Background
Again in 2012 the OMAFRA field crops team surveyed a range of corn fields to determine what impact the weather has had on the level of residual soil nitrate. Conditions have been almost polar opposite to last year when April was cool and May was wet. What impact has the warm and dry spring weather had on residual N supply?
To provide some insight on this question soil samples were taken from 65 fields on May 31 from across the province and included a range of previous crop and soil types.
Results
On the medium and light textured soils that had not received fertilizer nitrogen or manure soil nitrate levels were somewhat higher than normally observed. The average from these soils this year was 12.2 PPM, compared to an historical average of 11.0 PPM and last years average reading of 9.5 PPM. These levels correspond to approximate sidedress N recommendations of 95 (12.2), 105 (11.0) and 118 (9.5) lbs of N/acre.
This year the heavier soils (clay loams and clays) do not seem to be responding as one might expect. Survey readings on these soils are averaging 8.3 PPM, where they traditionally run in the 9-10 PPM range. So while the heavier soils are showing more residual N than last year (average was 7 PPM) they are not above normal like many of the medium textured fields.
Table 1. PSNT Soil N Test Levels For Fields Requiring Side Dress Nitrogen Application May 31, 2012
# Fields Sampled |
Sample |
Soil |
Soil Nitrate N Level |
16 |
0-30cm (0-12”) |
Clay, Clay Loam |
8.3 |
35 |
0-30cm (0-12”) |
Loam, Silt Loam, Sandy Loam |
12.2 |
Some other fields were sampled that had received manure or early spring applications of nitrogen, predominantly UAN sprayed on with herbicide. These fields showed significantly higher nitrate values than average supporting the idea that mineralization of nitrogen from organic sources has been significant this spring and that losses from dentriciation or leaching have been low.
Recommendations
1) In general terms on the medium textured soils there appears to be an opportunity to trim N side-dress N rates slightly (5-15 lbs N/acre) compared to average. This does not appear to be the case on heavier textured soils
2) We remind side-dressers that the numbers presented here are averages and that there is considerable variability across sites. Taking your own pre-sidedress N test in order to fine tune application rates is strongly recommended.
3) Producers with manure or other organic sources should be sure to give adequate credit for the nitrogen provided to the crop. This can be done through soil N testing or by evaluating manure analysis, rate and timing factors.
4) Some stakeholders feel compelled to apply nitrogen at rates higher than normal based on experiences in 2011. This approach is not supported by the soil nitrate data gathered in this survey.
Acknowledgements
Soil samples were collected from a range of fields across southwestern Ontario. OMAFRA would like to acknowledge the contributions to this survey by the many farm co-operators and industry stakeholders who participated in this effort. Appreciation is also extended to SGS Agri-Food Laboratories for rapid soil sample analysis.
Table 2. 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).