GROWERS are being encouraged to consider the type of fertiliser applied and its separation from the seed when they sow their crops to avoid fertiliser toxicity that may hinder establishment and early crop performance.
This advice is one of several tactics growers can take to reduce the likelihood of fertiliser toxicity occurring.
Some fertilisers are more damaging than others, with current advice largely based on the fertiliser salt index first developed in the 1940s.
University of Adelaide PhD student Jacinta Dockerill is challenging the salt index approach, which relates a fertiliser's toxicity risk with its impact on salinity measured in a laboratory.
Ms Dockerill, whose studies have been supported through a GRDC Research Scholarship, said her work was focused on directly measuring plant responses to fertiliser toxicity, rather than predicting the likely impact based on a laboratory test.
"We're trying to develop a method where we look at the overall crop response including both salt and toxicity effects," she said.
Toxicity can occur when the fertiliser product is not rapidly diluted by soil moisture or rain. As the fertiliser granules dissolve, they temporarily increase the soil salinity around the seed which impairs its uptake of water and the germination process.
In addition to this salt effect, some fertilisers, such as urea, also develop compounds that are highly toxic to germinating seeds.
As part of her Honours research, Ms Dockerill examined the effect of 12 different fertilisers on canola in an alkaline and a low pH soil.
Small seeds, such as canola were particularly susceptible to fertiliser burn due to the effects of salinity and toxic compounds. Evidence of fertiliser toxicity is seen as patchy or delayed emergence, and weak seedlings with poor early root and shoot growth.
"We found that there's a few fertilisers that have a very different toxicity response than indicated from their salt index," Ms Dockerill said.
"For instance, urea has a salt index of 75, which is lower than sodium nitrate with a salt index of 100. That tells us urea should be less toxic than sodium nitrate, but we found that urea was actually our most toxic fertiliser in the experiment."
Ms Dockerill said sulphate of ammonia also had a larger, more toxic effect than expected based on its salt index.
"With urea and sulphate of ammonia fertilisers we saw a response that suggested ammonia was adding to their toxicity," she said.
"We found that with both of those treatments, the toxic response was quite large and readily reduced canola establishment and early growth.
"Through our work, we are also trying to predict a response from a number of soil and crop factors that influence toxicity and hope to take away the guess work for growers so they can reduce potential crop losses."
Recommended approaches to avoid fertiliser toxicity are outlined in the GRDC Fertiliser Toxicity Fact Sheet.
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