Legumes

Plants of the legume family (peas, beans, lentils) together with the cereals and grasses have been the foundation of food production throughout agriculture. Legumes produce a wider range of products and have more ecological functions than the cereals and grasses. They fix nitrogen from the air directly into their own organs, which later release the nitrogen to the soil as a 'fertiliser' that other plants may use. The foliage and grain of legumes are rich in protein when harvested or grazed. The cereals and legumes together provide a staple human diet. Europe has imported legume products such as gum arabic and indigo for centuries, but at present relies also on imports for most of its legume-based, high protein food and livestock feed.

The panel to the right shows (left, top to bottom) mixed grain legumes, broad bean in pod, mung beans, tamarind 'brick' (and right) bowl of brown split pea, indigo-dyed cloth, pinto beans and logwood chips used for dying. Only one of them is not imported.

 

Functions and products

The cultivation of legumes is certainly ancient. Their ability to penetrate and fertilise the soil has been acknowledged and made use of in cropping sequences and rotations. They fertilise the soil mainly by adding nitrogen to it. They do this through a symbiosis between the legume plant and types of  bacteria - collectively called rhizobia - that live in 'nodules' on the plant's roots. The legume and bacteria together fix nitrogen from the air and convert it to compounds that each may use. When they die, the nitrogen is returned to the soil. What further sets the legumes apart from most other plant families is the number of different products that can be got from them. What else could give us chana dal, mushy peas and peanut butter; the dye plants indigo, brazilwood, logwood and dyer's greenweed; firewood and quality timber; oils for the kitchen and light industry; decorative plants and cut blooms; gums for cooking (arabic, tragacanth, guar); grazing and feed for cattle; a wide range of extracted medicinal products. 

The discovery of N-fixation by legumes was made only 125 years ago. For 5000 years or more before then, farmers used legume crops on the basis of their experience that the legumes enriched the soil. Not long after the discovery of N-fixation, i.e. by the end of the 1800s, chemists and engineers were looking for ways to manufacture nitrogen fertiliser directly by industrial processes. In the early 1900s, the Haber Bosch process for manufacturing nitrogen fertiliser came into operation. One of its consequences was to reduce the role of legume crops in farming. Manufactured nitrogen fertiliser transformed agriculture, raising yields and allowing farmers to do away with fertility-building breaks in a sequence of cereal crops. Not all agriculture used the manufactured fertiliser - some growers preferred not to, while some were too poor to buy it. Nevertheless, the area sown with legume crops decreased in many parts of the world including Europe.

More recently, several countries, such as Brazil, the USA and Australia, have appreciated the contribution of legumes to trade and to the nitrogen balance, and have set in place major programmes to improve legume crops and forages and to enhance the symbiosis.

Legume crops in Scotland

In contrast to these global developments in legume production, legumes now have few uses and occupy little of the farmed land surface in Scotland. They occur as combinable crops, as 'vegetables' and as constituents of managed grassland. Field beans Vicia faba (see photographs below) and peas Pisum grown for harvesting by combine now cover less than 1% of the arable regions of east Scotland. They tend to be grown by specialists with assured nearby markets, such as freezing plants for table peas. Several species of legume are also recorded in the annual census among vegetables which as a whole occupy only around 1% of the arable land. A few legumes, mainly white clover Trifolium repens are part of managed grass swards, either to be grazed directly by farm animals or to be harvested and processed as animal feed. These forage legumes contribute to the quality and productivity of the pasture, but the amount of nitrogen they fix is uncertain.

In contrast, much legume protein is imported for animal feed, mainly processed from soya bean grown elsewhere in the world. The low proportion of home-grown legumes, and the reliance on imported protein, constitute one of the great anomalies of agriculture in Scotland and in much of norther Europe too. Also imported are many products of legumes, including tropical pulses, that simply cannot grow here because it is too cold or too wet.

Were things different in the past? Wild legumes, as natural habitants of grassland, will have been eaten by the livestock of the stone age settlers, and will have moved into cultivated fields and cohabited with the cereals and other crops, enriching the soil. The legume used most as a grain (or food) crop since agriculture began in northern Europe is the broad bean or field bean, recorded in England since the stone age (Dickson & Dickson), still a staple of kitchen gardens, though having declined as a broadacre crop. Varieties of pea also have a long history.  Forage legumes cultivated in managed swards in the past included species such as sainfoin Onobrycis viciifolia, lucerne a subspecies of Medicago sativa, and the common vetch Vicia sativa. These soil-enriching forages can still be seen in parts of Europe but are now very rare in northern Britain.

 Plants of field bean at flowering (lower) and (top, left to right) mature beans (2 cm long), foliage and pods with beans (Living Field collection).

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Habitats, status and role of wild legumes in Scotland

Maybe because of their nitrogen-fixing and their great variety of form, wild legumes are found throughout Britain's croplands and semi-natural habitats. Scotland has around 60 species of wild legumes.

Whin and broom First described here are those two legumes that many people have seen - whin and broom. Heathland and derelict pastures, walls and hedges, waste ground and old industrial sites, farm tracks and highways all provide a habitat for these two yellow-flowered perennials. Whin, or gorse, Ulex  europeaus covers acre on acre of old sheep pasture in Scotland, in flower saturating the air with a smell of shredded coconut or coconut milk - no tropical coconut grove ever smelt so coconuty. Broom Cytisus scoparius quickly occupies disturbed land in both town and country.  It spectacularly colonised the roadside banks of the A9 between Perth and Inverness for years after the road was built, and is still in abundance there, having died away in some places but recolonised others.

Troublesome weeds A few legumes have given farmers and smallholders their share of weed problems. The common restharrow Ononis repens has strong stems and roots that in a mass were difficult to cut through before modern ploughs took control of the field. In their blend of colour, the flowers bring to mind a primitive sweet pea. The plant has a stickyness and pungency - so you can imagine how it tainted milk when it grew and was eaten in grazed pastures - and the roots do taste a little like liquorice root, as they are said to! The hairy tare Vicia hirsuta has been another troublesome weed, this one clinging and dragging down a crop rather than hindering soil cultivation. Both have been tamed by heavy machinery and chemical herbicides.

Few legumes, including these two, are now found in the cultivated area of fields. Whether weeds or not, legumes have been driven out of cultivation and have been reduced in pastures. The reasons are not clear-cut - their demise may be nothing to do with nitrogen fertiliser hindering their N-fixing ability, but rather a sensitivity to chemical herbicides, just like many other innocuous or beneficial plants. The resulting loss to nitrogen fixation in agriculture of the many legumes that cohabited with crops has never been estimated. White clover Trifolium repens is now the most common in-field legume, and is likely to come from sown grass-legume forages. None, other than white clover, were in the 100-most-abundant, in-field seedbank species found in major surveys of the past 10 years.  Perhaps the commonest weedy legume, after white clover, is volunteer field bean Vicia faba whose origin is the seed dropped from recent bean crops.

Banks, hedges and ditches Surprising then, perhaps, is the abundance of wild legumes in the parcels of cropland between fields, woodlands, tracks and roads. Here, a single species or several may occur, sometimes cohabiting in dense stands, sometimes dominant over other vegetation. Among these are meadow vetchling Lathyrus pratensis,  common bird's-foot-trefoil Lotus corniculatus, greater bird's-foot-trefoil Lotus pedunculatus, black medic Medicago lupulina, tufted vetch Vicia cracca, hairy tare Vicia hirsuta, common vetch Vicia sativa and bush vetch Vicia sepium, joined occasionally by kidney vetch Anthyllis vulneraria, whose inflorescences (incidentally) are among the strangest of the legumes, and which 'was known throughout Europe as a wound herb, or vulnerary' (Grigson). These legumes add colour, they support pollinating insects, and they must add nitrogen to the soil, but (as indicated below) no one knows how much.

Wild legumes of dunes, woodlands and mountains Many of those legume species of bank, hedge and ditch in farmland also live with other legumes that live only away from cultivation and intense human disturbance. These legumes that shun human influence exist in conditions of low fertility, dryness, salinity and cold, variously along coasts, in woods and on mountains. Such rarer species are outwith the scope of this project on crops and weeds, but are being examined in related work on Scotland's Wild Legumes which examines their distribution, ability to fix nitrogen and prospects (contacts for SWL are Euan James and Pete Iannetta).

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Legumes in the Living Field garden and surrounds

Legumes are now uncommon in much of arable farmland. The Living Field has been encouraging those wild legume species that can still tolerate farmed habitats. As a result, about 12 legume species (depending on the year) are now thriving in the Living Field Garden and on the various paths, tracks and unfertilised grass strips around the farm. Such an abundance of legumes is now rare to find in the intensive croplands of north east Britain, especially in so small an area. 

Following construction of the garden in 2004, most of the plants in the meadow were annuals and biennials (vegetative first year, flowering the next). Few legumes were present. Over the years since then, the meadow and other undisturbed areas have changed. No fertiliser has been given to these parts of the garden. The annuals and biennials have been replaced by perennials, among which many legumes are growing in abundance, dominating in some areas and cohabiting in others with perennials such as wild carrot, meadow clary and field scabious. These legumes fix nitrogen into the soil-plant system  and provide food for pollinators and other important functional groups in the arable food web. Legumes seen growing in the garden in 2010 are (thanks to Euan James):

  • Anthylis vulneraria - kidney vetch
  • Lathyrus pratensis - meadow vetchling
  • Lotus corniculatus - bird's foot trefoil
  • Lotus pedunculatus - big trefoil
  • Medicago lupulina - black medic
  • Trifolium dubium - lesser trefoil
  • Trifolium pratense - red clover
  • Trifolium repens - white clover
  • Vicia cracca - tufted vetch
  • Vicia sativa - common vetch

 

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Future of cultivated and wild legumes

Legumes will continue to have an important role in all low-input and subsistence agriculture. The resurgence of legumes in industrialised agriculture in some countries has been made possible by a committed effort by science and farming to improve the performance and yield of legume crops. Not least has been an acceptance that the most effective symbiosis will not necessarily result from those legume varieties presently grown and the bacteria presently resident in the soil. A thorough study of the symbiosis, followed by selection of strains of the bacteria and the matching of plant and bacterial strain, are needed for progress.

At present in the UK, beans and peas as broadacre crops tend to be cultivated by specialist growers, but are more widely judged to have a low potential and an unreliable yield, and too often fall foul of disease. There are two main reasons for attempting to improve the yield, acreage and status of legume crops. The first is that legumes remain the main alternative to manufactured nitrogen fertiliser. The manufacture and the application of fertiliser to fields is now known to be by far the main contributor to greenhouse gas emissions in arable cropping, in Scotland as in all countries where intense agriculture predominates  The second reason is that great quantities of legume-based animal feed, mostly from soy bean Glycine max, are imported. Few farm animals or farmed fish are fed from home-grown legume protein in Scotland and most of Europe.

Things may change. Legumes might not continue their decline. If nitrogen fertiliser continues to increase in price or if the environmental damage done by its manufacture and use become fully appreciated, then legumes crops could increase in area. In addition, the potential of wild legumes to add nitrogen and high-protein food sources to both cultivated and semi-natural ecosystems is still barely appreciated.

Research and extension at SCRI and more widely through the Legume Futures project (see references) is endeavouring to change perceptions, to design effective legume-based rotations in agriculture and to understand the crucial role of legumes in Scotland's habitats.

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Sources, references and contacts

Grigson G. 1958. The Englishman's flora. Paperback published 1975 by Paladin, St Albans, UK. 542 pages.

Hutchinson J. 1955. British wild flowers. Volumes I and II. (Pelican) Penguin Books. 947 pages.

Ranson F. 1949. British herbs. (Pelican) Penguin Books. 203 pages.

Stace C. 1997 New flora of the British Isles, second edition.Cambridge university Press. 1130 pages. ISBN 0 52158933 5 (plastic covers)

Thirsk J. 1997. Alternative agriculture. A history from the Black Death to the present day. Oxford University Press. 365 pages. ISBN 0-19-820662-3

Legume crop web sites

Contacts

Contacts and contributors for this page: Geoff Squire, Euan James, Janet Sprent, and Pete Iannetta.

[Page began 22 June 2010.  Last update 23 July 2010.]

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