Wisconsin’s Pre-sidedress Soil Nitrate Test

The pre-sidedress soil nitrate test (PSNT) is a tool available to Wisconsin corn growers for improving the efficiency of their nitrogen (N) fertilizer applications. Not to be confused with routine soil tests, soil N testing allows N fertilizer recommendations to be adjusted to field-specific conditions that can influence corn N need. Two soil nitrate tests are available to Wisconsin corn growers: a preplant test and a pre-sidedress test. Both have the potential for saving fertilizer dollars and by eliminating over applications of N.

 

The PSNT is most useful for confirming legume and manure N credits and providing a site-specific estimate of soil N availability. The test is particularly important when insufficient information is available to assess these credits using standard techniques, such as when previous manure application rate and/or nutrient content are unknown or when the stand density of a previous alfalfa (or other legume forage) crop is unknown. Another situation where the PSNT is of particular value is when abnormal weather conditions occur that may impact N mineralization rates (cool weather) or loss of inorganic N (excessive rainfall).

 

Soil samples for the PSNT are taken after planting when corn is 6 to 12 inches tall. By this stage of the growing season most of the conversion of organic N sources to plant-available forms of N has usually occurred. PSNT soil samples are collected to a depth of 1 foot at a sampling density of 15 soil cores collected randomly over 20 acres. Cores are mixed to obtain a 1-cup composite subsample for submitting to the soil testing laboratory. PSNT samples should be kept cool until taken to the lab. If samples are to be stored for more than two days, they need to be either air-dried or frozen to prevent changes in nitrate content during storage. The PSNT, as well as the preplant soil nitrate test, is not recommended on sands and loamy sands.

 

Because of the rapid growth of corn at this stage of the growing season, it is imperative that PSNT soil sampling, lab analysis, and supplemental sidedress N applications (if needed) be completed within one to two weeks.

 

Results of the PSNT are interpreted using a critical value of 21 ppm nitrate-N. Fields testing above 21 ppm N are not likely to respond to additional N. University of Wisconsin N recommendations for corn are shown in Table 1. The amount of N to credit against the N application rate recommended for corn (Table 1) at various PSNT values is shown in Table 2.

 

   

Table 2. Nitrogen credits for the pre-sidedress soil nitrate test (PSNT).  

	Soil yield potential 1
	Very high/high	Medium/low
PSNT value	Nitrogen credit
- - ppm N - -	- - - lb N/a - - -
> 21	-- 2	-- 2
18-20	100	80
15-17	60	80
13-14	35	40
11-12	10	40
< 10	0	0

  

Other considerations when using the PSNT:

1)      Corn following alfalfa - Abnormally cool spring weather can delay the release of N from legume residues. If a PSNT result for corn following alfalfa is less than 21 ppm nitrate-N, apply no more than 40 lb/a of supplemental N.  

2)      Corn following soybean - The PSNT does not work well in assessing the N contribution from a previous soybean crop and should not be used for this purpose. Follow the N rate recommendations from Table 1 – which account for the effect of soybean on corn N need.

3)      Second year N credits - Second year N credits from manure or legumes should be assessed separately using standard techniques. However, the PSNT will also measure any N released by manure or legume residues during the second cropping year. Do not do use both methods. N recommendations based on PSNT results should not be separately adjusted for second year manure or legume credits.