NITROGEN FERTILIZERS ARE OFTEN applied in the fall instead of the spring for spring-planted crops. Reasons include:
1. Fertilizers can be easily applied prior to fall bedding.
2. To minimize the number of practices that need to be performed in the spring.
3. To get a jump-start on planting in the spring, which can potentially lengthen the growing season and increase yields.
4. To take advantage of lower fertilizer prices.
5. To spread out farming costs evenly throughout the year.
While these justifications are understandable, the benefits of waiting until spring to apply nitrogen fertilizers outweigh the benefits of early fall applications.
The primary concern of fall-applied nitrogen fertilizers is the loss of nitrogen through nitrate leaching, immobilization, ammonium volatilization and other pathways. A common misconception is that nitrates will not leach in cool weather. If the ground is warm enough for fall tillage practices, it is warm enough for water to move nitrates through the soil system.
Nitrates can also leach during freeze/thaw cycles over the winter and in the early spring as temperatures slowly rise above freezing. Keep in mind that soil temperatures do not fluctuate as air temperature does, and will often be warm enough for leaching at lower soil depths when the air temperature is below freezing.
Another misconception is that ammonium and urea compounds will not contribute to winter leaching of nitrates; therefore, fertilizers that contain ammonium and urea are safe to apply in the fall.
Urea, in fact, is highly soluble until it is hydrolyzed to ammonium. It is true that ammonium is a positively charged cation that can easily attach to negatively charged soil surfaces, which does limit leaching potential.
However, ammonium from ammonium sulfate or hydrolyzed urea can be converted to nitrate through the microbial process of nitrification, creating negatively charged nitrate anions that will freely leach through the soil system. While nitrification does occur at a slower pace during the winter compared to warmer months, it does still occur.
Losses of nitrogen are costly, both financially and environmentally. Pre-plant nitrogen rates have a major impact on crop yield and quality. Nitrogen losses can significantly lower yields, thus lowering a grower's financial return from that field. Growers can easily apply the same amount of nitrogen fertilizer in the spring and achieve greater yields than from fall applications.
In addition to yield losses, practices such as fall applications of nitrogen fertilizers may be contributing to degradation of water quality in agricultural regions in southern Idaho.
Urban areas that rely heavily on groundwater wells and are surrounded by agricultural areas are at the highest risk. Twin Falls and Ada/Canyon areas are at the top of the Idaho DEQ 2008 nitrate priority area list.
For more information on nitrates and Idaho water quality, refer to "Ground Water in Idaho: Degraded Water: Nitrate" on the Idaho DEQ website (www.deq.state.id.us). Click on "Programs and Issues" on the "Water" menu. Under "Ground Water," click on "Degraded Ground Water: Nitrate."
If you have no choice but to apply N fertilizers in the fall, apply in November...
instead of September or October, as the warm early fall temperatures will significantly increase nitrate losses compared to late fall.
However, your best option is still to take a closer look at your nutrient management program and determine how to eliminate winter time lag between nitrogen fertilizer applications and planting.