"In the last two weeks, I've heard of more than a dozen field fires in three counties," says Mark Licht, an Iowa State University Extension field agronomist at Carroll in western Iowa. "It's been really dry and little time has been given to making sure combines, stalk choppers and bailers are working properly. I tell farmers to take their time and make sure a fire hazard is not missed."
If a field fire does take off, there are questions of how much of the crop nutrients are lost due to burning of the crop residue. Burnt material results in ash which contains the phosphorus and potassium, explains Licht. The ash is susceptible to blowing and therefore there is risk of some phosphorus and potassium to be lost from the field. Any nitrogen in burnt the material will be lost through volatilization. How much phosphorus or potassium or nitrogen is lost or is susceptible to loss depends on how long after the crop was mature that the fire occurred.
Keep in mind that going into maturity the plant relocated nutrient to the grain. As the plant dies the nutrient concentration in the stalk and leaves starts to decrease as the plant weathers. Potassium and nitrate are especially susceptible to leaching out of stalk and leaf tissue. For more information on this topic read Dry fall leads to field fires by John Sawyer, ISU Extension soil fertility specialist. Click on that headline and it will lead you to the online article written by Sawyer.
Hold off fall anhydrous application until soil is colder
Mark Licht and other ISU Extension field agronomists are getting questions from farmers who are finished with harvest and are wondering if they can safely apply anhydrous ammonia yet. Or should they wait until soils cool down? "If you're done with harvest it's tempting to go ahead and apply the nitrogen now, but that is very risky because of the potential for nitrogen loss to occur," says Licht.
Fall anhydrous ammonia applications are common in Iowa largely due to time constraints in the spring. Regardless of what the calendar says or when you finish harvesting, the slogan "Don't go until 50 or below" still holds true, says Licht. You want to wait until soil temperatures at the 4 inch depth fall below 50-degrees Fahrenheit and stay in a downtrend before you begin to apply anhydrous ammonia in the fall. In central Iowa that generally occurs during the second week of November.
You can click on this link to get the previous days soil temperatures as supplied by ISU Extension. Licht also points out that it's probably not a bad idea to keep an eye on the 6 to 10 day weather forecast.
Why is the thumb-rule 50-degrees—why not 60 or below?
So why is the slogan "Don't go until 50 or below," why not 60 or below? Anhydrous is the ammonium form which is stable in the soil. Under warm conditions with available moisture in the soil, anhydrous quickly converts to nitrite and then to nitrate. Nitrate is not only available for plant uptake, but also extremely susceptible to leaching. A full subsoil moisture profile combined with normal spring rains could make early applied anhydrous subject to field losses.
How much fall anhydrous ends up being lost? There is no good answer to this question. It depends on sustained fall soil temperature, spring rainfall, spring temperatures and field drainage. With little rainfall and slow conversion, very little nitrogen loss will be realized. With fast conversion, high rainfall and well drained soils, losses of N could be high. Learn more by reading Remember 50 degrees an article in the ISU Integrated Crop Management online newsletter. The article is written by John Sawyer, ISU Extension soil fertility specialist; and Elwynn Taylor, ISU Extension climatologist.
Be careful interpreting fall cornstalk nitrate test results
Making sense of the fall cornstalk nitrate results is the next step once you have the results back from the lab where you sent the stalk samples. Don't miss the opportunity to fine-tune your nitrogen management just because harvest is underway and you can't change the past," advises ISU field agronomist Mark Licht. "The fall stalk nitrate test provides a glimpse into how much nitrate-nitrogen was in the lower stalk when the plant reached maturity. Based on a considerable amount of ISU research over the years, this plant available nitrogen can be an indicator of feast or famine--too much or too little nitrogen being applied."
Optimal test results indicate that the plant-available nitrogen met the corn plant's needs for grain production. If the test shows that there was "excessive" nitrate in the stalks, it means that the plant-available nitrogen exceeded grain fill needs, says Licht. If you get the test results back several years in a row and they always show excessive nitrate in the stalks—without droughty or low yield conditions—it would suggest that a reduced nitrogen application rate is needed. In other words, you've been applying too much nitrogen per acre on that field.
While excessive test results are more definitive, low and marginal results can lead to incorrect assumptions of inadequate plant available nitrogen. Making decisions on low to marginal results should be coupled with knowledge of growing season conditions, nitrogen application rates and grain yields.
A low test result coupled with high grain yields would indicate enough nitrogen was applied. However, a low test result coupled with low yields and excessive precipitation may mean there was not enough plant available nitrogen present in the field--but in this case, since there was a lot of rain in 2010, it indicates nitrogen movement in the soil may be the problem versus an inadequate nitrogen application rate. For more information, read an online article at Corn stalk nitrate interpretation written by John Sawyer, ISU Extension soil fertility specialist.