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Corn+Soybean Digest

Rains And Potential For N Loss

You may have heard that loss of nitrogen (N) from the soil is associated with excess moisture – rainfall or irrigation. With rainfall during the last two weeks of May being much above normal, there is concern that some of the N fertilizer applied in the fall of 2003 has been lost. If so, is there a need to apply supplemental N during the growing season? If there was N loss, how much N should be applied? Is N loss affected by the source of N applied last fall? These are some of the questions that will be asked when soils dry.

Because of the transitory nature of N in soils, it’s difficult, to predict losses caused by excessive rains. There’s a general understanding that loss by leaching is a major concern for the sandy soils. However, compared to several years ago, most of the N fertilizer applied for corn grown on sandy soil is used as a sidedress treatment. Therefore, loss from leaching in sandy soils should not be a major concern at this time.

Loss from denitrification can be a concern where soils have a silty clay loam or clay loam texture. Yet losses due to denitrification have not been predicted with any reliability. This loss mechanism is highly dependent on soil temperature and the length of time that the soil remains saturated. Since the root zone in much of Minnesota was not recharged when the rains started, a considerable amount of the rainfall could be absorbed before soils became saturated. With this fact added to the below normal temperatures, loss due to denitrification should not have been substantial.

Looking back, we faced a similar situation in southern Minnesota in the spring of 2001. Depending on location, April through June 8 rainfall was 3.3 to 8.9 in. above normal. Naturally, there was concern about the fate of fertilizer N applied in the fall of 2000.

In an attempt to measure loss, soil samples were collected to a depth of 24 in. from 17 fields in southern Minnesota. The samples were analyzed for nitrate-nitrogen. The amount of nitrate-nitrogen measured was then compared with the rate of fertilizer N applied with allowances for residual N from the previous soybean crop. After looking at the results from the 17 sites, there did not appear to be a substantial loss of fall applied N.

This year, there was considerable storage space in the soil for much of the May rain in a large part of the state. This fact, coupled with low soil temperatures, reduces the concern for loss by denitrification. The information collected in the spring of 2001 also reduces the concern.

Is there a sampling procedure that can be used to predict the amount of in-season N needed for corn if N loss is suspected? This question has been asked in the past and probably will be on the minds of many this year. Unfortunately, there is not one test that is reliable. The condition of the corn crop is probably the best guide.

Mike Schmitt developed a scorecard for corn several years ago. A decision to apply in-season N was based on a combination of weather and the appearance of the corn. There are three questions that should be scored. Theses questions and associated scores follow.

Question #1.
When was the N applied? (Points)

a) 4 in. deep in the fall with soil temps above 50 ° F (5)
b) 4 in. deep in the fall with soil temps below 50 ° F (4)
c) 4 in. deep in the fall with N-serve and soil temps above 50 ° F (4)
d) 4 in. deep in the fall with N-serve and soil temps below 50 ° F (3)
e) in early spring (late March or April) (3)
f) in May (2)

Question #2.
What was the predominate spring (May) soil condition? (Points)

a) normal or drier than normal (1)
b) wetter than normal (3)
c) there was/is water standing in the fields (4)

Question #3.
How does the crop look? (Points)

a) taller than 12-16 in., but chlorotic (5)
b) shorter than 12-16 in., but chlorotic (3)
c) shorter than 12-16 in.; normal, green color (2)
d) taller than 12-16 in.; normal, green color (1)

Add the numbers for the answers to the three questions. With a score of 7 points or less, the N program is fine and additional N is not needed. With a score of 10 or more, supplemental N fertilizer is recommended. A score of 8 or 9 falls into a gray area, and it is recommended that the calculation be done again in one week because corn height and/or color is likely to change. The “re-evaluation” option only applies if there is enough time to sidedress corn.

If supplemental fertilizer N is suggested, rates of 30-60 lbs./acre are probably appropriate. Suggested rates may be higher (80 lbs. N/acre) with scores of 12 or greater.

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