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III. Agronomic Impacts of Crop Rotation
D. L. KARLEN ET AL.
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).
Crop Rotation Effect on Corn Grain Yield
in Northeast Iowa
LSD(O.05) = 0.3
cv = 4.3%
LSD(O.05) = 0.2
CROP ROTATIONS FOR THE 2 1st CENTURY
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).
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
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).
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
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
CROP ROTATIONS FOR THE 2 1s t CENTURY
rotation is whether the nitrogen contribution from legume fixation is responsible
for much, if not all, of the beneficial rotation effect. Bullock (1992) reviewed
several studies focusing on the fertilizer replacement value as the method for
assessing nitrogen contributions from legumes grown in rotation with nonlegume
crops such as corn or grain sorghum. He reported that this method overestimated
the nitrogen contribution by legumes and underestimated the rotation effect. For
example, soybean is given a fertilizer replacement value of 25 to 40 kg ha-1 in
many midwestern states. The actual nitrogen contribution by the soybean crop is
often much less or even negative. In the midwestern United States, soybean in a
2-year corn and soybean rotation may acquire only 40% of its nitrogen from
dinitrogen fixation, while the remaining 60% is taken up from the soil (Heichel,
1987). When the grain is harvested and removed, there is an estimated net loss of
84 kg N ha-1 due to the large nitrogen content of soybean grain: The nitrogen
contribution from alfalfa in rotation with maize in the upper midwestern United
States is also less than suggested by fertilizer replacement value methodology.
Fertilizer recommendations for corn following alfalfa in most midwestern states
credit the alfalfa crop with a nitrogen contribution of 100 to 125 kg N ha-'
(Bruulsema and Christie, 1987; Fox and Piekielek, 1988) based on fertilizer
replacement methodology. However, the actual contribution measured with '5N
methodology was only 24 kg N ha-1 (Harris and Hesterman, 1990). Based on
these studies, Bullock (1992) concluded that rotation with legumes does not
provide as much nitrogen as fertilizer replacement methodology estimates and
that much of the yield benefit which has been credited to nitrogen contribution is
actually due to other factors.
Jensen and Haahr (1990) also concluded that with winter cereals, the rotation
effect of pea was probably more important than the residual nitrogen effect. For
winter oilseed rape (Brussicu nupus L.), the residual nitrogen effect from pea was
equivalent to 30 to 60 kg N ha-1 if applied following oats. Removal of the
above-ground pea residues, which contained less than 1% nitrogen, had no effect
on the residual nitrogen value.
2. Phosphorus, Potassium, and Other Nutrients
There is very little direct evidence that crop rotation affects phosphorus relationships (Bullock, 1992). Karlen and Sharpley (1994) concurred, but suggested
that appropriate selection and use of a crop with a higher affinity for phosphorus
may reduce soil phosphorus stratification and increase phosphorus-use efficiency,
particularly if the nonharvested portion of the crop is returned to the soil. They
suggested that selection of crops which can more efficiently utilize residual soil
inorganic and organic phosphorus may be economically viable for farmers and
enhance the sustainability of soil phosphorus fertility.
Vivekanandan and Fixen (1991) reported that corn sampled at the six-leaf
D. L. KARLEN ET AL.
growth stage had a higher phosphorus concentration when following soybean
than when following corn. Similarly, Copeland and Crookston (1992) observed
that corn in a 2-year rotation with soybean accumulated significantly more phosphorus than did corn in continuous monoculture. This suggested that corn yield
increases associated with crop rotation may have been due to improved general
plant nutrition. However, in the same study, total phosphorus content of soybean
was not affected by crop rotation except for the very early vegetative stages.
Similarly, there was no consistent increase in leaf phosphorus concentration
when sorghum was grown in rotation with cotton (Brawand and Hossner, 1976).
They concluded that although there was a rotation effect, it could not be attributed to improved phosphorus nutrition.
Copeland and Crookston (1992) reported that K and total micronutrient content increased for corn in a 2-year rotation with soybean as compared to continuous monoculture. They proposed that general improvement in plant nutrition
may have been due to an improvement in corn root function and that causal
agents such as mycorrhizae may have played a role. Following a similar argument, the same research group (Copeland et al., 1993) reported that increased
water use by first-year rotated corn or increased water use efficiency of rotated
soybean, as compared to continuous monoculture, demonstrated that rotation
increased root surface and/or root activity which in turn improved water relations
and increased grain yield. Inclusion of legume cover crops into a crop rotation in
the southeastern United States also resulted in a beneficial redistribution of
potassium to the soil surface from deeper in the soil profile (Hargrove, 1986).
Extractable calcium and magnesium levels do not appear to be affected by crop
rotation. Increased availability of micronutrients including iron, copper, and zinc
because of microbiologically enhanced chelation may also be a beneficial effect
of crop rotation and cover crops (King, 1990).
Crop rotation is a fundamental tool of integrated pest management. Francis et
al. (1986a) coined the term “biological structuring” to describe the use of crop
rotations, management alternatives, biological phenomena, environmental conditions, and interactions of these factors to manage crop pests such as disease,
weeds, and insects. Crop rotation affects pest pressure in various ways, but in
general the literature supports Francis and Clegg (1990) who stated that “the
greater the differences between crops in a rotation sequence, the better cultural
control of pests can be expected.” While crop rotation does reduce pest pressure
it should be noted that even when pest pressure is minimal, the rotational effect
still exists. This suggests that pest control is a contributor to the benefit of crop
rotation, but is not responsible for the rotation effect itself. However, it should be
CROP ROTATIONS FOR THE 2 1st CENTURY
recognized that we are unaware of all pests which detrimentally affect crops and
thus it can be hypothesized that much of the rotational effect is due to alleviation
of unknown pests.
Crop rotation is an effective tool against certain pests, and efficacy may contribute to the rotation effect, but rotation does not control all pests (Bullock,
1992). Pests which are controlled by crop rotation have the following characteristics (Flint and Roberts, 1988). First, the pest inoculum source must be from the
field itself. Crop rotation does not control highly mobile pests since they have the
ability to invade from adjacent fields or other areas. Pests which can be controlled by rotation include soil and root-dwelling nematodes, soilborne pathogens
(if they do not produce airborne spores), and vegetatively propagated weeds such
as nutsedge (Cyperus). Second, the host range of the pest needs to be fairly
narrow or at least must not include plants which are reasonably common in a
given area. Third, the pest must be incapable of surviving long periods without a
living host. In other words, the pest populations must decrease substantially
within a year or two of removing a living host plant.
Weeds can reduce crop yields provided their densities reach a biological
threshold. Most research indicates that biological thresholds are greater than zero
(Aldrich, 1987), but there are scattered arguments in the literature that biological
thresholds are zero (Cousens, 1985).
A combination of crop rotation, smother crops, and mechanical cultivation
were used to control weeds prior to the introduction of the synthetic herbicide
2,4-D [(2,4-dichlorophenoxy)acetic acid]. Crop rotation alone was not sufficient
to control weeds; all three methods had to be used as an integrated program with
a primary goal of preventing weed reproduction (Regnier and Janke, 1990).
Crop rotation helps control weeds because they thrive and increase in crops
which have similar growth requirements to their own. For example, grasses
thrive in continuous corn, while broadleaf weeds thrive in continuous soybean.
In Nebraska, rotating corn and grain sorghum with a broadleaf crop is an effective method of controlling shattercane (Sorghum bicolor L.) because it allows for
the use of herbicides which are phytoactive on cereals (Francis and Clegg, 1990).
Similarly, Dale and Chandler (1979) reported that a corn and cotton rotation
enabled growers to control johnsongrass (Sorghum halepense L.) much better
than a continuous corn rotation because grass-specific herbicides could be used
during the cotton phase of the rotation.
Crop rotation introduces conditions and practices that are not favorable for a
specific weed species and thus growth and reproduction of that species are
hampered. For example, Forcella and Lindstrom (1988) found 25 weed seeds
m-2 in a continuous corn field, but only 4 weed seeds m-2 where corn was
D. L.KARLEN ET AL.
grown in rotation with soybean. Not all crops are equal in their competitiveness
with weeds. Van Heemst (1985) ranked 25 crops for their ability to compete with
weeds based on a mean reduction in yield. Wheat was considered the most
competitive and given a rank of first. Soybean ranked fourth and corn seventh.
Regnier and Janke (1990) suggested that factors such as rate and extent of canopy
development, plant spacing, and life cycle all contributed to a crop’s competitiveness. It has also been noted that cultivars within a species also compete
differently with weeds. Bullock (1992) cited several references suggesting this
difference may be attributable to production of allopathic compounds, especially
if small grains such as rye, wheat, oats, or barley are included in the rotation.
The importance of crop rotation was diminished with the advent of synthetic
herbicides. However, there is ample evidence confirming that crop rotation irnproves weed control even with synthetic herbicides (Bullock, 1992). For example, after 7 to 8 years of standard chemical and mechanical weed control from
1500 to 3000 weed seed m-2 were found with continuous corn, while in a cornsoybean rotation, the soil had from 200 to 700 weed seed m-2 (Forcella and
Lindstrom, 1988). Withholding herbicides for 1 year reduced continuous corn
yield by 10 to 27% but did not reduce corn yield in the 2-year corn and soybean
Interest in using crop rotation to control weeds is gaining popularity, especially
among those persons focusing on sustainable agriculture. For example, compared to growing continuous corn, growing corn in a 2-year soybean and corn
rotation or a 3-year soybean, wheat, and corn rotation reduced giant foxtail
(Seturiafuberi Herrm.) seed at the 0- to 2 . 5 , 2.5- to lo-, and 10- to 20-cm
depths (Schreiber, 1992). Similarly, Ball ( 1992) reported that cropping sequences
were the most dominant factor influencing species composition in weed seed
Temporal diversity achieved through crop rotation and spatial diversity
achieved through intercropping can markedly reduce weed population density
and biomass production (Liebman and Dyck, 1993). Among 26 comparisons
between monoculture and rotation cropping systems, they found that emerged
weed densities with rotations were lower in 21 studies, higher in 1 study, and
equivalent in 5 studies. They concluded that the success of rotation systems for
weed suppression appears to be based on the use of crop sequences that create
varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable or inhospitable environment that prevents the proliferation of a particular weed species. Liebman and
Dyck (1993) concluded that the relative importance and most effective combinations of various weed control tactics have not been adequately evaluated and
recommended, therefore three research thrusts should be addressed. These included (1) determining effects of crop rotation and intercropping on weed population dynamics including weed seed longevity, weed seedling emergence, weed
CROP ROTATIONS FOR THE 2 1st CENTURY
seed production and dormancy, agents of weed mortality, resource competition
between cultivated crops and weeds, and allelopathic effects; (2) determining
how to combine specific components of rotation and intercropping strategies that
may be important for weed control; and (3) designing and testing new integrated
approaches for weed control at the scale of complex farming systems.
Insect pests which have specific or at least narrow host ranges and which are
incapable of extended migration are particularly susceptible to crop rotation
(Ware, 1980). An example of this is the control of northern corn rootworm
(Diabrotica sp.) in the central United States. In a monoculture corn production
system, rootworm reaches an economic threshold about 30% of the time, but in a
2-year corn and soybean rotation, the economic threshold is reached less than 1%
of the time. However, increased use of a 2-year corn and soybean rotation
throughout much of the northern corn belt has resulted in selection for corn
rootworms with a 2-year rather than a 1-year diapause. Therefore, reports exist
of economic damage for corn grown in a short, 2-year rotation (Ostlie, 1987).
For some insect species, crop rotation is not an effective control practice. For
example, Johnson et al. (1984) reported that black cutworms (Agrotis ipsilon) are
more of a problem when corn is rotated with either soybean or wheat than when it
is grown continuously. Apparently, black cutworm moths are less attracted to
corn residue than to either soybean or wheat residues for oviposition (Busching
and Turpin, 1976).
Crookston (1 984) suggested that decreased crop yields associated with monoculture cropping systems were caused by increases in some unknown soil pathogen. Although attractive and frequently used to justify crop rotation as a method
for preventing fungal diseases (Curl, 1963), Crookston’s suggestion is not universally accepted (Roder et a f . , 1988). It is not necessarily clear to what extent
disease prevention contributes to the rotation effect. Bullock (1992) found that
monoculture wheat has problems with fungal diseases, in particular take-all
(Gaeumannomyces graminis var. tritici), but the severity of fungal diseases in
continuous wheat often decreases within 3 to 5 years. The reduction in severity is
known as “take-all decline,” and as a natural control of the disease it is effective.
The mechanisms responsible for take-all decline are not completely understood,
but changes occur in the microflora and microfauna in soils where take-all fungus
is established. Part of the take-all decline is due to a build up of competitive and
predatory microorganisms which control the take-all fungus (Crookston, 1984).
Curl (1963) suggested that in some cases the control mechanism may be a pest-
D. L. KARLEN ET AL.
predator type of relationship, while in others, the organisms are simply competing for limited resources. Crookston et al. (1991) postulated that a buildup of
beneficial organisms which help to control detrimental organisms might explain
why second-year yields for a continuous corn cropping system often show a
greater decline compared to yields of rotated corn than that observed during the
later years. Meese et al. (1991) reported that withholding corn for 1 year is
sufficient to obtain the maximum rotation effect, but Crookston et al. (1991)
reported that withholding corn for more than 1 year would increase the rotation
effect slightly. Both reported that soybean requires more than 1 year of absence to
negate deleterious soybean yield effects. The exact nature of the agent responsible for the deleterious effect of continuous soybean is not clear, but Whiting and
Crookston (1993), working in the northern U.S. corn belt, have reported that
plant diseases are not playing a major role. Thus, it is reasonable to conclude that
the yield increase observed for soybean in a soybean and corn rotation is not
necessarily due to a reduction in seventy or incidence of plant diseases.
Time does not decrease the seventy of all diseases (Bullock, 1992). Studies by
Stromberg (1986) showed that gray leaf spot in corn (Cercosport zeae-maydis) in
the southeastern United States becomes a severe problem if corn is grown continuously using no-tillage production practices. However, a rotation in which corn
is absent for at least 1 year prevents the disease from becoming an economic
The use of crop rotations to control Meloidogyne and Heteroderu glycines
species of plant parasitic nematodes on tobacco and soybean crops in North
Carolina was established during the 1950s and 1960s (Barker, 1991). This approach for control is currently increasing in importance once again because many
chemical nematicides are no longer available (Flint and Roberts, 1988). Negative
impacts of nematodes on crop production are decreased by crop rotation because
changing plant species generally reduces population levels of most plant parasitic
nematodes (Dabney e f ul., 1988; Ferris, 1967; Edwards et al., 1988).
A reduction of nematode pressure may account for most of the rotation benefit
for soybean in the southeastern United States since cyst nematodes (Heteroderu
glycines Inchinohye) in soybean can generally be controlled by crop rotation
(Dabney et al., 1988). Bailey et al. (1978) reported similar conclusions with
regard to root knot nematodes. Sasser and Uzzell (1991) reported that soybean
yields were improved most by increasing the number of years during which a
nonhost crop was grown.
In other nematode studies, a 1-year rotation with barley (Carter and Nieto,
1975), clean fallow (King and Hope, 1934), or planting a resistant processing
tomato cultivar (Flint and Roberts, 1988) were effective in controlling the cotton
root knot nematode (Meloidogyne incognita).
CROP ROTATIONS FOR THE 2 1st CENTURY
The economic impact of 3-year cotton and soybean rotations in soils with
varying population densities of Hoplolairnus Columbus was estimated by Noe et
al. (1991). They calculated maximum yield losses to be 20% for cotton and 42%
for soybean. Maximum nematode population densities at harvest were estimated
to be 182 per 100 cm-3 of soil for cotton and 149 per 100 ~ m for
They projected net incomes to range from a loss of $17.74 ha-1 for a soybean,
cotton, and soybean rotation to a profit of $46.80 ha-' for a cotton, soybean, and
cotton sequence. A range of economic assumptions and management conditions
are considered in this study.
Crop rotation research was de-emphasized in the early 1960s as priorities
shifted to the development of resistant cultivars and the evaluation of nematicides
(Schmitt, 1991). Resistant soybean lines (Brim and Ross, 1965, 1966)performed
well in cyst-infested fields and gave the maximum yield potential for the environment in which they were grown. As these and other nematode resistant cultivars
became widely grown, often in monoculture, nematode races shifted and, consequently, the use of resistant cultivars as a management tool is now limited.
Schmitt (1991) reported that following 2 or more years of a nonhost crop,
nematode populations were at low or undetectable levels and that soybean yields
were not affected. Those results suggested that a more prudent use of resistant
cultivars grown in rotation with nonhost crops would increase their longevity in
fields infested with cyst nematode races 1, 3, or 4.
Allelopathy occurs when one plant species releases chemical compounds,
either directly or indirectly through microbial decomposition of residues, that
affect another plant species. Liebman and Dyck (1993) stated that including
allelopathic plants in a crop rotation or as part of an intercropping system may
provide a nonherbicide mechanism for weed control. They found few studies that
focused on use of allelopathy in rotations, but management of allelopathic cover
crops for weed control has been extensively investigated (Bullock, 1992). Results of those studies are directly applicable to crop rotations.
Liebman and Dyck (1993) found that exudation of allelochemicals from living
roots of barley and oats have been suggested, but most studies of allelopathy
have been conducted with cover crops or dead crop residues associated with notillage production practices. Studies such as those by Yakle and Cruse (1983,
1984) are typical of those efforts to understand the effects of this complex
process, especially for monoculture corn production. With respect to weed suppression, Putman er al. (1983) found that compared to unplanted control treatments, residues of several fall-planted cereal and grass cover crops significantly
reduced growth and dry matter production by several weed species during the
following summer. Rye, wheat, and barley, which survived the winter and were
D. L. KARLEN ET AL.
subsequently killed by nonselective herbicides, had greater a~l~lopathic
and suppressed weeds much more than oats, grain sorghum, or sorghum-sudan
grass (Sorghum urundinaceum [Desv.] Stapf var. sudunense [Stapf] Hitchc.),
which were killed by winter conditions. Shading and cooling of the soil may
have contributed to this control, but several other studies have shown suppressive
effects that cannot be attributed to the physical presence of mulch. Identification
of the mechanisms governing the differential effect of cover crop residues on
weed and crop species provides a major challenge for persons studying and using
IV, SOIL QUALITY EFFECTS
The need to reduce negative on- and off-site impacts of agricultural practices
will probably provide one of the s~ongestincentives for reintr~ucingcrop
rotations into farm management plans. Kay (1990) reached a similar conclusion
in stating that a major goal for agricultural research will be to identify and
promote cropping systems which sustain soil productivity and minimize deterioration of the environment. To assess the effects of soil and crop management
practices such as crop rotation on both factors, several projects focus on the
concept of soil quality as an assessment tool (Karlen el a / . , 1992; Doran and
Parkin, 1994; Karlen and Doran, 1993; Karlen and Stott, 1994). Using different
crop rotations may improve soil quality by more closely mimicking natural
ecosystems than current farming systems (Karlen et al., 1992). This woutd occur
because temporal and spatial diversity across the landscape would be increased.
Furthermore, management strategies that maintain or add soil carbon have good
potential for improving the quality of our soil resources.
Critical factors being included in most soil quality assessments include measurements of soil structure, aggregation, bulk density, water infiltration, water
retention, soil erosivity, and organic matter (Karlen and Stott, 1994).All of these
factors are influenced by crop selection and rotation. Therefore, it is logical to
examine the effects of crop rotations on the various soil quality indicators as we
assess the need to re-emphasize rotations in 2 1st century farming systems.
Kay (1990) stated that the characteristics of plant species being grown, the
sequence of different species, and the frequency of harvest were ail aspects of
cropping systems that affected soil structure by influencing the formation of
biopores by plant roots and soil fauna. The network of biopores subsequently