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Corn+Soybean Digest

Dig in Deep

With fertilizer and other input costs still high compared to grain prices, knowing how much nitrogen (N), phosphorus (P) or potassium (K) to apply and where they're needed most can increase production efficiency. And deep soil tests may be needed to get down and dirty on cutting costs in the right place.

Jason Houghtelling knows it and counts on deep and shallow soil testing to keep corn in the 190-200-bu. yield range. He farms with his father Ken and grandfather Lloyd Frandsen on a fourth-generation crop and livestock operation in Cambridge, NE.

With help from a consulting agronomist, Houghtelling says strong yields are generated in the strip-till program. “Random deep soil testing the past few years has easily paid for itself,” says Houghtelling, also an agricultural lender at a local bank.

The southwestern Nebraska region is semi-arid, with annual rainfall under 20 in. Most corn is grown under pivot irrigation. Some dryland is planted in wheat stubble. Most soil is a heavy loam. But Houghtelling also has a more sandy loam on Republican River basin bottomland. “But deep soil testing is beneficial in both cases,” he says.

John Sawyer, Iowa State University Extension soil fertility specialist, says “drier production areas, such as the Great Plains,” can benefit from deeper soil tests to measure residual nitrate than more humid areas such as the eastern Corn Belt where rainfall amounts don't lead to accumulation of nitrate in the profile. The exception would be dry seasons and when corn will follow corn, he says.

“Where profile nitrate sampling is practiced, soil samples can be 3-5 ft. deep depending on the recommended depth for different areas,” he says. “Since there isn't excess precipitation to leach nitrate out of the soil, the profile samples collected in the fall or spring will reflect N that remains in the rooting profile and available for the next crop.”

Brent Bean, Texas A&M University Extension agronomist for High Plains growers in the northern Panhandle, says deep samples should pay. “Historically we have taken soil tests from 0-6 in. or 0-8 in.,” he says. “But roots pull N from 4 ft. deep and there can be a lot of variability in N availability. There may be 60 lbs. of N in deeper soil samples.”

KNOWING HOW MUCH of that N is available is essential, says Brad Farr, a Crop Production Services, Inc. crop consultant who works with Houghtelling. The operation features a one-pass strip-tiller, which runs in front of the planter.

“We run a deep chisel and plant strip-till in the chisel mark,” says Farr. “We till 8 in. to 1 ft. deep. We're going to the second foot with soil tests because it's important to get the benefits out of the deep root zone.”

Nitrogen and P fertilizers are applied about 8 in. deep at planting via chiseling. Starter fertilizer is also applied about 2 in. beside the seed.

Basic soil tests cost about $29 each, no matter what depth, says Farr. They are taken randomly across a field.

“We may be able to back up some on fertilizer applications,” says Farr. “And we've been able to identify the need for more N in the tighter soil as well as the bottomland more sandy soil.”

A savings of $20-30/acre on N or P costs improves the bottom line, says Houghtelling, as well as the higher yields from having more N where it's needed.

Sawyer says that due to rainfall greater than evapotranspiration seen in Iowa and across much of the Midwest, profile sampling for N isn't generally recommended. It can be an accepted practice with some soils in the upper Midwest where soils freeze and stay frozen all winter, he says. In Iowa, when sampling for soil nitrate, those samples are collected in late spring to a 1-ft. depth, usually around June 1.

“For P, K and pH, a 0-6-in. sample depth is recommended because that's the depth where our soil tests are calibrated,” Sawyer says.

Gary Hergert, University of Nebraska (UNL)

“Soils should be sampled for residual N to a 3- or 4-ft. depth before corn, wheat, sunflower and sugar beets and to a 30-in. depth for dry beans and millet,” he adds.

Hergert stresses that growers should take advantage of residual fertilizer nutrients. “Properly credit other nutrient sources, such as legumes, manure and irrigation water,” he says. “The cost to collect and analyze water samples for nitrate or manure samples for nutrient content has increased little. The value of the nutrients these resources contain has gone up substantially.

“Until retail fertilizer prices drop to match the recent downturn in grain prices, the importance of accurate soil sampling information cannot be overemphasized. It is critical that investments of this size be made with the best information possible, based on careful soil testing after accounting for all sources of nutrients,” stresses Hergert.

He says one problem with obtaining accurate fertilizer recommendations is the different soil test interpretation philosophies. “The way different laboratories interpret how much fertilizer to apply at a given soil test level, compared to what original soil test calibration recommended, is still debated.”

Jason Houghtelling, Cambridge, NE, says deep soil testing has easily paid for itself the last few years in low-rainfall areas and sandy soils.


Sawyer reminds growers to take soil samples after harvest and before winter. Spring is an option, but try to sample at the same time with each sampling to help with consistent results. Here are some tips for sampling:

Divide the field into sampling zones on the basis of soil survey maps, soil color, texture, major differences in observed crop yields and uniform histories of cropping, fertilization and liming. Intensive grid sampling is another viable alternative.

Use a soil probe and obtain an adequate number of cores, so the sample correctly represents each area. Twelve cores are recommend across a field and at least 20 if a field has been banded with fertilizer or had a manure application. “Make sure that your samples contain soil from the depth soil tests are calibrated,” says Sawyer.

Don't bias samples by including cores from areas like farm lanes, field borders and small, severely eroded spots.

Crop advisers can provide the most accurate recommendations if you provide cropping history and intended crops for the upcoming seasons, realistic yields, fertilization practices, past (especially recent) lime applications and history of manure applications.

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