Of the 720,000 tonnes of fertiliser applied every year to New Zealand pastoral and arable farms, 25% to 35% is blended, containing nitrogen (N), phosphorus (P) and potassium (K) in the one product.

But while this may seem to offer time and cost savings to farmers, it can also result in uneven crop yield through uneven spread.

A three-year project looking at spreading blended fertiliser is now offering some hard data and answers about what’s happening.

“The issue we’re trying to solve is there has been repeated striping or streaking in fields of crops, where the fertiliser hasn’t distributed uniformly,” says project leader and Lincoln Agritech Agronomist Allister Holmes. “This is a real concern, particularly in high-value crops like seed crops.”

Funded by a Sustainable Food & Fibre Futures grant, and co-funded by the Fertiliser Quality Council, Groundspread New Zealand, and Environment Canterbury, the project has involved field tests in Waikato, Canterbury, and Otago and analysis of the contents of around 20,000 trays of fertiliser. It has 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 Allister.

“The testing has 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,” says Allister.

And what have they found?

“We’ve found that the nitrogen and phosphorous in blends spread quite uniformly, but the potassium didn’t.”

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%.

“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.”

The project team is now working on recommendations and practical solutions.

“The most likely practical solution will be to spread the nutrients individually rather than as part of a blend,” says Allister. “That will increase the price of applications, because you’ll be driving over a field three times instead of once, but you’ll have more control.”

That control will also mean more accurate environmental reporting, increasingly required by regional councils and customers. “More and more they require proof of what you are doing and how good your farming practices are,” says Allister. “That includes fertiliser application.”

The project team is now applying for a 12-month extension, to test more regional blends, at the request of industry.