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"New Era in N Management"

Wednesday, February 15, 2006
filed under: Fertility

Dave Franzen recalls that when he started in the fertilizer business in 1976, the price of anhydrous ammonia was about $125/ton. Now, it’s about $500/ton or 30 cents/lb N, and the North Dakota State University extension soils specialist doesn’t see the price falling much from this plateau over the long-term.

He says ammonia and urea are being imported into the U.S. in record quantities, but the infrastructure is not great enough to handle the increased need, stressed further by the lingering effects of Hurricane Katrina which disrupted barge traffic. But it’s the increased cost of energy which is the real lynch pin to the trend in higher N costs – more than 90% of the cost of making anhydrous ammonia now is the cost of natural gas.

“We are entering a new era in N management,” he says. This means more attention to soil testing to accurately account for residual N, more interest in zone soil sampling and variable rates. “And should N rates be the same regardless of the price of N and the crop? Probably not,” says Franzen.

Recommendations in the past were developed in a time when N was cheaper, and crop prices were higher from an inflationary perspective. But with $500+/ton ammonia fertilizer, there will likely be more attention given to economic optimum N rates.

NDSU, for example, is exploring adjustments to its recommended crop N rates to reflect increased costs – generally, as crop prices decline and N costs increase, N rates may be adjusted, albeit slightly. Certainly, cutting back on fertilizer N too much will affect yield and quality of most crops. But generally, yield response to N, and economic return compared to the cost of N, tends to level off at high N rates.

Franzen recommends crop producers know their existing soil N in fields through soil sampling analysis, and establish reasonable yield goals based on past experiences and historical average yield. Use the historical yield average for a field and current fertilization recommendations to arrive at a base N rate. Then from the recommended total N rate, consider decreasing the rate by about 10% for 30-cent N, and 15% for 40-cent N.

More research data is needed to better resolve the issue of economic N rate and yield, but Franzen says the 10% to 15% reduction in rate when N is 30 cents/lb or more may be considered for most crops, including sunflower. The fact that sunflower in many cases overshot 2,000 lb yield goals by hundreds of pounds last year is not only a testament to the optimal weather conditions, but also makes the case that a 10% to 15% reduction in applied N may be considered in sunflower when fertilizer prices are high, without affecting crop yield or quality.

But again, before reducing rates, make sure you know the amount of residual N already in the soil. Soil scientists generally recommend that you’ll need 50 pounds of soil N plus fertilizer N in the top 2 feet of soil for every 1,000 pounds of expected sunflower yield. The only way to know for sure is soil testing to a two-foot depth in the fall or early spring.

In fact, consider a deeper probe to get a better idea about N further down in the soil that might be there for sunflower’s taking. Then you’ll know for sure how much N is needed, or if you can get by with less N than you thought. Deeper soil samples typically require a pickup-mounted hydraulic probe designed for sampling down to four feet.

As one of the deepest rooting crops long recognized as an “N scavenger,” sunflower may well be a crop that fits into a new era of N management. On-farm observations and university research validate the sunflower plant’s ability to nab soil N too deep to be used by other crops. A three-year Kansas State University study conducted near Tribune in the late 1980s demonstrated that sunflower roots could extend into the soil about 9.9 ft deep, while grain sorghum rooted to about 8.3 ft deep—nearly a 2 ft difference.

Franzen points out a nutrient analysis in a field near Williston, N.D., where N levels from 2-4 feet were over 200 lb/ac because of heat and drought in the recent past, leaving high amounts of residual N behind in a non-leaching environment. And in the High Plains, on fields that have been on irrigated corn over multiple years, it’s not at all uncommon to find 200 to 400 lbs/ac of nitrate below the corn root zone, typically three or more feet deep in the soil. Sunflower will root down and extract that residual N, provided there’s not a compaction zone to inhibit root growth, and if subsoil moisture is adequate to encourage root growth.

A study at the USDA-ARS Central Great Plains Research Station in Akron, Colo., in the late 1990s analyzed recovery of N fertilizer placed deep in the soil profile with different placement methods.

The Akron researchers found that sunflower recovered half the fertilizer N placed two feet deep. They measured 23% recovery from fertilizer N placed four feet deep, and 12% recovery at five and a half feet deep.

Growers are encouraged to consult with their local agronomists or extension experts for more information on managing N rates, as well as sampling at deeper soil depths. – Tracy Sayler

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