Source: Ontario Ministry of Agriculture, Food and Rural Affairs
With the spring of 2017 arriving cooler than normal, and Pre-Sidedress Soil Nitrate (PSNT) tests coming in lower than what is typically observed, many were wondering if 2017 was going to be a more responsive year for nitrogen (N).
OMAFRA has traditionally tried to gauge year to year differences in expected soil nitrogen supply by conducting an annual PSNT survey, where several dozen fields across the province are sampled for background (non-fertilized) soil nitrate levels.
The PSNT test involves pulling 12″ soil nitrate samples from unfertilized (starter only) corn fields just prior to sidedress timing. Because they are non-fertilized fields, these samples are only measuring N that has mineralized from the soil, and give an indication of the relative N supplying capability of that field. PSNT recommendations combine the soil nitrate measurement with yield expectation to provide an N recommendation (visit bit.ly/1SNkEM8 for more information). For example, using an expected yield of 170 bu/ac, each increase of 5 ppm in the PSNT reduces N recommendations by around 30 lb/ac (Table 1).
Table 1. Example of the change in nitrogen recommendations at a range of PSNT values, with an expected yield of 170 bu/ac
From 2015 to 2017, OMAFRA has been conducting an “N-Sentinel” project with funding from Grain Farmers of Ontario and Growing Forward 2, which takes the traditional survey a step further with additional goals of:
- Tracking background soil nitrate levels from May-July across a range of geographies and soil types
- Measuring nitrogen yield response at each location
- Determining if including a rainfall factor could increase the accuracy of nitrogen recommendations
When compared across years, the PSNT survey helps identify years where soil N supply may be lower or high than normal. For instance, soil nitrate levels were lower in 2017 than what has typically been observed in past PSNT surveys (Figure 1), possibly due to slower soil N release during the cooler spring. On the other hand, soil N supply appeared much higher than normal in 2015.
Figure 1. Soil nitrate values for various sample dates from 2011 to 2017 (alternate text)
Table 2 gives a breakdown of soil nitrate and yield measurements so far across the N-Sentinel project.
Table 2. “N-Sentinel” summary across all locations, 2015-2017
Zero-N Yield is the yield where no N was applied, with the exception of up to 30 lb/ac of starter N. Zero-N yields reflect the natural N supplying ability of the soil, so the more N that mineralizes from soil organic matter, the higher zero-N yields will be. The PSNT can be a proxy for estimating zero-N yields: the higher the PSNT, the higher the soil N mineralization, and the higher zero-N yields are expected to be.
Non-Limiting N Yield is the yield where a non-limiting rate of N was applied (i.e. 200+ lb/ac). This is important as it reflects the maximum yield attained when nitrogen is no longer a limiting factor.
Delta Yield is the difference between the zero-N and non-limiting N yields. This is important because this is the yield we are trying to capture with fertilizer N, the difference between what we receive naturally and what we need to meet our yield potential. Delta yield is generally proportional to N need, so the larger the difference, the more N that is required. Similarly, a smaller delta yield means less N is required to meet yield goals.
This concept has been incorporated into the 2015 revised PSNT recommendations which includes a yield expectation component. The PSNT value helps suggest what our zero-N yield will be, while our yield expectation will determine how much yield (i.e. delta yield) we need to capture with fertilizer N. Of course the challenge with N management is that we don’t know what our actual delta yield will be at the time of N application, as zero and non-limiting yields may vary year to year even within a field.
Estimated MERN is the estimated Most Economic Rate of Nitrogen (MERN) using the N-Rate Evaluator tool at www.gocorn.net (Figure 2). Calculating actual MERN requires an N response curve with several N rates. This tool estimates MERN using only zero-N yield and non-limiting N yield based on relationships derived from the Ontario Corn Nitrogen Database.
Figure 2. “N-Rate Evaluator” tool icon from www.gocorn.net.
Relationships in 2015 and 2016
It’s surprising how much yield can be made with the starter-only (zero-N) treatments; 162 bu/ac in 2015 and 143 bu/ac in 2016 (Table 2). These zero-N yields also reflect the average PSNT measurements across those years.
It’s also interesting to note the importance of yield expectation in the revised PSNT recommendations. For example, soil nitrate levels at the test sites in 2015 were very high (20 ppm), therefore using the traditional PSNT approach (no yield expectation factor), the recommendation would only have been 30 lb/ac of additional nitrogen. Non-limiting N yields at the 2015 test sites were exceptionally high however (234 bu/ac), so delta yield (+72 bu/ac) and estimated MERN (158 lb-N/ac) were actually still relatively large, despite the high PSNT. This compares to 2016 where PSNT values were closer to the long-term average (12 ppm), which using traditional PSNT recommendations would have suggested more N than the 20 ppm in 2015. In 2016 however, non-limiting N yields were also much lower (192 bu/ac) which actually resulted in a smaller delta yield (+49 bu/ac) and smaller estimated MERN (135 lb-N/ac) than 2015.
The 2017 PSNT value is 8 ppm which is lower than normal, suggesting zero-N yields may be lower than typical. If we were to still expect average yields, 2017 may have been a more N responsive year than average. Of course we won’t know until we complete harvest of our plots, which is currently underway. Stay tuned for yield results this winter.
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|Author:||Ben Rosser, Corn Specialist/OMAFRA|