Features Agronomy Soil
Where has all the nitrogen gone?

After soil testing and using the recommendations to fertilize to meet the needs of the crop, some Manitoba growers and extension staff saw a dramatic drop in fertility available to the plant late in the season in 2007. It came as a shock to many and now the question are raised on why and how it happened and if there is something that can be done to correct the problem.

Spring soil tests will indicate what is really available to the crop.

At a series of in-field meetings, growers were invited to walk through one acre fields of potatoes at several sites. The staff that set up the plots used best management practices and tracked the progress of the crop throughout the growing season. Each meeting had a focus, such as ‘emergence’, ‘tuber initiation’, ‘early tuber bulking’ and ‘late tuber bulking and maturation’, but, no matter what the main topic, fertility was high on the list of items to be covered each day.

“Fertility wasn’t all we discussed, but it was a major part of the meeting,” says Tom Gonsalves, a potato specialist with Manitoba Agriculture, Food and Rural Initiatives. “As the season progressed, we noticed at some of the sites that fertility levels were dropping more than we expected. This was a concern, so I started talking to other consultants, growers and researchers and found our results weren’t unique. It turns out what we were seeing is common to potato growing areas in Manitoba.”

Gonsalves wondered why the nitrogen levels seemed to be declining so dramatically. Anecdotal observations across the province caused him to begin investigating what might have happened and whether it could be corrected.

“When we talk about fertility, we should have a complete field history with a spring soil test to indicate the levels of nitrogen in the soil,” Gonsalves explains. He says he prefers spring soil tests to fall soil tests because he believes the information is closer to what is really available to the crop in the spring. “In our plots we were using the same production practices that most growers use and we had field histories on all our plots. We were producing Russet Burbank potatoes for processing on irrigated land the same as other growers. We also did petiole sampling to measure what nitrogen was in the plant using the standard accepted procedure.”

He says comparing soil test data and petiole sample results gives a good picture of what is happening with the plant. Soil tests will indicate what nitrogen is available to the crop and the petiole test tells how much nitrogen is in the plant. He says in-season petiole and nitrogen tests are useful indicators of what is happening to the crop and
in the soil.

He says a spring soil test provides a benchmark of the fertility a grower is beginning the growing season with. Growers also should take periodic tests throughout the season. “Having more tests throughout the season enables growers to track trends and then you can react if required,” he explains. “It’s important to stay consistent with your soil and petiole tests. Take your soil tests at the depth your laboratory recommends.” In rough numbers, he continues, a grower could expect to see 90 to 100 pounds of nitrogen in mid June, dropping to 50 to 60 pounds in late July and ending around 40 to 60 pounds in August. However, when the tests showed that fertility levels were at 20 to 30 pounds in July, Gonsalves not only wondered why but what could be done to repair the problem.

The ‘why’ could be the sandy soils common to some of the potato growing areas of Manitoba that can allow the nutrients to leach or the nitrogen could be tied up in saturated soils, or there could be denitrification. Without research to answer the question ‘why’, all Gonsalves could do was speculate on the answer. What concerned him more, however, was what to do about it.

“Traditionally, growers would put down nitrogen prior to planting and top dress at hilling,” Gonsalves explains. “There is a movement now to ‘spoon feed’ the crop which could involve
fertigation through the pivot which is an efficient way to get nitrogen to the crop throughout the season. If growers are using the fertigation route, I would put on approximately 10 pounds per acre pre-application. But growers could also apply nitrogen when they are applying their fungicide, although they are limited to the amount that can be put on this way because there is a danger of burning the plant if applying too much nitrogen.” He adds that a ground application of dry product follows, then watering it in, is also an option. Ultimately, he says, growers should use the method that works best for them as long as the fertility reaches the plant.

“If you are adding nitrogen in any of the accepted methods, it would be rare to see a quick spike in the nitrogen levels in the soil,” Gonsalves adds. After a nitrogen application, the soil tests may never show an increase as the crop is using the nutrient. What growers may see is a plateau in soil nitrogen levels instead of an increase.

For 2008, Gonsalves is planning to monitor a number of fields to determine what management decisions make a difference. “I might cut back on nitrogen to one third of the recommendation up front, put on 30 to 40 percent at hilling and apply the remainder a couple weeks after hilling, all the time planning to be done by the end of July.” With this process, he suggests, if a grower needed to put 190 pounds of nitrogen on the crop, the applications could be split to 75 pounds at planting, 75 pounds at hilling and four applications of 10 pounds to round out the requirements to the end of July. “I see spoon feeding as a viable way to improve the crop, especially on soils that are prone to leaching,” Gonsalves says. This could also solve the drop in fertility he has noted later in the season.

However, despite the numbers being much lower than desired on some of the demonstration plots, the resulting crop yield and quality were reasonable. Gonsalves says the yield and quality were still up to standard, which was unexpected given the low fertility numbers. “I only have one year of anecdotal observations,” he says. “In order to bring some clarity to the question, I plan to monitor the fields in 2008 more closely.” This would, hopefully, include monitoring in actual field situations which might help clarify the issue.

Gonsalves cannot explain definitively why fertility is dropping dramatically in some fields in various potato production areas across the province, but he believes that spoon feeding nitrogen to the crop would help keep the soil nitrogen at the required levels during the season. He also suggests that regular soil nitrogen and petiole tests throughout the growing season will help identify the problems sooner and remedial action can be taken before yield and quality might be affected. -end-