LONG-TERM TRENDS OF CORN YIELD
AND SOIL ORGANIC MATTER
IN DIFFERENT CROP SEQUENCES
AND SOIL FERTILITY TREATMENTS
ON THE MORROW PLOTS
Susanne Aref and Michelle M. Wander
Department of Crop Sciences and Department of Natural Resources
and Environmental Sciences
University of Illinois at Urbana-Champaign
Urbana, Illinois 61801
I. Introduction and History of the Morrow Plots
B. Morrow Plots Treatments and Their Historical Context
C. Records and Data from the Morrow Plots
II. Corn Yield
A. Experimental Effects
B. Significance of Technology, Planting Date and Weather
C. Connection with Illinois State Average Corn Yield
II. Soil variables: Soil Organic Matter, pH, P and K
A. Soil Organic Matter
B. pH, P and K
C. SOM: Interaction Between Corn Yield and Soil Fertility
III. Conclusions: Lessons from the Morrow Plots
Advances in Agronomy, Volume 62
Copyright © 1998 by Academic Press. All rights of reproduction in any form reserved.
I. INTRODUCTION AND HISTORY
OF THE MORROW PLOTS
1. The Morrow Plots and Long-term Experiments
There are four field experiments in existence the United States that are over 100 years
old. These include the Sanborn Field in Missouri, started in 1888, the Magruder Plots in
Oklahoma, started in 1892, the Old Rotation Plots in Alabama, started in 1896, and the
oldest experiment of all, the Morrow Plots in Illinois, started in 1876. The Sanborn Field
and the Morrow Plots experiments are both rotation, fertility and manure experiments
with corn as the primary crop. Wheat is grown in the Magruder Plots and cotton is
grown in the Old Rotation Plots. All four experiments have been designated National
Historic landmarks. The focus of these old experiments has been the productivity and
sustainability of continually cultivated land. For more information about long-term
experiments see Mitchel et al., 1991 and Paul et al., 1997.
2. Management of Long-term Experiments
The Morrow Plots are better known for their historic significance than for the
scientific information they have produced. Experiments like the Morrow Plots that are
National Historic Landmarks cannot be ended. Whether or not they merely survive as
historic places or continue to be viable experiments will depend upon how they are
managed and perceived.
In long-term studies treatments are applied for a long enough time period to assess
management impacts on the resource base; this aspect of long-term studies demands a
certain permanence to the design and, in some ways, makes them inflexible. Considered
judgment must be used with regard to their management. During the life of historic
experiments, some practices lose their practical relevance. Rash change can terminate
treatments under study before valuable information is gained and/or confound subsequent
findings. It is the viable long-term experiment that can be modified to include new
questions as time passes without sacrificing the original experiment’s intents. Despite
some limitations, such an experiment is found in the Morrow Plots. Through cautious
action, the Morrow Plots experiment has been modified only a few times. Remarkably,
this was done without ruining the initial set-up or losing sight of the original questions.
The experiment has evolved to study not only the effects of rotations but also timely
Stewards of the Morrow Plots have embellished the initial design, adding or
expanding questions at historically appropriate points in time. The original ten plot
experiment was intended to determine if corn yield would remain consistent after years of
planting the same land (Hopkins et al., 1908). This soil exhaustion study began in 1876
with three different crop rotations and three fertility treatments. By 1904 the experiment
had been reduced to only three of the original rotation plots, continuous corn (CC), corn-
oat (C-O) and corn-oat-hay (C-O-H), all of which had not been fertilized. At this time,
the three rotation main-plots were split and manure, lime and phosphorus were added to
their southern halves. Until 1955, the southern half of rotation plots were then managed
in accordance with what was known as the Illinois system of permanent soil fertility.
This was a system or ‘philosophy’ of soil management advocated by University of
Illinois soil scientists who had worked diligently to identify practices to support
agriculture on a permanent basis (Smith, 1925). The use of hybrids started in 1937,
reflecting agronomic practices common at that time. Use of commercial fertilizers
became common during the middle of the century. In 1955, when all six main plots were
in corn, a nitrogen-phosphate-potassium (NPK) treatment was added to part of the plots.
This third phase of the trial would determine if, and how rapidly, productivity could be
restored to previously untreated plots and how treatment history (untreated or manured)
impacted yield response. At this time, seeding rates were adjusted to match soil fertility
levels. The fourth phase of the trial was begun twelve years later, in 1967. Soybean
replaced oat in the two-year C-O rotation and a very high-level NPK treatment replaced
manure applications on some plots. This last modification brought the C-O rotation in
line with regional cropping practices and would determine how very high-levels of
fertilizer application impacted productivity.
3. Relevance of the Morrow Plots
Overall trends and cumulative impacts of management systems are best studied
through long-term experiments (Peck, 1989; Mitchell et al., 1991; Barnett et al., 1995).
In the Morrow Plots, changing agricultural norms have competed with experimental need
to sustain treatments long-term. The result has been three surviving rotation plots that
have been reduced in size and repeatedly subdivided. Even with their limitations, long-
term experiments like the Morrow Plots, provide us with the only reasonable empirical
basis upon which we can evaluate agricultural sustainability (Steiner, 1995). The
changing nature of the Morrow Plots’ treatments reflect the agricultural history of the
region and show that managers of the plots have been interested in both short and long-
term aspects of productivity.
This review paper will describe the history of the Morrow Plots and the impacts of
their experimental treatments on corn yield and soil variables. We report on all data
available at this time. For the first time, statistical analyses of the complete yield and soil
organic matter (SOM) record are presented. We consider trends in response to rotation
and fertility treatments applied during the main phases of the Morrow Plot’s history.
Yield responses simultaneously reflect changing soil productivity levels and the
immediate impacts of improved technology and weather. By analyzing soil variables,
soil organic C, total N, pH, P and K, throughout the trial’s history, we study the long-
term impacts of treatments on the system’s productive potential.
B. MORROW PLOTS TREATMENTS AND THEIR
1. Phase 1: 1876 - 1904 - Life and Times during Plot Establishment
a. History of the University
The University of Illinois was founded as the Illinois Industrial University in 1867 to,
as Dean George E. Morrow put it, 'secure a wider and better education for the industrial
class', noting it was not to be 'simply and only a collection of farms and work shops in
which the manual labor of the farmer and mechanic is shown and practiced'.
b. Establishing Treatments, 10 Plots
The Morrow Plots experiment began in 1876 to produce results 'suggestive to the
practical farmer’ (Illini Week, Oct. 26, 1989). C. W. Silver’s promptings directly
inspired this kind of trial. Silver (1875) advocated that Illinois emulate trials that had
been going on in Europe for a generation using American crops. Taking the suggestions
of Silver, Manley Miles, then professor of Agriculture, established the experiment that
may have become the Morrow Plots. It is not certain that Miles, who left the University
after only one year, established the trial (Experiment no. 23) that became the Morrow
Plots. It is known that the management of Experiment no. 23 fell to Morrow, who had
also been inspired by Rothamsted during a 1879 sabbatical trip. Morrow successfully
fought an up-hill battle for the College of Agriculture against the prevailing view that one
did not go to a University to study agriculture. Morrow also had to overcome the fact
that this experiment was seen by farmers as too 'academic' (The Illinois Agriculturalist,
The Morrow Plots are located at 40° 06’ 15” north latitude and 88° 13’ 32” west
longitude, on a prime piece of real estate on the University of Illinois Campus. Little is
known about the history of the site prior to plot establishment. Furthermore, no records
were kept between 1876 and 1887. The site was part of the North Farm which had been
in agricultural use for some time, possibly 40 years or more (Darmody and Peck, 1997).
The original trial consisted of five acres divided into ten plots. The initial
experimental set-up was nearly balanced. There were three continuous corn (Zea mays
L.) rotation plots (CC), one corn-oat (Avena sativa) rotation plot (C-O), and six corn-oat-
hay rotation plots (C-O-H). Hay crops included sweet and red clovers (Melilotus alba
and Trifolium pratense), with occasional inclusion of soybean (Glycine max) or cowpea
(Vigna ungiculata). Even though this was primarily a rotation experiment, fertilizer
treatments were used on two of the CC plots. Plot 1 received manure and plot 2 received
P, K and sulfurous ammonium (PKS) amendments. The third CC plot received no
amendments. The fourth plot was the only plot in C-O rotation so corn was only grown
every other year. The remaining six plots were devoted to the C-O-H rotation which was
initially planted in a six year cycle: corn-corn-oat-hay-hay-hay. These plots were offset
so that each part of the cropping cycle was represented every year (1, phase 1). All plots
were moldboard plowed in the Fall. There is some indication that catch crops were
sporadically grown in the CC and C-O rotations.
In August of 1895 an observatory (now also a National Historic Landmark) was built
on two of the fertilized CC plots (1 and 2), leaving only the rotation portion of the
original trial. Lack of interest in the experiment resulted in a breakdown in crop
sequence on the C-O-H plots (5 through 10) in 1897. Plots 6 through 10 were planted for
the last time in1901. In 1903, almost 30 years after the initial experiment was begun,
plots 6 through 10 were seeded to lawn.
Treatments in the four phases of the Morrow Plots
N - S W E
A B C D
Phase 1: 1876 – 1903a
Plot 1 CC M
Plot 2 CC PKS
Plot 3 CC U
Plot 4 C-O U
Plot 5-10 C-O-H U
Phase 2: 1904 – 1954b
Plot 3CC N U U
S M with LrP M with LbP
Plot 4 C-O N U U
S M with LrP M with LbP
Plot 5 C-O-H N U U
S M with LrP M with LbP
Phase 3: 1955 – 1966c
Plot 3 CC N U U-NPK U U
S M M-NPK MPS M
Plot 4 C-O N U U-NPK U U
S M M-NPK MPS M
Plot 5 C-O-H N U U-NPK U U
S M M-NPK MPS Mc
Phase 4: 1967 – 1996d
Plot 3 CC N U U-NPK U U
S H-NPK M-NPK MPS M
Plot 4 C-O N U U-NPK U U
S H-NPK M-NPK MPS M
Plot 5 C-O-H N U U-NPK U U
S H-NPK M-NPK MPS M
aPhase 1, then called Experiment no. 23, included ten plots with three crop rotations: continuous corn
(CC), corn-oats (CO) and corn-oats-hay (C-O-H). Plot 1 was amended with manure (M), plot 2 was
amended with P,K and sulphorus ammonium (PKS). Plots 3 through 10 were unamended (U).
bDuring Phase 2, plots 3, 4,and 5 survived and were divided into NW, NE, SW and SE plots. Manure lime
and phosphorus were applied to all south plots. Beginning in 1904 and ending in 1919, phosphorus was
applied to SW plots as rock phosphate (M with LrP) and to SE plots as bonemeal (M with LrbP).
Beginning in 1920, P was added as triple superphosphate.
cDuring phase 3, plots were subdivided in the east west direction (A, B, C, D). Nitrogen, phosphorus and
potassium were applied to formerly unamended (U-NPK), manure amended (M-NPK plots). At this
time, the seeding density of all NPK fertilized plots and some manure amended plots (MPS) were
dIn phase 4 a high NPK treatment was applied to some formerly manured plots (H-NPK) and the two year
corn-oat rotation was changed to a corn-soybean rotation.
2. Phase 2: 1904 - 1954: Modification of the Experiment
a. Reduction of Size and Number of Plots, Installation of Tiles
In 1904, the three remaining plots, numbered 3, 4 and 5, were reduced in the west -
east direction to one-fifth acre plots. All other agricultural or horticultural use of the land
around the Plots had been suspended due to the growth of the University. Additionally,
before the 1904 field season, tile lines were installed between the north and south sub-
plots of all rotation treatments (Odell et al., 1984a).
b. Introduction of Manure-Lime-Phosphate Treatment
A fertility treatment was added to the Morrow Plots to make them reflect agronomic
practices of the time (Table I, phase 2). This modification allowed study of the effect of
application of manure, lime, and phosphate (Hopkins, 1911; DeTurk, 1938; Stauffer et
al., 1940; Lee and Bray, 1949). The three rotation main-plots were each divided in
fourths in the north-south and the west-east directions and a manure, lime and phosphate
treatment was added to the south plots. There was no difference between the two north
plots (NW and NE) in each rotation, while 600 lbs rock phosphate was applied to the SW
plot and 200 lbs steamed bonemeal was applied to the SE plot.
Manure was applied before corn was grown. Roughly four tons of cured manure
were applied annually to the CC plots and applied every other year to the C-O plots. Six
tons of manure were applied to the C-O-H plots every three years, again when corn was
grown. Barnyard manure was obtained from Animal Sciences stockpile and contained
mostly (>90%) cattle and some horse or poultry manure. It included some bedding
material since much of the stockpiled waste came from the University’s dairy operation.
Phosphate was applied with manure on all plots until 1909. From then on, manure was
still applied every year on the CC plots. Phosphate was applied every other year to the
CC and C-O plots and every third year to the C-O-H plots.
Initially limestone application rates were set at 200 lbs/A with this amount to be
applied every ten years. This was changed to a regular lime application of 3 tons/A,
applied once every 6 years along with phosphate. Rock phosphate and bonemeal were
last applied in 1919 (Odell et al., 1984a).
c. Introduction of Hybrids
In 1937 corn hybrids were introduced to the plots. Until 1955 several hybrids were
used each year. Between 1955 and 1989 only one hybrid was seeded at a time (except in
1964), with varieties being changed about every eight years. The frequency of hybrid
replacement increased after 1990.
3. Phase 3: 1955 - 1966 Addition of N and K Fertility
a. Introduction of NPK Treatment
Fifty years after the first addition of a fertility treatment, the Morrow Plots were
updated again. Commercial N, P and K fertilizers, then an accepted practice, were added
to the Plots. Nitrogen was added as urea, P as triple superphosphate and K as muriate of
potash. This modification brought the Morrow Plots in line with agronomic practices of
the day. With this experimental alteration it could be determined whether, and how fast,
soil fertility was restored to depleted land and how manuring influenced productivity
(Bruce, 1955; Russell, 1956; Bartholomew and Kirkham, 1960).
The implementation of the new treatment required further division of the plots. They
were split in the west-east direction, plot NW became NA and NB, plot NE became NC
and ND, SW became SA and SB, and SE became SC and SD (Table I, phase 3). All B
plots, with and without a history of manure application, now received the NPK treatment.
Manure was no longer added to SB plots. Additionally, crop residues were now returned
to the B plots after harvest (Odell et al., 1984a).
b. Number of Kernels per Hill
Seeding density was adjusted to match soil fertility levels. The untreated north
sections of all plots had always been planted at a lower density than the manure-amended
south plots. For the CC plots, the density was 8,000 plants/acre (p/A) in untreated plots
versus 12,000 p/A in manured plots. For the two rotations, C-O and C-O-H, all plots had
a planting population of 12,000 p/A. All fertilized plots were planted at the higher rates
of 16,000 p/A. One manured plot in each rotation was changed in 1957 to the higher
planting density of 16,000 p/A (Table II).
Alteration of fertility and seeding rates in phase 3 resulted in a total of five treatments
(untreated plots (U), manure amended (M), manured with higher seeding density (MPS),
and NPK applied to previously untreated plots (U-NPK) and to previously manured plots
(M-NPK)), which were applied to all crop rotations.
4. Phase 4: 1967-1996 Addition of High Fertility
and Substitution of Soybean
a. Introduction of High-level NPK Treatment
The ‘green revolution’, which occurred in the late nineteen fifties and early sixties,
inspired the introduction of a very high level fertilizer (H-NPK) application in 1967
(Table I, phase 4). This sixth fertility treatment has been studied by several authors
(Jones and Hinesley, 1972; Cescas and Tyner, 1976; Welch, 1976; Omueti and
Treatments Applied Since 1967 to North and South Regions of the Morrow Plots
(amounts in pounds per acre)
Location Treatment A B C D
plants/acre 8,000 24,000 8,000 8,000
1 none 200 none none
2 none >45 none none
3 none >336 none none
Manure4 none none none none
plants/acre 24,000 24,000 16,000 12,000
N 300 200 none none
P >112 >45 none none
K >560 >336 none none
Manure none none 600 600
C-O and corn-soybean rotation
plants/acre 12,000 24,000 12,000 12,000
N none 200 none none
P none >45 none none
K none >336 none none
Manure none none none none
plants/acre 24,000 24,000 16,000 12,000
N 300 200 none none
P >112 >45 none none
K >560 >336 none none
Manure none none 600 600
plants/acre 12,000 24,000 12,000 12,000
N none 200 none none
P none >45 none none
K none >336 none none
Manure none none none none
plants/acre 24,000 24,000 16,000 12,000
N 300 200 none none
P >112 >45 none none
K >560 >336 none none
Manure none none 900 900
1 Nitrogen was applied as urea.
2 Phosphorus, applied as triple superphosphate since 1920, had been added systematically (40 lbs/A) since
1955. In 1967, application was changed to a maintenance based approach. The NPK and H-NPK plots
testing less than 45 or 112 were amended with 49 or 98 lbs/A P.
3 Potassium, applied as muriate of potash, was also changed from a systematic (300 lbs/A) to a
maintenance based approach. During phase 4, the NPK and HNPK plots testing lower that 336 and 560
were amended with 93 and 186 lbs/A K, respectively.
Mortvedt, 1986; Jones, 1992). The H-NPK treatment was applied to the SA plots
(previously manured plots) in each rotation. In addition, a switch was made from
systematic fertilizer application rates to application based upon soil test values. The NPK
treatments (B plots) with soil test levels less than 45 or 336 lbs/A received 49 and 93
lbs/A of P and K, respectively. The H-NPK treatment (SA) plots with soil test levels less
than 112 and 560 received 98 and 186 lbs/A of P and K, respectively. The planting
population in H-NPK plots was increased from 12,000 p/A to 24,000 p/A. The other
NPK plots planting density was increased from 16,000 p/A to 24,000 p/A some years
later. From 1967, crop residues were returned to all plots.
b. Other Alterations
To reflect current rotation practices in the region, the two year rotation was changed
from a corn-oat to a corn-soybean rotation. Until 1967 manure was applied on the basis
of crop removal. After 1967, wet manure (estimated at 30 to 40% moisture content) was
supposed to be applied on a 3 ton per-acre per-year basis. The timing and amount of
manure applied has been inconsistent and records are limited. During the last decade
manure has been applied to all rotations once every three years during the fall prior to
corn production in the C-O-H rotation.
C. RECORDS AND DATA FROM THE MORROW PLOTS
The earliest phases of very old experiments are rarely adequately recorded. The
Morrow Plots are no exception in this respect. First, though the Plots have been
cultivated since 1876, there are no records of yield before 1888 and no soil samples
available for the period before 1904. This means that the first 12 years of yield data is
missing and that 28 years passed before any soil samples were taken.
Fortunately, the Illinois Agricultural Experiment Station was started in 1887. This
meant that from 1888 data from the Plots appeared in annual reports published by AES.
The trial was then identified as Rotation Experiment no. 23. Handwritten records in the
Morrow Plots field book provide information about the earliest phases of the trial. The
first entry in the Morrow Plots field book mentions the Observatory, which had been
built in 1895. Whomever recounted the period from 1876 to 1912 must have copied
some of the earliest records from another source. Morrow recorded the Morrow Plots
treatment structure and layout in 1879; his record was copied into the Morrow Plots field
book. Field practices and yield have continued to be recorded in field books. No other
formalized record keeping system has evolved.
a. Plant Production Variables
Crop row widths have been 40 inches with a 20 inch spacing between hills. The plots
have been plowed, fertilized and manured mostly in the Fall and disked just before
planting. Initially weeds were controlled by hand pulling and later by application of
herbicides. Corn has been harvested by hand and until recently, was planted by hand
with an extra kernel added to each hill to permit thinning. Planting densities were
adjusted as described previously (sections I,B,3,b and I,B,4,a). The yield record began in
1888 and has continued.
Extensive weather records were available for the period from 1888 to 1996. The
Urbana Weather Station was established in 1888 as an independent project to study air
and soil temperatures. This trial, located 700 feet north-west of the Morrow Plots was
terminated in 1897. The weather station was moved to a location adjacent to the plots
and maintained until 1984. After this, the equipment was moved two miles south-south-
west of the Plots.
b. Soil Variables
The soil, which is a Flanagan silt loam, (fine, montmorillonitic, mesic Aquic
Argiudoll) developed under tall grass prairie in Peoria loess lain over Wisconsin age,
calcareous loam glacial till (Ferenbacher et al., 1984; Darmody and Peck, 1997).
Soil carbon contents were available for samples collected from west and east plots in
1904, 1911, 1913, 1923, 1944, 1953, 1955, 1967-69, 1973, 1974, 1980 to 1995. Organic
Carbon was determined by a modified Schollenberger method. The method involves
oxidation at 175 C° for 90 seconds using a solution of concentrated sulfuric
acid/potassium dichromate (Schollenberger, 1945). Nitrogen values, believed to be
based on macro-Kjeldahl analyses, were available for the same samples through 1973.
Soil samples collected in 1969 were analyzed using a LECO CNS-2000 with N
calibration based on a NIST coal standard. The N concentrations of samples analyzed in
1968 via Kjeldahl matched combustion based determinations of samples collected in
1969. The N content of soils collected in 1969, 1973, 1974, 1980, 1986, and 1992 were
then determined with the LECO CNS-2000.
Soil pH, P and K values were assessed beginning in 1967, since the H-NPK was
introduced and fertilizer application rates were based on test levels. Soil pH was
determined with a glass electrode (1:1 soil:water), P with Bray P1, and K with
ammonium acetate extraction and atomic absorption.
2. Statistical Analysis
The Morrow Plots experiment lacks the randomization and replication that has long
been expected in field experiments. In long-term experiments original randomization
remains in place; hence, spatial differences that exist between plots become apparent
over long time periods. With the Morrow Plots’ data, we investigate these differences as
well as differences resulting from treatments.
The four experimental phases were treated as a factor. Phase 1 included data
collected before 1904. Even though hybrid adoption did not change the experimental
focus or design, this shift in practice constituted a large technological advance.
Therefore, phase 2 was divided into sub-phases 2A and 2B to distinguish between
periods before and after hybrid introduction. Phase 2A was the period 1904 to 1936,
phase 2B was the period 1937 to 1954, phase 3 was the period 1955 to 1966, and phase 4
was the period 1967 to 1996.
Unless otherwise indicated, analyses were performed using SAS proc mixed version
6.11 (SAS Institute Inc., SAS Campus Dr., Cary, NC 27513). Year and interactions with
year were random effects, rotation and phase were fixed effects. Other fixed factors
(fertility treatment, north and south plots, and A, B, C and D blocks) varied with models
according to experimental question.
Yearly averages of variables from all phase, rotation, and treatment combinations
were used to assay overall effects on corn yield, and soil pH, P and K. Yield analysis
was based on data collected between 1888 through 1996. Soil pH, P and K analyses were
based on data collected annually between 1967 and 1995.
Different analyses were carried out to determine the effects of spatial location on corn
yield, soil C and N contents and C/N ratios. Rotation and individual plot were used to
analyze corn yield in each phase. To assess spatial impacts on soil C, N and C/N ratios,
plots were analyzed using both north and south direction (which coincides with manure
application) and east and west effects. Soil C and N contents and C/N ratios were
analyzed using data collected from 0-15 cm in 1904, 1911, 1913, 1923, 1933, 1943,
1953, 1961, 1973, 1974, 1980, 1986, and 1992.
To assess yield stability, mean yield and standard deviation (stdev) were considered.
Yearly corn yield from each phase, rotation, and treatment combination were averaged.
These averages were then used to get means and stdev for each phase. Mean and stdev
were analyzed as factorials with three fixed factors, rotation, treatment, and phase using
SAS proc glm to determine the behavior of yield and yield variability.
Correlation analyses between Morrow Plots’ corn yield and individual weather
variables as well as Morrow Plots’ corn yield and Illinois state yield averages were
carried out using SAS proc corr.
II. CORN YIELD
A. EXPERIMENTAL EFFECTS
1. Overall Effects, 1888 - 1996
a. General Trends
The complete historical yield record for the Morrow Plots is shown in Fig. 1. Yield
of CC plots has always been much lower than yield of C-O and C-O-H plots. Manure
Introduction of hybrids
High level NPK adde
Introduction of hybrids
High level NPK adde
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
H-NPK M-NPK MPS M U-NPK U
Introduction of hybrids
High level NPK adde
Figure 1: Morrow Plots yield from 1988 to 1996 by the three rotations: continuous corn (CC), corn-
oat (C-O), and corn-oat-hay (C-O-H). Vertical lines indicate changes from one phase to the next. Phases
were 1888-1903, 1904-1936,1937-1955,1956-1966,1967-1996. Plots were U, untreated; M, manured;
MPS, manured with higher planting density; U-NPK, previously untreated, now NPK treated; M-NPK,
previously manured, now NPK treated; H-NPK, previously manured, now NPK treated;
addition increased yield. After an initial decrease in corn yield from phase 1 to phase 2,
yield was stable until the introduction of hybrids in 1937. Corn yield then increased in
the C-O-H rotation. Productivity did not immediately increase after hybrid introduction
in theCC or C-O rotations. Yield in the C-O plots started to increase in the late forties
and in the CC plots in the early fifties. Benefits of the longer rotation allowed first C-O-
H and then C-O rotations to utilize the higher yield potential of hybrids.
There were large statistical differences between corn yield of the fertilized and
manured plots, and even larger differences between yield of the untreated and all other
plots. In fertilized plots that had been manured (M-NPK and H-NPK), yield in the C-O
rotation equaled yield in the C-O-H rotation. Corn yield in U-NPK plots, which were
previously untreated, was 11 Bu/A less in the C-O rotation than in the C-O-H rotation.
Yield in the CC rotation was 15 Bu/A less in the U-NPK than in the M-NPK and H-NPK
plots. Higher planting density in manured plots increased yield compared to lower
density M plots; planting density-based differences in yield were 13, 18 and 25 Bu/A in
CC, C-O and C-O-H plots, respectively. Between 1950 and 1975, which included the
years of the green revolution, yield in the untreated C-O and C-O-H plots increased
linearly. Yield in these plots were extremely stable. Yield in the untreated CC plots and
all treated plots (CC, C-O and C-O-H), also increased at this time but was more variable.
Yield in the eighties and thereafter was highly variable in all plots. During the mid-
seventies yield began to plateau.
b. Rotation and Treatment Effects
Yield means of all three rotations were significantly different from each other.
The difference between the yield of the C-O and C-O-H rotations was only half of the
difference between the yield of the CC and C-O rotations. Based on the ANOVA F
values, the effect of rotation on yield was only half as large as the effect of treatment on
yield (Table III). The NPK treatment effects were not different from each other but all
Morrow Plots corn yield 1888 - 1996: ANOVA
Tests of Fixed Effects
ANOVA NDF Type III F Pr > F
Phase 4 24.59 0.0001
Rotation 2 108.82 0.0001
Treatment 5 171.60 0.0001
Rotation X treatmenta 10 11.89 0.0001
aX indicates an interaction effect. Table IV
Morrow Plots corn yield 1888 - 1996: Mean comparisonsa (in bushels per acre)
Rotation All U M MPS U-NPK M-NPK H-NPK
CC 87.82c 35.81c 64.60c 77.39c 106.22c 121.30b 121.05b
C-O 106.01b 50.49b 88.13b 106.22b 125.85b 129.12a 136.24a
C-O-H 116.08a 69.15a 98.63a 123.83a 136.95a 133.98a 133.94a
Phase All Treatment All CC C-O C-O-H
1 102.35bc U 51.82d 35.81e 50.49e 69.15d
2A 82.26d M 83.79c 64.60d 88.13d 98.63c
2B 96.39c MPS 102.48b 77.39c 106.22c 123.83b
3 111.87b U-NPK 123.19a 106.22b 125.85b 136.95a
4 123.64a M-NPK 128.13a 121.30a 129.12ab 133.98a
H-NPK 130.41a 121.05a 136.24a 133.94a
aValues within groups in columns followed by different letters are significantly different at the 5% level.
other treatment differences were very highly significant (Table IV). Yield of the U plots
was significantly lower than yield of all other treatments. Yield of the M plots was
significantly lower than yield of the MPS plots, which was significantly lower than yield
of the NPK plots.
There was a very highly significant interaction between rotation and treatment, but it
was only a tenth of the size of the rotation effect on yield. For U, M, MPS and U-NPK,
yield increased significantly with longer rotation (Fig. 2). This was not the case for the
M-NPK and H-NPK treatments, where yield was the same in C-O and C-O-H rotations.
The yield of M-NPK and H-NPK plots were not significantly different in each
rotation. Yield in the U-NPK plot was significantly lower (by 15 Bu/A) than yield in M-
NPK and H-NPK plots only in the CC system. The difference in yield response
associated with the different rotations was due to the residual effect of manure. The H-
NPK and M-NPK fertility treatments were applied to plots that were previously manured
as opposed to the U-NPK treatment, which was applied to untreated plots.
c. Phase Effects
The phase yield means were all significantly different except in phases 1 and 2B.
Yield decreased significantly from the first phase to the second (Table IV). Soil
exhaustion was reflected in declining yield until the introduction of hybrids shifted
production upward. Sustained increases in yield in later phases were due to alterations in
cultural practices that included the use of hybrids and the addition of fertility.
Figure 2 Average yield for each treatment and rotation. Averages are least-squares means from the
ANOVA for the whole data set. See Fig. 1 for abbreviations.
2. Yield by Phases
a: Phase 1: Original Experiment
Yield reported between 1888 and 1903 were used in the analysis of the first
experimental phase of the Morrow Plots (Fig. 3). Comparisons of yield in the CC plots
revealed that the untreated and PKS amended plots were similar and were significantly
lower than yield in the manured plot. In the manured CC plot, phase 1 yield was in the
same range as yield in the C-O-H plots (all of which were untreated). Statistical contrasts
indicated yield differences between the C-O-H rotation plots and the PKS CC, U CC, and
C-O plots were significant. Yield in the C-O plot fell between the manured CC plot yield
and the other two CC plots yield and was not significantly different from either. This
early attempt at commercial fertilization did not succeed. Yield response to manure, and
not P, K and S addition, indicated N was already limiting (Bogue, 1963; Odell et al.,
1984b). During the soil exhaustion phase of the trial, there was a slight downward trend
in yield in all the plots, although the correlation (-0.17) between yield and year, was not
Figure 3 Average yield for each of the 10 plots in phase 1, 1888-1903. Rotations were CC, C-O, and C-
O-H. Comparisons were among yield in the three CC plots, the C-O-H plot and the average of the C-O-H
b. Phase 2: Manure-Lime-Phosphate
i. IMPACT OF MANURE-LIME-PHOSPHATE TREATMENT. Addition of manure
immediately increased corn production. ANOVA results indicated that the relative
importance of treatment and rotation factors were similar in phases 2A and 2B (Table V).
Differences between yield in U and M plots were 20 Bu/A (phase 2A) and 40 Bu/A
(phase 2B) (Table VI and Fig. 4). These differences are comparable to the differences
between yield in the CC and C-O-H rotations, which were 24 Bu/A (phase 2A) and 40
Analysis of yield by phases, 2A, 2B, 3 and 4: ANOVA
2A 2B 3 4
_________________ ________________ ________________ ________________
Source DF F P>F F P>F DF F P>F F P>F
Rotation 2 39.69 0.0001 68.99 0.0001 2 128.69 0.0001 97.65 0.0001
Plot 3 127.37 0.0001 199.29 0.0001 7 140.48 0.0001 221.00 0.0001
Rot*plot 6 3.85 0.0015 15.05 0.0001 14 7.85 0.0001 8.19 0.0001
Analysis of yield by phases, 2A, 2B, 3 and 4: Mean comparisonsa
(in bushels per acre)
2A 2B 3 4
CC 32.14c 41.91c 77.19c 90.51c
C-O 44.48b 63.41b 95.01b 117.49b
C-O-H 56.30a 80.47a 108.09a 132.78a
NW 31.80d 36.72c NA 51.60e 74.00d
NE 36.61c 43.93b NB 122.37ab 149.51a
SW 52.85b 81.64a NC 53.19de 73.60d
SE 55.97a 85.42a ND 60.50d 78.13d
SA 103.27c 153.06a
SB 128.16a 154.15a
SC 120.15b 121.36b
SD 108.20c 104.95c
a1Values within groups in columns followed by different letters are significantly different at the 5% level.
2B). Within each phase, there was no significant yield difference between the manured
SE and SW plots (different P sources); however, there was a small but statistically
significant yield difference between the untreated NE and NW plots. Since the plots
were treated the same the difference was due to spatial variation. Even though plot
differences of 5 to 7 Bu/A were observed, the larger differences of 20 and 40 Bu/A
between the north and south (untreated and manured) plots indicated these differences
were due to treatment.
ii. BONEMEAL AND ROCK PHOSPHATE TREATMENTS. The ANOVA of yield in
phases 2A and 2B indicated plots SW and SE were not significantly different, suggesting
the source of phosphorous (rock phosphate versus bonemeal) had no effect on crop yield.
Note, phosphorous was applied with manure, which had a great impact on yield,
preventing separate assessment of the P sources.
c. Phase 3: Nitrogen-Lime-Phosphate-Potassium Treatment
i. MANURE, ROTATION AND PLOT DIFFERENCES. Recall that during phase 3, the
NPK treatment was applied to the B block and the MPS treatment (increased planting
density) was applied to the SC plots. There was a 50 Bu/A yield difference (not
NA NB NC ND SA SB SC
SD NA NB NC ND SA SB
SC SD NA NB NC ND SA
SB SC SD NA NB NC ND
SA SB SC SD
CC C-O C-O-H
2A 2B 3 4 2A 2B 3 4 2A 2B 3 4
A B C D
Figure 4: Phase and treatment yield means for each rotation. The NA, NC, and ND plots were
untreated, the SD plot was manured, the SC plot was manured in all phases and had higher planting
densities in phases 3 and 4, the NB plot was untreated, the SB plot was manured in phases 2, and 2B. Both
plots received NPK in phases 3 and 4. The SA plot was manured in phases 2A, 2B, and 3 and received
high-level NPK in phase 4. The NA and NB yield were recorded as one NW yield and the NC and ND
yield were recorded as one NE yield in phases 2A and 2B. South plot needs were similar.
B nor SC) between north (untreated) and south (manured) plots, which was larger than
the 40 Bu/A difference between north and south plots in phase 2B, indicating manure
addition had an additive effect with benefits to yield increasing with time (Table VI and
Fig. 4). The 30 Bu/A yield difference between the C-O-H and CC rotation observed in
phase 3 was less than the 40 Bu/A difference in phase 2B, indicating diminished impact
of rotation when fertility treatment are considered. For the individual plots with the same
treatment, there was a significant yield difference (9 Bu/A) between the NA and ND
plots, exhibiting the same spatially based trend observed in the untreated plots in phases
2A and 2B. The untreated plot yields were ranked: NA≤NC≤ND. As in earlier phases,
yield in the manure amended SA and SD plots did not differ.
ii. IMMEDIATE EFFECT OF NPK TREATMENT. Yield in previously untreated plots
increased immediately after introduction of NPK fertilizers. ANOVA of the 1955 data
showed that in one year, fertilizer application increased yield in previously untreated
plots to the same level as manured plots. Untreated plots yielded an average of 50 Bu/A,
which was significantly less than the mean yield produced by any of the other treatments.
Corn yield in fertilized treatments, which was 102 Bu/A for M-NPK plots, 95 Bu/A for
U-NPK plots, and 94 Bu/A for M plots, were statistically similar. Yield in the CC plots
(75 Bu/A) was significantly lower than yield of the C-O and C-O-H rotations (87 and 93
iii. LONG-TERM EFFECT OF NPK TREATMENT. Within a few years yield differences
between M, MPS, U-NPK and M-NPK fertility treatments became apparent. Yield in the
untreated plots remained the lowest overall. The higher planting density in SC plots
(MPS) produced significantly higher yield than manured plots with lower densities.
Yield was highest in M-NPK, intermediate in U-NPK and lowest in MPS plots. Only the
difference between M-NPK and MPS plots was significant. The planting density in this
period was the same for M-NPK and MPS; hence, the observed yield difference of 8
Bu/A was due to either NPK application or spatial variation in the plots (Table VI and
d. Phase 4: High-level NPK Treatment
i. COMPARISON OF H-NPK, M-NPK, AND U-NPK. The first year the high-level soil
fertility treatment was applied, associated yield was lower than yield of the other NPK
treatments (in all rotations). The second time the H-NPK treatment was applied, which
occurred one, two, and three years later in the CC, C-O, and C-O-H rotations, the
associated yield equaled or exceeded yield in the M-NPK and U-NPK plots. After this
and until the late seventies, corn yield in H-NPK plots was higher than M-NPK and U-
NPK yield (Fig. 1). This was due to the differences in plot planting density. During this
time only the H-NPK treatment had a density of 24,000 p/A while the M-NPK and U-
NPK plots had 16,000 p/A. In the late seventies, planting density in all NPK plots was
raised to 24,000 p/A. Over the last 20 years, within rotation yield in the H-NPK and M-
NPK plots have been similar. With similar manure histories, the high fertility treatment
(H-NPK) failed to increase yield over the M-NPK treatment. Despite the initial planting
density advantage in H-NPK plots, mean comparisons for the entire phase showed no
difference between H-NPK, M-NPK and U-NPK (Table VI and Fig. 4). In phase 4, the
M-NPK and U-NPK plots improved corn production by 27 Bu/A, while the H-NPK
brought the yield from the previous M plot from 103 Bu/A to 153 Bu/A.
ii. IMPACT OF SUBSTITUTION OF SOYBEAN FOR OAT. There was no obvious
difference in corn yield before and after the change from oat to soybean. Corn yield
increased steadily in all three rotations during phase 3 and during the first part of phase 4
(Fig. 1). Note, even though soybean was now in the rotation we continue to use the C-O
notation to identify the two-year rotation.
iii. IMPACT OF CHANGING MANURE APPLICATION PRACTICES. In phase 4, yield in
M and MPS (higher planting density) plots did not change, while yield in all other plots
(even in untreated plots) increased significantly. Yield increases ranged between 20 and
27 Bu/A (Table VI and Fig. 4). In phase 3 yield in the C-O M and CC U-NPK plots were
the same, indicating comparable productivity (Fig. 4; phase statistics not shown).
Likewise, yield in the C-O-H M, the C-O MPS, the CC M-NPK, the C-O M-NPK and U-
NPK plots were the same. In phase 4, yield in the C-O-H M, the C-O MPS and the C U-
NPK plots were the same, as were yield in the C-O-H MPS and the CC M-NPK plots.
Records indicate that manure application frequency declined in CC and C-O rotations
during phase 4. Nutrient supply may have limited yield in M and MPS plots.
Interestingly, the difference between U and M yield declined from 50 Bu/A in phase 3 to
30 Bu/A in phase 4. The overall difference between C-O-H and CC plot yield was 40
Bu/A in phase 4.
iv. SIGNIFICANCE OF WEATHER. Besides experimental inputs and soil condition,
yield was dependent on weather. As mentioned earlier, the H-NPK plots were not more
productive than the M-NPK plots. In fact in very dry years, yield in H-NPK plots was
very low (Fig. 1). During the 1988 drought, the untreated C-O-H plot yield exceeded the
H-NPK plot yield. Osmotic stress associated with high fertilizer rates may have been the
3. Yield Stability
Mean yield and stdev representing rotation by treatment combinations within phase
are shown in Fig 5. Throughout the course of the trial, yield stdev has declined and then
increased, indicating periods of high and low yield stability. Yield means were discussed
in sections II,A,1 and II,B,2 and are not discussed further here. For reference, results
from ANOVA for yield mean are included with those for stdev in Tables VII and VIII.
Treatments had the greatest impact on yield stdev. Rotation effects were only significant
through interaction with experimental phase. The phase effect on stdev was highly
significant; the corresponding mean square was almost twice that of the interaction with
rotation. Only the phase and treatment main effects on stdev are discussed here. The
contributions to stable yield made by adoption of various technologies during phases 2B
and 3 cannot be separated from the excellent weather conditions that prevailed at this
time. The years of the green revolution had an average maximum temperature of 85.2˚F
and an average precipitation of 4.13 inches in July. This was 0.9˚F lower and 0.5 inches
higher than the averages of the other years of the Morrow Plots’ existence. The variances
of these weather variables were also lower during this period than in all other years.
Relatively consistent and slightly lower temperatures and higher precipitation in July
may be associated with the desirable weather conditions that are generally believed to
have occurred during the green revolution years in the Midwest. The reduction in yield
stability during phase 4 was associated with higher planting densities in some plots and
by more variable weather.
Averaged over years, higher phase means had higher standard deviations. However,
residual analyses of the yield data indicated that there was no relationship between yield
Figure 5 Treatment effect on phase yield a) and yield sd b) in each rotation. Rotations were CC, C-
O, and C-O-H.
size and stdev. In general, yield stdev increased with the amount of fertility applied. The
stdev of plots was ordered: U < M ≤ MPS < U-NPK ≤ M-NPK < H-NPK. The very high
Overall yield mean and standard deviation: ANOVA
Yield mean Yield stdev
Source DF MS F Value Pr > F MS F Value Pr > F
Phase 4 2129.55 22.51 0.0001 50.18 4.66 0.0052
Rotation 2 3597.84 38.03 0.0001 16.76 1.56 0.2289
Phase X Rotation 8 31.10 2.88 0.0178
Treatment 5 5440.75 57.51 0.0001 332.44 30.84 0.0001
stdev of H-NPK plots yield indicates that not only did this treatment fail to increase yield,
it lowered yield stability significantly.
B. SIGNIFICANCE OF TECHNOLOGY, PLANTING DATE
1. Impact of Hybrid Introduction
Use of hybrids and other technologies resulted in a general increase in yield during
phases 2B and 3 (Fig. 1). Correlation between year and yield for U and M plots within
Overall yield mean and standard deviation comparisonsa
Phase Rotationb Treatment
______________________ ___________________ _________________________________
Id mean sd Id mean Id mean sd
1 48.33d 24.41a CC 83.78c U 48.94d 11.21d
2A 44.31d 23.00ab C-O 102.86b M 84.55c 17.25c
2B 61.93c 20.79b C-O-H 113.36a MPS 101.28b 18.85c
3 71.92b 19.50b U-NPK 117.04a 25.59b
4 93.81a 24.27a M-NPK 122.19a 25.24b
H-NPK 126.00a 36.22a
aValues within groups in columns followed by different letters are significantly different at the 5% level.
bRotations were not significantly different for stdev.
rotation was used to statistically corroborate the effects of hybrids. Since hybrids were
started in the Morrow Plots in 1937 and the quality of manure was changed in 1967, only
data within this period was used in the analyses. All correlations between yield and year,
except for the U plot in the C-O-H rotation (n=10), were highly significant. This
indicates there was a strong positive linear relationship between yield and year. The
correlation coefficients between yield and year of U plots in CC, C-O and C-O-H
rotations were 0.62***, 0.70** and 0.45NS, respectively. Coefficients of M plots in CC,
C-O and C-O-H rotations were 0.71****, 0.76***, and 0.78**, respectively. While
hybrid introduction increased yield in both U and M plots, the higher correlation between
yield and year in M plots demonstrated how fertility magnified the benefits of hybrid
adoption. Within treatment differences between rotations are obscured by variable
numbers of observations.
2. Impact of Planting Date
During phase 1 planting date varied between 120 and 147 Julian days (Fig. 6). The
range in date narrowed during phase 2A from 125 to 140 Julian days. In phase 2B the
planting dates were progressively later. During subsequent phases, planting occurred
earlier and earlier until the end of phase 4. The trend toward very early planting dates
was dramatically reversed in the late eighties. Planting dates have been progressively
delayed during the nineties.
A positive relationship existed between planting date and precipitation in May and a
negative relationship existed between planting date and May maximum temperature.
Figure 6 Morrow Plots planting date record, 1888-1996.
Correlations between April weather and planting date were not significant. When
planting was not completed by early May, too much moisture and to a lesser degree low
temperatures delayed planting. There was no significant correlation between planting
date and average yield; however, years with very late planting dates tended to have low
3. Important Weather Components: Correlations with Yield
Correlation analysis was used to study the relationship between weather and corn
yield. The weather variables considered were temperature, modified growing degree
days (GDD), precipitation, the product of temperature by precipitation (TxPPT), and
snow fall. Monthly averages of these variables were obtained from the previous Fall
through the growing season (September to September).
During the growing season, there were highly significant correlations between the
yearly average of treatment yield means and temperature (negative), GDD (negative),
precipitation (positive) and TxPPT (positive) in July and temperature (negative) in
August. Though planting date was significantly correlated with May weather, May and
June weather variables were not significantly correlated with yield. Table XI contains
Yearly Average Yield Correlations with Weather Variables Using
108 Years of Data (1888-1996)
Temperature r Precipitation r Interaction of precipitation r
Maximum As rain With mean temp.
January -0.245f Decembera 0.200e January -0.200e
July -0.347g Totalb 0.258f With minimum temp.
August -0.335g Totalc 0.236e Novemberd 0.248f
Minimum July 0.421h Totalb 0.251f
April 0.192e As snow July 0.431h
Growing degree days January 0.246e Total 0.312g
Total 0.224e Total 0.212e
a Previous December
b Previous year’s total
c Total of 2 years’ prior
d Previous November
e Significant at the 5% level.
f Significant at the 1% level.
g Significant at the 0.1% level.
h Significant at the 0.01% level.
the significant correlations. Similar observations have been made by other authors
(Smith, 1914; Thompson, 1969; Offutt et al., 1987; Dixon et al., 1994).
Pre-season monthly weather variables were not as highly correlated with yield as
were July and August variables. April minimum temperature was the only spring
variable that had a significant correlation (positive) with yield. In January there were
significant negative correlations between average yield and temperature and TxPPT as
well as a positive correlation between average yield and snow fall. Highly significant
positive correlations existed between yield and the previous year’s November TxPPT and
Total yearly GDD, snow fall, and TxPPT, were all significantly and positively
correlated with yield. The previous year's totals of precipitation and TxPPT were
positively correlated with yield as was total precipitation from two years earlier. The
correlation coefficients for precipitation totals from one and two years earlier were of the
same magnitude. While growing season variables are most often considered in yield
models, this data shows that weather in the previous winter, fall and even year can
influence yield. Van der Pauw (1966) indicated that not only growing season weather
had an impact on yield but also noted the impact rainfall in previous periods on soil
C. CONNECTION WITH ILLINOIS STATE AVERAGE CORN YIELD
To use the Morrow Plots as a model of yield trends in Illinois, total yield for the state
was compared to Morrow Plots mean yield for each phase/rotation/treatment combination
(Fig. 7). In general, there was a positive correlation between these mean yields and corn
yield in Illinois (Table X). Treatment correlations varied from phase-to-phase. We
assume the treatments with the highest correlation in each phase best reflect agronomic
practices in use in Illinois at that time.
During phase 1, when all plots were untreated, only yield in the CC plots was
significantly correlated with Illinois yield. In phase 2A, U plot yield in all three rotations
was more highly correlated with Illinois yield than was yield in M plots. This changed in
phase 2B, at this time yield in U plots was not correlated with Illinois yield. Yield in M
CC and C-O-H plots was significantly correlated with production in Illinois. These
results suggest that prior to hybrid adoption, production in untreated plots and Illinois
farms was similar. By the end of phase 2, Illinois corn production was most like
production in manured plots seeded with hybrids.
The impact of commercial fertilizer application on yield correlations was assessed by
combining data from phases 3 and 4. Between 1955 and 1995, the correlation between
NPK treated plot yield and Illinois yield became the most significant; these very highly
significant correlation coefficients ranged between 0.631 and 0.891. A graph of Illinois
and NPK treated plots’ yield shows how similar production has been in the last 40 years
(Fig. 8). Interestingly, yield in M and MPS plots were not significantly correlated with
0 20 40 60 80 100 120 140 160
Illinois yield (Bu/A)
MPS M U-NP
Illinois yield (Bu/A)
Figure 7 Comparison of Morrow Plots yield and Illinois average yield, 1888-1996.
Correlation between Illinois and Morrow Plots yield in
each phase, treatment and rotation
Rotation Phase n U M MPS U-NPK M-NPK H-NPKa
CC 1 16 0.685d
2a 33 0.530d 0.386c
2b 18 -0.064g 0.504c
3-4 41 0.208g 0.051g 0.117g 0.631f 0.701f 0.734e
C-O 1 9 0.279g
2a 16 0.623d 0.413g
2b 9 0.167g 0.419g
3-4 21 0.647d 0.084g 0.189g 0.828f 0.827f 0.738d
C-O-H 1 4 0.345g
2a 11 0.745d 0.732c
2b 6 0.661g 0.927d
3-4 14 0.818e 0.198g 0.485g 0.891f 0.888f 0.760d
aData from H-NPK plots occurred in phase 4 only.
bPhases 3 and 4 were combined, U-NPK and M-NPK correlation coefficients in phase 4 alone were 0.606
and 0.611e in CC, 0.743d and 0.783e in C-O, and 0.878e and 0.866d in C-O-H, respectively.
cSignificant at the 5% level.
dSignificant at the 1% level.
eSignificant at the 0.1% level.
fSignificant at the 0.015% level.
gNot significant at the 5% level.
Illinois yield. When phases 3 and 4 were analyzed separately, there were positive but
non-significant correlations in phase 3. During phase 4, yield stagnated in M and MPS
plots, failing to keep pace with increasing State yield. Additionally, yield in U plots was
not correlated with Illinois yield in the CC rotation, was highly correlated with yield in
the C-O rotation and was very highly correlated with yield in the C-O-H rotation. Even
yield was much lower in U plots, trends in C-O and C-O-H rotations were similar to
trends in Illinois.
In phases 3 and 4, yield of the Morrow Plots NPK treatment have been higher than
Illinois yield, with a few exceptions (Fig. 8). In 1980, 1988 and 1995, yield in CC plots
was lower than Illinois yield. In 1980 and 1988, drought occurred and in 1995 planting
was delayed by excess precipitation in May which was followed by dry conditions in
June and July. In 1993 yield in the U-NPK CC plot was substantially lower than Illinois
yield. The cause for this is unknown; however, it has been suggested that gray squirrels
damage may have been greatest in this plot, which is close to a large grassed area.
Figure 8 Comparison of Morrow Plots yield and Illinois average yield, 1955-1995.
III. SOIL VARIABLES: SOIL ORGANIC MATTER,
pH, P AND K
A. SOIL ORGANIC MATTER
1. Changes in C, N and C/N
a. Phase Effects
The soil C and N contents of the Morrow Plots have declined during the history of the
trial (Fig. 9). The C and N record begins with samples collected in 1904 when phase 1
had ended. The initial C and N contents of the plots is unknown. The C and N contents
of the adjacent grass boarder, which has never been disturbed, ranged between 39.4 and
26.7 g kg-1 in 1990. These values are probably lower than those found in the initial plots
(Darmody and Peck (1997). We used data from 1904 (Phase 1), 1913, 1923, 1933 (Phase
2A), 1943, 1953 (Phase 2B), 1961 (Phase 3), 1973, 1980, 1986, and 1992 (Phase 4) to
assess overall rotation, phase, and plot effects on soil C, N and C/N ratios. Statistically
significant C losses occurred through all phases of the experiment (Table XI).
Significant losses of N occurred between phases 1 and 2A and between phases 2A and
2B. During phase 3, N did not change. Soil N then increased significantly during phase
4 of the experiment. This could have been caused by addition of samples collected from
B and D plots (see section II,A,1,c), by change in N analyses methods (see section
II,A,2,a) and/or by soil erosion. Darmody and Peck (1997) recently reported that the
Plots are now 15 cm lower that the adjacent grass border. While aggradation of boarders
probably contributes to some of this difference, erosion may be lowering the plow layer
into subsurface horizons which have substantially higher clay and fixed ammonium
Changing soil C and N contents are associated with significant changes in soil C/N
ratios. Initially soil C/N ratios increased, the phase 1 C/N ratio was significantly lower
than the phase 3 ratio. During phase 4, C/N ratios decreased significantly, falling below
12.0 for the first time. Decreased C/N ratios could be associated with increased
humification of soil organic matter (SOM), increased abundance of fixed N (Stevenson,
1994), and/or inconsistent analyses, sampling or tillage patterns.
b. Impact of Rotation
After experimental phase, crop rotation has had the most significant impact on soil C
and N contents (Fig. 10; Table XI). Average organic matter contents were highest in the
C-O-H, intermediate in the C-O, and lowest in the CC rotations. Rotation has had no
impact on average soil C/N ratios. The positive effect of rotation length on SOM
contents has been reported before but to our knowledge has not been analyzed
statistically (Odell et al., 1984a&b; Darmody and Peck, 1997). The interaction between
rotation and experimental phase will be discussed in section (II,A,2,a).
1900 1920 1940 1960 1980 2000
NA NB NC ND
SA SB SC SD
1900 1920 1940 1960 1980 2000
Carbon Content (g C per kg soil
1900 1920 1940 1960 1980 2000
Soil Carbon and Nitrogen Contents and C/N Ratios: 0-15 cma.
Effect ID (g kg-1 soil) (g kg-1 soil) C-N ratio
Phaseb 1 25.6a 2.06a 12.42a
2A 24.4b 1.92b 12.72ab
2B 22.6c 1.74cd 12.91b
3 21.8d 1.72c 12.69ab
4 20.1e 1.75d 11.42c
Rotation CC 19.2a 1.55a 12.36a
C-O 23.0b 1.84b 12.46a
C-O-H 26.5c 2.12c 12.48a
Direction North 20.9a 1.68a 12.38a
South 24.9b 2.00b 12.48a
West 20.8a 1.20a 12.23a
East 25.0b 1.98b 12.63b
Plotc A 18.6a 1.66a 11.16a
B 19.7a 1.76a 11.22a
C 21.6b 1.84ab 11.68b
D 23.5c 1.96b 12.06c
aValues within groups in columns followed by different letters are significantly different at a 5% level or
bUnless specified, data includes values from soil samples collected in 1904, 1911, 1913, 1923, 1933,
1943, 1953, 1961, 1973, 1974, 1980, 1986, and 1992 from west and east plots only.
cValues in plots A, B, C and D are from phase 4 (1973, 1974, 1980, 1986, and 1992) only.
c. Spatial Heterogeneity
In a non-replicated trial like the Morrow Plots, care must be taken to investigate and
acknowledge inherent variability of the experimental unit. Aware of spatial variability in
the field, researchers have discussed C and N contents associated with individual plots,
noting rotation and treatment impacts on loss or gain occurring over time. Odell et al.
(1984a) and Darmody and Peck (1997) both reported a west-east gradient in soil C and N
contents. Our analysis confirm this trend, indicating C, N and C/N ratios all
Figure 9 Trends in Morrow Plots soil carbon contents: 1904 to 1992. Soils (0-15 cm) were collected
from north (N) and south (S) plots on west (A and B) and east (C and D) sides of the field. Rotations were
CC, C-O, and C-O-H.
CC C-O C-O-H
Carbon (g N per kg soil
CC C-O C-O-H
Nitrogen (g N per Kg soil
Figure 10 Average carbon contents of soils under continuous corn, corn-oat, and corn-oat-hay rotations,
during different experimental phases of the Morrow Plots trial.
significantly from western A plots to eastern D plots (Table XI). The wider soil C/N
ratio of eastern plots indicates SOM in plots D and C was less humified than SOM in
plots A and B. This spatial difference in C/N ratios was also noted by Odell et al.
(1984a). Fortunately, the spatial gradient in soil C and N contents is perpendicular to,
and therefore does not confound, the main rotation treatments.
Manure application to the southern half of all plots, which occurred immediately
after the soil record began, also created a spatial pattern in soil C and N contents (Tables
XI and XII). Manure application increased average C and N contents in S plots relative
to N plots but had no effect on soil C/N ratios. Like rotation, impacts of manure
application were not obscured by the west-east gradient.
Effect of Phase and Manure Application on Soil C, N, and C-N ratios
CC C-O C-O-H
______________________ ______________________ ____________________
Phase North South North South North South
Carbon (g kg-1 soil)
1 22.5a 22.2 25.0 25.6 28.7 29.3
2A 19.6 23.6 22.2 26.5 25.5 29.0
2B 15.3 20.5 20.2 26.3 23.8 29.7
3 15.1 20.6 19.3 25.1 22.7 28.1
4 14.7 18.8 18.5 22.0 22.2 27.5
Nitrogen (g kg-1 soil)
1 1.79 1.90 2.07 2.02 2.25 2.35
2A 1.54 1.82 1.77 2.06 2.02 2.29
2B 1.26 1.57 1.54 1.99 1.80 2.27
3 1.21 1.56 1.52 2.00 1.79 2.21
4 1.28 1.61 1.59 1.93 1.97 2.40
1 12.6 11.0 12.3 12.7 12.7 12.4
2A 12.7 12.9 12.6 12.9 12.6 12.7
2B 12.1 13.0 13.1 13.0 13.1 13.0
3 12.5 13.2 12.7 12.5 12.6 12.7
4 11.5 11.6 11.7 11.6 11.3 11.4
aAll samples were collected from 0-15 cm. After phase 1, manure, lime, and phosphate were applied to
south plots. Only samples collected at least 6 years after phases began were included in phase means
analyses. Samples numbers from each phase by treatment and treatment and location (north and south)
combination varied considerably, preventing meaning use of least-squares means for mea comparison.
The west to east increase in soil organic matter complicates the interpretation of
phase effects. Unfortunately, soil samples available from phases 1 through 3 were
collected from west and east sides of the field, not from A, B, C and D plots. Only the
specific plot origins of samples collected during phase 4 are known. Phase effects on N
were analyzed using only A and C plots to avoid spatial effects. Phase 4 increases in N
were still statistically significant (Table XI). If the increases in N were due primarily to
the inclusion of samples from the eastern plots, the C/N ratios should have increased, and
2. Trends in C, N and C/N Ratios
a. SOM Interaction Between Rotation and Phase
While most of the variation in soil C and N contents was explained by phase and crop
rotation, the interaction between these factors was also significant. Fig. 10 shows the
relative changes in SOM content within rotation and phase and reveals that longer crop
rotations lost less C and N. The CC rotation led to the largest and most rapid loss of C
and N; overall this rotation lost 5.9 and 0.39 g of C and N per kg soil, respectively,
compared to losses of 4.8 and 0.27 and 4.1 and 0.11 from the C-O and C-O-H soils.
Moreover, the cumulative loss since 1904 of C was 26.5, 18.9 and 14.1% and of N was
21.3, 13.3 and 4.8%, in the CC, C-O and C-O-H soils.
The amount of C and N lost between phases 1 and 2A from all three rotations’ soils
was similar (Fig. 10). Losses of C and N occurring between 2A and 2B were
significantly larger from the CC than the C-O or C-O-H soils. The most rapid loss of
SOM from the CC soil coincided with the adoption of corn hybrids. The C contents of
the CC plots remained constant during phases 2B and 3 and then decreased significantly
again in phase 4. Even though less C was lost from the C-O and C-O-H soils than from
the CC soils, their C contents declined significantly during all phases of the trial. Like
the CC soils, C-O soil N contents remained unchanged after phase 2B. High levels of
above ground productivity in the CC plots were not reflected by SOM contents; while
soil C losses did cease during phase plots phase 3, they accelerated again during phase 4
even as yields continued to increase. The magnitude of C loss during phase 4 from the
C-O plots was similar to that lost from the CC plots. The return of crop residues to all
plots which began in phase 4, should have curtailed SOM losses. Declines could have
been associated with the substitution of soybean for oat in the C-O rotation and/or the
application of the H-NPK treatment to previously manured plots as well as reduced
manure application frequency. As previously noted, overall soil N concentrations
increased significantly during phase 4. This increase in N occurred in CC and C-O-H
soils; however, only the N increase in C-O-H soils was statistically significant (Fig. 9b).
Again, this may have been due to erosion. The C-O-H plot, which lies on the south edge
of the field, may have suffered more soil loss than the other two rotation plots. It has the
highest elevation and drains, along with all the other plots, toward the northwest edge of
the field (Darmody and Peck, 1997). Another explanation may be the methods used to
determine N. Even though combustion based analyses produced average soil N contents
that were similar to values obtained by wet oxidation techniques, there may have been an
interaction between SOM contents and N recovery. Work is underway to clarify this
b. SOM: Interaction Between Treatment and Phase
When rotation plots were split in two at the end of phase 1, the average SOM
contents of north and south plots were similar (Table XII). The average soil C and N
contents of phase 2A manure-amended plots (south) were already notably higher than
untreated (north) plots. Thereafter, a steady loss of C and N from CC soils was apparent
in both north and south plots. Similar losses of SOM from C-O or C-O-H soils occurred
only in untreated plots. The combination of longer rotation and manure application
stabilized or even increased soil C and N contents. During phase 3 southern B plots, and
during phase 4 southern A plots, ceased to be manured. After this, means of south plots
included data from manured and previously manured sub-plots. Accordingly, average
soil C contents of the southern C-O and C-O-H plots begin to decline in phases 3 and 4.
The C/N ratios of north and south plots remained similar during all phases of the trial,
suggesting manure application did not have a systematic effect on SOM composition.
B. pH, P AND K
All pH, P and K values are based on samples collected during phase 4 of the
experiment. Both rotation and fertility treatment had significant impacts on soil pH
(Table XIII). The average soil pH of the CC plots was significantly lower than that of the
C-O and C-O-H plots. Overall, the pH of untreated soil, which was similar in all three
rotations, was significantly lower that the pH of all amended soils. In addition, the pH of
the M soils was significantly higher than the pH of the M-NPK amended soils. The
acidifying effect of fertilizer application was most expressed in the CC H-NPK plots.
These soils had an average pH value of 6.01 compared to values of 6.32 and 6.37 in
comparably fertilized C-O and C-O-H plots.
Fertility treatments had the greatest impact on soil P content (Table XIII). The H-
NPK soils’ P levels were the highest, followed by the M and M-NPK, MPS and U-NPK,
and then the U soils. Soil P levels decreased with increasing rotation length (CC > C-O
> C- O-H), reflecting the frequency of fertilizer application. One exception to this
ranking of rotations resulted from their interaction with the M-NPK fertility treatment; P
levels were significantly higher in the C-O-H than in the C-O and CC rotation soils.
Treatment impact on soil pH, phosphorus and potassium levels during phase 4a
Treatment All rotations CC C-O C-O-H
All treatments 6.12Ab 6.20B 6.20B
H-NPK 6.24Ac 6.01ad 6.32c 6.37c
M 6.39B 6.36c 6.35c 6.45cd
M-NPK 6.24A 6.23bc 6.20b 6.29bc
MPS 6.47B 6.50d 6.51d 6.39cd
U 5.48C 5.40cd 5.55e 5.50e
U-NPK 6.23A 6.19b 6.29bc 6.22b
P (lbs per acre)
All treatments 63.0A 49.9B 46.7C
H-NPK 97.9A 104.5a 100.5a 88.7b
M 56.5B 89.9b 42.4e 37.2e
M-NPK 56.9B 54.0d 50.5de 66.0c
MPS 48.2C 65.5c 43.0e 36.0e
U 11.3D 13.5f 13.0f 8.1f
U-NPK 48.4C 50.7de 50.7d 43.9e
K (lbs per acre)
All treatments 297.9A 281.8B 240.8C
H-NPK 352.4A 388.3a 373.6a 295.6cd
M 270.0B 312b 274.1de 223.2h
M-NPK 277.4B 290cd 293.3cd 248.1fg
MPS 257.2C 289d 258.3f 224.4h
U 214.7D 223.7h 204.7j 215.6i
U-NPK 269.1BC 283.0d 286.6de 237.8gh
aValues are means from samples collected (0-15 cm) annually between 1969-1995.
bMeans for rotation treatments within “All treatments” row that are followed by
different capitalized letters are significantly different at the 5% level.
cMeans for fertility treatments within “All treatments” column that are followed by
different capital letters are significantly different at the 5% level.
dMeans for treatment by rotation interactions that are followed by different lowercase
letter within row or column are significantly different at the 5% level.
Soil K levels were influenced most by soil fertility treatments (Table XIII). As
expected, the H-NPK treatment significantly increased K test levels. The K contents of
soils amended with manure and/or NPK were similar. The average K levels of non-
amended soils was 215 lbs/A, a rather high value for soils cropped continuously for over
a century. Crop rotation had the same effect on K as it had on P; soil K contents
decreased in order of increasing rotation length (CC > C-O > C-O-H).
C. SOM: INTERACTION BETWEEN CORN YIELD
AND SOIL FERTILITY
1. Yield and SOM
A century of data from the Morrow Plots reveals that corn yield have increased as
SOM contents have declined; this does not indicate that yield and SOM contents were
negatively correlated. Throughout the experiment, yield in plots with higher SOM
contents have been higher than yield in plots with low SOM contents. Yield trends have
shifted upward in concert with technology adoption (hybrids, pesticides, and commercial
fertilizers). Technology based increases in yield potential can occur despite losses in
soils inherent productive capacity.
Cassman and Pingali (1996) argue that diminishing soil productive capacity can
reduce yield increases caused by improved technology adoption. To assess the impact of
Morrow Plots fertility practices on soil productivity, we considered the changes in corn
yield and soil C contents associated with the phases of the trial (Fig. 11). Both the loss of
fertility and the benefits of technology were manifest in U plot yield and soil C trends.
Initially losses in SOM, and therefore soil nutrients, were reflected by decreased yield.
We speculate that introduction of hybrids and improved disease, weed and insect control
led to later increases in the yield of U plots. After this, corn yield increased despite
continually declining SOM contents. Yield during phase 2A, which was already higher,
increased markedly in C-O-H but not in CC or C-O plots. During phase 3, the lagging
yield response was overcome to some degree by untreated CC and C-O plots. During
phase 4, average yield in untreated C-O and C-O-H increased while CC plots did not.
This may indicate that once again, soil productivity is limiting yield in the U CC plots.
Manure amendment led to immediate increases in SOM content and to yield increases
in both the C-O and C-O-H rotations. Initially, corn yield did not respond in the CC
plots, indicating factors other than nutrition limited productivity in this rotation. During
phase 2, soil C contents dropped or remained the same in CC, C-O and C-O-H plots as
yield increased. Changes in soil C contents, which may be explained by altered
manuring practices, were not reflected by decreasing yield response. During phase 3,
a. Untreated Plots b. Manured Plots
Figure 11 Changes in corn yield and soil carbon contents by phase. (a) Untreated and
previously untreatedplots; (b) manured, manured with higher planting density, and previously
manured, now NPK and H-NPK treated plots. Number adjacent to symbols identify means from
the four phases of the trial.
continued to increase and higher planting densities (MPS) increased relative yield
response. There was no change in yield during phase 4 in M and MPS plots. At this
time, soil data indicated SOM increased in M and decreased in MPS plots. Changes in
SOM contents probably reflected plot effects more than anything else since M plots were
on the east and the MPS plots on the west side of the field.
Fertilizer application led to greater phase 3 yield response in U-NPK plots than in M-
NPK plots. This was most notable in the CC rotation which was the most nutrient
stressed. During phase 4, CC yield response lagged behind response in the C-O and C-
O-H rotations in U-NPK, M-NPK and H-NPK plots. It is possible that the relatively low
SOM contents of CC plots hindered yield response in that rotation. While not
significantly different, the H-NPK treatment was associated with the greatest C losses.
2. Yield and pH, P and K
Information about soil pH, P and K was only available for phase 4 of the trial. In
general, low pH, P and K test values were associated with low yield. However,
differences in yield of the three rotations was not explained by overall differences in soil
chemistry. The pH of the C-O and C-O-H rotations was higher while P and K contents
were lower. We conclude that larger yield in longer rotations was tied to SOM-
dependent benefits and not mineral nutrition.
IV. CONCLUSIONS: LESSONS FROM
THE MORROW PLOTS
The changing nature of the Morrow Plots’ treatments reflects the agricultural
history of the Corn Belt. During the existence of the Plots, the dominant agricultural
practices in the region have evolved from low intensity systems that exploited highly
fertile virgin soils, to systems that relied on rotation, manure, lime and phosphorus
application, to more intensive systems with simplified rotations that include hybrids
grown in higher densities with increased commercial inputs. Substantive changes in the
agricultural norms of the region have been mirrored by changes in the Morrow Plots.
The treatments that were once focal, now serve as invaluable controls. The continuous
corn treatment, which began as the most extreme treatment in the soil exhaustion trial,
was compared with what was then considered to be the best management practice, crop
rotation. Now it is the manured C-O-H rotation that is an extreme treatment, returning
much more organic matter and nutrients to the soil than the mineral fertilized continuous
corn or corn-soybean systems.
Initially Morrow Plots researchers were concerned about the depletion of native
soil fertility. In the mid-to-late 1800s Jethro Tull and others observed sustained crop
yield on newly plowed soils. This lead the United States Bureau of Soils to promulgate
the theory that “practically all soils contain sufficient plant food for good crop yields (and
that) ... this (nutrient) supply will be indefinitely maintained” (Hopkins, 1906).
University of Illinois scientists argued vehemently against this assertion (Davenport,
1908). Results from phase 1 of the Morrow Plots and other experiments had already
proved that soil fertility, N in particular, was limiting production in Illinois (Bogue,
1963). During phase 1 of the trial, corn yield and SOM contents were directly correlated
and both were decreasing.
At this time, researchers wanted to determine whether and how productivity could
be maintained on a permanent basis. The fertility treatment that was added during Phase
2 of the experiment included the key elements of the Illinois System of Permanent Soil
Fertility. While not an absolute system, Illinois scientists then recommended the use of
raw rock phosphate, crushed limestone, nitrogen from legume crops and the liberation of
K and other mineral nutrients from soils through biological processes (Smith, 1925). The
phosphate trial that was applied to the west and east sides of the plots did not indicate
that yield was influenced by source of P (rock phosphate or bonemeal). However, P
treatments were only maintained from 1904 to 1912 and their effects on yield in
untreated plots were overshadowed by the dramatic effect on yield caused by manure and
lime application. In phase 2, the manure, lime and phosphate treatment immediately
increased corn yield in all rotations. During this phase, manure treatment slowed SOM
losses in the CC and C-O rotations and halted SOM losses in the C-O-H rotation. Corn
yield in the manured CC plots has generally been comparable to yield in the untreated C-
O-H plots indicating that manure application or rotation alone can sustain reasonable
levels (corn yield greater than 100 Bu/A) of productivity long-term.
When hybrids were introduced during phase 2, corn yield and soil productivity
were to a degree, decoupled. The dramatic benefits of improved technologies were
demonstrated as yield, which was still the highest in the C-O-H and lowest in the CC
rotation, increased dramatically in all plots. Differences in timing of yield response
exhibited by the three rotations reflected the non-nutrient based benefits of higher SOM
Researchers now wondered about the limits to productivity. During phases 3 and
4 they would seek to determine whether application of commercial fertilizers and
amendments and the intensified planting of improved hybrids would not only sustain, but
also increase productivity thresholds. Application of commercial fertilizers immediately
increased yield in all three rotations. Soil condition continued to influence yield but the
impact was not as dramatic as the impact of fertilizers. Soil properties did significantly
influence yield potential in continuous corn. During phase 4, the CC U-NPK plots
yielded 12 Bu/A less than the previously manured CC H-NPK plots and 14 Bu/A less
than CC M-NPK plots. In C-O and C-O-H rotations, yield in U-NPK, M-NPK and H-
NPK plots was similar, indicating that rotation alone improved soil condition sufficiently
to allow the full benefit of fertilization to be realized.
The higher planting densities that were adopted during phases 3 and 4 increased
yield when fertility was adequate. However, higher densities were associated with
reduced yield stability when factors like bad weather limited productivity. Corn was
planted as early as possible, a strategy that was most successful during the seventies and
eighties. In the nineties, years with very wet springs delayed planting. When planting
was not completed by early May, the delay was generally caused by too much moisture
and/or low temperatures. Weather in July and August was very highly correlated with
yield. Weather from the previous Fall and Winter also impacted yield.
The very high levels of fertilizer introduced in 1967 did not improve production
and reduced yield stability. While the H-NPK treatment is no longer seen as a relevant
treatment, it serves as an extreme. Results show that in overfertilized systems, yield is
not increased and is less stable than plots fertilized in accordance with university
recommendations. The negative environmental and economic costs of the H-NPK
treatment make its continuation undesirable.
Ironically the findings of these plots, which were initially seen as too academic,
have not been published widely in academic journals; they have however, been used
frequently in anecdotes. Despite the fact that next to nothing has been reported on
Morrow Plots’ yield, production trends have been used as a sort of benchmark, reflecting
conditions in the state. The early correlation between U plot yield and Illinois yield
coincided with a period of soil mining. It was not until 1937 that yield in manured plots
was more highly correlated with Illinois yield than yield in untreated plots. This may
indicate that it took a while before there was widespread adoption of rotation and/or use
of inputs like manure and lime. The high correlation between Illinois yield and yield in
the NPK plots during the last 40 years suggests these plots reflect State norm. Special
attention should be paid to the continuous corn and corn-soybean rotation because these
systems are so widely used. The U-NPK fertilized plots may best reflect field conditions
in the region. Declining soil condition is indicated by production trends; during phase 4,
yield in U-NPK CC and C-O plots were 41 and 15 Bu/A less than yield in the U-NPK C-
O-H plot. Yield in the fertilized corn-soybean rotation may have exceeded yield in
comparable CC plots because of immediate rotation effects or because of its history of
rotation with oats. The long-term impact of the switch to soybean from oat in the C-O
rotation will not become clear until more time has passed. Decreases in soil C contents
accelerated during phase 4 in both the CC and C-O rotations despite the return of
residues to all plots during this phase. Reduction in the frequency of manure application
to the M plots may have been a contributing factor.
The questions asked during the various phases of the trial indicate how issues and
expectations in agriculture evolve. When viewed as a whole, the story chronicled by the
Morrow Plots is that of the effect of management practices and technological innovations
on corn yield and SOM. Fertilizer application and pest control measures have increased
corn yields. In all but the longest rotation, SOM levels continue to fall. However,
declining inherent productivity has not been noticed; even in the most SOM depleted
soils, technological innovations have continued to increase yield. Despite the fact that
yield responses have been greater where SOM is conserved, long crop rotations and
manure are not widely used. When crop yield is the sole factor considered, use of these
kinds of soil building practices may not be competitive. If the relationship between SOM
and soil quality, which includes soils ability to regulate water flow and/or its ability to act
as an environmental filter, is considered, maintenance of organic matter and all it
represents may become an imperative.
Changes in the Morrow Plots’ treatments have provided an empirical basis upon
which we can evaluate the various phases in our agricultural history. Findings have
shown that production is greater where soil condition is maintained, that use of mineral
fertilizers have off-set to a large extent losses in soil productivity, and that excessive use
of mineral fertilizers is undesirable from a production stand point alone.
CC Continuous Corn.
C-O Corn - Oat rotation.
C-O-H Corn - Oat - Hay rotation.
NPK N, P, and K treatment.
PKS P, K and sulphorous ammonium treatment.
Phase 1 1888 - 1903. Original ten-plot experiment (Experiment no. 23).
Phase 2A 1904 - 1936. Comparison of manure, lime and phosphorous prior to
hybrids were introduced. Plots were split in the north - south direction
for manure application and in the west - east direction for source of
Phase 2B 1937 - 1954. Comparison of manure, lime and phosphorous after hybrids
Phase 3 1955 - 1966. Fertilizers and higher planting density were introduced.
Plots were split again in the west - east direction with B plots receiving
Phase 4 1967 - 1996. H-NPK was added to a previously manure plot (SA).
NW North-west subplot was untreated from 1888 and was split into NC and
ND in 1955.
NA Western sub-subplot remained untreated after 1955.
NB Eastern sub-subplot. NPK was added in 1955.
NE North-east subplot was untreated from 1888 and was split into NC and ND
NC Western sub-subplot remained untreated after 1955.
ND Eastern sub-subplot remained untreated after 1955.
SW South-west subplot was manured from 1888 and was split into SA and SB
SA Western sub-subplot was manured 1955 - 1967 and received high level
NPK from 1967.
SB Eastern sub-subplot. NPK was added in 1955.
SE South-east subplot was manured from 1888 and was split into SC and SD
SC Western sub-subplot remained manured with higher planting density after
SD Eastern sub-subplot remained manured after 1955.
U Untreated plots.
M Manure treated plots.
MPS Manured plots with higher planting density.
U-NPK NPK treatment applied to previously untreated plots.
M-NPK NPK treatment applied to previously manured plots.
H-NPK High level NPK treatment applied to previously manured plots.
We thank Ted Peck for his invaluable assistance. He has devoted a great deal of effort to the
preservation of the Morrow Plots records and samples. Without these materials, this paper could
not have been written. Dr. Peck also provided insights into the history of the Plots that would
have otherwise been missed. Additionally we thank Bob Dunker for providing yield records and
information about plot management and Bob Darmody for comments on the first draft. We thank
Audrey Bryan and Wayne Wendland of the Illinois State Water Survey for providing us the
weather data. Finally, we acknowledge Xueming Yang for his work on the soil sample inventory
and his assistance in the laboratory.
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