Watertown, Wis. – Nitrogen management is one of the most important prongs of any fertilization strategy. The relative scarcity of the nutrient means it gets readily used up by any living thing in nature - weeds and crops alike. Nitrogen’s chemistry also means that it doesn’t hang around in the soil just waiting for the crops to find it.

“Ammonia is lost to the air and nitrate is washed below the root zone by rain,” explains Dustin Sawyer, Rock River Laboratory agronomy specialist and lab manager “It can be difficult to stick to the 4Rs [right place, right time, right source, right rate] without a little help.”

Thankfully, as Sawyer goes on to explain, there are analysis tools available that can help mitigate these nitrogen management challenges.

Pre-plant Nitrate Test

Different forms of nitrogen overwinter differently. “Whether the field had fall-applied anhydrous or manure, there’s no need to guess how much nitrogen is still in the soil,” states Sawyer.

The Pre-Plant Nitrate Test (PPNT) is a specialized tool that tests the amount of nitrate in the soil profile down to a depth of three feet. “Collecting a sample for this test is a bit more intense than a typical soil sample in that it’s necessary to collect a 12 inch core, then a second core from the 12 to 24 inch depth,” explains Sawyer. “Those two depths do not get mixed together – they get tested separately. This helps to not only see how much nitrate is in the soil, but also where in the soil it is.”  

Sawyer shares that most labs will also provide an estimate of how much nitrate is available in the 24 to 36 inch portion of the soil profile as well, based on the nitrate results from the two different depths of soil. “Using the analysis results, the lab should also be able to provide a credit that can be plugged into the nitrogen recommendation to decrease the amount of nitrogen fertilizer that needs to be applied prior to planting.”

As the name suggests, this test is recommended to be completed shortly after thaw and before planting.

Pre-sidedress Nitrate Test

There are situations when a crop simply needs more nitrogen than other times. This is when sidedressing helps to dial in nitrogen applications with precision.

“The fertilizer applied during sidedressing is going to be used almost immediately by the plant,” describes Sawyer. “Just as with the PPNT, the Pre-Sidedress Nitrate Test, or PSNT, is a great way to know how much nitrate is in the soil so that credits can be applied against the fertilizer additions.”

PSNT samples are collected in a more traditional manner, as only one depth is required. Cores are typically collected to a depth of 12 inches, and 10 cores from the field make up one sample.

Sawyer adds that there are also important caveats on the laboratory end to ensure accurate information is delivered to the user. “It’s important that the results of a PSNT are returned quickly so that the fertilizer application can happen as soon as possible. Because nitrate moves so quickly in the soil profile, the PSNT is a snapshot of the available nitrate and the results should be acted upon within a few days of the test.”

He goes on to stress the importance of stabilizing the nitrate in the samples within 24 hours of the sample collection. The best way to do this is to get the sample to the lab right away. “If that can’t happen, the samples should be frozen prior to shipping to the lab,” suggests Sawyer. “This will slow or stop the microbial activity that would otherwise convert the nitrate.”

Plant tissue analysis

Not all of the fertilizer that’s applied makes its way into the plant. Some nutrients are always lost to the environment. “The percentage of nutrient that does get into the plant is referred to as the Nutrient Use Efficiency or NUE,” shares Sawyer. There are many factors, most of which cannot be controlled, that impact the NUE for any given cropping system.

“Plant tissue analysis is the best way to track NUE and be sure that in-season fertilizer additions are working,” says Sawyer. “Simply collecting samples before and after applications will show definitively that the crop is seeing benefit from the fertilizer dollars.”

More traditionally, plant tissue analysis is used to confirm a nutrient deficiency in a plant that is suspected based on visual diagnosis. While using plant tissue analysis this way will help to correct a deficiency, according to Sawyer, it’s better to test the plant before the visual symptoms appear.

Late Season Corn Stalk Nitrate Test

The final analysis opportunity for the season is the Late Season Corn Stalk Nitrate Test (CSNT). This isn’t used to make any corrective action, rather it’s used to look back at the season and assess how the nitrogen fertilization strategy worked.

“The idea is that the corn stalk should have a certain amount of nitrate left in it at the end of the growing season,” shares Sawyer. “Too little, and the plant could have benefitted from more fertilizer. Too much, and fertilizer dollars were probably wasted.”  

He also adds the most important point for the CSNT: that the sample needs to be collected in a very specific way at a very specific time. “As the name suggests, the part of the plant being collected is the corn stalk, but it’s precisely the 8 inches of the corn stalk that is between 6 and 14 inches above ground. The samples should be collected one to three weeks after black layer.”

Nitrogen management – specifically controlling those variables that growers and their consultants can influence - is what programs like 4R Nutrient Stewardship are all about. And for nitrogen, in particular, there are many ways that laboratory analysis can help with that control. Fertilizer strategies can seem daunting, but assessing nutrient levels before making the plan can help best navigate the use of the amendments available.