Pre Side-Dress Nitrate Test (PSNT) for Corn

Corn plant nitrogen (N) requirements are large during the rapid growth phases (V8 thru V14) and greatest during grain fill stages. N applied during the preceding fall and early spring, that is much before these important plant growth stages, is subject to more loss through volatilization, denitrification and leaching. This year some growers applied N in March and April when the soil was very dry and field operations were easy to accomplish. The lack of rain to incorporate the surface broadcast N did not occur for many weeks.

Determining Nitrogen availability for plants
The largest factor for urea conversion to ammonium (a cation held by the soil cation exchange capacity) is water. If soil surfaces are dry, the urea pellet can lay on dry soil for many days with minimal volatilization losses. In other parts of the state, N has been subject to leaching because of excessive precipitation. What is the best way to determine if the applied N is still present in the soil and available for plants? The check book accounting method for determining if the amount of plant available N is sufficient in the soil will not work. The reason it will not work is that the N cycle is very complex.

N can be present in the soil in many different forms that include organic and inorganic. N is constantly transforming from one pool to another. Just because we add inorganic N to the soil does not mean that it will remain in this form. Microbial transformations and immobilizations of N will cause the check book accounting method of inorganic N to fail. This is why soil fertility researchers have struggled with the golden method of N recommendations for corn and other crops. However, very controlled experiments across many different growing environments are the best approach for determining a N rate. Currently the N rate calculator uses yield goal, a coefficient, credits that include previous crop and soil test is the best field and infield method we have for South Dakota corn growers. However as this point in the growing season when N has been applied and questions about how much N might have been lost to volatilization or leaching arise; the N rate calculator is not the best option.

Pre-Side-Dress Nitrate Testing (PSNT)
Growers and agronomists might consider the pre-side-dress nitrate test (PSNT). This test uses a 0-12 composite sample of a field or zone for nitrate-N determination. Deeper soil samples from 12-24 inches could also be used that would improve the predictability of the model assumptions. The PSNT was determined from many research sites in the upper Midwest. The test works best from heavily manured, tilled and high organic matter fields. It probably works the least best in fields with previous N applications; however, it could still be a good tool.

Wisconsin and Iowa have their own interpretations of the model. In SD, we probably should use Iowa since it geographically closest to SD with somewhat similar soils; however, there are exceptions to every interpretation. The PSNT soil samples are obtained at the V2-V3 growth stage and submitted to the lab in preparation for the side-dress field operation. The threshold nitrate level is 25 ppm nitrate-N in the soil sample. For every ppm below 25, 8 lbs N/a are recommended. For example, if a 0-12 inch soil sample nitrate-N result was 15 ppm, the resulting recommendation would be 25-15 x 8 = 80 lbs N/a. For more information about the PSNT, please visit the University of Nebraska-Lincoln CropWatch website.

Source: Anthony Bly, South Dakota State University