Issues with nitrogen fertilizer: Fall 2013
The N rate calculator located at http://extension.agron.iastate.edu/soilfertility/nrate.aspx remains our best “range-finder” for guiding the process of determining N rates. For central Illinois corn following soybeans, and with NH3 priced at $680/ton ($0.41/lb N) the current guideline rate is 167 lb N/acre (204 lb of NH3), and the range is 154 to 183. In northern Illinois, the most profitable rate under these same prices calculates to 149 lb N/acre, with a range of 137 to 163. As we have noted before, the ratio of N to corn prices tends to stay relatively constant over time, with a bushel of corn equal in value to about 10 lb of N (in the form of NH3). Changes in fertilizer prices by next spring are likely not to greatly change the guideline rates, but of course adjustments in total N applied can be made in the spring.
Cover crops and N?
Cover crop seed has been dropped or drilled into a lot of Illinois fields this fall. Ongoing dry weather has meant delays in germination in many areas, and temperatures down into the low 20s last week might have damaged some small cover crop plants, especially in harvested fields with cover crop plants more exposed. Growth of the cover crops will hopefully pick up now, but delays at the start may mean less growth before cold weather sets in.
We expect that a cover crop with vigorous growth and a good root system will take up some N left in the soil in the fall, and more N next spring if the cover crop overwinters. Once taken up into cover crop biomass, N is less likely to be lost to tile lines. From a crop standpoint, the N in the cover crop will be of value only if the next crop is one like corn that requires N from outside sources. The breakdown and release of cover crop N and its supply to the next crop is a biological process that depends on the weather. Cover crop residue in the spring can also affect soil temperature and water content, and so can affect the planted crop in ways other than through N supply.
One idea that has been floated is that fall cover crops can help take up fall-applied N, thus keeping it safe from loss and preserving it until the corn crop is up and growing next spring. This may happen to some extent, but fall uptake will usually be limited if the cover crop starts to grow well only after harvest of the crop it’s planted into, and if fall N is applied only after soil temperatures reach 50 degrees; this will typically give only a few weeks of uptake, or even less. Applying NH3 into growing cover crops will cause some damage to roots, and when soil temperatures are cool and dropping, it’s not likely that such roots will be able to take up much of the fertilizer N up before soil temperatures get low enough to halt root activity.
Uptake of fall-applied N increases as soils warm in the spring, but vigorous cover crops like cereal rye need to be killed in March, and soil temperatures by that time are usually still cool. As an example, average soil temperature at the 4-inch depth reached 61 degrees near Champaign in mid-March of 2012, one of the warmest Marches on record, but reached only 46 degrees by the end of March in 2013 and did not reach 60 this year until the end of April. So while we think that cover crops can have beneficial effects on N nutrition, we don’t expect this to be consistent over years. We certainly cannot afford to get sloppy with fall N thinking that cover crops will bail us out to prevent loss.