A New and Improved Soil Nitrate Test for Corn!

What's New?

Measuring the amount of nitrate-Nitrogen in the surface 30 cm (12.in.), usually in the first two weeks of June has proven to be a reasonably good indicator of potential soil N supply for a corn crop. This test is commonly referred to as the Pre-Sidedress Nitrogen Test or PSNT and was first introduced into Ontario in 1993.

Over the last twenty years we have examined the PSNT nitrogen fertilizer recommendations for corn and now, in 2015, we have arrived at a superior recommendation approach using this test. This new approach or calibration will be significantly more accurate than the older recommendation especially in higher yielding corn fields.

What's Different?

The main difference in the new PSNT recommendation system is that sidedress fertilizer N rates will hinge not solely on the soil nitrate value (i.e. 15 PPM) but will also factor in the expected yield for the field or part of the field.

In the former PSNT approach this was always a concern and some of you have called over the years to express this concern. How could a 15 PPM soil nitrate test mean the same amount of recommended nitrogen regardless of whether it came from a field where the yield expectation was 135 bu/ acre or from a field that was going to yield 235 bu/acre? Thanks for those calls; they inspired us to keep working at improving the PSNT.

So now Table 1 illustrates the new recommendations for sidedress N application rates based on both the soil N test and yield expectations.

What's Not Different?

The actual testing procedure has not changed just the recommendation system. So all of the old rules still apply:

  1. Samples must be from a depth 30 cm (12 in.).
  2. It is important that all cores in a field be taken to the same depth.
  3. It is very important to ensure that sampling avoids any previously broadcast nitrogen fertilizer or starter fertilizer bands.
  4. Sampling should occur when corn is 6-12 inches (15 - 30 cm) tall, and usually in the May 25 - June 15 window.
  5. Soil nitrate samples tend to be less accurate when sampled after periods of significant rainfall. If possible allow a day or two for the soil to dry before taking PSNT samples.
  6. Place soil cores in a clean plastic pail, crushed by hand and well mixed.
  7. Microbial action in the sample can change the nitrate content quickly if it is not handled properly. Chill or freeze samples as soon as possible. For shipping, pack samples with insulating material to keep them cool and send them by courier to ensure quick delivery to the lab.

Table 1. Pre-Sidedress Nitrogen Soil Test (PSNT) fertilizer N recommendations based on soil nitrate concentration (PPM) and expected yield (bu/acre).

Soil Nitrate

(PPM)

Expected Yield (bu/ac)
120 143 167 191 215 239
Sidedress Nitrogen Fertilizer Recommendation (lb N/acre)
0
176
197
218
240
261
282
2.5
163
184
205
225
246
267
5
151
171
191
211
231
252
7.5
138
158
177
197
216
236
10
126
144
163
182
201
221
12.5
113
131
149
168
187
206
15
99
117
135
153
172
190
17.5
83
102
120
138
156
175
20
57
86
105
123
141
159
22.5
0
60
88
107
126
144
25
0
0
63
90
110
128
27.5
0
0
0
66
92
111
30
0
0
0
0
68
93
32.5
0
0
0
0
0
69
35
0
0
0
0
0
0

Other Considerations

Spilt applications of nitrogen are generally considered to offer environmental and profitability advantages over approaches where all the N is applied at planting. However, we are convinced that the real advantage to split applications comes when sidedress rate fine-tuning is accomplished. Measurement of soil nitrate-nitrogen concentrations is one tool for accomplishing this. Our research to date suggests that properly conducted PSNT testing combined with this new recommendation schedule can result in N recommendation improvements compared to the general N recommendations derived from the N Calculator.

The other exciting option that now exists with this tool is the potential for site specific variable rate N applications. A field that is divided into yield zones could then be PSNT sampled by those zones, a yield expectation applied to each zone, and then a variable rate prescription derived from the above table. Something great to try in 2015!


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca