Summary
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Background
OMAFRA Field Crop staff lead an annual PSNT survey across Ontario to measure nitrate mineralized from the soil as an indication of year-to-year soil nitrogen supply. Each spring, 80-100 soil samples are collected from fields that have not received N. Samples are collected just prior to traditional sidedress time, typically V3-V4 stage.
PSNT Survey
Nitrogen mineralization is favoured by warm soil temperatures with adequate soil moisture and aeration. Excess moisture can slow mineralization or increase the potential for losses through denitrification (especially under warm conditions) or leaching.
Spring of 2021 progressed similar to 2020. Sunshine and little rainfall produced great soil conditions through April and May. A period of cold temperatures dominated for a 1-2 week period early May, which was followed by seasonal to above seasonal temperatures . Variability in temperatures brought into question how N mineralization may be affected in 2021.
From May 31 to June 2, 93 samples were collected across Ontario from a range of previous crops, soil textures and geographies. A V3-V4 stage is typically targeted for PSNT sampling, but given the start of sidedressing and warm temperatures in the forecast, the survey was slightly ahead of schedule with most corn ranging V1-V3. Where corn fields without preplant N could not be found, soybean or processing vegetable fields were sampled if rotations were representative of grain corn (following wheat or soybeans).
Survey Results
The average PSNT value for the 2021 survey was 13 ppm. This is similiar to 2020 (14 ppm) and slightly above the long-term average of 12 ppm (2011-2020, Fig 1) suggesting similar to slightly higher than normal mineralization by sidedress stage in 2021. For historical reference, these levels compare most to years 2012, 2018 and 2020 (Fig 1). The 5 point lines in Figure 1 reflect a previous GFO funded project at 25 farms where soil N samples were collected throughout the spring. Years with single points reflect the traditional provincial soil N survey where single sampling times target the V2-V4 crop stage.
Recommendations
PSNT values are similar to the long-term survey average, suggesting soil nitrogen release may be similar to slightly ahead of where it normally is by the traditional sidedress stage. This suggests adjustments from a soil nitrogen supply perspective may not be required in 2021.
In General:
- Every 5 ppm change in soil nitrate usually changes PSNT N recommendations by 25-30 lb-N/ac
- Every 1 bu/ac change in yield expectation usually changes OMAFRA N recommendations (N-calculator and PSNT recommendations) around 0.80 lb-N/ac.
How Do I Know if My N Rates are Correct?
If you don’t measure, you don’t know. A simple way to evaluate N rates is to apply test strips 30 lb-N/ac above and below your normal N rates to get a feel for how corn yields respond on your fields. A history of N rate strips can provide an indication if your N rates are close to right, or could be adjusted up or down, and may provide guidance for changing rates for different environments (fields, years).
Application To Your Fields
This report provides a crude indication only and should not be used as a recommendation for nitrogen rates for any given field. Soil N supply is highly influenced by the environment (soil texture, organic matter, local weather, previous management). For instance, if you are in an area that has received significantly more rainfall than other parts of the province, you may have experienced less mineralization or more loss than is reflected in these results. The only way to know soil nitrate concentrations on your own farm is to pull samples from your own fields.
PSNT Sampling
Collect several 12″ soil samples across a field using a soil probe. Sample parts of fields separately if there is reason to suspect differences in N content (different field history, soil type, topography etc.). Make a well-mixed representative sub sample (1 lb) to fill a lab box or bag. Samples should be chilled to prevent further N mineralization and sent to a lab as soon as possible, keeping them cool during transport.
OMAFRA PSNT recommendations are only calibrated for where soil nitrate samples are measuring naturally mineralized N and should not be used where broadcast N fertilizer has been applied. A modest amount of N applied banded (eg. 30 lb-N/ac) or in furrow as starter is OK provided sampling is done mid-row to avoid these bands. Due to differences in N mineralization timing, recommendations in corn fields with manure, red clover or alfalfa in the previous year are more accurate when using nitrogen credits published in OMAFRA Pub 811, Agronomy Guide for Field Crops.
OMAFRA revised PSNT recommendations in 2015 to include both soil nitrate measurement and expected yield (Table 1). Using your field specific PSNT values and yield expectation in Table 1 provides an optimal target for total fertilizer N rate needed to achieve estimated yield expectations.
Acknowledgements
Sincere thanks to the growers, industry and organization participants (Alliance Agri-Turf, Agronomy Advantage, Benjamins Agronomy Services, FS Partners, Grain Farmers of Ontario, Maizex Seeds, Sullivan Agro, University of Guelph Ridgetown Campus and many individual producers) and OMAFRA staff who provided fields for sampling or collected soil nitrate samples, and to Grain Farmers of Ontario and SGS Agri-Food Laboratories Guelph for financial support of the annual PSNT survey.
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