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IV. Break Crops for Improving Soil Structure
ROBSON et al.
hemp provides complete ground cover by about the third week after seedling
emergence, thus protecting the soil surface from capping and erosion (Bosca and
Karus, 1998; Section II.E). Adequate nutrients and water must be available if the
rapid early growth is to be sustained, and the soil must be relatively well structured
and uncompacted to allow optimal yield.
Hemp has a high leaf turnover rate and is prone to considerable density-induced
mortality (Van Der Werf et al., 1996). The leaf litter beneath the very dense canopy
acts like a mulch, reducing water loss from the soil by minimizing air turbulence
and evaporation (Marshall et al., 1996). Mulches also have a positive effect on soil
structure through protection from wind and rain. Retting (controlled decomposition
prior to processing) of hemp stalks in the ﬁeld provides large inputs of organic
matter for the soil at the end of the season. In addition to this, the stubble and other
debris on the soil provide a protective cover against structural damage from rain
over winter (Bosca and Karus, 1998; Scheifele, 1994).
Hemp has exceptionally long roots which can penetrate to a depth of 2–3 m
under good soil conditions. It can therefore extract residual nutrients from deep in
the soil (Bosca and Karus, 1998; Low, 1995; Scheifele, 1994). This can increase
the nutrient use efﬁciency of the cropping system and can also help soil structural
development by extracting water from deep in the soil proﬁle, creating cracks and
the formation of soil peds.
Hemp improves the yields of subsequent crops including cereals (Anon., 1996).
Yield increases of 10–20% in winter wheat crops have been reported when grown
after hemp, primarily due to improved soil structure and weed suppression (Bosca
and Karus, 1998). Due to the very rapid early growth and biomass accumulation,
strong weed suppression is almost guaranteed, and no weed control measures are
required in the organic crop (Bosca and Karus, 1998). The high degree of weed
suppression beneﬁts both the hemp and the succeeding crop. For example, the weed
seed bank was reduced following a hemp crop, and consequently weed competition
was reduced in the subsequent crop (Bosca and Karus, 1998).
Hemp is a self-tolerant plant, which is susceptible to few serious pests and
diseases (Bosca and Karus, 1998; Partland et al., 2000; van Der Werf et al., 1996).
The lack of insect predation may be due to the lack of close wild relatives of
hemp, which would have the same pests (Bosca and Karus, 1998). There is also
anecdotal evidence that even the low concentrations of THC found in hemp has
an insect-deterring effect (Bosca and Karus, 1998). Gutberlet and Karus (1995)
found that pests simply prefer other crops to hemp. This minimal susceptibility
to pests and diseases could promote the use of hemp as a pest and disease break
in cereal rotations, perhaps providing the necessary break for take-all. There is
also evidence that hemp can restrain some types of nematodes, which could be
important in the cultivation of potatoes (Bosca and Karus, 1998) and cereals.
The company Hempﬂax (The Netherlands) reports that the soil in hemp ﬁelds
dries faster and warms up more rapidly earlier in the year than soils supporting other
AGRONOMIC AND ECONOMICAL POTENTIAL
crops (Bosca and Karus, 1998). This may be an advantage for the early sowing
of the next crop. However, in a particularly dry season, the subsequent crop may
become water stressed causing a depression in yield. For similar examples in other
crops, see Hamblin et al. (1993) for lupins, and Kirkegaard et al. (1994) for oilseed
Hemp has a high nutrient requirement, and prior to harvest, the hemp extracts
more nutrients per hectare than grain crops, removing 2 to 3 times as much N, 3 to 6
times as much P, and 10 to 20 times as much K (Anon., 1999). These requirements
are due to fast biomass production. However, up to 70% of the nutrients taken
up are returned to the soil through fallen leaves during the season, mechanically
stripped leaves and ﬂowers at harvest, and the retting process (Anon., 1999; Bosca
and Karus, 1998). It is recommended in an organic rotation that hemp is grown
after a grain legume or clover to ensure adequate nutrition (Bosca and Karus,
1998). In an organic arable rotation, a cereal would usually occupy this optimum
position in the rotation. However, on a nutrient-rich soil, or with adequate nutrient
addition from manures, the hemp crop could be grown as a break between cereal
crops (Tables II–V).
Since hemp is sown in late April to early May, it can easily be used as an
intermediate crop in a rotation. Harvest dates for hemp depend on the variety and
climate. Between 50 and 55 degrees north latitude, hemp is harvested between
the end of August and the ﬁrst 10 days in September (Bosca and Karus, 1998).
Hemp can be sown with a cereal drill, so no specialized machinery is needed.
For harvesting, the hemp crop can be mowed or chopped. Chopping makes for
simpler management in the ﬁeld and enables existing harvesting machinery to turn
and press the chopped hemp plants, ready for transport to a processing plant. The
Kemper cutter (John Deere International) is suitable for chopping hemp, and a
range of new machinery suitable for hemp production and harvest is becoming
available throughout Europe and North America (Hemp Industries Association,
2001; Hemptech, 2001).
There is a growing market for both conventional and organic hemp and their
many products. High quality ﬁbers are being used for clothing, and both are currently produced in Germany at prices competitive to those of cotton. Fibers are
also being used for thermal insulation, although they cannot, at present, compete economically with synthetic ﬁbers (Bosca and Karus, 1998). Hemp is also
used in the production of speciality pulp and paper, which could be an important
use in the long term (Schlegelmilch, 1995). Hemp ﬁbers are increasingly taking
the place of wood ﬁbers in the manufacture of press-molded interior panels in the
automotive industry, due to their low weight and high tensile strength (Bosca and
There is a market in the selling of hemp hurds (the nonﬁber portion of the stalk)
which are produced at every stage of mechanical processing. These have various
uses, for example, animal bedding especially for horses. There are additional
ROBSON et al.
markets for hemp seed and oil, which can be used to produce body-care products
such as soaps, detergents, oils, dyes, and as a source for the essential nutrient
gamma-linolenic acid. There is also a market for hemp seeds as a high quality
nutritious food (Bosca and Karus, 1998).
The area of organic hemp grown in the UK doubled in 2000. A license is required in order to plant the crop which must be grown under contract. Crop quality
in the UK is currently unsatisfactory for the rapidly growing organic clothing
market, therefore no organic premium was assumed in this review. Organic hemp
has one of the lowest net margins of any of the crops assessed in this review
(Table X). Only EU subsidies make organic hemp cultivation a proﬁtable venture
under current economic conditions in Europe. The point where ﬁber costs and market prices meet dictates the subsidy level, so that the costs of producing hemp are
just covered and no more. However, cost reductions in hemp processing techniques
can be expected in the near future due to higher crop yields from new varieties
combined with lower harvesting costs due to new machinery (Bosca and Karus,
1998). This, along with the fact that hemp contributes positively to soil structural
development and suffers from few pest, disease, and weed problems, means that
is has signiﬁcant potential for use within temperate organic rotations.
B. OILSEED RAPE (Brassica napus SUBSP. oleifera)
Oilseed rape has been grown since the 16th century in Europe, but it is only since
the 1960s that it has become a major world crop (Kimber and McGregor, 1995).
In global terms, conventionally grown oilseed rape is the second most important
vegetable oilseed after soybean, accounting for 14% of total oilseed production
at 40 Mt in 2000 (Weiss, 2000). Fifteen megatons of rapeseed oil was produced
in 2000, and this constituted 15% of the global consumption of vegetable and
marine oils (USB, 2000). According to Scarisbrick and Ferguson (1995), brassica
crops will play an increasing role in supplying the world’s need for human and
animal foodstuffs and industrial oils. At present, very little oilseed rape is produced
organically in any temperate region.
The beneﬁts of planting oilseed rape before a cereal in a rotation are widely
reported (e.g., Angus et al., 1991; Cresswell and Kirkegaard, 1995; Gregory, 1998;
Kirkegaard et al., 1997; Pouzet, 1995). However, the exact mechanisms of such
beneﬁts are still under investigation. The effects include subsoil amelioration,
leading to increased nutrient and water uptake and disease suppression in the
following cereal (Kirkegaard et al., 1994).
The main beneﬁt which oilseed rape brings to a rotation is its effect on soil
structure. Actively growing root systems have the potential to ameliorate subsoil
under poor physical conditions by “biological drilling” (see Section II.E) and taprooted species such as oilseed rape are generally considered superior to grasses in
AGRONOMIC AND ECONOMICAL POTENTIAL
their ability to ameliorate poorly structured soils (Lal et al., 1979). Some workers
have reported that oilseed rape roots were more affected by soil impedance than
cereal species. For example, Cresswell and Kirkegaard (1995) found no signiﬁcant
subsoil structural differences after two seasons of growing oilseed rape, but that
biological drilling may occur if the crop was retained for longer. Unfortunately the
pest and disease implications for growing oilseed rape for more than 2 years would
be too great, particularly in organic systems (Cresswell and Kirkegaard, 1995).
Wheat yield increases following oilseed rape of 10–103% have been reported in
Australia (Gregory, 1998; Kirkegaard et al., 1994, 1997), along with an increase in
protein content of wheat grain (Kirkegaard et al., (1994)). The wheat N requirement
was reduced by 30 kg ha−1 after an oilseed rape crop, and regardless of season,
there was sufﬁcient residual N after oilseed rape to supply winter wheat until
spring (McEwen et al., 1989). Increases in wheat yields after oilseed rape often
depend on seasonal conditions. In dry areas, oilseed rape can limit the growth of
the subsequent cereal by leaving insufﬁcient available soil water (Kirkegaard et al.,
1994). The efﬁcacy of oilseed rape as a break crop in a cereal rotation is reduced
where levels of disease in the cereals are low (Kirkegaard et al., 1997).
Kirkegaard et al. (1994) reported that the plant population of a winter wheat
crop following a brassica break was reduced, and that this was due to the signiﬁcant allelopathic effect that brassica residues have on wheat seedlings. The
allelopathic effect depends on the type of residue and the state of decay. Fresh
residues resulted in a reduction of emergence and growth of wheat. However, decayed residues, particularly of oilseed rape, signiﬁcantly increased growth (Purvis,
1990). The reduction in wheat seedling density was more than compensated for by
the stimulation of seedling growth after brassicas, which on average was increased
by 29%. Further work is required to determine the reasons for increased cereal
growth following rape, since it cannot be explained by residual N, or reduced
disease incidence alone.
There is considerable evidence that compounds present in oilseed rape residues
have a controlling effect on cereal pathogens. For example, isothiocyanates (ITCs),
which are formed in brassica plants during the hydrolysis of glucosinolates, inhibit
the growth of some soilborne fungi (Walker et al., 1937). Angus et al. (1994)
found that dried, irradiated brassica roots were consistently effective in reducing
the growth of G. graminis, but inhibition by young live roots was not consistent.
Brassica residues can also inhibit the activity of beneﬁcial and plant pathogenic
nematodes in the ﬁeld (Mojtehedi et al., 1991).
Rape is susceptible to pests such as slugs (Deroceras reticulatum and other
species), cabbage stem ﬂea beetle (Psylliodes chrysocephala), virus vector aphids
such as Myzus persicae, and cabbage root ﬂy (Delia radicum) (Ekbom, 1995). It is
also susceptible to a number of diseases including sclerotinia stem rot (Sclerotinia
sclerotiorum Lib.), stem canker (Leptosphaeria maculans [Desm.] Ces. & de Not.),
and alternaria leaf and pod spot (Alternaria spp.) (Rimmer and Buchwaldt, 1995;
ROBSON et al.
Evans and Gladders, 1981). Weeds, too, cause yield losses and are acknowledged
to be the most important limiting factor in rapeseed production in Canada (Orson,
1995). The choice of cultivar and crop husbandry methods limit the impact of
pests, diseases, and weeds in rape (Ekbom, 1995; Pouzet, 1995), and this will
be particularly important in organic systems, where the choice of prevention and
control methods is limited.
Oilseed rape is combinable and is therefore mechanically compatible with a
cereal rotation. However, organic production of oilseed rape is difﬁcult due to its
very high nutrient requirements and its susceptibility to pests and diseases, many
of which can prove difﬁcult to control under organic regulations (Tables VI–IX).
If oilseed rape were to be included in an organic rotation, it would ideally follow a
clover ley or similar crop which leaves high levels of residual nitrogen. Winter rape
is a good crop to grow prior to winter cereals because its extensive rooting and early
harvest leave the soil in good condition for sowing in the autumn (Lampkin, 1990;
Pouzet, 1995). The spring rape crop is also compatible with a cereal rotation and
is proposed by some as contributing toward a more environmentally responsible
rotation (Fisher et al., 1996). The stubble from the preceding cereal can be left
over winter to provide a feeding ground for birds (Fisher et al., 1996). It also acts
as a protective measure against soil erosion and nitrate leaching in common with
other winter cover crops. Spring oilseed rape has smaller nutrient demands and
lower incidences of pests and diseases than winter sown crops (Fisher et al., 1996).
Top dressing with slurry or liquid manure would be beneﬁcial in late spring and/or
summer during the period of maximum nutrient demand. Long rotations (4–6
years) are necessary between oilseed rape and other crucifers due to susceptibility
to similar diseases such as sclerotinia (Pouzet, 1995).
There is a large and well-established market for conventionally produced oilseed
rape, predominantly as an edible oil (Holmes, 1980). There is also an increasing
diversity of cultivars with specialized fatty acid proﬁles for niche markets. In
addition, new cultivars offer alternative or new sources of raw materials for edible, industrial, and pharmaceutical use (Pouzet, 1995). There is also a market for
rapeseed cake for livestock feed (Scarisbrick et al., 1989). With a 50% premium
assumed, the net margin for organic oilseed rape, however, is low, at £285 ha−1
(Table X). Some processors are currently interested in making health products
from the crop and may be prepared to pay a higher premium, but this market
is not yet established. An organic processing capacity is required in countries
interested in producing the crop before farmers will begin to consider it as a
serious option for organic systems. (Fowler and Lampkin, 1999). The advent
of genetically modiﬁed rape in conventional agriculture may provide an incentive to establish a larger alternative organic market for rape, since GMOs are not
permitted as organic animal feed and since an increasing number of consumers
are choosing to avoid genetically modiﬁed products (Food and Drink Federation,
AGRONOMIC AND ECONOMICAL POTENTIAL
V. BREAK CROPS FOR WEED MANAGEMENT
A. POTATOES (Solanum tuberosum)
The potato originated in South America and has become a major dietary staple in
almost all temperate countries. Conventional world production is of the same order
as the major cereals. Organic potato production has increased dramatically in the
past 10 years in Europe. For example, 731 ha of root crops (a large percentage of
which was potatoes) was produced in EU countries, the Czech Republic, Norway,
and Switzerland in 1993 (Lampkin and Foster, 2000). By 1998, the area of land
used for organic root crop production had increased to 5733 ha. In the United States
1970 ha of organic potatoes was produced in 2000 (Economic Research Service,
USDA, 2001). At present, many European countries including the UK are net
importers of organic potatoes, and there is great potential for further development
of the industry in these countries (Soil Association, 2000a).
Potatoes provide a valuable weed, pest, and disease break for cereal rotations.
They are the only common member of the Solanaceae family grown outdoors in
the cooler temperate climates and they therefore provide a useful pest and disease
break from crops in other important agricultural crop families including Poaceae,
Brassicaceae, Fabaceae, Chenopodiaceae, and Apiaceae. Tobacco also belongs to
this family, but it is frost tender and is rarely grown in areas that are subject to
late frosts, hail storms, or high winds. Although potatoes are often quoted as being
a useful break crop, most of their break functions have not been quantitatively
The production of potatoes under most organic systems requires deep, thorough
cultivation. This, along with postplanting weed control treatments and the fact
that the crop rapidly produces a dense canopy, means that weeds are generally
out-competed (Litterick et al., 1999). Weed numbers in following cereal crops
are consistently lower after potatoes than after cereals (A. Litterick, unpublished;
Rankin, pers. comm.; Halder, pers. comm.; Rose, pers. comm.).
The incidence and severity of damage by several cereal pests can be signiﬁcantly reduced through the introduction of potatoes into the rotation. For example,
leatherjackets (Tipula oleracea) are most commonly associated with damage to
spring barley and grassland (Coll and Blackshaw, 1996); however, large numbers
can damage other cereal crops such as winter wheat. One method of cultural control used in the UK is the use of early potatoes lifted in late July or early August
(Wiseman et al., 1993). The ﬁeld is cultivated at harvest in August to prevent the
eggs being laid, avoiding the subsequent problem of over-wintering eggs emerging
as voracious larvae in the spring. Slug damage in cereals is more likely to occur
following leys, cereals, or brassica seed crops than following fallows, potatoes, or
sugar beet (Glen et al., 1996). Cereals or brassica crops provide good surface cover
ROBSON et al.
over much of the season under which slugs are more likely to increase in contrast
to less cover over the season afforded by carrots or sugar beet. Soil disturbance
associated with root crop (including potato) harvest is also likely to reduce slug
numbers (Glen et al., 1996).
The use of potatoes in cereal rotations also helps to control a number of diseases
prevalent in cereal crops. For example, potatoes have the potential to aid the control
of take-all, which is prevalent throughout Europe where wheat and barley are grown
(Prew and Dyke, 1979). After harvest, the fungus remains alive on the stubble and
root residues of the infected crop. Volunteer plants after harvest are attacked by
mycelium surviving on the crop debris and can be so badly affected that they die.
More often, however, they survive and carry the disease through to the next year.
If a nonsusceptible crop such as potatoes is grown, the pathogen has no host, and
the fungal populations decline. A single nonhost year can reduce populations of
G. graminis to levels at which wheat or barley can be economically viable again
(Butterworth, 1989; Werker et al., 1991).
An increasing number of organic producers are choosing to specialize in potato
production. In these cases, potato is not considered as a break crop, and rotations
are based on the need to optimize potato quality and yield. In some cases, specialist
potato growers rent land from local organic cereal growers on an annual basis and
place the potato crop in the host farmer’s rotation at a point which is mutually
suitable (Redpath and Rankin, pers. comm.) The fact that dedicated machinery is
required for land preparation, planting, harvest, and packing means that organic
potatoes will increasingly be produced by specialist growers who can afford to
make the investment based on the high returns which successive potato crops can
bring (Haward, pers. comm.).
Early potatoes, and sometimes maincrop potatoes, can be harvested sufﬁciently
early to allow a winter cereal to be planted afterwards. If potatoes follow a cereal,
however, the land is often left without a crop until February–April (Wiseman
et al., 1993). In this case, the cereal stubble could be left over winter to protect the
soil from erosion and possible nitrate leaching (Fisher et al., 1996).
There are three main challenges in organic potato production: provision of adequate nutrients, prevention and control of potato late blight (P. infestans), and weed
control (EAGGF, 2000). Potato crops are ideally grown after a fertility building
phase in the rotation due to their high demand for nutrients and poor nutrient use
efﬁciency (Soil Association, 1998). There may be beneﬁts in applying manure,
particularly if soil analysis and past cropping suggest that soil K or N are deﬁcient.
To protect against blight, organic farmers are advised to plant resistant varieties
early, using clean, high quality seed, and using a permitted copper fungicide if
absolutely necessary (EAGGF, 2000; Soil Association, 1998). Approval for the
use of copper fungicides in organic farming systems in Europe will be revoked
in August 2002, and until effective alternative strategies for blight control are
devised, organic potato production may be difﬁcult or impossible in mild wet