Special Edition   April 24, 2009

FERTILIZER CONSIDERATIONS AFTER FLOODING AND DELAYED FIELD CONDITIONS

There are many considerations for growers following field flooding or at least delayed field conditions due to wetness. These include logistics, delayed application, shallow application, nutrient transformations, and nutrient leaching.

Logistically, this spring was shaping up to be challenging even before the current inundation. The planting season calendar is now even more compressed. On the good side, fields will not all dry at once, so that will help distribution of fertilizer on a regional and local scale. On the not-so-good side, road conditions are terrible, and that will most certainly slow transport of nutrients to both retail locations and to the farm. At the warehouses, the supply of fertilizer appears to be good. Fertilizer costs are lower compared to last spring with the exception of potash.

Hardly any fertilizer was applied in the Valley region last fall, when in most years more than half is applied. More than half of the fertilizer nitrogen in the state is normally anhydrous ammonia; however, with wet soils, it will be impractical to apply ammonia due to the mucky conditions in medium and heavier soils this spring. Therefore, I expect that many growers will choose to broadcast urea, band urea, and in some cases even broadcast or band liquid 28-0-0. In many years, the cost of 28-0-0 (UAN) is substantially higher than urea, but this year costs are often very similar. Because of the small difference in costs there might be UAN used, particularly if custom-applied by the retailer who is set up with a sprayer and nurse equipment.

An option for row crops would be to delay ammonia application until side-dress season, when corn and rowed sunflowers are at 4-6 leaves. If the weather continues to be wet through this period and anhydrous is still impractical to use, rigging an applicator with coulters and dribbling UAN into the shallow slit made by the coulter, or even dribbling UAN between the rows on the soil surface is often an acceptable method of application.

Application of urea to sugar beets preplant should be no more than 75 lb N/acre. Last year under dry conditions, reduced stand was seen in a Crookston, MN study even at levels this low. Because of the restriction on preplant N to sugarbeets, split application may need to be conducted. Applying half of the N on preplant and then side-dressing or dribbling between rows later on will provide the N required.

Small grains need their N at seeding or before. Small grains grow so quickly that the logistics and weather cooperation required to make delayed N application work often is thwarted. Dry weather following a top-dress application can result in N not getting to the plants early enough for full benefit. A series of studies across the state several years ago showed that using top-dress N application was not as consistently efficient as preplant N applications. Canola would not require the N on as soon as small grains, but the weather is still against us. It is wet now, but it could be dry during a critical stage of growth and the top-dressed N would be rendered inefficient as a result.

Growers will probably be able to seed small grains faster than fertilizer will be supplied logistically. If this happens to a grower, it is more important to seed than to wait for fertilizer. The grower should definitely apply a small amount of starter P with the crop, but beyond that the planting delays are critical to avoid. If a grower is forced to seed and fertilize later, urea or UAN can be used. If urea is used and the forecast for at least ˝ inch of rain is less than 90% chance in the next couple days, have the urea impregnated with Agrotain® to have about 10 days of assurance that the urea will not be lost. Another option is to stream-bar UAN as soon as possible after seeding. Half of the UAN is ammonium nitrate, which cannot be lost by volatilization of urea, and the urea half is aided by the concentrated effect of surface banding, which reduces the rate of urease enzyme activity and subsequent urea loss.

If fall anhydrous was applied last year after soils cooled (this would after October 15 in the SE part of the state in 2008) then any flooding to date has had little if any effect on residual N. Also, soils in the SE part of the state did not lose their surface frost until about April 10. A grower can access local information by going to the NDAWN website and clicking on bare soil temperature for the nearest NDAWN station. This means that any residual N that was present before freeze-up (Thanksgiving week, 2008) and April 10 was frozen in place. If flooding was present after that date, then that N will start to change into nitrous oxide gas through the activity of denitrifying microorganisms and may be lost. Also, even if water does not cover the soil, but the soil remains saturated denitrification losses could occur.

Because of the wetness last fall from lighter soils including sandy loams and loamy sand textures were subject to nitrate and sulfate leaching. Any continued wetness from about April 10 on in SE North Dakota and whenever the frost leaves the rest of the state will result in substantial nitrate and sulfate leaching. Growers may benefit from the application of sulfate forms of sulfur on sandier soils less than 3% in organic matter, especially on hilltops and slopes this spring. Heavier soils in depressions and footslopes and soils higher than 3% organic matter are far less likely to need S even this spring. Any crop grown in the region will respond to S if it is short. Growers with sandy soils should also consider that any residual N tested for in early fall 2008 will probably not be as high when planting finally takes place and rates may need to be higher than originally planned for. Loam soils or heavier will probably not require enhancement. I would expect our spring organic matter/residue N mineralization rates to be much higher this spring than last year. This should fill in any gaps in leaching from early fall through this pre-plant spring period.

Dave Franzen
NDSU Extension Soil Specialist
david.franzen@ndsu.edu


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