Potassium’s not always a super spreader

Imagine applying blended fertiliser to your high value arable crops only to find one key element wasn’t landing where it wassupposed to be but instead was potentially compromising yield or quality.

Trials underway in Oamaru.

Researchers based in Canterbury say this is exactly what happened when they tested the distribution and spreading patterns of 25 different mixes of nitrogen, phosphorus and potassium.

Nitrogen and phosphorus in blends spread quite uniformly when field tested in different regions. But potassium didn’t, explains Lincoln Agritech agronomist Allister Holmes.

“In some cases, the variation in coverage uniformity between the potassium being spread individually and being spread as part of a blend was around 40 per cent. If you put the three on together with one application you are not going to get a uniform distribution of the potassium, and in some crops this leads to a lower yield, or quality issues.”

It’s the first time such work has been done in New Zealand.

The results come from a three-year study funded by MPI Sustainable Food and Fibre Futures, and co-funded by the Fertiliser Quality Council, Groundspread NZ, and Environment Canterbury.

Holmes says plenty of previous work has been done here on the spreading characteristics of single products, like urea and DAP, and some has also been done on blended fertilisers in their complete form.

“But what hasn’t been done until now is breaking down the blends to look at the ballistics of each different component on its own.”

Perhaps surprisingly, there hasn’t been a lot of previous research done on this overseas either, he says.

The answer to why one element in a three-way blend is so much more irregular in its placement than the other two most likely lies in its chemical nature.

“The consensus is that the ammonium sulphate and di-ammonium phosphate granules in the blends we tested tend to be quite rounded and uniform in shape, whereas the potassium chloride granules tend to be angular and irregular.”

The project involved field tests in Waikato, Canterbury, and Otago and analysis of the contents of around 20,000 trays of fertiliser.

It tested 25 different lines of blended fertilisers from multiple companies, broadcast over fields by eight different spreaders.

“The blends had quite different physical characteristics in the North Island and South Island, because, even though they may have the same brand name, they are blended locally,” says Holmes.

The testing involved getting fertiliser trucks or spreaders on tractors to drive over a series of trays, then weighing the material that lands in the trays and analysing the blend of compounds in them.

Funding for the project is due to finish soon, but the research team has applied for a 12-month extension from MPI to test another type of fertiliser mix—namely urea and superphosphate.

“In the past 12–18 months there has been quite a lot of talk about issues with these blends. The products have a chemical reaction when they are mixed, meaning they have to be handled with care and spread quite quickly otherwise they build up on the spreading equipment which can lead to poor spreading performance.”

Meantime, the team has completed an extensive round of testing of four different manufacturers of truck ground spreaders near Oamaru and is soon to complete a final round of spread testing of tractor-mounted spreaders in the Waikato.

The data from these tests will be combined with the data from the first rounds of testing, and then recommendations will be made for the fertiliser industry, ground spreaders and growers who use blends of nitrogen, phosphorus and potassium.

Of the 720,000 tonnes of fertiliser applied every year to NZ pastoral and arable farms, 25 to 35 per cent is blended, creating blends that may contain nitrogen, phosphorus, potassium, sulphur and/or other elements in one product.

Ostensibly, applying two or more nutrients at once saves farmers and growers time and money. But that assumption is based on the prediction that it spreads evenly.

The study arose out of repeated striping or streaking in fields of crops, where the fertiliser hasn’t distributed uniformly.

“The initial discussion started about five years ago, when there was some concern from both groundspreaders and the Fertiliser Quality Council, that we didn’t have enough knowledge to be confident about the spreading properties of blended fertilisers.”