Growers more proactive with N at seeding in 2013
WATERLOGGING drained about 80 kilograms a hectare of nitrogen from soils in parts of southern New South Wales in autumn last year. That’s the equivalent of $120/ha on urea at $1.50/kg.
This caused lower yields and lower protein in crops harvested, which has driven many growers to want to know more about last year’s N story and whether it will have any implications for this year.
The level of soil total N is extremely sensitive to moisture - in lower rainfall areas, changes in total soil N are equal to the N removed by crops but in wetter areas, denitrification means more soil N is lost than crops remove.
The main reason for low N in 2012 was denitrification – the conversion of nitrate to N² and N²0 and loss to the atmosphere – during the March floods.
This was widespread in southern NSW in autumn 2012, where waterlogging led to more than 50kg/ha of N disappearing from soils as microbes responsible for denitrification found perfect conditions in warm and anaerobic soil conditions after floods.
Grassroots Agronomy’s Greg Condon, based at Junee, held a farm field day for clients just before seeding with CSIRO researcher Dr John Angus to outline the story behind low protein wheat and where the N went.
The extent of denitrification resulting from last year’s waterlogging in early autumn, in which soils stayed wet until July, is considered an anomaly. This year, some tests before seeding have showed 80-100 kilograms of N in the top 0-60cm of soil.
But even so, Greg says growers are being more strategic in using nutrient evaluation tools such as deep soil testing, paddock history and Yield Prophet, perhaps mixed in with a bit of ‘gut feel’ at times.
“We haven’t seen low soil N as a general trend this year but we’ll be doing more testing after seeding and later in the season,” he said.
“We’ve found the general rule of thumb that N will be half the depth of the wetting front to be true.”
He says the lesson from 2012 was timing of N and whether growers should be considering a more proactive approach, such as putting on one-third of N upfront in wet conditions rather than more later.
“When N is lost to the system like it was, we can be more proactive with N upfront, particularly on canola where we can get the plants set up earlier in the season,” he said.
For canola, this might mean putting on 15-20kg/ha of N at sowing, or for wheat 10-15kg/ha of N, where growers have the ability to split N fertiliser from the seed at sowing to avoid issues with crop emergence.
“The timing is highlighted when you have extreme rainfall events, we had a profile of moisture which was great for yield but the N wasn’t there for protein.”
But the question is being considered in a broader context of the sustainability of rotations without legumes.
“With continuous cropping, it’s quite a big issue at the moment in terms of the whole N story and how to deal with N for canola, wheat and barley without growing pulse crops.
“There has been a lot of interest in pulses for grain and brown manuring for their ability to have a slower release form of N in the system.”
Greg says growers need to be conscious that 2013 has lower subsoil moisture levels which means yield potential, particularly for canola, will be reduced.
“We don’t have that buffer in medium and low rainfall areas and that has a big influence over N requirements because yield potential is less so the risk is higher without that moisture.
“We’ll be relying on in-crop rainfall this year so yield potential hence the N requirements won’t be there if the profile doesn’t fill up in the winter.”