Top tips on growing your own legumes

PUBLISHED: 14:15 10 September 2014 | UPDATED: 14:15 10 September 2014

Michael points out the nodules on the roots of a legume

Michael points out the nodules on the roots of a legume


Growing legumes like peas and beans in your veg plot not only provides a feast for the table, but also a feast for your crops as it enriches the soil with a natural fertiliser

Of all the vegetables growing in my garden, an increasing amount of space is being taken over by legumes. They provide a year-round feast that ranges from the delicate sweetness of spring and early summer peas to the earthy-flavoured French beans of mid-summer and autumn, followed by late-harvested dried beans made into stews and casseroles that lift the spirits during the long nights of winter. Not only is more garden space devoted to vegetable legumes, but I am also learning to appreciate the green manuring value of the forage legumes such as tares and clovers. When dug into the ground, these plants enrich the soil and improve the yields of the crops that follow them.

A team approach

Though most of the air around us is nitrogen (one of the most important plant nutrients), plants cannot exploit this gaseous source directly. Instead, the nitrogen must first be extracted from the air and converted to a form that satisfies the plants’ needs. The process of extraction and conversion – known as nitrogen fixation – is done naturally by soil-inhabiting bacteria called rhizobia. Unfortunately, the rhizobia can’t fix nitrogen by themselves but must form an intimate relationship with the roots of legumes. Once the relationship is established, however, the bacteria and legumes work as a team, setting up miniature fertiliser factories that can save gardeners considerable money.

To initiate the relationship that leads to nitrogen fixation, rhizobia infect the young legume roots as they emerge from germinating seeds, causing the formation of nodules that become their home. As soon as house is set up, a symbiotic swap takes place: the plants supply life-sustaining carbohydrates to the rhizobia, which in turn provide much-needed nitrogen to the plants.

A complicated relationship

The relationship between rhizobia and legumes is not always a compatible one as rhizobia are naturally divided into different groups, each of which will infect some legumes but not others. For example, there is a ‘pea and vetch group’ that infects garden peas, broad beans and tares, while the ‘clover group’ infects red and white clovers. Fortunately, both of these groups are native to British soils, and the legumes infected by them will normally form nodules and fix nitrogen when grown in the garden.

French and runner beans are different matter, and the ‘bean group’ of rhizobia they require is not native to Britain. Being exotic, however, doesn’t mean that they are absent from the soil, and these vegetable stalwarts often become well-nodulated when grown in long-established gardens. It is my guess that the rhizobia responsible for the nodulation accidently came into the garden attached to seed sown in past years. The bacteria population then gradually increased with successive crops until it became high enough to produce plenty of nodules on the roots.

If there is a lack of nodules on either your French or runner beans, it might be worth checking the plants of your gardening neighbours. If their roots are covered with nodules, try putting some of their soil in the bottom of the furrow where your beans will go to help nudge nodulation along.

Despite its importance to food production, nitrogen fixation is a fickle gardening ally that is reduced by drought, soil acidity and cloudy days. Nevertheless, the nitrogen is free and shouldn’t be taken for granted. When the fossil fuel runs out, it may be all that we have left to feed ourselves.

Checking for nitrogen fixation

Gardeners concerned with the efficiency of nitrogen fixation need only to inspect the nodules of their legumes. Carrying out an inspection is a simple matter of carefully digging up some plants and washing the soil from around the roots. First check to see if nodules are present on the roots. Then cut the nodules in half to check the colour - if it is pink, then this is a sure sign that nitrogen is being efficiently fixed. A white colour is another matter and indicates that the roots have been infected by inefficient rhizobia that just aren’t up to the job.


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