BURLEY — Years like 2016 throw everything farmers and crop consultants think they know about nitrogen fertilizer management into doubt.
The first rule of nitrogen management is to take a soil sample to determine how much nitrogen is available. Then set a yield goal and fertilize to reach that goal.
But thanks to unusual weather patterns during the 2016 growing season, soil tests didn’t always give growers a good idea of how much nitrogen was available plus many growers underestimated their yield goals.
“Statewide, we didn’t put on as much nitrogen on all our crops because our yield goals turned out to be too low,” explained Paul Stukenholtz. “There was a lot of hard red spring wheat that had low proteins this year. Even some malt barley got rejected for too low protein and that’s unheard of.”
A warm spring, followed by a cool mid-summer, then warm temperatures again in the fall meant that every crop grown in southern Idaho reported good to excellent yields. That’s rare because years when cool season crops do well, generally means warm season crops suffer and vice versa. Plants also kept growing later into the fall.
“Our total use of nitrogen grew up until the end of the season,” said Stukenholtz. The crop consultant who also manages a soils laboratory spoke during a panel on nitrogen management during the University of Idaho Snake River Sugar Beet Conference.
Sugar beet plants need nitrogen early in the season to grow well and produce tonnage. But if nitrogen is applied too late or if nitrogen held in the soil becomes available late in the season, sugar beet plants may be enticed to keep growing rather than storing sugar. Growers are paid on both tonnage and the amount of sugar extracted from their beets which makes timing of nitrogen fertilizer more critical.
Many beet growers were faced with the agonizing decision to apply more nitrogen in July to yellowing beets that were stunted and not closing the rows. The gamble paid off, at least this year.
Company-wide, Amalgamated Sugar reported average recoverable sugar of 18.04 percent, a new company record surpassing the 2015 record of 17.4 percent. Growers didn’t just break the record, they shattered it, said Greg Dean, a sugar grower who moderated the panel.
Dean likes to put all of his nitrogen on for beets before the plants reach the fourth to sixth true leaf stage.
“It’s a drag race. I’m trying to catch all the energy I can to get to full canopy as quickly as possible,” he explained. “I drive top growth with nitrogen to capture energy for root growth and sugar accumulation.”
But others fear putting all the nitrogen on too early may lead to leaching losses. Some prefer to put on a small amount of nitrogen early, then use soil tests or tissue samples during the growing season to spoon feed the plant additional nitrogen.
If the only nitrogen growers had to account for was what they applied, managing nitrogen would be easy. But nitrogen is relased naturally from the soil throughout the growing season and figuring out how much will be available and when is an uncertain science.
Many consultants do not apply any nitrogen in the spring following a potato crop or corn crop because of the high rates of nitrogen applied to those crops. On the flip side, it is common to recommend applying half to two-thirds of the nitrogen before planting when following a wheat crop with heavy residue.
Even though alfalfa is a legume that can contribute nitrogen to the soil, it’s not uncommon for former alfalfa fields to be low in available nitrogen the following year.
“You’ve got to put on nitrogen to get a good stand,” Stukenholtz said.
But by the second or third year, growers will start to see more nitrogen become available. Often, that nitrogen shows up in soil tests taken in August and September, after temperatures have warmed and the mineralization rate picks up. That’s true also for fields that have received manure or compost in the past, or fields where cover crops have been grown.
David Tarkalson, a soil scientist with the Agricultural Research Service in Kimberly, has seen evidence of that in field used for a manure study between 2005 and 2009. Soil tests taken this spring in the plots that received manure during that time frame had 200 to 300 units of available nitrogen compared to 120 units in the non-manured plots.
“In that case, the soil test is pretty accurate. I still think there is a large reservoir of flushable nitrogen in the manured plots,” Tarkalson said.
Given how many fields have received manure and the growing popularity of cover crops, it is difficult to predict how much nitrogen may be mineralized throughout the growing season. Add to that changes in climate that many growers have been observing and nitrogen management becomes that much more difficult. That’s why it’s so important to do the most important key to nitrogen planning, Dean said.
“Take a moment this time of year to see how your plan all came together for this field in this year,” he explained. “Look at the numbers at the end of the year. It’s one key to nitrogen planning that is often overlooked.”