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Advances in agronomy volume 53



Advisory Board
Martin Alexander

Eugene J. Kamprath

Cornell University

North Carolina State University

Kenneth J. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the


American Society of Agronomy Monographs Committee
P. S. Baenziger
J. Bartels
J. N. Bigham
L. P. Bush

M. A. Tabatabai, Chairman
R. N. Carrow
W. T. Frankenberger, Jr.
D. M. Kral
S. E. Lingle

G. A. Peterson
D. E. Roiston
D. E. Stott
J. W. Stucki


D V A N C E S I N

ono
V O L U M5E3
Edited by

Donald L. Sparks
Department of Plant and Soil Sciences
University of Delaware
Newark, Delaware

ACADEMIC PRESS
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Copyright 0 1994 by ACADEMIC PRESS, INC.
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PRINTED IN THE UNITEDSTATES OF AMERlCA
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5

4

3 2 1


Contents
CONTRIBUTORS
...........................................
PREFACE.................................................

vii
ix

CROPROTATIONSFOR THE 2 1 s CENTURY
~
D . L . Karlen. G. E. Varvel. D . G. Bullock.
and R . M . Cruse
I. Origin of Crop Rotations ...................................
I1. 2 0 t h Century Crop Rotations ................................
I11. Agronomic Impacts of Crop Rotation .........................
IV. Soil Quality Effects ........................................
V. Biological Diversity .........................................
VI. Economics of Crop Rotation ................................
VII . Policy Impacts on Crop Rotations ............................
VIII . Summary and Conclusions ..................................
References ................................................

2
5
11
22
30
32
33
36
37

ROLEOF DISSOLUTION
AND PRECIPITATION
OF MINERALS
INCONTROLLING
SOLUBLE
ALUMINUMIN ACIDICSOILS
G. S . P. Ritchie
I . Introduction ..............................................


I1 A Framework for Understanding Mineral Dissolution and
Precipitation in Soils .......................................
I11. Factors Affecting Dissolution and Precipitation of AluminumContaining Minerals .......................................
Iv. Modeling Soluble Aluminum ................................
V. Aluminum in Acidic Soils: Principles and Practicalities ..........
References ................................................

.

V

47
50

51
64
77
80


CONTENTS

vi

MANAGINGPLANTNUTRIENTS
FOR
OPTIMUM
WATERUSEEFFICIENCY
AND WATER
CONSERVATION
Jessica G. Davis
I. Introduction ..............................................
11. Conserving Water Supply by Optimizing Water Use Efficiency . . .
111. Conserving Water Quality through Nutrient Management . . . . . . .
Iv Needs for Further Research .................................
References ................................................

INTERPARTICLE
FORCES:
A BASIS FOR THE INTERPRETATION
OF SOILPHYSICAL
BEHAVIOR
J. P. Quirk
Introduction ..............................................
Interparticle Forces ........................................
Soil Water Relations: Swelling and Shrinkage . . . . . . . . . . . . . . . . . .

85
86
92
108
109

Iv. Swelling of Sodium Clays ...................................
v. Swelling of Calcium Clays ..................................
VI. Surface Area and Pore Size ..................................
VII . Water Stability of Soil Aggregates ............................

VIII. Sodic Soils and the Threshold Concentration Concept . . . . . . . . . .
Ix. Concluding Remarks .......................................
References ................................................

122
124
143
146
152
161
166
169
176
177

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

185

I.
11.
111.


Contributors
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

D. G. BULLOCK (l), Department ofAgronomy, University of Illinois, Urbana,
Illinois 61801
R. M. CRUSE (I), Department of Agronomy, Iowa State University,Ames, Iowa
5001 I
JESSICA G. DAVIS (85), Department of Crop and Soil Sciences, University of
Georgia, Coastal Plain Experiment Station, Tifton, Georgia 3 1 793
D. L. KARLEN (I), National Soil Tilth Laboratory, United States Department of
Agriculture, Agricultural Research Service, Ames, Iowa JOOl I
J. P. QUIRK (1 2 l), Department of Soil Science and Plant Nutrition, School of
Agriculture, The University of Western Australia, Nedhnd, WesternAustralia
6009, Australia
G. S. P. RITCHIE (47), Department of Soil Science and Plant Nutrition, School
of Agriculture, The University of WesternAustralia, Nedlands, Western Azlsh-alia 6009, Australia
G. E. VARVEL (l), Soil/Water Conservation Research Unit, United States Department of Agriculture,Agricultural Research Service, University of Nebraska,
Lincoln, Nebraska 68583

vii


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Preface
Volume 53 contains four excellent reviews that cover a broad spectrum of important advances and topics in the plant and soil sciences. Sustainable agriculture is
one of the most discussed issues and venues for research in agronomy at the
present time. The first chapter comprehensively reviews the history of crop
rotations and future directions in this important area. Topics that are covered
include twentieth century rotations, agronomic impacts of crop rotations, effects
of rotations on soil quality, economics of crop rotations, and policy impacts. The
second chapter provides a thorough discussion on how dissolution and precipitation affect soluble aluminum in acid soils. The author reviews factors that affect
dissolution and precipitation, ways to model soluble aluminum including thermodynamic and kinetic approaches, and the effects of aluminum on aspects of
acid soils.
Water quality and conservation are of paramount importance in protecting and
preserving our environment and are among the most active areas of research in
agronomy. The role that nutrient management has on optimal water use efficiency
and conservation is the topic of the third chapter. Discussions on conserving
water supplies via optimization of water use efficiency and preservation of water
quality through nutrient management are thoroughly covered. The fourth chapter
is a definitive treatise on how interparticle forces affect soil physical behavior
which, of course, has immense effects on plant growth and yield. Topics that are
discussed include interparticle forces, soil water relations, swelling of clays,
surface area and pore size, water stability of soil aggregates, and sodic soils.
I thank the authors for their comprehensive and timely reviews.

DONALD
L. SPARKS

ix


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CROP R~XITONS
FOR THE 21sr C

~

D. L. Karlen,' G. E. Varve1,z D. G. Bullock,3 and R. M. Cruse4
'National Soil Tilth Laboratory
United States Department of Agriculture
Agricultural Research Service
Ames, Iowa 50011
*Soil/Water Conservation Research Unit
United States Department of Agriculture
Agricultural Research Service
University of Nebraska
Lincoln, Nebraska 68583
3Department of Agronomy
University of Illinois
Urbana, Illinois 61801
4Deparunent of Agronomy
Iowa State University
Ames, Iowa 5001 1

I. Origin of Crop Rotations
11. 20th Century Crop Rotations
A. Pre-World War I1
B. Post-World War I1 Developments
C. 2 1st Century Outlook
111. Agronomic Impacts of Crop Rotation
A. Crop Yield
B. Water Use Efficiency
C. Nutrient Use Efficiency
D. Disease and Pest Interactions
E. Allelopathy
W. Soil Quality Effects
A. Soil Structure
B. Aggregation
C. Bulk Density
D. Water Infiltration and Retention
E. Soil Erodibility
F. Organic Matter
V. Biological Diversity
A. Effects on Wildlife
B. Alternative Land Uses
VI. Economics of Crop Rotation
VII. Policy Impacts on Crop Rotations
VIII. Summary and Conclusions
References
1
Advance in A p n q , Vdume 53

Copyright Q 1994 by Academic Press, Inc. All rights of reproduction in any form reserved

Y


2

D. L. KARLEN ETAL.

I. ORIGIN OF CROP ROTATIONS
The practice of crop rotation or sequentially growing a sequence of plant
species on the same land (Yates, 1954) has been in existence for thousands of
years. As noted by Parker (1915), crop rotation developed primarily from the
experiences of mankind relative to soil productivity. MacRae and Mehuys ( 1 985)
stated that it was practiced during the Han dynasty of China more than 3000
years ago. Early agriculturists experienced low yields that resulted from continuous cropping, and throughout history, crop rotation was found to be necessary to
maintain productivity. However, it was seldom, if ever, understood why.
Early writers noted that crop rotation was in use in ancient Greece and Rome.
Pliny mentioned the use of four rotational schemes, including two for rich soils
and one each for second- and third-quality soils (White, 1970a,b). For rich soils,
the two rotations mentioned included three-crop sequences of barley (Hordeurn
vulgare L.), millet (Panicurn rniliaceurn L.),and turnip (Brussica r a p L.) or
wheat (Triticurn aestivurn L.), millet or turnip, and emmer (Triticurn dicoccon
Schrank), and 4 months of fallow followed by spring beans (Varafaba L.) or no
fallow with winter beans (Hiemalis faba L.). For second-quality soils the suggested 2-year rotation was wheat and beans or another legume and for thirdquality soils it was emmer and beans or legume followed by a fallow period.
Pliny also recommended that when fallow is not an option, the field should be put
down to lupins (Lupinus albus L.), vetch (Vicia sativa L.), or beans. These crops
could then be incorporated as green manure in preparation for growing emmer.
Columella recommended similar crop rotation practices to those described by
Pliny (White, 1970a,b). Systems cited by these ancient writers generally described the typical legume and cereal crop rotation in which some form of
legume [pea (Pisurn sativurn L.), bean, vetch, or lupin] was alternated with a
cereal crop.
The actual use of crop rotations by the Romans has been debated at length by
both medieval and classical historians, some of which insist the practice of
rotation was rare. According to White (1970b), these historians argued that
rotation systems were widely recommended by Roman agronomists, but in practice were not used extensively by farmers of the time. However, it is interesting
to note accounts mentioning crop rotation by other writers not considered
agronomists, which included Virgil’s detailed account of crop rotations as alternatives to traditional fallowing in his poems. Whether crop rotations were in
widespread use is not known, but the practice was used and its benefit was
known.
According to Brehaut (1933), in his translation of Cato’s De agricultura, other
writers, including Cat0 the Censor, indicated the use of rotations was prevalent.
Cat0 noted the beneficial effects of lupins, beans, and vetch and indicated the


CROP ROTATIONS FOR THE 2 1st CENTURY

3

likely use of this crop rotation was in a legume-cereal system. In Italy during the
first century B . c . , Varro also noted the importance of a green manure crop,
especially legumes, in cropping systems prevalent at that time.
Despite the beneficial effects of crop rotation, the practice fell out of favor with
the demise of Roman power throughout Europe (White, 1970b). Less use of crop
rotation and a return to the old crop and fallow system appeared to occur with the
return to a more rural civilization as the more urban civilization prevalent in the
Roman Empire disappeared.
Throughout the Middle Ages, little mention is made of crop rotation and as
noted earlier, the prevalent practice was probably the crop-fallow system. One
exception mentioned for this period was the practice of alternating 2 years of
wheat and 5 years of grass in a system called ley farming (crop rotation). This
sequence was used by the Monks of Couper around 1400 in Britain (Franklin,
1953). Crop rotation was probably used to some extent during this period, but it
appears that a crop-fallow system, with the use of manure, was the general
system in use.
Crop rotations, as we now know them, are often traced back to the Norfolk
rotation. This was popular in England about 1730 (Martin et al., 1976). The
Norfolk rotation, which was widely used at the time, consisted of turnip, barley,
clover, and wheat in a 4-year sequence. The Norfolk and many other similar
rotation systems were in use throughout the 18th century, but little was actually
known about the specific benefits of rotating crops. The prevailing thought was
that each of the crops in the rotation obtained their nutrients from different zones
or parts of the soil. This perception was used to explain why a sequence of
different crops yielded better than a single crop grown year after year.
Between 1730 and 1840 the practice of crop rotation and the use of artificial
manure (lime and other soil minerals) to supplement animal manure had become
almost universal in England (Parker, 1915). One early English agricultural writer, Arthur Young, was not necessarily a proponent of this system. Young was a
great apostle of mixed farming. He lauded the value of legumes, the use of crop
rotation, and the feeding of livestock on the farm and the return of the manure to
the land. Young insisted grass land and grazing were of primary importance and
management of arable land of secondary importance to English agriculture.
However, he did emphasize the importance of crop rotation and animal husbandry to agriculture at the time (Parker, 1915).
As would be expected because of the heavy influence of English and Scottish
settlers, most early agriculture in the United States was based on English customs. Several letters between Thomas Jefferson and George Washington (Bureau
of Agricultural Economics, 1937) support this statement and indicate that crop
rotation was also the prevalent practice in the United States. Jefferson wrote in a
letter addressed to President Washington in 1794 that he was going to have to use
a milder course of cropping because of the ravages brought about by overseers


4

D. L. KARLEN E T A .

during his absence. His rotation was first year, wheat; second, corn (Zea mays
L.), potatoes (Solanum tuberosum L.), or pea; third, rye (Secale cereale L.) or
wheat, according to the circumstances; fourth and fifth, clover or buckwheat
(Fagopyrum esculentum Moench); and sixth, something he described as folding
or buckwheat if it had not been used in the fourth or fifth years. Another letter,
dated 1798, indicated he was using a triennial rotation of 1 year of wheat and 2
years of clover in his stronger fields or 1 year of wheat and 2 years of pea in the
weaker fields followed by a crop of Indian corn and potatoes between every other
rotation. Jefferson commented in both cases that he felt these types of cropping
systems, with the addition of some manure, would help his fields recover their
pristine fertility at Monticello. In later years, after retiring from the presidency,
Jefferson returned to Monticello and noted in a letter to C. W. Peale in 1811 that
his rotations were mainly corn, wheat, and clover; corn, wheat, clover, and
clover; or wheat, corn, wheat, clover, and clover. It was apparent that he knew
well the benefit of rotation with legumes by the prevalence of clover in each of
these systems.
In some of his letters and papers, George Washington described a good crop
rotation plan that he found in use on Long Island in 1790. It consisted of corn
with manure, oats (Avena sativa L.) or flax (Linum usitatissimurn L.), wheat with
4 to 6 pounds of clover and 1 quart of timothy (Phleum pratense L.), and
meadow or pasture. From 1800 to 1810 this same rotation with some slight
modifications came into quite general use in Pennsylvania. However, in Virginia, a rotation similar to that of Jefferson’s was used by many farmers (Parker,
1915).
Jefferson, Washington, and many other progressive farmers of the time used
rotations and manure extensively in an attempt to regain productivity levels
similar to those when the virgin soils of the United States were first broken out.
In other parts of the world, it was apparent crop rotations and other systems
similar to the Norfolk rotation were in extensive use by farmers during the 19th
century. Despite their extensive use of rotations, agriculturists of the time, such
as Baron Justis von Liebig (1 859), believed that although crop rotation improved
the physical and chemical condition of the soil, all plants would eventually
exhaust the soil. Liebig felt that unless soils were heavily manured, all fields
would eventually lose their fertility, regardless of crop rotation.
Hall (1905) presents an excellent summary of the prevailing thoughts and
experiments concerning crop growth and production during the 19th century. It
was during this time period that researchers discovered legumes had the ability to
assimilate and utilize nitrogen from the atmosphere, which enlightened researchers regarding the benefit of growing crops in rotation with legumes. As
described by Hall (1905), this discovery provided an explanation as to the benefit
of existing crop rotation studies and led to new investigations on crop rotations
during the 19th century at Rothamsted, England (the world’s first agricultural


CROP ROTATIONS FOR THE 2lst CENTURY

5

research station). These studies further identified that some of the nitrogen fixed
by the legumes in a cropping system becomes available for succeeding crops and
clearly identified at least part of the beneficial effects of crop rotations.

11. 20th CENTURY CROP ROTATIONS
A. PRE-WORLD WAR 11
The discovery in the latter part of the 19th century that legumes could fix
nitrogen from the atmosphere was a major reason rotations remained popular into
the early part of the 20th century. Nitrogen was the major limiting nutrient for
most crops and it could only be supplied by the addition of manure or by
incorporating a legume of some type in the cropping system. Use of crop rotation
during this period, similar to patterns established throughout history, was greatly
dependent on the amount of new or virgin land available for crop production. If
cheap and plentiful amounts of fertile land were available, crop rotations were
not extensively used. Only as land became more expensive and less plentiful
were crop rotations utilized more extensively.
Johnson (1927) presented examples of rotation experiments conducted in several different areas of the United States during the early part of the 20th century.
In Georgia, the suggested rotation was corn, cowpea (Vigna unguiculata L.),
oat, and cotton. Cowpea was sown during the last cultivation. The corn was
harvested for grain and the cowpea was worked into the soil. Oat was sown in
late fall and harvested in late May or early June. Cowpea was sown again as a
green manure crop to be incorporated the next spring just before planting cotton
(Gossypiurn hirsuturn L.). This crop sequence increased cotton yields as much as
100% after the first series of the rotation and even greater increases in productivity were maintained in successive rotations.
Rotation experiments at the University of Missouri that began in 1888 included a 6-year corn, oat, wheat, clover, timothy, and timothy rotation and a
3-year corn, wheat, and clover rotation. According to Johnson (1927), after 30
years, yields of corn were increased 60.4%, oat 3%, and wheat 32% in the 6-year
rotation and 30.8% for corn and 40.8% for wheat in the 3-year rotation over the
yields of the corresponding continuously cropped areas. Manure applications
averaged 6.8 tons annually in both rotation and continuous cropping systems.
Results from an Ohio experiment were similar (Johnson, 1927). The main
difference between the Ohio and Missouri experiments was the use of fertilizers
instead of manure. Yields of corn, oat, and wheat in rotation were increased
29.9, 30.8, and 42.5%, respectively, above yields of those crops in continuous
culture. In Delaware, corn grain yields increased 156.9% in a rotation of corn


6

D. L. KARLEN ET AL.

(including a cover crop of rye and vetch), soybean [(Clycine m u (L.) Merr.)],
wheat, clover, and timothy as compared to continuous corn when no fertilizer or
manure was used. With nitrogen, phosphorus, and potassium applications corn
grain yields still increased 24.4% in rotation as compared to the continuous corn
system (Johnson, 1927).
Cover crops were widely recommended for many cropping systems in the
early 20th century (Johnson, 1927). Among these systems was one including two
crops of kale (Brassica oleracea var. acephala DC.), three of cabbage (Brassica
oleracea var. capitata L.), three of potatoes, three of sweet potatoes [(Zpornoea
batatas (L.) Lam.)], and one of German millet [(Setariaifulica (L.) P. Beauv.)].
These 12 crops were grown in rotation during the 9-year period from 1912 to
1921 both with and without cover crops at the Virginia Truck Experiment Station. Similar to the experiments just described, both rotations received the same
amount of fertilizer, nitrogen, phosphorus, and potassium. Rotation again increased yields, but yields were increased to an even greater extent with the use of
cover crops. Yield increases ranged from 12.5% for kale to 62.5% for millet with
cover crops. In this experiment, rotation was considered a major factor contributing to disease control in truck crops and probably contributed greatly to the yield
increases. Other popular rotations for truck farmers in Virginia at this time
included a 3-year rotation of potatoes, corn, and rye grown during both the first
and second year, and sweet potatoes and rye grown during the third year. In
Norfolk County, Virginia, early potatoes were grown as a spring crop. This was
followed by a cover crop of native grass, which was cut for hay, or legumes such
as soybean or cowpea, which were incorporated in the fall. Cabbage was then
planted in November and harvested the following April or May just before
planting a corn crop. Sometimes, soybean was planted directly in the row with
the corn and rye was sown between the rows at last cultivation as intercrops.
When the corn was harvested, the cycle, starting with early potatoes, was repeated. This Norfolk County, Virginia, rotation consisted of two main crops
(potatoes and cabbage), two catch crops, the hay, and corn crop (Johnson, 1927).
With small modifications, this was similar to most of the truck crop rotations
used throughout the eastern United States.
In the same symposium, Lyon (1927) described the effects of legumes and
grasses in different crop rotations. Most of the systems he described were similar
to a corn, oat, wheat, and hay rotation, where the hay was usually either a
legume or a grass such as timothy. He concluded that with few exceptions,
experiments conducted at eight experiment stations in the humid regions of the
United States generally showed legumes to be superior to grasses for increasing
yields of the following crops. In the drier parts of the country, however, grasses
were generally superior to legumes because they usually did not deplete soil
moisture as extensively as legumes.
Crop rotation was not a widely accepted practice in the United States corn belt
during the early 20th century. The soils were extremely fertile and after the virgin


CROP ROTATIONS FOR THE 2 1st CENTURY

7

sod was plowed, they sustained corn yields at sufficiently high levels for many
years. However, even on these extremely fertile soils, crop rotation greatly
increased yields at several locations compared to growing monoculture corn
(Wiancko, 1927). Despite the superiority of rotated corn yields, none of the other
crops in the rotation produced net returns anywhere close to that of corn. Therefore, farmers wanted to grow continuous corn even though its production had
greatly reduced the fertility of many soils. Wiancko (1927) concluded that corn
was the principle crop of the corn belt and that fact had to be recognized and
considered in crop rotations proposed for general use in the region.
Crop rotations in the southern and southeastern United States usually revolved
around the staple crops of cotton, tobacco (Nicotiana tabacum L.), rice (Oryza
sariva L.), and peanut (Arachis hypogaea L.). Parker (1915) discussed several
rotation schemes used for these crops during the late 19th and early 20th centuries. He presented rotations for both livestock and mixed grain and livestock
farms. They usually had corn, oat, wheat, clover, and meadow in various combinations and sequences, with the main emphasis being on feed for livestock.
Rotations for tobacco were usually tobacco, wheat, and clover; tobacco, wheat,
and cowpea; or tobacco, wheat, red clover (Trifolium pratense L.), meadow, and
corn. Cowpea was generally included where the legumes were used as a green
manure to maintain the soil humus supply. Cotton rotations were similar to those
of tobacco in that emphasis was placed on one crop, while other crops in the
system were selected for maintenance of soil humus levels and/or their potential
as livestock feed. Crops in the cotton-based rotations included corn, wheat, oat,
peanut, cowpea, and crimson clover (Trifolium incartum L.) in 2- and 3-year
sequences with cotton. Rice was most often grown continuously, but progressive
farmers of the time were becoming aware of crop rotation benefits, and if possible they used a rice, rice, rice, fallow, corn, and pea or bean (as green manure)
rotation.
Western regions of the United States also utilized crop rotations extensively
during the early part of the 20th century. Crop rotations varied widely because of
large growing season precipitation differences across the region. In more humid
parts of this region, crop rotations were similar to those discussed for the corn
belt states to the east. Drier areas of the Great Plains used cropping systems
developed for the region with respect to water conservation. Parker (1915) presented several of the rotations used during this period for what he termed grain
farming and mixed grain and livestock operations (Table I). In these rotations,
grain was Durum wheat (Triticum durum Desf.), winter wheat, rye, emmer,
awnless barley, or 60-day oat. The specific selection depended on local conditions. Green manure/fallow referred to growing crops such as Dakota vetch,
Canadian field pea, sweet clover, common millet (Panicurn miliaceum L.), or
Hungarian millet [Setaria italica (L.) P. Beauv.], which were plowed under in
early summer, and then allowing the land to rest for the remainder of the season.
The term cultivated crop referred to such crops as Indian corn, Kafir corn (Sor-


Table I
Qpiieal3- to 7-YearCrop Rotations Used for Grain Farming and Mixed Grain and Livestock Operations
in the Western United States during the Early 20th Centuryn

Farming system
Grain only

Option

Year 1

1

Grain

2

Grain
Grain
c-P
c-crop
c-crop
Grain
c-crop

3

c-crop

Grain

4

c-crop
Grain
Grain

g-m-f
s-clovere
s-clover

2
3
4

5
Grain and livestock

1

5
6
After Parker (1915).
Green manure/fallow.
Cultivated crop.
Bromegrass (Brornus srerilus 1.).
Melilotus oficinalis Lam.

Year 2

g-m-f
Grain
Grain
Grain
Bromed
Grain

Year 3

Year 4

g-m-fb
Grain
g-m-f
g-m-f
g-m-f
Brome
Pea or
vetch hay
g-m-f

Grain
Grain

Grain

c-crop
Grain

csrop
c-crop

Grain
Grain

c-crop
Millet or sudan grass
Grain

Year 5
c-cropc
Grain
g-m-f
g-m-f
Grain
Pea or
vetch hay
-

Year 6
-

g-m-f
-

Grain

g-m-f

Year I


CROP ROTATIONS FOR THE 21st CENTURY

9

ghum bicolor L. Moench), durra (Sorghum bicolor L. Moench), and proso millet
(Panicurn miliaceum L.), which again depended on local conditions and the
particular needs of the individual farmer.
In western areas, where irrigation was available, crop rotations differed greatly
from those used for dryland situations. The emphasis in these cropping systems
was usually on some form of high value cash crop, similar to those in the more
humid areas of the East and South. The high value cash crops were mainly
potatoes and sugar beet (Beta vulgaris L.), usually in some form of rotation with
a legume and grain crop.
The importance of crop rotation was evident even with irrigation (Powers and
Lewis, 1930). Substantial increases in soil nitrogen and total carbon were found
where irrigation and crop rotation or use of manure occurred. They reported
striking differences in crop yield, water use efficiency (yield per acre inch applied), net profit per acre, and water cost per unit of dry matter. Powers and
Lewis (1 930) suggested that settlers on newly irrigated arid land should utilize a
crop rotation to improve the nitrogen and organic matter content of their soils and
that such practices would help ensure economical use of water and establishment
of profitable crop production under irrigation.
Examples of irrigated crop rotations used in different parts of the West included a 5-year rotation in Utah that consisted of sugar beet, oat and pea for hay,
sugar beet, oat, and a fifth field in alfalfa (Medicago sativa L.); and a 5-year
rotation used in Colorado that consisted of oat, pea, potatoes or sugar beet,
barley or wheat, and a fifth field in alfalfa (Parker, 1915). These and many other
similar cropping systems were used throughout irrigated areas of the western
United States. The emphasis in most of the rotations was on some form of staple
crop and an adaptable soil renovating crop, such as the alfalfa in the rotations just
described. Alfalfa was well adapted to irrigation, provided an excellent source of
forage, and as a legume, it contributed greatly to rebuilding the fertility of the
soil in these irrigated systems.
As noted earlier, selection of the specific cropping system during the latter part
of the 19th and early part of the 20th centuries was based on the individual needs
of the farmer, selection of crops adapted to a particular region, and the climatic
limitations of the area. The other basic requirements in a crop rotation during this
time period, regardless of the region, were the need for some sort of legume to
provide nitrogen for successive crops and different forms of green manure crops,
which were used to maintain fertility of the soils.

B. POST-WORLD
WARI1 DEVELOPMENTS
Cropping systems before World War I1 changed little, but following this world
event, crop rotations that included legumes were de-emphasized. Increased avail-


10

D. L. KARLEN ETAL.

ability of nitrogen from industrial sources during the 1950s and early 1960s
hastened this change throughout the United States (Tyner and Purcell, 1985).
Plentiful and inexpensive nitrogen fertilizer following World War I1 devalued
legume rotations except for farmers with livestock systems that required the
legume as a feed source (Olson and Sander, 1988). Post-World War I1 research
reports document a consensus that synthetic fertilizers and pesticides could forever replace crop rotation without loss of yield (Aldrich, 1964; Benson, 1985;
Shrader et al., 1966). Melsted (1954) concluded that to achieve maximum production with minimum soil deterioration, an adequate supply of fertilizer nitrogen was essential.
Increased availability of nitrogen fertilizers, herbicides for weed control, and
pesticides for insect and disease control reduced the use of extended rotations
(Rifkin, 1983; Crookston, 1984; MacRae and Mehuys, 1985). These changes
have resulted in extensive monoculture corn throughout most of the corn belt,
with generally increasing yields. The introduction of improved crop varieties was
a major factor (Power and Follett, 1987), but mechanization (i.e., replacement of
draft animals, which required feed and land devoted to its production, with
tractors and combines) also contributed to the general decline among farmers and
researchers in perceived need for, and therefore use of, crop rotations. Mechanization and adoption of short, 2-year corn and soybean rotations or continuous
monocropping enabled farmers to benefit from the economy of scale by specializing their operations, improving marketing practices, and having to invest in
fewer pieces of equipment (Bullock, 1992; Colvin et al., 1990; Power and
Follett, 1987). Furthermore, many of the government production control and
income stabilization programs limited rotation options and forced farmers to
abandon extended crop rotations (Francis and Clegg, 1990).

c. 2 1ST CENTURY OUTLOOK
Intensive monoculture cropping has increased throughout the United States
since World War 11, and crop rotations have diminished. However, in many
areas, crop rotation has steadfastly remained the major cropping system. Current
consensus is that crop rotation increases yield and profit and allows for sustained
production (Mitchell et al., 1991). In many areas, including several different
types of crops remains the most economical and feasible method for crop production because it is one of the most effective disease and pest control systems. More
recently, increased energy costs resulted in renewed interest in crop rotations as a
source of nitrogen (Tyner and Purcell, 1985). However, interest in rotations as a
source of nitrogen is present only when energy and fertilizer costs are high. Both
are uncertain at this time.
Post-World War I1 abandonment of extended crop rotations, in favor of short
rotations and monocropping systems, has generally been profitable. However,


CROP ROTATIONS FOR THE 2 1st CENTURY

11

the change has had negative consequences, especially if on- and off-site environmental consequences are considered. Many effects are site specific, but they
include decreased soil organic matter content, degraded soil structure, increased
soil erosion, increased sedimentation of reservoirs, increased need for external
inputs, and increased surface and groundwater contamination. Long-term effects
of not using crop rotations are not clear, but it is reasonable to question if the
substitution of capital, energy, and synthetic chemicals is sustainable (Bullock,
1992). These questions are raised as we look toward the 21st century, because, as
stated by Hauptli et al. (1990), “Modern agriculture is a very recent development, when considered in the context of evolution or even human history.”
Crop rotation has not been abandoned in the United States. Approximately
20% of the corn is grown in continuous monoculture, but most of the remaining
80% is grown in a 2-year rotation with soybean or in short (2- or 3-year) rotations
with alfalfa, cotton, dry beans, or other crops (Power and Follett, 1987). The
primary crop rotation change involves use of pasture and green manure crops.
Few are included in current crop rotations.
Many of the rotation factors, processes, and mechanisms responsible for increased yield remain unknown. Increased nitrogen supply is sometimes responsible (Russelle et al., 1987), but improvements in soil water availability (Benson,
1985; Roder et al., 1989), soil nutrient availability (Bolton et al., 1976; Higgs et
al., 1976; Peterson and Varvel, 1989a,b,c), soil structure (Barber, 1972; Dick
and van Doren, 1985; Griffith et al., 1988), soil microbial activity (Cook, 1984;
Williams and Schmitthenner, 1962), and weed control (Bhowmik and Doll,
1982; Slife, 1976); decreased insect pressure (Benson, 1985), nematode populations (Dabney et al., 1988), and disease incidence (Dick and van Doren, 1985;
Edwards et al., 1988); and presence of phytotoxic compounds and/or growthpromoting substances originating from crop residues (Barber, 1972; Benson,
1985; Bhowmik and Doll, 1982; Welch, 1976; Yakle and Cruse, 1983, 1984)
have also been identified as contributing factors. Currently, no amount of chemical fertilizer or pesticide can fully compensate for crop rotation effects, and
analysis of these individual factors generally does not explain the entire yield
response associated with crop rotation. Determining how the factors associated
with crop rotations interact and contribute to the currently undefined “rotation
effect” will apparently continue to provide a major research challenge.

111. AGRONOMIC IMPACTS OF CROP ROTATION
A. CROPYIELD
Increased yield may be one of the most practical justifications for reintroducing crop rotations (Wikner, 1990; Karlen et al., 1991). Several studies showing


D. L. KARLEN ET AL.

12

that corn, grown in a 2-year rotation with soybean, yields 5 to 20% more than
monoculture corn have been published (Strickling, 1950; Welch, 1976; Kurtz et
al., 1984; Voss and Shrader, 1984; Peterson and Varvel, 1989c; Crookston et al.,
1991). Data from a 15-year study in Iowa (Table 11) show the typical response.
Crookston et al. (1991) reported that annually rotated corn yielded 10% more
than continuous corn, and that first-year corn, following 5 years of soybean,
yielded 15% more than continuous corn. Based on these results, they suggested
Minnesota farmers consider using longer crop rotations. However, yield response to rotations greater than 2 years may (Crookston et al., 1991) or may not
(Lund et al., 1993) occur.
Increased emphasis on crop residue management to reduce soil erosion may
also encourage crop rotations because they can largely eliminate corn yield
decrease observed between no-tillage and conventional tillage production practices (Karlen et al., 1991). This response is particularly evident on poorly
drained soils (Dick et al., 1991). Furthermore, because many cropping systems
have a small profit margin, a 5% yield increase for corn may result in a 50%
profit increase (Crookston, 1984).

Table I1
Crop Rotation Effect on Corn Grain Yield

in Northeast Iowa
Year

Continuous

Rotation

Mg ha-l
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992

8.0
9.5
9.5
9.9
7.5
5.3
5.5
7.6
10.2
8.1
5 .O
6.6
10.6
8.3
9.1

9.4
10.2
9.7
10.4
7.7
6.8
7.3
8.8
10.8
9.0
6.3
8.0

8.0

9.0

11.2
9.4
10.0

LSD(O.05) = 0.3

cv = 4.3%

15-year average
LSD(O.05) = 0.2


CROP ROTATIONS FOR THE 2 1st CENTURY

13

Crop yield increases due to rotation are not limited to corn. Grain sorghum in
rotation with soybean (Brawand and Hossner, 1976; Clegg, 1982; Gakale and
Clegg, 1987; Peterson and Varvel, 1989b; Roder et al., 1988; Langdale et al.,
1990) or corn (Robinson, 1966) showed increased yield compared to continuous
grain sorghum. Soybean yield also increased when grown in a rotation with corn,
grain sorghum, or simply following a fallow period (Crookston, 1984; Dabney er
al., 1988; Peterson and Varvel, 1989a).

B. WATERUSEEFFICIENCY
The need to develop more water-efficient crop management practices may be
one of the strongest incentives for adopting crop rotations. Crops should be
managed in a rotation sequence so that complementary root systems fully exploit
available water and nutrients (Karlen and Sharpley, 1994). Sadler and Turner
( 1994) suggested “opportunistic cropping” as a means for increasing agricultural
sustainability through water conservation or by increasing productivity from
applied water. Opportunistic cropping is not crop rotation in the typical sense,
but this management practice requires farmers to remain sufficiently flexible to
adapt their farming practices to utilize rainfall and/or irrigation water as efficiently as possible. Therefore, opportunities to rotate crops spatially and temporally may become increasingly important.
Roder er al. (1989) evaluated yield and soil water relationships for a sorghum
and soybean cropping system. They found that crop rotation increased soybean
yield, but that nitrogen fertilization did not. The soybean yield advantage from
rotation decreased as the amount of spring rainfall increased.
Increasing temporal and spatial diversity by using different crop rotations may
mimic natural ecosystems more closely than current farming practices. This
change may lead to increased agricultural sustainability (Karlen et al., 1992).
One example is in semiarid areas where saline seeps began to develop about 30
years after cultivation began, and especially after about 10 years of an alternateyear, crop-fallow rotation (Ferguson er d.,1972). Formation of saline seeps
gradually became a problem as production agriculture disrupted annual crop
growth associated with native plant communities in semiarid regions. Ferguson
and Bateridge (1982) found that 50 years of crop-fallow farming significantly
reduced soluble salt content of some soils. Although this was beneficial from an
edaphic perspective, they found that up to 90 Mg ha-1 of salt was moved toward
the water table where it resulted in groundwater salinization and became a source
of salts for saline seeps.
Undoubtedly, some water moves below the root zone of native vegetation, but
the quantity is not large. Native vegetation is diverse with varying growth habits
and rooting depths. Therefore, most precipitation infiltrating the sod is transpired


14

D. L. KARLEN ETAL.

(Ferguson et al., 1972; Halvorson and Black, 1974). With cultivation during
periods of above-normal annual precipitation, and with improved soil water
storage and conservation during fallow, increased use of summer fallow enhances percolation of water below the root zone and thus contributes to formation
of saline seeps (Halvorson and Reule, 1976). By using flexible crop rotations
involving small grains, grasses, deep-rooted crops, and a minimum amount of
summer fallow, soil water loss by deep percolation could be prevented and
development of saline seeps could be alleviated (Halverson and Black, 1974).

C. NUTRIENT
USEEFFICIENCY
1. Nitrogen

Increased use of crop rotations may be mandated to improve nutrient use
efficiencies and reduce losses of nitrogen to surface and groundwater resources.
Crop rotation per se is important, but the sequence with which crops are grown
may be more important (Carter et al., 1991; Carter and Berg, 1991). Karlen and
Sharpley ( 1994) reviewed several studies showing how crop sequence could
influence nitrogen movement through the soil profile and ultimately into groundwater resources. Several studies showed that soybean and alfalfa, which do not
require supplemental nitrogen inputs, can effectively use or “scavenge” residual
nitrogen remaining in the soil from previous crops (Johnson et al., 1975; Mathers et al., 1975; Muir et al., 1976; Olson er al., 1970; Stewart et al., 1968).
Alfalfa roots may grow to depths greater than 5.5 m in some soils, and
research has shown that nitrate can be utilized by the crop from any depth where
soil solution is extracted by plant roots. Mathers et al. ( I 975) reported that alfalfa
removed nitrate from the soil profile at a depth of 1.8 m during the first year of
establishment and to a depth of 3.6 m during the second and third years. Olson et
al. (1970) found that crop rotation reduced soil solution nitrate concentrations at
a depth of 1.2 to 1.5 m by 34 to 82% compared to continuous corn. They found
that the decrease in solution nitrate was directly proportional to the number of
years in oats, meadow, or alfalfa production, and attributed this to combined
recovery of nitrate by shallow-rooted oat crops followed by deep-rooted alfalfa
crops.
Soybean can also effectively scavenge residual soil N (Johnson et al., 1975;
Havlin et al., 1990; Karlen e t a l . , 1991), but in Wisconsin, soybeans were not as
effective as alfalfa because of their more shallow rooting depth (Jackson et al.,
1987). This finding was supported by Olson et al. (1970), who also concluded
that recovery of subsoil nitrates by deep-rooted legumes such as alfalfa will
probably be more effective on medium and heavy textured soils than on sands.
One of the persistent nutrient management questions associated with crop


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