May 23, 2012

The unusually warm spring has presented challenges and opportunities for nitrogen management. One of the unknowns when determining nitrogen rate is how quickly nitrogen is being released from soil organic matter. Soil temperature, moisture, and aeration influence this rate. Usually most nitrogen is released from the soil in late spring when temperatures increase and conditions are best for release. This past winter we had unusually warm conditions, leading to the potential for increased soil nitrates this spring in many soils.

The High Plains Regional Climate Center has summarized the average soil temperature at a 4-inch depth for the last 20 years and for the October 2011– April 2012 period for several Nebraska sites (Table 1). Across the state the average increase was just under 3 degrees Fahrenheit, with eastern Nebraska generally seeing above average levels. These temperatures indicate a likelihood of increased soil nitrates this spring as compared to previous years. However, how much more is unknown since mineralization requires moisture as well as warmer temperatures.

Most ground planted to corn probably has had some N applied. If only a moderate amount of N has been applied and the remainder is to be applied sidedress or with irrigation water, soil sampling now can help determine soil nitrate levels. You also may want to consider a real-time, sensor-based N application to determine crop need. With the high price and restricted availability of nitrogen fertilizer, this could improve profitability in the weakening corn market. If you have already accounted for the likelihood of above average N in the soil this spring, you may not need to conduct the PSNT test.

UNL has not published specific recommendations for the PSNT, but guidelines developed by Iowa State University may be used. These guidelines are available in Nitrogen Fertilizer Recommendations for Corn in Iowa, ISU Cooperative Extension publication Pm-1714.

Using the Pre-sidedress Nitrate Test

Sample corn when it is 6-12 inches tall or in late May to early June. Sample field areas that are similar and 10-20 acres in size. Sample either 1) 0-1 feet or 2) 0-1 feet and 1-2 feet and run nitrate tests on the samples. This test works best if you avoid previous fertilizer application bands, including starter and anhydrous ammonia bands. Take 15-20 cores per sample.

Interpreting PSNT Results

While Nebraska has not published specific instructions for this corn growth stage, other states have published the following:

Iowa Method. Iowa State University has the most specific PNST-based N recommendations. They are based on the price of corn and nitrogen, amount of spring rain, and the PSNT results. For fields in continuous corn or corn following soybeans, ISU recommends subtracting the soil nitrates from 25 ppm (critical level) and multiplying the difference by 8. For example, with a soil test of 18 ppm nitrate-N, the nitrogen recommendation would be: 25-18 = 7 x 8= 56 lb N/acre.
 

Wisconsin Method. The University of Wisconsin uses a different system. They base their recommendation on the yield potential of the soil. Their critical level is 21 ppm nitrate-N. The amount they recommend per ppm below this critical level is 15 lb for high producing soils and 8 lb for low producing soils. They recommend the same nitrogen rate for all soils below 10 ppm nitrate.

Recommendations

Sample fields with a strong likelihood of high nitrate concentrations. This includes fields that had adequate but not excessive moisture last fall, those that would have been tilled in the fall (darker, warmer), and fine-textured, south facing slopes. Manured fields and those with alfalfa last year are excellent candidates for the PSNT test.

The ISU publication has a specific table for this situation. If soil tests are over 23 ppm, additional nitrogen is probably not needed. The UNL nitrogen algorithm is not designed for use with the PSNT numbers. The UNL recommendation (see Fertilizer Recommendations for Corn) is based on preplant nitrogen tests taken to 3 feet; however, the algorithm does give 8 lb of nitrogen credit for each ppm nitrate. The correct value for Nebraska is probably between the Iowa and Wisconsin numbers of 8 lb and 15 lb of nitrogen for each ppm nitrate under the critical value.

Does the PSNT Test Work?

UNL researchers were part of a multi-state effort to verify the validity of the PSNT test. Soils were sampled before the season and at recommended spring PSNT sampling times. Nitrogen was applied and yields were measured. After the season researchers assessed whether the preplant or pre-sidedress treatment recommendations were correct. Dan Walters, former UNL soil scientist, summarized the results in Comparing the Performance of Preplant and Presidedress Soil Nitrate Tests for the North Central Region.

Walters grouped the results into two types of errors. Type A errors were those where too little nitrogen was recommended; Type B errors were those where too much nitrogen was recommended.

Results collected from 200-300 sites indicated a failure rate of 18-35%, depending on the test used and the depth. Most of the errors were Type B where the test over-recommended nitrogen. This data indicates that when using either the preplant or the presidedress nitrate test, there is a low likelihood of under-recommending nitrogen.

Charles Shapiro
Extension Soils Specialist
Northeast REC, Haskell Ag Lab
Gary Hergert
Extension Soil and Nutrient Management Specialist
Panhandle REC, Scottsbluff
Richard Ferguson
Extension Soils Specialist, Lincoln