If you didn’t fertilize last fall, soil sampling can be one of the best fertilizer management tools this spring. “If you don’t soil sample every year and you didn’t sample last fall, this is the year to do it, says Gary Hergert, University of Nebraska soil and nutrient management specialist.

“Farmers can control input costs; they can’t control the weather. But if they soil-sample, they will know what is out there and that will pay off. It’s not too late to sample this spring.”

Early spring sampling will also reflect any changes in nitrogen (N) from winter precipitation. And a look at drought history tells a tale.

“There are parallels between the droughts of 1934 and 2012,” says Drew Lerner, president of World Weather, Kansas City, Mo. “Last year joins the four previous major droughts in 118 years of data collection: 1906, 1934, 1936 and 1988.”

The resulting low yields in many areas may mean a higher than normal nutrient carryover, especially N. That’s why soil sampling should pay off.

The Midwest P and K management system is based on soil testing, response-based fertilizer application for low-testing soils or removal-based fertilizer application to maintain desirable soil-test P and K values.

“On most soils in the Corn Belt, P and K bond to soil minerals and don’t move very far from point of application,” says T. Scott Murrell, the Indiana-based north central director, International Plant Nutrition Institute (IPNI). “While there may be some loss from surface runoff, in most situations, P and K not taken up by the crop carry over for use in future years.

“Lower yields caused by the drought mean less P and K were removed with grain harvest. However, if corn intended for grain harvest was instead cut for silage, and that silage contained grain, P and K removal will be greater than planned,” he says.

Now is not the time to guess about P and K if you don’t have recent test results as the starting point for estimates, says Hergert. “If you don’t know what’s out there, don’t guess, soil-test.”

“When you take soil samples, it gives you a chance to evaluate where you are with P and K,” says Kyle Freeman, manager, new product development at the Mosaic Company in Plymouth, Minn. “Then you can adjust your nutrient management in the spring if needed. Based on fall soil tests, we’ve seen lower yields don’t necessarily mean you’ll see a huge jump in soil-test P.

“With or without soil sampling, plan for success by using best practices,” Freeman urges. “If you are planning on a 225-bushel corn crop, make sure P and K needs are taken care of.”

“If you have soil sample results from last fall, compare them with your previous test,” says John Sawyer, Iowa State University Extension soil fertility specialist. Sampling with dry late-summer and fall soil conditions may have affected results. “Do they look unusual? If so, you may want to resample in the spring or use previous trends in soil test results. If your normal sampling frequency was to not sample last fall, and your crop yields were not drastically affected by dry conditions last summer, don’t worry; apply what has been normal for recent years. If your crop yields were significantly affected by the dry conditions, then you can sample in the spring and adjust application rates in the spring or fall, if needed.”

If fall or spring soil test results look different than expected, Sawyer adds, “remember that reduced yields will have less nutrient removal. If corn was cut for silage instead of for intended grain then, especially for K, there will be more removal with the silage than for grain only because there is a lot of K in the plant vegetation. We’d like to maintain an agronomic optimum soil test level for the long term and best match inputs to removals.”

Murrell concludes, “If soil tests don’t change as expected based on past applications, drought-induced changes in soil chemical reactions are likely a significant part of the explanation.”

Moisture affects nutrient use

 “Moisture has the biggest impact on nutrient use,” explains Fabian Fernandez, University of Illinois soil fertility specialist. “Plants take nutrients from the soil solution. If it is dry, the plants are not growing and nutrients can’t be used.”

In Illinois, the Council for Best Management Practices funded a fall soil-sampling project for residual N lead by Emerson Nafziger.“There wasn’t a lot of information out there,” says Fernandez, one of the team members who analyzed the results. “We wanted to know where we are since there was the potential for a lot of N left in fields.”

Nearly all of the samples were taken following corn in 2012 and many were from fields where yields were low from dry weather. Survey results indicate high levels of residual N are common in many fields.

Since moisture was the limiting factor for yields in 2012, the amount of N used in a field and corn yield in that field didn’t match very well, according to the report by the Illinois team. The relationship between the amount of N applied and the amount found in the fall was quite poor. Plus, low yields were not associated with high amounts of leftover N among the fields sampled.

The report concludes, “We think this simply shows how complex the N interactions are in the soil. In a year like 2012, there is little N loss, N uptake ends early as the crop stops taking up water…Late summer and fall rainfall can produce new flushes of mineralized N long after crop uptake stops. It’s possible that soil moisture drives both yield and the amount of N in the soil and that these two things were more or less independent of each other.” This report confirms the need for an early-spring soil-test before applying N, Fernandez points out. Low yields don’t necessarily mean high residual N.

While there appears to be residual N in Illinois, there is wide variability in Minnesota, according to Gyles Randall, a recently retired University of Minnesota soil scientist. “Some Minnesota soil tests from last fall show no residual N.”

Recommendations for spring

When spring arrives, and if you didn’t apply fertilizer in the fall, “follow the strategy you planned if you have soil tests,” Hergert says. “If you don’t have soil tests, get them. If you have a problem getting a good sample in dry soils, use a screw auger. Get samples down to 2 or 3 ft.”

For corn following corn, “N loss really happens in the spring. Until you get close to planting, you really don’t know what you have. If you suspect high levels of residual N, test your soils in the spring,” says Fernandez.

“If you don’t sample, consider using the low end of the Corn Nitrogen Rate Calculator,” Sawyer says. “Use the bottom end of the range if it is dry and you expect average rainfall. You can make an upward adjustment if you receive well above-normal rainfall in the spring.”

Hergert adds, “For irrigated fields, you have a projected yield. For rain-fed corn, shoot for average. Split the N between preplant and a sidedress application. Then, you can manage the nutrients effectively.”

Another tool for evaluating residual N is the Adapt-N online computer model. “Even if you didn’t use the program during 2012, you can sign up and evaluate the season after the fact and determine how much N remained in the soil profile at the end,” says Harold Van Es, professor of soil science at Cornell University. “This tool simulates water and N dynamics in a corn field.”

Van Es led the development team for the model that was available for the Northeast, Iowa, Minnesota, Wisconsin, Illinois, Indiana and Nebraska in 2012. “In the late winter, we’ll activate the 2013 season information. We hope to add all states by the coming growing season.”

In general, for corn following low-yielding soybeans, “we recommend that producers use the same rates as their normal application program,” Sawyer concludes. “Iowa research shows that soybean yield in the prior year does not relate to optimal corn N fertilization rate or the differences between optimal rate for continuous corn and corn following soybean.”

Drought slows fertilizer shipments

The 2012 drought made it a challenge to get P and K into position for fall and spring application. As the Mississippi River water level drops, some rock formations are now much closer to the surface. That means barge traffic between Cairo, Ill. and St. Louis has moved more slowly than normal during the last quarter of 2012.

“We have experienced extended transit times,” explains Rob Plosz, senior director supply chain for the Mosaic Company, a major supplier of P and K. “It has sometimes taken 10 days longer to get from point A to point B” on the river.

“Things change every day, Plosz says. “We have light-loaded barges and switched to rail from barges to be safe, in some cases. We are mindful of getting product into position.”

While the U.S. Army Corps of Engineers moved up the timeline to begin removing the rocks from February to December, the project won’t be completed until sometime during the winter.