- Waiting too long to understand Nitrogen status of your field can cost yield and operational efficiencies

- N-Watch is a valuable tool for assessing N status and helping to make management decisions prior to additions of "in crop" nitrogen applications

- The next N watch sampling regimen should be taken when corn is 6-12 inches tall (V3-V5)

- Iowa State PM 1714 is a tool to assist growers interpreting soil test nitrate levels at the 6-12 inch growth stage of corn

If you wait too long to see deficiency symptoms in corn, you may have already experienced yield loss and the N application window may close down due to crop size or weather related issues, conversely over application of N can lead to reduced profits and negative environmental impact when N is not needed and over-applied.  N-Watch can be a very good tool to help estimate soil Nitrate- N concentration and to assess any management actions needed.  As a guide to help interpret N-Watch data output refer to the Iowa State publication regarding (PM-1714 CLICK HERE). 

PM 1714 testing procedure is calibrated to assess soil nitrate-N sufficiency in the 0-12 inch depth.  Sample time should be synchronized to 6-12 inch corn height (Using the droopy leaf method), which equates to approximately the V3 – V5 growth stage depending on the corn hybrid, consider turnaround time for lab analysis, evaluation of results and logistics related to N application. The critical level of Nitrate – N in the top 12 inches of soil for corn following corn and corn following soybean is 25 ppm, when nitrate – N levels fall below this critical value an N management decision should be considered.

Example from PM 1714.  A soil test of 15 ppm and a critical concentration of 25 ppm results in a recommendation of 80 pounds per acre to be applied (25 - 15) x 8 =80

Example from PM 1714.  A soil test of 35 ppm and a critical concentration of 25 ppm indicates that the soil already has approximately 80 pounds of N more than needed (25 – 35) x 8 = (80) No N application needed

Remember that N-Watch and PM 1714 are tools where interpretation of soil test N data should be considered as part of the whole system of tools and not just an easy number that triggers a nitrogen application. The best use of N Watch and the most accurate interpretation of soil test numerical values come when there has been an on-going N testing program that leads to a more robust situational awareness of soil N trends and any given soil test regarding N status in a field.  Soil tests may underestimate PAN where 1.) Inhibitors have been used, 2.)More than 150 lb.N/acre of Nh₃ were applied or 3.) More than 150 lb.N/acre of manure was injected.  Also, first year corn following alfalfa and second year corn after alfalfa tends to mineralize more plant available N after soil sampling.  Excess rainfall events prior to soil sampling can also have a confounding effect of soil nitrate levels.