The Nitrogen Fixing Myth Exploded

04 October 2011 | Posted in Gardening by The Marshalls Family

Here at Marshalls, we’ve always advocated leaving pea and bean plants in the ground, even over winter to provide green manure, add nitrogen and improve soil. However, last Friday, Chris Beardshaw on BBC, debunked the long-standing myth about nitrogen-fixing pea shoots, consigning it to nothing more than an old wives’ tale.

It seems that, according to new research, that yes, the Nitrogen fixing plants, or legumes which include all peas and beans, work together with nitrogen fixing bacteria called Rhizobia, to "fix" nitrogen.

The Rhizobia chemically convert nitrogen from the atmosphere to make it available the plants.

It’s a symbiotic relationship between Legume plants and the nitrogen fixing bacteria - the Rhizobia living in the plants’ root nodules, looking after their nitrogen needs. So, zero or minimal nitrogen fertiliser is not required.

In addition, we always believed that once the crop is harvested and the plant cut back to ground level, the root nodules should release all the valuable fixed nitrogen for following crops.

What Chris Beardshaw pronounced on Gardeners’ Question Time is this – the nodules do indeed fix nitrogen – BUT – once plants flower that nitrogen is used by the plant leaving only 3% of the total pre-flowering level by the time harvesting is complete. So, perhaps it is no surprise that Chris for one will not be leaving his pea and bean roots in the garden this winter.

For me – I’m leaving my bean plants in this autumn to add organic matter and wait and listen to the arguments in the gardening press these next few weeks.


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