DHI Agronomy Turf Food & MilkWater & EnvironmentalAbout AgSourceLocations News Contact
Nitrate Analysis

Nitrate Analysis

Late Spring / Pre-Side Dress Nitrate Testing

Optimize Nitrogen Applicationsmall-corn-with-hand.jpg

To ensure your fields have adequate nitrogen levels for the best production and highest yield, utilize a late spring nitrate test.

Why Test?

This is one way to screen out fields that might not respond to nitrogen. Others growers use this test to determine how much supplemental nitrogen is needed. The PSNT can be a valuable tool for a grower wanting to confirm the amount of N credited from manure of previous legume crops.  Click here to view the AgSource Spring Nitrate Test tech bulletin.

The following conditions make late spring nitrogen testing a good investment:

  • Little or no nitrogen was applied right after harvest.
  • Small amounts of nitrogen were applied (< 125 lbs).
  • Nitrogen was applied at sub-optimal conditions (temp<55°F) limiting effectiveness.
  • Small amounts of manure were applied or incorporated after soil temperatures drop below 55°F.
  • Early spring nitrogen application (cold soil temperatures) will not affect measured nitrate. 

* Due to the biological process which converts ammonium-nitrogen into nitrate-nitrogen, nitrogen levels may be uncertain. The rate of this conversion is dependent by soil temperature, as well as other factors. When the soil temperature is less than 50 degrees most of the applied ammonia will be held in this form and not converted into nitrate.

How to Take Soil Samples

  • Divide fields into small management zones of 6-10 acres.
  • Take samples when corn plants are at about 6 to 12 inches tall, usually four to six weeks after planting. (measure from the ground surface to the center of the whorl)
  • Sample cores at a depth of 0 to 12 inches.
  • Collect a minimum of 15-20 cores for the best representation (more is better).
  • Thoroughly mix together (laboratory needs a minimum of 1 pound of mixed soil).
  • Ship as soon as possible: samples should be sent the same day as collected.

Corn Nitrogen Recommendations
Based On The Pre-Sidedress Soil Nitrate Test (PSNT)

N (ppm)
Very High
Application Rate (lb/a)
 > 210  0  
 20-1860  40  
 17-25100  40  
 14-13125  80  
 12-11150  80  
 < 10160***  120***  

*Note: When corn follows alfalfa, the maximum N recommendation is 40 lb N/a for all PSNT results less than 21 ppm N

**To determine a soil's yield potential for a specific soil type, consult the UWEX publication "Soil Test Recommendations for Field, Vegetable and Fruit Crops" (A2809)

***No adjustment made to original Corn N recommendations

For more information, reference "Nitrogen Fertilizer Recommendations for Corn in Iowa," an informational piece developed Iowa State University's University Extension in Ames, Iowa.  

Time of Sampling

This time of sampling is late enough to reflect the effects of spring weather conditions and early enough that more N can be added if needed. Soil nitrate concentrations may substantially increase or decrease during the spring due to nitrogen release characteristics of the soil.

The increases are caused by nitrification of N from fertilizers, plant residues, animal manures, and soil organic matter. The decreases are caused by leaching, de-nitrification, or immobilization of N into organic forms that are not available to plants. Soil nitrate concentrations decrease with plant uptake after corn is 12 inches tall. Results from samples taken before or after corn is 6 to 12 inches tall should be used with caution because critical concentrations of soil nitrate change with time of sampling.

Depth of Sampling

Concentrations of soil nitrate in late spring usually decrease with increasing depth within the normal rooting zone of corn. Therefore, optimal concentrations of soil nitrate vary with thickness of the layer sampled. The late spring nitrogen soil test is based on concentrations of nitrate-nitrogen (NO3N) in the top 12 inches of soil.

Concentrations of nitrate in the surface foot of soils in late spring tend to be proportional to the amounts of nitrate in the rooting zone of corn. This means the soil test tends to reflect nitrate below the depth of sampling. Recommendations are based only on 12-inch sample depths for the side-dress test. Sampling deeper than one foot may be advisable on sands.

How to Sample

The most appropriate sampling strategy varies with location. Collection of samples as suggested above provides information that can be used to identify future sampling strategies that are more efficient for the fields being tested.

Care should be taken to ensure the soil samples are collected in a manner that is not biased by the presence of corn rows or bands of fertilizer. At least 24 cores should be collected in fields having more than 50 lb. N/acre applied as anhydrous ammonia. Our results with ammonia treated fields have not been as consistent as other management.

Sampling bias can be minimized by collecting soil samples in "sets of eight" cores that have various assigned positions relative to corn rows. By this method, the person doing the sampling moves in a random pattern within the test area to select approximate positions for collecting cores. Each time a core is collected, however, its exact position is selected relative to the two nearest corn rows. The first core is collected in the row. The second is collected one-eighth of the distance between any two rows after moving to another part of the test area. The third is collected one-quarter of the distance between any two corn rows after moving to another part of the test area. The process is continued until the eighth core is collected seven-eighths of the distance between any two corn rows.

The soil from all cores should be crushed and thoroughly mixed in a clean pail before a sub-sample is removed for analysis.

AgSource Laboratories - Bonduel, WI715-758-2178 bonduel@agsource.com 
AgSource Laboratories - Ellsworth, IA 515-836-4444ellsworth@agsource.com 
AgSource Laboratories - Lincoln, NE 402-476-0300 lincoln@agsource.com 
AgSource Laboratories - Umatilla, OR541-922-4894 umatilla@agsource.com