There are several advantages of applying N at planting time. Typically, the price of N is more cost-effective in the fall than in the spring of the following year. If the nitrogen requirements of the crop can be applied at the time of seeding, the additional time and expense of a second pass over the field can be eliminated. Also, it ensures that there are nutrients available to the crop early in the spring, a critical time in establishing yield potential. Spring applications can get delayed due to poor weather or adverse field conditions, thus limiting the roots’ access to available N.
In a study done by Alberta Agriculture, it was discovered that when planting winter wheat in stubble fields low in soil N, the additional N fertilizer that was applied improved stand establishment and overwintering ability and did not reduce winter hardiness, plant populations, or yield. In this same study it was determined that N applied at the time of seeding was generally as effective and often more effective than spring broadcasting.
Possible disadvantages of N applications at seeding time are risk of seedling damage and risk of significant N losses. If placing N in the seedrow, safe rates of up to 30 lbs/ac can be applied, but may vary with moisture conditions, soil type, type of opener, and row width. Seedling damage can largely be overcome with openers, which place fertilizer away from the seedrow. If warm and moist soil conditions persist for a long period following seeding, risk of N losses due to denitrification or leaching can be substantial. Slow release N products can create more options for producers wishing to place higher rates of N with the seed and can also decrease the risk of these N losses.
Topdressing in late fall has also attracted the attention of some growers. With the cooler temperatures that the Prairies typically experience in late fall, N losses are usually minimal. Applying urea to cold (temperatures below 10°C approximately), but not frozen soils is the primary way to minimize losses. Precipitation, either as rain or wet snow, is always needed to move surface-applied urea into soil so it does not volatilize. On warm soils having temperatures above 15 degrees C, it is essential that precipitation occur within a day or two of application to minimize losses. On cold soils, the critical window for rain or snow is a little wider at five to seven days after application. To minimize N losses, spread urea fertilizer when rain or snow is in the forecast or when the chances are good for substantial showers soon after application.