Nitrogen and phosphate application periods are starting to drift closer together with more regional council pressure on timing, Ground-spread Fertiliser Association president John Shultz said.
“In Canterbury we have had the regional council advising no phosphate be applied in August when we would traditionally start applying in early July-August. There’s a real reluctance to start until September.”
Meantime, those applications start to push into nitrogen applications in dairying regions, in particular, while cropping fertiliser applications are also coming on stream.
“Then you add in the pressure that comes from the weather, for example, down in Southland this year where the weather means nothing can happen until it dries out a bit more.”
But Shultz said the workings of Overseer also do little to ease pressure on timing and demands.
“Down south, in particular, no early nitrogen application will go on. It shows up graphically on Overseer if it is in winter, which is fair enough if the risk of leaching is there.
“But Overseer operates on assumptions.
“It is calendar driven rather than condition driven and until July 31 it is classed as winter, whereas August 1 is okay, regardless of what conditions are actually like.”
Schultz said for some farmers later August tends to be a wetter period than late July but the message through Overseer is that application is less risky in August.
“But, theoretically, Overseer is a very good system. It operates to determine the losses of nutrients out the bottom of the system. There is certainly an awareness there from our clients to do the right thing. None is saying we wish we could go back to where we were.”
Schultz acknowledged phosphate losses have tended to be overshadowed by nitrogen in recent years but for some catchments phosphate-sediment issues are more significant for farmers to manage.
“We have lived with nitrogen application changes out of sensitive periods for some time but this year is the first we have noticed it for phosphate. It is not a 100% shift but even a 10% shift out of usual application periods puts pressure on supply and application.”
He wants Overseer modified to take more account of actual conditions at application time.
“No one is saying we want to put more on. It’s just sometimes months like July can be good for application.”
Overseer chief executive Caroline Read said it appears the software is being confused as a real-time application measurement tool when it is based on an assessment of a 30-year climate profile rather than a dry month.
“While there is no actual hard cut-off in the model, the model is set up with regional variations based on Niwa data. So areas where there might be a significant difference in average rainfall between July and August might see larger changes in drainage.
“Where users have been informed there is a hard change in modelling between July 31 and August 1 in the year, this is not true, but the modelling reflects the long-term climate for a particular region not the specific climate for a specific year.”
Overseer is not designed to advise farmers on the exact timing of fertiliser applications in a specific year. It is designed to describe the impacts of seasonal fertilising for their farm generally.
“Overseer should not be relied on for day-to-day decisions on-farm. It should be relied on to assess the impact of different farm management approaches over the long term.”
Determining if the conditions are suitable for applications in autumn or winter is best done by assessing time-specific environmental data from the farm.
A fertiliser researcher said the model is based on a steady-state basis and a single application of fertiliser should have only a relatively minor effect on nutrient losses over time.
“What Overseer has done is make farmers aware of where their losses may be and adjusting application time is one of the things they can do to reduce those.”