Variable Rate Nitrogen Application on Row Crop Spinach

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Variable Rate Nitrogen Application on Row Crop
Spinach
2005 ASAE Annual International Meeting
Paper Number 051119

• C. L. Jones Research Engineer
• N. O. Maness Professor -
  Hort./Landscape Architecture
• M. L. Stone Regents Professor,
  ASAE Fellow
• J. B. Solie Sarkeys Professor, ASAE Fellow
• D. Zavodny Research Engineer

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Nitrogen Mismanagement causes

• Nitrogen Use Efficiency (NUE) 40-70
• Leaching and groundwater contamination
• Ozone depletion from gaseous losses as N2
• Loss of profit
• Health hazard due to nitrate build-up in foliage

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Current Best Practices

• 50 to 110 kg N/ha applied at planting
• Side dress application after emergence
• Rate depends..
• soil type
• seasonal variation
• cultural practice
• Soil testing limited by economics, resolution,
  time

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Objectives

• Answer the following
• When should we apply post-emergence N fertilizer
  in spinach?
• How should we determine in-season N application
  rates?
• Does variable rate application of N in row crop
  spinach work?

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Past Spinach Research

• Readily responsive to nitrogen
• NDVI (normalized difference vegetation index) is
  a good predictor of chlorophyll content
• Chlorophyll content and NDVI may be used to
  estimate photosynthetic capacity and productivity
• NDVI may be used to estimate nitrogen needs in
  spinach

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Equipment

• GreenSeeker active multispectral sensors (NTech
  Industries, Ukiah, Calif.)
• Handheld
• Applicator mounted
• LEDs emit red and NIR light
• Photodiode receives reflectedlight, produces a
  voltage
• Data used to calculate NDVI

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Determining In-Season Response

• Non-N-limiting strips

Stamp Calibration Plots
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Experiment Design

• Oklahoma Vegetable Research Station, Bixby,
  Oklahoma
• Spring, 2003 Spring, 2004 Fall, 2004
• San Juan and Fidalgo varieties
• Overhead sprinkler irrigation
• Severn very fine sandy loam, prior year fallow,
  preplant herbicide application

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Spring 2003
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Spring 2003 App. Rates/Results
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Spring, Fall 2004
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2004 App. Rates/Results
Sprayed plots averaged 10 kg/ha biomass better
than hand application
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Comparison VRT vs. Non-VRT
2003 for plots yielding approx. 16,000
kg/ha non-VRT plot 132 kg N/ha VRT plot
81 kg N/ha Fall 2004 VRT plots
treated with approx. 75 kg N/ha total 21,000
kg/ha non-VRT plot treated with 75 kg N/ha
13,000 kg/ha
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Conclusions

• When should we apply post-emergence N fertilizer
  in spinach?
• Once approximately 45 days post seeding (50
  ground cover stage)

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Conclusions

• How should we determine in-season N application
  rates?
• Compare NDVI of plot to best NDVI in stamp or
  calibration plots

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Conclusions

• Does variable rate application of N in row crop
  spinach work?
• Yes! Examples
• Spring 04 43 improvement
• Fall 04 53 improvement over non-VRT plots
• -VRT plots were visibly more uniform in color,
  texture, and size

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QUESTIONS?

• Acknowledgments
• Support through funding from the USDA Special
  Research Grant Number 2003-06134

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