Mr VegHead says...

Vegetable musings from Sea Spring Seeds
29
Nov

Growing legumes, Rhizobia and Nitrogen Fixation

Teepees of climbing french beans and runner beans.

Of all the vegetables growing in my garden, an increasing amount of space is being given over to growing legumes.

Something for nothing

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.

Dwarf French beans var. Cantare.

Dwarf French beans var. Cantare. Legumes provide a year-round feast that the earthy-flavoured French beans in mid-summer and autumn.

 

Sugar snap peas var. Delikett

Super snap peas var. Delikett. Legumes in the garden include the delicate sweetness of spring and early summer peas.

 

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.

Winter tares belong to the legume family and the rhizobium in nodules on their roots will fix nitrogen

When dug into the ground green manures, such as these winter tares, enrich the soil and improve the yields of the crops that follow them.

Red clover has a symbiotic relationship with rhizobium in which nitrogen from the air is incorporated into the soil.

Red clover leaves. Clover is another legume green manure, that will increase the nitrogen content in the soil.

 

 

A team approach – rhizobia and legumes

One of the most important plant nutrient is nitrogen; but though most of the air around us is nitrogen, it is one of the ironies of nature that plants cannot take advantage of this gaseous source directly. Instead, the nitrogen must first be extracted from the air and converted to a usable form in the soil where plant roots can take it in. The process of extraction and conversion – known as nitrogen fixation – is done naturally by soil-inhabiting bacteria called rhizobia. Unfortunately, the rhizobia can fix nitrogen only when they form an intimate relationship with legumes, though once the relationship is established, the bacteria and legumes work as a team, setting up miniature fertilizer factories that can save gardeners considerable money.

To initiate the relationship that leads to nitrogen fixation, rhizobia infect the young 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. Instead, it is complicated by the fact that the 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 a ‘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.

Sowing broad bean seeds

Sowing broad bean seeds

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 you are starting a garden from scratch and find that 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 the roots are covered with nodules, try putting some of their soil in the bottom of the furrow where your beans will go – you just might be able to transfer enough of their rhizobia into your garden to nudge nodulation along.

 

To have or to have not

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.

A young runner bean plant with large nodules on the roots formed by rhizobia bacteria.

The roots of a young runner bean plant with active nodules.

Roots of mature runner beans showing the nodules formed by rhizobia bacteria

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. These roots were on mature runner bean plants pulled up at the end of the season

 

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.

Rhizobium nodules should be cut in half to see how well they are fixing nitrogen.

To see effective nodules are they should be cut in half. If they are pink, then this is a sure sign that nitrogen is being efficiently fixed.

Value for money

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.

 

Pea and bean seeds

To buy seed of peas, French beans and runner beans go to www.seaspringseeds.co.uk