Legumes – best known here as the beans, peas, vetches and clovers – have sustained cropland and wild habitats by their ability to fix nitrogen from the air. Nitrogen is an essential nutrient needed by plants in large quantity for their amino acids and proteins. Without nitrogen, plants would not grow.
Legumes form a relationship – a symbiosis – with bacteria of the general name ‘rhizobia’, and it is the symbiosis in the nodules formed on roots that fixes the nitrogen. Nitrogen is released back to the soil when plants die or break down and when grazing animals, having eaten plants, drop dung and urine.
Through much of agricultural history, legumes and animal waste were the main forms in which nitrogen was made available to crops. Either legumes were grown with or before a crop such as a cereal, or animals were grazed widely on uncropped land and brought back to drop their waste near or on cropped fields. But about 100 years ago, industrially made nitrogen began to replace legumes, a move that had major consequences for both food production and pollution (further information near bottom of page).
Main legume groups and species in the garden
Legumes are of the plant family Fabaceae (Papilionaceae). The garden grows grain legumes such as beans and peas, encourages wild legumes and keeps a collection of forage legumes that are now rare in the croplands. Examples are in the panel below.
Images: (top) white melilot with bee, kidney vetch, (mid) sainfoin, field bean, lucerne, (bottom) red clover and milk vetch in seed (Living Field collection)
Most of the legumes are invaluable to the insect food web – growing legumes is probably the most direct and immediate way to enhance insect wildlife. Here are some of the plants.
Weeds of cropland Wild legumes are now so scarce among crops that it may be hard to image that several of them were once troublesome agricultural weeds. Two can be seen in the garden. Common restharrow Ononis repens must have gotten its name from its tough thick, stems and rhizomes that once made ploughing by hand or horse such effort. It lives in the sown legume collection in the west garden, where it grows luxuriant, unlike the short plants of the same species that occupy scrub and coastal grazings. Restharrow is still found in a few places in farmland within a few miles of the garden. The hairy tare Vicia hirsuta looks nondescript, small leaves and small mauve flowers, but grows fast and hangs on to crops with its many tendrils, twines among them, dragging them down, and bringing problems at harvest. Grows all over the place in the garden.
Grain legumes Peas and beans are grown in the garden some years. They have been grown in the croplands for thousands of years. Both have foliage and grain that are high in nitrogen and vegetable protein, and would probably be much more widely grown if they could be bred to mimic more closely in composition the imported soya bean, now used so extensively as feed for stock and farmed fish. The main cropped bean is the field bean or faba bean Vicia faba which is grown for human food (much of it for export) and animal food. Unlike most of the genus Vicia it stands upright by itself, and looks like the more familiar gardener’s broad bean, the same species, but which tends to have fatter, longer pods and bigger seeds. Beans of the genus Phaseolus are also well known to gardeners in the form of the french bean and runner bean, for example, but they are hardly grown in the north as a field crop. The other main cropped legume is pea Pisum sativum, which grows shorter than the faba, and has tendrils that link plants to give them some cohesive strength. Harvested peas are sold for fresh produce, canning and processing.
N-fixing shrubs Several leguminous shrubs are common throughout the lower cropland and higher grazing land, yet few people appreciate they are nitrogen-fixers. Whin or gorse, two common names for Ulex europeaus is the spiny shrub that you might think would be the last thing that farm animals would eat, but when suitably pulped in a whin mill, was once a nutritious addition to their diet, and gave clearing whins more of a purpose. The plant gives a yellow dye. Broom Cytisus scoparius is about the same size as whin and commonly grows in the same places, but lacks the spininess. There are usually some plants within a 100 m of the garden. Dyer’s broom or dyer’s greenweed Genista tinctoria grows in the dye plants bed. It was raised from seed with some difficulty, but now withstands even severe winters. Its flowers and foliage are similar to those of broom.
Forages – clovers Of the forage legumes, those grown with grass and other plants in hay meadows to be grazed or cut and stored for feed, the clovers are perhaps the best known today. White clover Trifolium repens is one of the hardiest, is still widely sown with grass mixtures and reappears in many arable fields as a hangover from the time when the fields were probably pasture. Red clover Trifolium pratense is larger but less persistent and hardy, and typifies the traditional meadow-clover to many people. The other clover kept in the legume collection is alsike Trifolium hybridum which finds it hard to survive the winter in the garden. Variously, bees like the first two, but if both were in flower together, they preferred the red.
Forages – vetches and vetchlings Plants of the genera Vicia and Lathyrus have been grow as forages at various times. The commonest in the garden, both turning up wherever there is the space, are tufted vetch Vicia cracca, growing into a mass of leaves and tendrils supporting blue strings of flowers, and common vetch Vicia sativa, a small plant not hard to miss. A packet of seed bought as something else turned out to be hungarian vetch Vicia pannonica which grew vigorously for a year, but reappeared for a couple of years. Yellow vetchling Lathyrus pratensis, scrambling with yellow flowers and milk vetch Astragalus glycyphyllos are both grown in the legume collection.
Forages – melilots The white melilot Melilotus albus and the yellow flowered ribbed melilot Melilotus officinalis are both plants of waysides, waste places, railway sidings and dunes, but mainly in the south of Britain and mainland Europe. They grow almost into small shrubs 1 m or more tall, and hold their flowers on upright branches in racemes about 10 cm long. The melilots were tried as forages in the north as recently as the mid-1900s but are no longer found. Bees much preferred white melilot over the yellow. See photographs at Bee plants.
Forages – medics and others Perhaps the main legume forage still grown in parts of Europe is lucerne Medicago sativa, which does well here, 0.5 m tall, winter hardy and vigorous, flowers blue-mauve to deep purple. Others tried in the north and growing in the garden include sainfoin Onobrychis viciifolia winter-hardy with the spikes of pink-red flowers, kidney vetch Anthillis vulneraria with the characteristic ‘ball’ heads, wooly beneath the protruding flowers, and the yellow-flowering common bird’s-foot-trefoil Lotus corniculatus and greater bird’s-foot-trefoil Lotus pedunculatus.
Management in the garden
Several of the legumes are local and found their way into the garden over the years – among which are red and white clovers, common bird’s foot trefoil, hairy tare, common vetch, tufted vetch and yellow vetchling.
These wild, local legumes are not managed in any way. They most likely found a niche as the fertility of the soil declined over the first few years without fertiliser. Once the legumes begin to nodulate, they fix their own nitrogen from the air, and release some of it for other plants.
Grain legumes are raised from bought seed, but not every year. The legume forage collection is kept on raised beds in the west garden. Seed of each species was sourced from various suppliers of natural forages. Sainfoin, alsike clover, greater bird’s-foot-trefoil, black medic, milk vetch, kidney vetch, the two melilots and lucerne were all raised in this way.
Most except the melilots appear to be perennial in this environment, but a few of them die away in the winter and do not return. Kidney vetch and red clover self-seed prolifically.
Legumes vs industrial nitrogen manufacture
The rate at which nitrogen could be supplied to land by legumes began to limit yield as crops were improved, and remained limiting until early in the 1990s when nitrogen that was industrially fixed by the Haber Bosch process began to be made into fertiliser. It was cheaper and less troublesome to buy bags of fertiliser than to grow legumes, and so the areas sown with legumes for grain (peas and beans) and forage (vetches and clovers) declined in Europe.
However, the change from legumes to bags of mineral fertiliser had massive global consequences. Nitrogen fertiliser is now recognised as a major global pollutant of air and water both during the making process and when applied to land. Following restrictions on when and where nitrogen fertiliser can be put on fields, the total nitrogen used has declined, yet it remains very high.
The croplands rely heavily on imports, both of the nitrogen fertiliser itself and nitrogen-rich protein needed to feed farmed animals and fish, which is bought in from the Americas. Without these imports, agriculture as now practiced would collapse. There is therefore a renewed interest in legumes, their products and their contribution as a fertiliser.
Links to related material on the Living Field web
- Fixers 1 – coastal wild legumes
- Kidney vetch and the small blue
- Fixers 2 – restharrow on the dunes
- Bee plants – many legumes support bumble bees