Whenever we read about growing legumes one phrase that always pops up is

legumes fix nitrogen from the air into the soil

but is that necessarily the case?

Legumes fix nitrogen by having nodules on their roots which hold certain specific bacteria, it’s the bacteria which fix the nitrogen and pass it along to the beans in exchange for sugars from the beans. Production of these sugars is an extra burden on the plants resources.

There’s a few scenarios that this phrase often pops up in so I want to put down my ideas about these.

Overwintering Broad Beans/Peas/Green Manures

One thing about winter is that its cold.  Cold and shorter days.  OK that’s two things,  but two things that don’t help plants grow much.

The times when a plant needs the nitrogen is when it’s growing, flowering or fruiting in order to produce the proteins needed. In the short winter days overwintering legumes aren’t going to be making fantastic spurts of growth, if any growth at all and they’re certainly not flowering never mind fruiting.  All this energetic, nitrogen guzzling growth is going to happen in the spring

So if they’re not going to be using up much nitrogen for growth  why go the extra energy of producing the sugars for the bacteria to fix nitrogen – there’s probably enough available in the soil for the growth they do make.

As the temperature drops bacteria often becomes dormant.  In the middle of winter when brass monkeys daren’t go out doors is the bacteria actually active enough to lock in much nitrogen even if the legume is trying to bribe it with sugars.  It may be that both sides decide to have a break and get back to it in spring when they don’t need their thermals on.

The legume can then play the field and grab any loose nitrogen that’s about – locking it up so it doesn’t get washed away.  There’s less competition from weeds and other plants and it doesn’t need loads at this time of year

Leave Legume Roots To Rot As They Contain Nitrogen

You’ll often be told something like

Legume roots add extra nitrogen to the soil when they die

or

Plant your brassicas after your beans as they will benefit fron the extra nitrogen

OK, leaving the roots to break down in the soil is a great way to open up the soil and add organic matter but do legume roots contain that much more nitrogen than the roots of other plants?

All plants will contain some nitrogen in their roots – nitrogen is needed in the proteins that make up the roots  and allow them to do what they do.

The nodules on a legumes root are where the nitrogen fixation occurs – but whats the point of bribing all the bacteria to do the work if you’re just going to leave all that produce sitting in the warehouse going off?

In a growing legume plant the majority of the nitrogen will be in the leaves and stems of the plant. The nitrogen is used to make the proteins that make up the plant structure and the chlorophyll needed to create sugars and in the high protein seeds they produce.   The nodules are the factories and will have some additional nitrogen but I assume that gets transported to the bits of the plants that actually need it asap.

You get the benefit of any extra fixed nitrogen by composting or digging in the rest of the plant so that it all add into the soil in the end.

Do Legumes Actually Fix Nitrogen

Read up a “How To Grow Beans” tutorial, especially a runner bean tutorial, and you’ll be instructed on how to build a bean trench.  This will usually have you digging a trench during winter and filling it full of green waste, kitchen waste or manure so that there is added water retaining organic matter as beans love moisture.

That’s fine but these also happen to be the things used to add nitrogen to a compost pile so that when you do plant your beans you end up planting them into a trench of nitrogen rich compost.

If there is an abundance in the soil of nitrogen then the plants can just use this nitrogen without having to spend extra resources on manufacturing the sugars to feed the bacteria. If there isn’t any competition for the available nitrogen then they may not need to fix their own.

Is This The Case

OK, i don’t know if the above scenarios are whats happen but I think that they are as likely, or even more likely, than the bean will always produce sugars to feed the bacteria who fix the nitrogen, even when they can get it elsewhere or they don’t need it.

Of course in the long run it doesn’t matter if legumes fix nitrogen or not so long as there’s enough nitrogen available for whatever you grow in the soil and compost you add.