Springfield, Ill. – Making nitrogen recommendations from a soil test is often more art than science. In Western Canada, recommendations are based on available soil nitrogen, nitrogen release by the soil and the amount of nitrogen required to grow a target yield of a specific crop.
In parts of the U.S. corn belt, researchers are basing nitrogen recommendations on yield response, rather than just target yields.
Paul Fixen is the director of research for the International Plant Nutrition Institute, based in Brookings, South Dakota.
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At the recent InfoAg precision farming conference in Springfield, Illinois, he discussed this new concept of developing nitrogen recommendations in corn crops.
“Why do we want to change from yield based to yield response based nitrogen management?” asked Fixen.
“In one data set in corn from central Illinois, collected from an Illinois State University regional data base for crop fertility, to produce the same optimum yield of 200 bushels per acre on the same field, the optimum N rate varied from less than 50 pounds of N per acre to 250 lb. per acre.
“So this suggests there might be an issue with yield based N recommendations.”
Fixen defines delta yield as the difference between the yield where no fertilizer nitrogen was applied and the yield where the optimum amount of nitrogen was applied so nitrogen was not a limiting factor.
“There seems to be a fairly robust relationship between delta yield and nitrogen need. The relationship between delta yield and optimum N doesn’t move around very much,” said Fixen.
“If you can tell me what your response is going to be in the delta yield, we can do a reasonably good job in predicting what nitrogen need there might be.”
Fixen said delta yield is a promising approach for incorporating yield information into nitrogen recommendations, where soil nitrogen supply is not being measured by soil tests.
“It requires not just an estimate of your attainable yield, but also of your check yield. The huge challenge is estimating what yield we would get if we didn’t apply any fertilizer N. But there are ways to get around that,” he said.
“Retailers who work with groups of farmers and who have access to nitrogen-use technology could generate local databases of check yields sorted by crop, type of year or whatever the case might be.
“That kind of information could be a valuable local asset.”
Fixen said nitrogen recommendation guidelines are best developed using sites with multiple years of optimum yield and optimum nitrogen rate determination.
He said within a region that has similar weather patterns, yield goal differences are primarily due to soil differences.
But when developing recommendations, cultural practices must also be taken into consideration.
A cultural example with corn would be the practice of increasing plant populations, which directly affects optimum yield and optimum nitrogen recommendations.
Fixen referred to an Ohio study where plant populations of 16,000 plants per acre had an optimum nitrogen requirement of 92 lb. per acre to produce an optimum yield of 125 bu. per acre, while 24,000 plants required 100 lb. per acre to produce 142 bu. per acre and 32,000 plants per acre required 118 lb. per acre to produce 155 bu. per acre.
Other Illinois trials indicated higher nitrogen requirements for higher plant densities, as well.
Fixen said the best approach would be to develop a local database of yields where no nitrogen is applied.
He used central and northern Illinois as examples, where typical average unfertilized corn yields following soybeans were around 100 bu. per acre and 125 bu. per acre, respectively. If it’s corn following corn, yields would be 15 to 35 bu. per acre lower.
Estimates would be lower for coarse textured soils, dry conditions or excessively wet soils.
They would be higher for high organic matter soils, fields with a history of manure application or fields with nitrogen carryover after droughts or other crop failures.
Fixen said most nitrogen fertilizer recommendations have a yield based component in them.
“One of the early goals of optimizing N fertilizer recommendations was to develop a rational basis for maintaining the levels of N fertilizer use, not only to optimize crop production but to provide a balance between N inputs and losses of nitrate to surface and ground water.
“In the 1960s it was common practice to associate attainable yield with a region based on climate and soils. The laboratory would look at where the soil sample came from, then make a recommendation based on typical yields in that area.
“With computers, there was a shift from regionalized recommendations to where a yield goal was part of the soil test application.”
Now, Fixen said there is starting to be a shift back to more regionalized nitrogen recommendations.
“In some regions, the goal is to do a better job of estimating attainable yields of specific fields, in specific management programs,” he said.
“The University of Nebraska has developed a crop simulation model called Hybrid Maize, with a sophisticated crop simulation program, but a user friendly interface for the farmer.”
Fixen said from 1964 to 2005, corn grain produced in the U.S. per unit of fertilizer nitrogen used moved from 0.76 to 1.15 bu. per lb. of nitrogen. Since 1975, with a 12 percent increase in nitrogen fertilizer use, corn yield has increased 60 percent and nitrogen efficiency has increased 51 percent. So the amount of fertilizer nitrogen needed should have less impact on attainable yield.
While establishing target yields or attainable yields is one aspect of determining nitrogen fertilizer recommendations, Fixen said using yield response may help further refine nitrogen recommendation rates.
