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IV. Break Crops for Improving Soil Structure

IV. Break Crops for Improving Soil Structure

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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



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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

rape.

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

Karus, 1998).

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



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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



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407



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;



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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,

2000).



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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

tested.

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



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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



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