ISSUE 2 May 8, 2003
TOPDRESS PLANNING FOR SMALL GRAINS
For growers with small grains in the ground before the rain, some of the wheat is looking very good. Cool weather, good emergence, reasonable tillering so far and good soil moisture are making what looked like an iffy year appear very optimistic. However, due to high N prices and conservative yield expectations the N status of the field may be low compared to todays yield prospects.
Growers who want to gamble and take advantage of higher yield potential may want to try topdressing their small grains. Topdressing after 3 leaf stage will help head size spikelet number and kernel number decision, all of which are yield component decisions that the wheat plants make. Hot and/or dry weather later on, particularly at 4-5 leaf stage and at pollination will modify any improvements that the N additions would make. Also, whatever topdressing strategy is employed, it must rain following application before the 6-leaf stage for the fertilization to be effective.
My favorite application would be liquid N (UAN, 28-0-0) applied with streamer bars about the 4-leaf stage. Do not broadcast the UAN, the burn will be great and may cause a yield reduction in some cases. Streamer bars concentrate the application into bands, which tend to drive most of the fertilizer to the soil surface rather than coat the leaves, where it could damage the leaf tissue and cause "burn". A concentrated band also slows the rate of urea volatilization from the urea portion (the N in UAN is about 50% urea) of the UAN. The ammonium nitrate half of the UAN solution is not subject to volatilization, making UAN a preferred fertilizer over dry urea for this type of application. Application with streamer bars most effectively avoids burning on calm days. The danger of burn increases as the wind increases. Under windy conditions, the wind breaks the stream apart and converts it into a poor broadcast application, coating leaves and increasing burn.
If dry urea is used, it might be wise to also have it coated with AgritainÒ, which is a tried, tested and proven urease inhibitor, and gives about 10 days of protection from volatility.
Whichever source is used, rain is required to move the N into the soil so that roots can utilize it. About 1/4 inch of rain is needed in bare soil to accomplish this, and with residue, the more rainfall is needed, particularly if there is a thatch cover. If rain does not fall until after the 6-leaf stage, yield will not be enhanced.
SOYBEAN PLANTING ON HIGH N SOILS
This spring, some fields are testing high in residual N, or received a high N application last fall in anticipation of small grain or corn seeding this spring. However, some growers need to plant soybeans on these fields, perhaps for economic or rotation reasons. Seeding some crops onto high N soils (flax (increased lodging), malt barley (high protein), sugarbeet (low sugar), dry bean (greater white mold development potential) can be a problem. However, there are no problems associated with seeding soybean on high N soils. In our area, white mold risk is not increased and production potential should still be high. Studies in the central US corn belt in the past have shown that fertilizer N is not superior to the soybean/rhizobium relationship. The studies also showed that production was not reduced due to fertilizer N. Although I consider N application to soybeans a waste of money in most situations (except fields new to soybeans or fields with a history of early seasons stress such as chlorosis or high salts/carbonates which will benefit from soil N plus fertilizer N up to 50 lb/acre), seeding onto high N soil should result in equivalent production to properly inoculated fields with lower N.
N APPLICATION AFTER A RAIN
During spring seeding, it is tempting to return to the field immediately after a rain to begin applying fertilizer and preparing for tillage and seeding. If the field is conventionally, or minimum till, application will not be possible until the soil surface is dry and application of urea can generally be made if tillage or rainfall will occur within about 48 hours. However, under no-till the fields may be firm enough to tempt an application of urea even when residue is damp. This is a worst case scenario for urea volatilization. When residues are damp, urea is quickly dissolved and dispersed onto the residue. Residue contains hundreds of times as much urease enzyme as bare soil, and the windy conditions that often accompany the passage of a weather front can act like a distillery, picking up the newly formed ammonia and carrying it far away. In no till, wait until the residue is dry before applying the urea, or use AgrotainÒ to protect it from conversion for about 10 days. Hopefully, rain will fall during this period and move the N into the soil so that volatility is no longer a threat.
NDSU Extension Soil Specialist