Soil nitrogen management has made its way into the limelight in recent years as regulations continue to tighten within nutrient management systems and growers work to fine tune nitrogen applications. Soil nitrogen’s susceptibility to fluctuations in precipitation poses a particular management challenge that growers and their agronomists are constantly battling, with ever-changing and ever-improving technology and tools. Nitrogen modeling programs claim to predict the amount of available nitrate in the soil, chlorophyll monitors guide variable rate technology (VRT) applications, and in-field systems analyze soil nitrate in just a few minutes. The collection of data is faster, easier, and cheaper than ever. But how do growers and their agronomists use the information from all these new tools and correctly interpret the results to accurately apply this new-found knowledge? An understanding of the processes within nitrate analysis can help in conquering nitrogen management information.

As with all good stories, we’ll start at the beginning; the definition of a soil test. A soil test, be it nitrate, phosphorus, or pH, consists of two basic steps: isolate the nutrients in question, and quantify them. The isolation step is completed by what is known as an extractant, which removes (or extracts) the nutrient from the soil. Extractants are typically liquid chemical solutions that are mixed with the soil for a set amount of time. The makeup of the extractant is such that mixing it with soil will gently remove nutrients from the soil, allowing them to be measured. Quantifying, or measuring, the amount of nutrients that were removed is called “detection,” and is performed by a physical instrument known as the detector. The combination of the extractant and the detector create the soil test. Extractants and detectors can be mixed and matched (within reason) to create variants of soil tests.

The aim of any soil test is to determine the amount of a given nutrient that the plant will find in the soil. These are known as “plant-available” or “exchangeable” nutrients. Plants, however, are not passive in their environment. They are active and can manipulate the chemistry of the rhizosphere, which is the soil environment immediately around the roots. Exactly how they manipulate their environments, however, is very complex and cannot be easily mimicked in a laboratory setting. As a result, the agronomic community does not have an exact definition of, or method to get to, a true “plant-available” value. This is where the extractant portion of the soil test comes into play.

Various researchers over time have developed different extractants of varying strength and composition in an effort to best approximate what the plant sees in the soil. Let’s use phosphorus (P) for an example, as many are familiar with the common P extractants, Bray-1, Olsen, and Mehlich III. Each of these extractants uses slightly different chemistry to remove “plant available” P from the soil. Because the extractants are different, they will yield different results. Though all three may use the same detector, the different extractant makes them different soil tests, and they cannot be direct substitutions for one another. Different soil tests will yield different results. This fact requires that we use the correct interpretation for the soil test that we are using. For example, a soil test P of 10ppm may be optimal if the Bray-1 test is used, but is low if Mehlich III is used. We need to know something about the test in order to know what the data mean.

These same principles apply to nitrate testing. Table 1 lists three samples analyzed by four different soil nitrate methods. Because different soil tests yield different results, each of the analyses is correct; there is none that is more correct than another. The results just need to be interpreted within the context of the soil method, and the end fertilizer recommendation should be about the same. The standard, laboratory-based pre-plant and pre-sidedress nitrate tests are both based on a test that uses potassium chloride (KCl) as the extractant with colorimetric detection. This is a different method than the in-field system, which uses both a different extractant and a different detector. Computer programs that try to predict soil nitrates are based upon complex algorithms, and will produce a different result yet.

Table 1: Comparison of soil nitrate test methods (ppm)

Sample ID

Standard method

Electrode

CTA

Ion chromatography

2005-110

33

24

37

35

2004-112

59

48

58

5

2005-119

62

43

71

76

Samples were analyzed by multiple laboratories for the North American Proficiency Testing program

The key to using nitrate testing to its fullest is to know exactly which testing methods are used. Further, be certain that the interpretation of the data is based upon the same soil test method. How can you be sure? Your laboratory should clearly state their methods on their reports. New in-field technologies also exist that allow nitrate testing in near real-time. These systems, while convenient, are not designed to replace the standard laboratory-based analysis. They yield an estimate of the result that the laboratory might produce, and the results should not be interpreted the same way as a laboratory result. The equipment manufacturers should be able to provide credible, peer-reviewed interpretations to the data that the system produces.

Our world is becoming increasingly overrun with convenience. Scientific analysis, where accuracy and precision have traditionally ruled, is not immune to this encroachment. Substitutions for laboratory analysis are here, and more are sure to come. To use a familiar analogy, saccharine is only a surprise when you’re expecting sugar. Likewise, computer programs and in-field nitrate systems will disappoint if we expect them to be a direct replacement for tried and true laboratory analysis. However, if we use these systems in their own context and apply the correct interpretations, they can become useful tools to approximate what is happening in the fields.

Rock River Laboratory provides production assistance to the agricultural industry through the use of advanced analytical systems, progressive techniques, and research-supported analyses.  Employing a team of top specialists in their respective fields, Rock River Laboratory is built on providing accurate, cost-effective, and timely analytical results to customers, while featuring unsurpassed customer service.