Drying seedbeds provide opportunity to address low nitrogen levels in the north
09 December 2022, AU: It’s hard to believe that after one of the wettest winters on record, some northern growers have had to pull up their planters due to seedbeds being too dry to plant summer sorghum.
While moisture profiles are strong across the region, NSW Department of Primary Industries (NSW DPI) Research Agronomist, Loretta Serafin, says it’s easy to forget how quickly the seed bed can dry out in warm, windy conditions and growers could be opting to wait for another rain event to plant.
“Growers in the north would be confident in their soil moisture profiles but they need a good link from a moist seedbed to the underlying profile to help sorghum seed get out of the ground and grow rapidly,” she says.
“The good news is that there’s still another five weeks left of the planting window, closing in early January, so if growers are wanting to take advantage of good moisture profiles and drive their systems in an opportunistic way, there’s still time to wait for a rainfall event to soak that first 5-6 centimetres of soil.
“One thing growers could be doing now while they wait is thinking about their crop nitrogen (N) requirements and how much N they might have in their paddocks, because any summer crops will need that support to deliver promising yields.”
This month, the Grains Research and Development Corporation (GRDC) conducted a webinar on nitrogen requirements for summer crops, as growers were concerned about low nitrogen levels.
You can watch a full recording of the webinar via the GRDC website.
The University of Queensland’s Chair of Tropical Agronomy, Professor Mike Bell, says in some cases, paddocks in the northern region would be depleted of nitrogen after two big winter crops and the ongoing wet conditions causing loss of mineralised nitrogen through denitrification.
“There are a lot of under-fertilised paddocks in the region and while moisture profiles are promising for a summer crop, growers’ nitrogen levels wouldn’t be where they usually are at this time of year. They’re going to have to do something to address that if they’ve planted sorghum, or are planning to,” he says
“The cost in terms of yield loss of not doing something to support the crop is large.”
Professor Bell says if growers currently had a dry seedbed, they could take the opportunity to apply nitrogen and hopefully receive follow up rain to start planting.
“The climate forecast is pretty strong for a wetter-than-average summer, so the chances of receiving follow-up rain are good,” he says.
Professor Bell says a big issue, which generated a lot of discussion in GRDC’s webinar, was that some growers have already planted their summer crops but didn’t have time to address potential low nitrogen levels at sowing.
“The big question for the people who have already planted crops into depleted soils is ‘how do we catch up?’,” he says.
“It’s a hard situation because growers are wondering whether they top dress nitrogen over their crops, which could pose a risk, especially if they have wet soils, because both volatilisation and denitrification risks are elevated.
Professor Bell says dry topsoil was the best opportunity for growers who have already planted crops to address nitrogen requirements.
“Whenever you get the top 10cm of soils to dry out enough to get it on, the next rain event will wash it in so the crop can use it,” he says.
“Crops won’t desperately need nitrogen until six weeks from planting, so if growers have a dry period before then, it’s not too late to apply it.
“Once you’re beyond the 6-7 week period after sowing though, growers should expect the response is going to be less than if they were able to apply it earlier.”
Growers were also asking whether now was a good time to get early fertiliser N into soils for next year’s winter crop.
“The answer to that is unequivocally no,” Professor Bell says.
“With a wetter than average summer predicted, the profile already near to full and no crop to use water or nitrogen, there’s a huge risk of that nitrogen being lost through denitrification.
“While growers will need to address this leading up to the winter cropping season, they shouldn’t be applying N to their fallows now.”
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