This presentation examines 30-year trends in Iowa corn yields and fertilizer use, and evaluates these trends through an experiment with consistent fertilizer application rates. The experiment found that Iowa's increasing corn yields were not driven by increasing fertilizer rates, as experimental yields also rose significantly with constant fertilization. Additionally, the increased experimental yields did not reduce nitrate loading in drainage water. The conclusions are that ongoing increases in nitrogen fertilizer are unnecessary economically or environmentally for Iowa corn production.

1. Consistent Fertilizer Application
Increased Corn Yields Without
Change in Nitrate Export
Chelsea C. Clifford & Matthew J. Helmers
Iowa State University
9/1/22
OK - @DitchOntologist & @ISUAgWaterMgmt

2. The Mississippi Basin has a nitrate problem.
Average annual fertilizer N inputs by county for the Mississippi River basin for 1997 to 2006.
David et al. 2010
Must we keep adding more fertilizer?
NOAA NCSS

3. Thus, this presentation will:
• Examine thirty-year trends in
average corn yields and
fertilizer inputs in Iowa (with
data from the USDA National
Agricultural Statistics Service).
• Evaluate these trends through
comparison with experimental
corn yields given consistent
fertilizer application rates over
the same period.
• Assess experiment’s yield
change’s effects on nitrate
loading.

4. Iowa’s average corn yield has risen over time
r2=0.67
+2.50 bu./ac./yr.

5. r2=0.70
...as has average fertilizer use (risen in Iowa).
+0.924 lb./ac./yr.

6. r2=0.70
...as has average fertilizer use (risen in Iowa).
+0.924 lb./ac./yr.

7. Iowa corn yield apparently rises with fertilizer
r2=0.66
+2.52 bu./lb.

8. … but has yield risen because of fertilizer?
N fertilizer (+)
Corn yield (+)
+
N fertilizer (+)
Corn yield (+)
Year (+)
Year (+)
+
+
N fertilizer (+)
Year (+)
Corn yield (+)
… so therefor…
+
+

9. … but has yield risen because of fertilizer?
Year (+)
N fertilizer (+)
Other changes, e.g.
• Cultivars
• Climate
• Soil Health
• …?
Corn yield (+)

10. … but has yield risen because of fertilizer?
Variable Slope t p
Year 2.00 2.79 0.012*
N fertilizer (average, lb./ac.) 0.978 1.49 0.156
Effects on Iowa average corn yields (bu./ac.):
Year (+)
N fertilizer (+)
Other changes, e.g.
• Cultivars
• Climate
• Soil Health
• …?
Corn yield (+)

11. The Experiment:
Agricultural Drainage Water Research & Demonstration Site
Gilmore City, IA
Lawlor et al. 2008
8 (/78) corn/soy
rotation plots
fertilized at
consistent high
rates (150-160
lb/ac./yr.) since
1989
We controlled for (kept
constant) fertilizer rate;
what happened to yields?
MN Pollution
Control Agency

12. Experimental yields rose (given consistent N)
r2=0.45
+2.16 bu./ac./yr.

13. Yield growth similar in experiment and state
+2.16 bu./ac./yr.
+2.50 bu./ac./yr.

14. Corn yield (+)
-
Drainage nitrate
Did yield increase reduce drainage N exports?

15. Did yield increase reduce drainage N exports?
r2<0.01
+0.215 lb./ac./yr.

16. Components
of nitrate
loading
(no persistent
temporal
trends)

17. Conclusions
• Iowa corn yield and nitrogen-based fertilizer application rates have
increased in recent decades.
• The yield increase in our consistently highly fertilized experimental
plots suggest that Iowa’s increasing fertilizer did not drive its
increased yields.
• Our experimental yield increase (with fertilizer levels at current levels
all along) did NOT reduce drainage nitrate loading.
• Ongoing increases in nitrogen fertilizer are not what Iowa needs,
economically or environmentally.

18. Acknowledgements
• Dozens of Iowa State University students and staff collected
experimental data and maintained the experiment over the years.
• This experimental data is part of Iowa Agricultural Drainage and Nutrient Studies which are a
cooperative project between the Iowa Department of Agriculture and Land Stewardship and Iowa
State University and have been supported, in part, through funds authorized by the Iowa
Groundwater Protection Act. In addition, research from 2011-2015 was part of a regional
collaborative project supported by USDA-National Institute of Food and Agriculture, award
number 2011-68002-30190, “Cropping Systems Coordinated Agricultural Project: Climate Change,
Mitigation, and Adaptation in Corn-based Cropping Systems.”
• Statewide data comes from the U.S. Department of Agriculture National Agricultural Statistics
Service, and gaps in our own precipitation data were filled with data from the U.S. National
Oceanic and Atmospheric Association weather station GHCND:USC00136719.

19. Questions?
Chelsea Clifford: cchelsea@iastate.edu, @DitchOntologist
Matt Helmers: mhelmers@iastate.edu, @ISUAgWaterMgmt
(Please contact me about informal
network of ditch researchers!)