Crop rotation or
Crop sequencing
is the practice of growing a series of dissimilar types of
crops in the same area in sequential
seasons for various benefits such as to avoid the build up of
pathogens and pests that often occurs when
one species is continuously cropped. Crop rotation also seeks to
balance the fertility demands of various crops to avoid excessive
depletion of soil nutrients. A traditional component of crop
rotation is the replenishment of
nitrogen
through the use of
green manure in
sequence with cereals and other crops. It is one component of
polyculture. Crop rotation can also
improve
soil structure and
fertility by alternating deep-rooted and
shallow-rooted plants.
Method and purpose
Crop rotation avoids a decrease in soil fertility, as growing the
same
crop repeatedly in the same place
eventually depletes the
soil of various
nutrients. A crop that leaches the soil of
one kind of nutrient is followed during the next growing season by
a dissimilar crop that returns that nutrient to the soil or draws a
different ratio of nutrients, for example, rice followed by cotton.
By crop rotation farmers can keep their
fields under continuous production,
without the need to let them lie fallow, and reducing the need for
artificial
fertilizers, both of which can
be expensive. Rotating crops adds nutrients to the soil, and
dirt.
Legumes, plants of the family
Fabaceae, for instance, have nodules on their
roots which contain
nitrogen-fixing bacteria. It therefore makes good sense
agriculturally to alternate them with cereals (family
Poaceae) and other plants that require
nitrates. A common modern crop rotation is
alternating
soybeans and
maize (corn). In
subsistence farming, it also makes
good nutritional sense to grow beans and grain at the same time in
different fields.
Crop rotation is a type of cultural control that is also used to
control pests and diseases that can become established in the soil
over time. The changing of crops in a sequence tends to decrease
the population level of pests. Plants within the same
taxonomic family tend to have similar pests
and pathogens. By regularly changing the planting location, the
pest cycles can be broken or limited. For example,
root-knot nematode is a serious problem
for some plants in warm climates and sandy soils, where it slowly
builds up to high levels in the soil, and can severely damage plant
productivity by cutting off circulation from the plant roots.
Growing a crop that is not a host for root-knot nematode for one
season greatly reduces the level of the nematode in the soil, thus
making it possible to grow a susceptible crop the following season
without needing soil
fumigation.
It is also difficult to control
weeds similar
to the crop which may contaminate the final produce. For instance,
ergot in weed grasses is difficult to separate
from harvested grain. A different crop allows the weeds to be
eliminated, breaking the ergot
cycle.
This principle is of particular use in
organic farming, where
pest control may be achieved without
synthetic pesticides.
A general effect of crop rotation is that there is a geographic
mixing of crops, which can slow the spread of pests and diseases
during the growing season. The different crops can also reduce the
effects of adverse weather for the individual farmer and, by
requiring planting and harvest at different times, allow more land
to be farmed with the same amount of machinery and labor.
The choice and sequence of rotation crops depends on the nature of
the
soil, the
climate,
and
precipitation which
together determine the type of plants that may be cultivated. Other
important aspects of farming such as crop marketing and economic
variables must also be considered when choosing a crop
rotation.
History
Old crop rotation methods were mentioned in
Roman literature, and referred to by several
civilizations in
Asia and
Africa. During the
Muslim Agricultural
Revolution of the
Islamic Golden
Age,
Muslim
engineers and farmers introduced a new modern rotation system
where land was cropped four times or more in a two-year period.
Winter crops were followed by summer ones, and in some cases there
was a crop in between. In areas where plants of shorter growing
season were used, ie.
spinach and
eggplants, the land could be cropped three or more
times a year.
According to some sources, in parts of
Yemen
wheat yielded two harvests a year on the same land,
as did rice in Iraq. Scholars such as
Andrew Watson have written of a Muslim
agricultural revolution as the Islamic world made significant
progress in developing a more "scientific" approach based on three
major elements: sophisticated systems of crop rotation, highly
developed irrigation techniques and the introduction of a large
variety of crops which were studied and catalogued according to the
season, type of land and amount of water they require. Numerous
farming encyclopaedias, with surprisingly great precision and
details, were produced.
From the end of the
Middle Ages until
the 20th century, the three-year rotation was practiced by farmers
in Europe with a rotation of
rye or winter
wheat, followed by spring
oats or
barley, then letting the
soil rest (leaving it fallow) during the third stage. The fact that
suitable rotations made it possible to restore or to maintain a
productive soil has long been recognized by planting spring crops
for livestock in place of grains for human consumption.
A
four-field rotation was pioneered by farmers, namely in the region
Waasland
in the early
16th century and popularised by the British
agriculturist Charles Townshend
in the 18th century. The system (
wheat,
turnips,
barley and
clover), opened up a fodder crop and grazing
crop allowing
livestock to be bred
year-round. The four-field crop rotation was a key development in
the
British Agricultural
Revolution.
George Washington Carver pioneered
crop rotation methods in the United States
by teaching southern farmers to rotate soil
depleting crops like cotton with soil enriching crops like peanuts and peas. [7593]
In the
Green revolution, the
traditional practice of crop rotation gave way in some parts of the
world to the practice of supplementing the chemical inputs to the
soil through top dressing with
fertilizers, e.g., adding
ammonium nitrate or
urea and restoring soil
pH with
lime in the search for increased
yields, preparing soil for specialist crops, and seeking to reduce
waste and inefficiency by simplifying planting and harvesting. Some
disadvantages of this type of
monoculture have since become apparent, notably
from the perspective of
sustainable agriculture and the risk
of catastrophic crop failure.
Effects on soil erosion
Crop rotation can greatly affect the amount of soil lost from
erosion by water. In areas that are highly
susceptible to erosion, farm management practices such as zero and
reduced tillage can be supplemented with specific crop rotation
methods to reduce raindrop impact, sediment detachment,
sediment transport,
surface runoff, and soil loss .
Protection against soil loss is maximized with rotation methods
that leave the greatest mass of crop stubble (plant residue left
after harvest) on top of the soil. Stubble cover in contact with
the soil minimizes erosion from water by reducing overland flow
velocity, stream power, and thus the ability of the water to detach
and transport sediment Soil Erosion and Cill prevent the disruption
and detachment of soil aggregates that cause macrospores to block,
infiltration to decline, and runoff to increase . This
significantly improves the resilience of soils when subjected to
periods of erosion and stress.
The effect of crop rotation on erosion control varies by climate.
In regions under relatively consistent climate conditions, where
annual rainfall and temperature levels are assumed, rigid crop
rotations can produce sufficient plant growth and soil cover. In
regions where climate conditions are less predictable, and
unexpected periods of rain and drought may occur, a more flexible
approach for soil cover by crop rotation is necessary. An
opportunity cropping system promotes adequate soil cover under
these erratic climate conditions . In an opportunity cropping
system, crops are grown when soil water is adequate and there is a
reliable sowing window. This form of cropping system is likely to
produce better soil cover than a rigid crop rotation because crops
are only sown under optimal conditions, whereas rigid systems are
sown in the best conditions available .
Crop rotations also affect the timing and length of when a field is
subject to fallow . This is very important because depending on a
particular regions climate, a field could be the most vulnerable to
erosion when it is under fallow. Efficient fallow management is an
essential part of reducing erosion in a crop rotation system. Zero
tillage is a fundamental management practice that promotes crop
stubble retention under longer unplanned fallows when crops cannot
be planted . Such management practices that succeed in retaining
suitable soil cover in areas under fallow will ultimately reduce
soil loss.
See also
- Dryland farming, a specific form
of crop rotation applicable to areas with limited
precipitation.
- The Dutch article on crop
rotation has excellent illustrations (use of an
internet-dictionary recommended).
- Set-aside is the modern name given to
the practice of fallowing agricultural land.
- Soybean management
practices refers to the different practices that are utilized
by soybean producers.
- Ley farming, an agricultural system where the field is
alternately seeded for grain and left
fallow
- Shmita, the seventh year of the Jewish
seven-year agricultural cycle, where the land is left to lie fallow
and all agricultural activity—including plowing, planting, pruning
and harvesting—is forbidden by Torah law.
References
- Andrew M. Watson (1974), The Arab Agricultural Revolution and
Its Diffusion, 700-1100, The Journal of Economic History, Vol. 34,
No.1, The Tasks of Economic History, pp. 8-35.
- al-Hassani, Woodcock and Saoud (2007), Muslim heritage in Our
World, FTSC publishing, 2nd Edition, pp.102-123.
- Unger, P.W., and McCalla, T.M. “Conservation Tillage Systems.”
Advances in Agronomy. Vol. 33. pg. 2-53. 1980.
- Rose, C.W., and Freebairn, D.M. “A mathematical model of soil
erosion and deposition processes with application to field
data.”
- Loch, R.J., and Foley, J.L. “Measurement of Aggregate Breakdown
under rain: comparison with tests of water stability and
relationships with field measurements of infiltration.” Australian
Journal of Soil Research. Vol. 32. pg. 701-720. 1994.
- Carroll, C., Halpin, M., Burger, P., Bell, K., Sallaway, M.M.,
and Yule, D.F. “The effect of crop type, crop rotation, and tillage
practice on runoff and soil loss on a Vertisol in central
Queensland.” Australian Journal of Soil Research. Vol. 35. pg.
925-939. 1997.
- Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R.,
and Hammer, G.L. “PERFECT. A computer simulation model of
Productive Erosion Runoff Functions to Evaluate Conservation
Techniques.” Queensland Department of Primary Industries. Bulletin
QB89005. 1989.
- Huang, M., Shao, M., Zhang, L., and Li, Y. “Water use
efficiency and sustainability of different long-term crop rotation
systems in the Loess Plateau of China.” Soil & Tillage
Research. Vol. 72. pg. 95-104. 2003.
External links