PSNT – does it mean anything this year?

Carrie Laboski, Extension Soil Scientist

Over the past week I’ve received a few inquiries regarding the validity of the pre-sidedress nitrate test (PSNT) for corn this year considering the cool weather this spring. This is a very valid question for many in Wisconsin. To answer it, we’ll look at research results and weather summaries.

The accuracy of the PSNT to predict N credits was assessed using data from 101 corn N rate response studies conducted throughout Wisconsin from 1989-1999. These data showed that on 59 % of the sites the PSNT underpredicted N credits from manure and legumes when the average May-June air temperature was more than 1°F below normal. Underpredicting N credits means that using the PSNT in this situation would have resulted in more N fertilizer being applied than was needed. The underprediction of N credits in cool springs is a result of slow early season N mineralization prior to PSNT sampling, followed by a typical flush of N mineralization once the soil warms up, ultimately resulting in more N being available for the crop than the PSNT predicted. It should be noted that when the average May-June air temperature was more than 1°F below normal, the PSNT accurately predicted N credits on 37% of the sites and over predicted N credits on 4 % of the sites.

So the question now is: How cold has it been? Using the departure from normal of the weekly average air temperature data published in the Wisconsin Crop Progress report we can see that it has been a cool spring. Table 1 shows the weekly departures for five cities in Wisconsin and the average of those departures from May through June. Air temperatures in May and June in Central and Northern Wisconsin are more than 2°F below normal, while portions of Southern Wisconsin are nearing the 1°F below normal threshold.

Table 1. Air temperature departure from normal for selected Wisconsin cities.

City	Average weekly air temperature departure from normal* for the week ending						Average air temperature departure from normal
	10-May	17-May	25-May	31-May	7-Jun	14-Jun	3-May − 14 Jun
Eau Claire	4	-3	4	-3	-6	-8	-2.0
Green Bay	5	-4	2	-5	-7	-7	-2.7
La Crosse	3	-4	3	-4	-5	-8	-2.5
Madison	5	-1	4	-2	-5	-3	-0.3
Milwaukee	7	0	5	-3	-7	-6	-0.7

* Normal based on 1971-2000 data.

Source: Wisconsin Crop Progress(http://www.nass.usda.gov/Statistics_by_State/Wisconsin/Publications/Crop_Progress_&_Condition/index.asp)

Thus for Central and Northern Wisconsin we can expect that the PSNT results will be low and result in a low N credit. In these locations, a better N credit may be obtained this year by using book value N credits for manure and legumes. For more information on book value N credits, consult your local County Extension Agent and/or Chapter 9 in Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin (UWEX publication A2809; http://learningstore.uwex.edu/Nutrient-Application-Guidelines-for-Field-Vegetable-and-Fruit-Crops-in-Wisconsin-P185C0.aspx ).

Based on past research and the current spring’s weather, the PSNT should perform adequately in Southern Wisconsin. If there is concern about the PSNT underpredicting N credits in Southern Wisconsin, then delaying PSNT sampling by a week or two, compared to normal, will allow more time for the soil bacteria to mineralize N and potentially reduce the risk of the PSNT underpredicting N credits.