Most years the OMAFRA field crops team surveys a range of corn fields to determine what impact the weather has had on the level of residual soil nitrate. With the wet conditions prevalent throughout the spring of 2011 there were two predominant questions:
1) Is there a trend for residual nitrate levels to be lower than normal?
2) If the full rate of nitrogen was applied before May 15 has there been a significant amount lost to denitrification or leaching from the abundant rains this spring?
This year soil sampling was conducted June 10-13 and was restricted to areas south and west of Toronto since areas to the east tended to have spring conditions that were closer to normal.
Results
Fields that had not received fertilizer nitrogen did tend to have soil nitrate levels that were lower that normally observed. This was most evident on the heavier textured soils (clay loams and clays). Traditionally the heavier soils have had soil nitrate readings of 10 ppm and this year the average was closer to 7 ppm (Table 1). This corresponds to an N rate recommendation of 144 lbs N/acre compared to 110 lbs N/ acre historically.
On the medium textured soils the differences were not as great. The average from these predominantly loam soils has usually come in at 11 ppm while this year the average was 9.5 ppm. These correspond to historical sidedress recommendations of 105 lbs N/acre compared to this year at 118 lbs N/acre.
Table 1. PSNT Soil N Test Levels For Fields Requiring Side Dress Nitrogen Application
# Fields Sampled |
Sample |
Soil |
Soil Nitrate N Level |
9 |
0-30cm (0-12”) |
Clay, Clay Loam |
7.0 |
21 |
0-30cm (0-12”) |
Loam, Silt Loam, Sandy Loam |
9.5 |
We also sampled fields that had nitrogen applied during the first half of May. The soil nitrate test has not been calibrated for these situations but our hope was that the test would give us a feel for the remaining N levels from the original applications. The medium textured fields had an average of 31 PPM where on average 148 lbs N had been applied, heavier soils scored 22 ppm where an average of 157 lbs N/acre had been applied and lighter soils showed 23 ppm were an average of 156 lbs N/acre had been applied (Table 2). The trend was clear that on soils that have tendencies to loose nitrogen from denitrification (clays) or leaching (sands), the soil nitrate levels in the top 12 inches were lower than the medium textured soils.
Table 2. PSNT Soil N Test Levels For Fields That Received Preplant Nitrogen
# Fields Sampled |
Sample |
Soil |
Avg. N Applied (lbs/ac) |
Soil Nitrate N Level |
4 |
0-30cm (0-12”) |
Clay, Clay Loam |
157 |
22 |
12 |
0-30cm (0-12”) |
Loam, Silt Loam, |
148 |
31 |
8 |
0-30cm (0-12”) |
Sand, Sandy Loam |
156 |
23 |
All of this begs the question; If I applied 150 lbs N acre on May 10 and a 12 inch soil nitrate test comes back lower than I expected is it really gone or just moved lower in the soil profile where the corn plant can still recover it? In this survey we sampled some research plots to both 12 inches and 24 inches (Table 3). It is clear from these results that some of the applied N was in the 12-24 depth layer and that sampling only to 12 inches would underestimate the amount of fertilizer nitrogen still available. On heavier soils this may not be case as N is lost due to denitrifaction and not to downward movement.
Table 3. PSNT Soil N Test Levels by Sampling Depth
Nearest Town |
Sampling |
Zero N Applied, |
150 lbs/ac N Applied, |
Elora |
0-30cm (0-12”) |
9 |
33 |
Elora |
30-60cm (12-24”) |
5 |
16 |
Bornholm |
0-30cm (0-12”) |
11 |
25 |
Bornholm |
30-60cm (12-24”) |
6 |
11 |
Woodstock |
0-30cm (0-12”) |
6 |
39 |
Woodstock |
30-60cm (12-24”) |
8 |
10 |
All sites with loam or silt loam soil |
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Recommendations
1) Sidedressers that have been fine-tuning rates downwards over the past decade may see 2011 as a year to apply more N. If however you traditionally apply 150 lbs N/acre we see little reason on average to boost this rate. Take the PSNT to be sure.
2) Nitrogen applied before May 15 may have been lost especially on heavy or light textured soils. In some cases where the 0-12 inch soil nitrate test is less than 25-30 ppm some additional sidedress nitrogen may be warranted. To be certain of your N status sampling should be done to a depth of 24 inches. We are less concerned about losses from N applied in late May through early June.
Table 4 converts the ppm values from the PSNT soil N test into recommended nitrogen application rates where there has not been nitrogen applied.
Acknowledgements
Soil samples were collected from a range of fields across southwestern Ontario between Friday June 10th through Monday June 13th. OMAFRA would like to acknowledge the contributions to this survey by the many farm co-operators who participated in collaboration with OMAFRA, Agris Coop, Belmont Farm Supplies, Hoegy Farm Supplies and Southwest Ag Partners, while SGS Agri-Food Laboratories supplied rapid soil sample analysis which was an important component of the project.
Table 4. OMAFRA Publication 811 Table 1-24. Nitrogen Recommendations Based on Nitrate-Nitrogen
Conversion Factors
To convert soil test results from kg/ha to ppm for a 30-cm (12-in.) sample, divide kg/ha by 4. For example, if the nitrate-nitrogen concentration of a sample taken from the top 30 cm (12 in.) of soil is 32 kg/ha, the nitrate nitrogen is 32 kg/ha ÷ 4 = 8 ppm.
1 Spring nitrate-nitrogen refers to samples taken within 5 days of planting (either before or after).
2 Pre-side-dress nitrate-nitrogen refers to samples taken when the corn is 15-30 cm (6-12 in.) tall (usually within the first 2 weeks of June).