Crop Scene Investigation - 36: The case of the disappearing nitrogen

Monday, January 2, 2012

by BERNARD TOBIN

Whenever you plant soybeans no-till into first-time soybean ground, there's a high probability that nodulation will not take place. That's a message that agronomist Paul Sullivan preaches every spring.

It certainly was a key point for Kelvin, an Ottawa area grower who converted a long-time hay and pasture farm to soybeans in the spring of 2011. In his conversations with Kelvin, Sullivan emphasized the need for scouting first-time soybean fields in early July and checking the roots for nodulation.

If nitrogen-fixing nodules are not found, supplemental nitrogen is required to produce those healthy, dark green plants and generate strong yield potential. Kelvin also followed Sullivan's recommendation to have his seed double-inoculated in anticipation of problems with nodulation.

Kelvin finished up planting his 15-inch soybeans on May 30, after working through some tough soil conditions. The seed slot had opened up following planting, but things worked out well when subsequent rains helped close the slot, encouraging the roots to drive downward.

In July, when Sullivan scouted the six new soybean fields on Kelvin's farm, he was not surprised to find strong healthy roots with no nodules. That signalled the need for nitrogen and Kelvin was keen to heed Sullivan's advice. So he set out to apply 110 pounds per acre of 46-0-0 treated with Agrotain to reduce volatilization.

Sullivan returned to the farm in mid-August to scout for soybean aphids. He noted significant aphid pressure – about 750 to 1,000 pests per plant.

As he walked the farm, he found that most of the soybean fields had responded nicely to the recommended nitrogen application, but he could see that one field appeared to be yellow and was likely suffering from nitrogen deficiency.

After consulting with Kelvin, Sullivan learned the field in question received nitrogen at the same time and rate as the other, greener fields. Kelvin said that he had followed Sullivan's advice without question and applied the recommended nitrogen to all fields. 

Sullivan then decided to take a closer look. "When I walked into the field, I discovered these 15-inch strips of healthy plants that stretched the entire length of the field. It wasn't visible from afar, but when you got into the field, you could see what looked almost like narrow check strips of healthy plants. The strips appeared to be about 50 feet apart."

The agronomist knew the field had some potash deficiency, but there's no way it would be distributed in such a pattern. Sullivan also considered the aphid pressure he had seen in the fields: "Aphids don't generally turn soybeans yellow," he mused.

Sullivan then quickly turned his attention to the fertilizer spreader. Kelvin told him that he had used a new four-ton spreader that was in excellent working condition. He then went on to describe for Sullivan how he covered the fields.

"The fertilizer apron on the spreader is controlled by the hydraulics on the tractor and the spinner is engaged with the power take-off," Kelvin said. "I just drove into each field with the spreader, turned on the apron to begin the fertilizer flow, and then started the pto.

This would prompt the spinner and fertilizer would be broadcast in a 50-foot pattern."

With Kelvin's spreading protocol fresh in his mind, Sullivan quickly pieced together what had happened in the field. The intended nitrogen application was indeed spread on the field, but the vast majority of the soybeans did not receive any.

Do you know what happened in Kelvin's soybean field? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993.

Correct answers will be pooled and one winner will be drawn for a chance to win a Wireless Weather Station. The correct answer, along with the reasoning followed to reach it, will appear in the next issue of Better Farming. BF