise to remain cautious.