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¶ Introduction
The “pre‐sidedress nitrogen test (PSNT)” can be used to help make more accurate nitrogen fertilizer recommendations. Originally developed by the University of Vermont, several universities have worked to determine the critical soil nitrogen levels to help identify whether a certain field would or would not respond to a sidedress nitrogen application. The PSNT can help assess the current nitrogen availability, but should be considered a guide, not a guarantee, to estimate nitrogen rates for in‐season application.
¶ A. Background
- PSNT compares soil nitrate test result (as ppm NO3‐N) to some critical value.
- Critical values range from 15 to 30 ppm NO3‐N.
- Result above the critical level; unlikely
- Is index or threshold, not necessarily a calibrated value.
- Can help assess when additional nitrogen might be needed.
- Not always clear how
- Soil nitrogen is a moving target.
- Nitrogen cycling and uptake occurs during entire growing season.
- Nitrate levels drop rapidly during early vegetative growth stage (7‐leaf to 8‐leaf).
- Soil nitrogen availability at any point in time is affected by field conditions, weather, timing, etc.
- Simple add/subtract of inputs and outputs won’t necessarily match up with soil nitrate result.
- Nitrogen cycling and uptake occurs during entire growing season.
- Most valuable when used to supplement preplant soil testing.
- Can be helpful when excess precipitation occurs after nitrogen application.
- PSNT results may be most useful when part of nitrogen is broadcast before planting, then followed by sidedress application.
¶ B. Sampling Requirements
- Nitrate can be highly variable across the field and even within the row, especially if nitrogen was banded.
- Nitrate does not move laterally across top soil.
- Collect a total of 30 to 45 soil core samples for each composite sample.
- Higher frequency of soil core samples most important with preplant banded nitrogen or if manure applied
- Sampling depths (see Fig. 1)
- Required: Surface soil core sample from zero to 12 inches (0‐30 cm)
- Optional: Separate subsoil core sample from 12 to 24 inches to verify subsoil nitrate concentrations
- 24 inches to verify subsoil nitrate concentrations. i. Recommendations under “normal” conditions not significantly improved with second foot
- Collect subsoil core samples from alternate surface core sample locations (i.e., “everyother” probe with coring tool)
- Each composite sample should represent field management zone of not more than 40 acres
- Critical level applies to 4‐leaf to 6‐leaf growth stage
- Typically occurs during late May through early June
- Soil nitrate levels drop rapidly during onset of rapid vegetative growth (after 6‐leaf stage)
¶ Figure 1. PSNT Sampling Depths
¶ C. Sampling Approaches
- Collecting adequate number of cores per sample is crucial.
- Nitrate does not move laterally through soil.
- Ammonium does not move laterally through soil
- Applied anhydrous ammonia (NH3) does move outward from injection point until hydrolyzed to ammonium form (NH4+) when soil moisture is present
- “Sets‐of‐three” approach (see Figure 2)
- Simplest approach, but considered less accurate than following methods
- Three (3) core samples required per location
- Sample 10 to 15 locations within field zone
- Take the first core sample right between two rows (e.g., 15 inches from either row in 30‐inch rows)
- Take second core sample 3 inches away from row to left
- Take third core sample at 3 inches from the row to right
- Move to next location and collect next set of core samples
- Composite all cores to prepare final sample from field management zone
- “Sets‐of‐five” approach (see Figure 3)
- Preferred method when nitrogen fertilizer band position is known.
- Five (5) cores required per sampling zone
- Sample 7 to 8 locations within field zone
- Take first core sample from band position
- Take next core sample one‐fourth (1/4th) of distance between bands and perpendicular to band direction
- Take next core sample one‐half (1/2) of distance between bands and perpendicular to band direction
- Repeat previous two steps on opposite side of band
- Move to next location and collect next set of core samples
- Composite all cores to prepare final sample from field management zone
- “Sets‐of‐eight” approach (see Figure 4)
- Is more complicated, but considered more accurate.
- Preferred approach when nitrogen fertilizer is broadcast or fertilizer band location is not known
- Eight (8) cores required per sampling zone
- Sample 4 to 5 locations within field zone
- Collect first core sample directly in row
- Move laterally to next row and collect second core sample perpendicular to row and 1/8th of row spacing distance
- Move laterally to next row and collect third core sample perpendicular to this row and twoeighths (2/8ths or 1/4th) of row spacing distance
- Move laterally to next row and collect fourth core sample perpendicular to this row and three‐eighths (3/8ths) of row spacing distance
- Repeat progressively until total of eight core samples are collected from location
- Move to next location and collect next set of core samples
- Composite all cores to prepare final sample from field management zone
- Is more complicated, but considered more accurate.
¶ Figure 2. “Sets of Three” Sampling Approach
¶ Figure 3. "Sets of Five Sampling Approach
¶ Figure 4. “Sets of Eight" Sampling Approach
¶ D. Required Laboratory Analysis
- Analyze nitrate‐nitrogen (NO3‐N) on all samples
- Additional analysis for ammonium‐nitrogen (NH4‐N) in selected situations
- Significant ammonium levels most likely if one or more of following conditions have occurred:
- Anhydrous ammonia was primary fertilizer source and soil temperature in application zone has been consistently 60°F or below following application
- Nitrification inhibitor was used a) e.g., nitrapyrin or dicyandiamide (DCD)
- Soil pH was below 5.5
- In these situations, low levels of soil nitrate may indicate limited conversion of ammonium to nitrate, rather than simply loss of nitrate
- Significant ammonium levels most likely if one or more of following conditions have occurred:
¶ E. Interpreting PSNT Results
- Accurate interpretation requires proper number of core samples in composite sample from field management zone
- No firm agreement on just how much nitrogen is needed when PSNT result is less than critical level
- Critical level of 25 ppm NO3‐N appears common for eastern, higher‐rainfall corn‐growing areas
- Typically have low subsoil nitrate levels at planting
- Colorado State research suggests critical level for western, lower‐rainfall corn‐growing areas may be about 15 ppm NO3‐N
- Typically have higher subsoil nitrate levels at planting than higher‐rainfall areas
- Nitrogen rate should be higher when PSNT result is closer to zero; lower rate needed when PSNT result is closer to critical value
- How much lower or higher is not always clear
- Expect that any additional nitrogen would be beneficial in at least nine of ten situations when PSNT result is below critical levels, according to research
- Recommended nitrogen rate is likely to be beneficial, but also likely to be more than needed for optimum yields in one of every three situations
- Critical level of 25 ppm NO3‐N appears common for eastern, higher‐rainfall corn‐growing areas
¶ F. Iowa State Recommendation Approach
- Probably most useful under “normal” conditions
- Following equation provides reasonable estimate of Iowa State nitrogen recommendations using PSNT:
- Nrec = [(25 ‐ PriceFactor ) ‐ SoilTestN ) * 8 ] * CropAdjustment, where:
- Nrec = lb N/ac, recommended in‐season nitrogen fertilizer application rate
- 25 = critical level as ppm NO3‐N
- PriceFactor = adjustment based on favorable or unfavorable grain/fertilizer ratio (see Table 1)
- SoilTestN = 0 ‐ 12" soil test nitrate as ppm NO3‐ N from PSNT samples
- 8 = assumes 8 lb. preplant N raises soil nitrate test by 1 ppm
- CropAdjustment = multiplier when manure or alfalfa is in rotation (see Table 1).
- Nrec = [(25 ‐ PriceFactor ) ‐ SoilTestN ) * 8 ] * CropAdjustment, where:
- Grain/fertilizer ratio
- Ratio of bushels of corn required to buy some amount of nitrogen fertilizer
- Favorable ratio
- 1 bushel buys about 7 lb. nitrogen
- About 100 to 120 bushels of corn required to buy one ton of anhydrous ammonia (82‐0‐0)
- About 35 to 40 bushels of corn required to buy a ton of nitrogen solution (28‐0‐0)
- Unfavorable ratio
- 1 bushel of corn buys about 15 lb. nitrogen
- About 200 to 250 bushels of corn required to buy a ton of anhydrous ammonia (82‐0‐0)
- About 70 to 90 bushels of corn required to buy a ton of nitrogen solution (28‐0‐0)
¶ Table 1. Recommendation Adjustment Factors |
|
| Price factor adjustment | |
| • Favorable grain/fertilizer ratio | 0 |
| • Unfavorable grain/fertilizer ratio | 5 |
| Crop Adjustment Multiplier | |
| • Corn following corn | 0.00 |
| • Corn following soybeans | 0.00 |
| • Corn following alfalfa | |
| • Favorable grain/fertilizer ratio | 0.65 |
| • Unfavorable grain/fertilizer ratio | 0.75 |
| • Manure applied in two of prior four years | |
| • Favorable grain/fertilizer ratio | 0.65 |
| • Unfavorable grain/fertilizer ratio | 0.75 |
¶ G. Purdue recommendation Approach
- Considered more appropriate for situations with severe potential for fertilizer nitrogen losses
- 25 ppm NO3‐N critical level may not be appropriate
- Collect samples no later than 6‐leaf growth stage
- Nitrogen cycling and uptake occurs during entire growing season
- Soil nitrate decreases rapidly after 7‐leaf to 8‐ leaf growth stage
- Separate surface and subsoil samples are recommended (0 to 12 in. and 12 to 24 in.)
- Assumes excess rainfall has depleted both surface and subsurface nitrogen
- About one‐third of fertilizer nitrogen typically found in lower root zone under “normal” rainfall
- Requires NO3‐N or NO3‐N plus NH4‐N results
- Required depth: Surface soil core sample from zero to 12 inches
- Optional depth: Separate subsoil core sample from 12 to 24 inches
- Analyze for nitrate only
- Used to verify subsoil nitrate concentrations
- Collect subsoil core samples from alternate surface core sample locations (i.e., “every‐other” probe with coring tool)
- Core sample #1 and subsequent odd‐numbered core samples: Both 0" ‐ 12" and 12" – 24"
- Core sample #2 and subsequent even‐numbered core samples: 0" – 12" only
- Following equation provides reasonable estimate of Purdue nitrogen recommendations using PSNT:
- Nrec = [ExpectedN ‐ SoilTestN ) * 5, where:
- Nrec = lb N/ac, recommended in‐season nitrogen fertilizer application rate
- ExpectedN = expected ppm NO3‐N or ppm NO3‐N+NH4‐N from Table 2 at original
- SoilTestN = ppm NO3‐N or ppm NO3‐N+NH4‐ N from soil sample results
- 5 = suggested fertilizer nitrogen rate; assumes 5 lb nitrogen required for each 1 ppm reduction of SoilTestN below ExpectedN
- Nrec = [ExpectedN ‐ SoilTestN ) * 5, where:
- Suggested nitrogen rate assumes corn is healthy and remaining growing season is expected to be “normal” or “typical”
¶ Table 2. Expected Soil Nitrogen Levels in Upper 12‐Inches for Different Rates of Applied Nitrogen Fertilizer |
||
| Fertilizer N rate applied before rains began |
ppm NO3‐N only* | ppm NO3‐N+ ppm NH4‐N** |
| 130 | 30 | 36 |
| 140 | 32 | 39 |
| 150 | 36 | 42 |
| 160 | 38 | 44 |
| 170 | 41 | 47 |
| 180 | 44 | 50 |
| 190 | 47 | 53 |
| 200 | 50 | 56 |
| 210 | 52 | 58 |
| 220 | 55 | 61 |
| 230 | 58 | 64 |
| 240 | 61 | 67 |
| *Assumes background level of ammonium at 6 ppm and “normal” levels of soil N below the 12‐inch sampling depth. | ||
| **Assumes “normal” levels of soil N below the 12‐inch sampling depth. | ||
¶ References
Magdoff, F., D. Ross, and J. Amadon. 1984. A soil test for nitrogen availability to corn. Soil Sci. Soc. Am. J. 48:1301‐1304.
Bauder, T. and R. Waskom. 2003. Best management practices for Colorado corn. Colorado State Univ. Ext. Bull XCM574A. pg. 58.
Spellman, D.E., et. al. 1996. Pre‐sidedress nitrate soil testing to manage nitrogen fertility in irrigated corn in a semi‐arid environment. Comm. Soil Sci. Plant Anal. 27:561‐574.
Blackmer, A.D. and R.D. Voss. 1997. Nitrogen Fertilizer Recommendations for Iowa. Iowa State Univ. Coop. Ext. Publication Pm‐1714. 4 pg.
Shapiro, C. & G. Hergert. 2013. “With heavy rains, how likely is N leaching” in Univ. of Nebraska Coop. Ext. Svc. “CropWatch” newsletter. 31 May 2013. https://cropwatch.unl.edu/heavy‐rains‐how‐likely‐n‐leaching‐unlcropwatch‐ may‐31‐2013 accessed 18March2019.
Sawyer. 2016. Late‐Vegetateive Corn Stage Soil Sampling for Nitrate‐N. Integrated Crop Mgmt. News. Iowa State Univ. http://crops.extension.iastate.edu/cropnews/2016/06/late‐vegetativecorn‐ stage‐soil‐sampling‐nitrate‐n, accessed 14July2016.
Camberato, J. and R.L. Nielsen. 2017. Soil Sampling to Assess Current Soil N Availability. Agronomy Dept., Purdue Univ., West Lafayette, IN. http://www.kingcorn.org/news/timeless/AssessAvailableN.html accessed 12March2019.