"Where will N loss have been the most likely and how much will have been lost?"
This is the corn fertility question on the minds of many as we add up the rainfall totals since the second week in June 1998. Unfortunately, science has not progressed enough to make this an easy question to answer, and making an estimate at the level of the individual field requires good knowledge of local conditions since the nitrogen was applied. When determining whether losses of pre-season applications are extensive enough to warrant supplemental applications answers must be found for the following series of three questions.
With high rainfall levels, nitrate is readily lost from very sandy, well drained soils by leaching and from heavy, saturated soils by denitrification. To answer Question 1 you need to consider both the form of N that was applied and the date of the application.
Anhydrous and urea contain no nitrate N whereas 28% contains a quarter of the N already in the nitrate N form and low pressure solutions may have up to a third of the N in the nitrate form. This N is susceptible to loss immediately upon application. Ammoniated N sources must be converted to nitrate before this portion of the fertilizer can be lost. Nitrification occurs most rapidly in warm soils (60 to 85oF) that are aerated. Under these conditions, most of the ammonium from fertilizers other than anhydrous will be converted to nitrate in one to two weeks. Conversion of anhydrous ammonia is slower because the material itself is toxic to the soil microorganisms and nitrification will be delayed 2 to 6 weeks. Use of a nitrification inhibitor with anhydrous can further delay nitrification.
The bottom line is that, without an inhibitor, all N applied in April and early May, regardless of original source, would likely have been in the nitrate form by the time the "Monsoons" descended in the second week of June.
Once N is converted to nitrate it can be lost during and after heavy rainfall by the process of either leaching or denitrification.
Leaching : On a sandy soil with rapid infiltration, one inch of rainfall moves nitrates down about a foot. If total rainfall has been less than 3 to 4 inches in the past few weeks then the nitrate will remain in a zone of the soil profile that is accessible to plant roots (assuming no tile lines at a shallower depth). If rainfall has exceeded 6 inches then consider all nitrate lost.
Denitrification: This occurs on soils that can become saturated and will be the primary loss mechanism around the state in the past few weeks. Research in the wetter regions of the cornbelt has demonstrated that in May and June 4 to 5 % of soil nitrate can be lost each day a soil is saturated. A soil is saturated when all the pore spaces are filled with water, but a field can be saturated without having a duck pond on top. One of the situations observed this year was that saturating conditions were followed by a rapid surface dry down. However, under the top crust of more poorly drained fields, soils remained "soupy" and conditions were good for denitrification for a period longer than would be expected from a windshield field tour. When estimating losses to denitrification, remember that no further nitrification of ammoniated N occurs once a soil is saturated and that this N is "protected" from loss.
The answer to this final question is as difficult to determine as the answers to the previous two. Experience and state yield records suggest that 20% yield reductions may be expected on poor stands that survive the current wet spell.
Nitrogen loss will have been mostly likely to occur on excessively well-drained sandy soils and on heavy textured poorly drained soils. Incidentally, these are soils where research has demonstrated that year-in, year-out it pays to sidedress (See Sidedressing Tips below). Given that coming up with an answer to each of the three questions above requires estimation of "unknowns" such as soil temperatures and length of saturation, the table below was created to allow estimation of N loss based on the N management practice and current visual observations of the crop. The critical factor to understand when determining the likely cause of a sickly, yellow stand is that drowning roots will not accumulate nutrients. Therefore shoot deficiency symptoms can occur even when ample amounts of N remain in the root environment.
| Determining supplemental N needs from the visual symptoms and preplant N management strategy | |||
|---|---|---|---|
|
Visual Symptoms |
Nitrogen Source and Management |
Source of Problem |
Action Plan and Comments |
|
Little chlorotic plants (4 leaf collars or less) |
All forms of N applied before May 15th & Non-anhydrous forms of N applied between May 15th and June 4th |
Nitrogen Loss |
Supplement with sidedress of 40 –70 lbs/A. Yield potential likely reduced so lower rates (40-55 lbs) may be sufficient. |
|
Anhydrous applied after May 15th & all other non-nitrate forms of N applied just prior to soil saturating rainfall† |
Poor root function due to waterlogging |
Supplemental N probably not needed. Reassess crop condition after root zone has been unsaturated for 4-7 days. Consider using the PSNT at this time to assure N availability †† |
|
|
Big chlorotic plants (5+ leaf collars) |
All forms of N applied before May 15th & Non-anhydrous forms of N applied between May 15th and June 4th |
Nitrogen Loss |
Supplement with sidedress of 40 –70 lbs/A. Yield potential not impacted so higher rates (55-70 lbs maybe needed. |
|
Anhydrous applied after May 15th & all other non-nitrate forms of N applied just prior to soil saturating rainfall |
Poor root function due to waterlogging |
Supplemental N probably not needed and plants should recover rapidly as soil dries. If plants still yellow, consider using the PSNT if you have access to high clearance equipment. If you will not be able to apply N to tall corn, you may wish to apply 20 to 40 lbs of N as an "insurance policy". |
|
|
Beautiful little green corn |
All forms of N applied before May 15th & Non-anhydrous forms of N applied between May 15th and June 4th |
No problem evident yet but warm spring weather may have resulted in higher than average N losses during April and May |
Supplemental N probably not needed but you may wish to use the PSNT as an "assurance policy". |
|
Beautiful big green corn |
All sources of N applied at any time |
No problems |
Buy your neighbor a cup of coffee and discuss your excellent N management |
|
† Estimate the loss of nitrate N fertilizer applied just before saturation as 5% per day. |
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|
†† See following section on "What else ca I do?" |
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Those making preplant N applications commonly ask questions relating to testing the soil to quantify fertilizer N persistence in the soil and supplemental N needs. Dr. Jay Johnson at Ohio State has suggested that testing the top foot of soil for nitrate could help a farmer make a decision on a supplemental application. Johnson recommends that a test value of 25 ppm or greater be interpreted as sufficient for crop growth and taken as an indication of minimal fertilizer N loss. However, the meaning of soil test values below this benchmark is less clear as they could indicate either that nitrate has been lost or just moved deeper in the rooting zone where it is still available. Therefore, a low test value cannot be used as a definite indicator of crop need. Those with rapidly growing corn that is already knee-high and chlorotic may not want to risk waiting for the results of the soil test before deciding if supplemental N is needed.
A more informative alternative to soil nitrate testing in this situation is chlorophyll meter readings. To use the chlorophyll meter, reference strips (strips through the field that receive 40-80 lbs/A additional N) must be established prior to the time when you want to assess crop N status. All bulk field readings are made relative to readings in the reference strips, and when the bulk field readings fall below 95% of reference strip readings, then an economic response to supplemental N can be expected. The N should be applied before tasseling at rates of 40 to 60 lbs/A (see below for tips on applying N to tall corn).
Not only have field conditions been too wet to escape N loss to leaching and denitrification, they have also been too wet to allow equipment back in the field for sidedressing. Many farmers who were planning to sidedress, as well as those who have determined their preplant application got washed away, are beginning to ask the "how late … how big" questions.
Corn needs to be able to accumulate large quantities of N during the grand growth stage. Nitrogen accumulation rates during the seedling and early vegetative stage are relatively low, but from the 8 leaf stage (8 leaf collars) through pollination corn typically accumulates between 4 and 8 pounds of N per day, totaling over 60 percent of its N requirement. Nitrogen accumulation rates fall off significantly during the reproductive stages and N accumulation is typically completed by 4 weeks past tasseling. Therefore, the objective of sidedressing N is to have applications made before plant demand is high, ideally before the 8 leaf stage with most of the N on prior to the 10 leaf stage. Supplemental applications (spoon-feeding) should be complete by or shortly after tasseling. Under conditions of severe N deficiency, a response to low rates of N (30 to 60 pounds) applied up to three weeks after pollination my be expected.
The rules for short corn apply for tall corn… N solutions should not be applied directly to foliage ("over-the-top"). Research from Minnesota has shown that the extensive foliar burn following solution applications "over-the-top" after the 4 leaf stage significantly reduces yields. Tall corn requires the use of high clearance equipment such as a high boy. Ideally the equipment is fitted with drop hoses that deliver the fertilizer in a solid stream on the ground. Hoses should be long enough to drag on the ground, and a weight attached to the end will help keep the hose between rows.
What about other materials for broadcast or aerial application? As with sidedress applications made in a timely fashion, granular fertilizers such as urea or ammonium nitrate can be used for late applications of N. Burn spots will result from granules that fall into the whorl but, while unsightly, yield reductions from this damage should be minimal.
For other information about corn, take a
look at the Corn Growers Guidebook on the World Wide Web at
http://www.kingcorn.org