A New Nitrogen Test That Works

Ellen Phillips, Crop Systems Educator, Countryside Extension Center, 708-352-0109, ephillps@uiuc.edu

For years soil scientists have been saying that there is no good soil test for nitrogen to use as a basis for nitrogen fertilizer recommendations. That may be changing with the promise of a new test being developed at the University of Illinois that can estimate mineralization.

The rule of thumb for nitrogen application has been 1.2 pounds for every expected bushel of corn yield. From there farmers would subtract out the credits for legume nitrogen and manure in order to come up with the final rate of nitrogen to apply to their field. The 1.2 pound rate is based on field studies where increasing amounts of nitrogen fertilizer is incorporated and is compared to the yield. The most economical rate of nitrogen was recommended.

At best, this rate method is a "best estimate" guess for a particular field in Illinois since it is not based on local soils or conditions. Applying too little nitrogen can result in significant yield loss. In other environments, this method might lead to applying too much nitrogen and result in environmental pollution.

Using a standard Mason jar, University of Illinois researchers are testing a new simple chemical test that can help farmers fine tune nitrogen applications. It is based on measuring a material in the cell wall of soil microbes called amino sugar nitrogen. This sugar is just one source of soil organic nitrogen.

It is a simple test that is now being refined to make it easy for soil labs to conduct. A soil sample is taken similarly to present soil sampling methods. In the lab, the soil is placed in a Mason jar with sodium hydroxide. By the lid is a container with boric acid indicator solution. The jar is then heated for five hours at 120 F. During this time the amino sugar nitrogen is converted to ammonia gas that is then trapped in the boric acid. Upon completion of the five hours, the indicator solution is titrated and the amount of nitrogen can be calculated.

According to Dr. Rich Mulvaney, University of Illinois Crop Science Dept., more field research is needed. Questions still being studied are: should we sample every year, how many samples are needed per acre, what is the right sampling depth, how should samples be handled prior to arriving at the lab, what affect does weather have on amino acids in soils and many more. It is believed that this test will be able to detect fields where no nitrogen is needed if normal climate conditions occur.

A recent study looking at 25 Illinois soils (0–12 inch samples) correctly identified fields as being responsive (< 225 mg kg-1) or nonresponsive (> 235 mg kg-1) to N fertilization for corn production. Another study currently underway indicates that manure applied years ago can still be providing substantial nitrogen to a crop.

It will be a couple of years before this test is available to soil test labs. In the mean time, current research is indicating that this test could reduce the amount of fertilizer applied to fields and hopefully improve the environment by minimizing excess nitrogen being applied.