Nitrogen fixing myth exploded?

A member sent this article from the website of Marshalls Seeds asking what we thought of it...

04 October 2011 | Posted in Gardening by Martin Harvey

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.

At the danger of boring you all with technical details, here are some further thoughts from the Chairman...

When deciding on fertiliser recommendations for crops, we take plant residues into account, based on previous crop species, soil type and over-winter rainfall. For peas and beans, we tend to assume now that they supply 2 – 2.5 g of nitrogen per square metre, which is less than used to be suggested (5 - 7.5 g per square metre). This has been estimated by looking at nitrogen uptake in the crop following peas (but with no fertilizer applied).

Therefore Chris Beardshaw is only partly right, that they supply less nitrogen than we once thought. However where he is perhaps wrong is to assume that the seeds grab all the nitrogen and there is none left in straw and roots. Typical concentrations of nitrogen in pea seeds on a fresh weight basis are:

Pea straw: 5.0 g of nitrogen per kg

Pea seeds: 12.6 g of nitrogen per kg

So, after harvest, about 30% not 3% of the above ground nitrogen should be present in straw (and 70% in the seeds).

The amount in the roots is not really known. A series of experiments are taking place now, in the field, to try and measure these residues. Experiments are looking during and after crops of peas and beans, with and without haulm ploughed in, but it is too early to report any results.

What nitrogen is present in the roots (in the nodules), is present in organic matter, but can be easily lost over winter: For example on the allotment, where you are likely to lose the nitrogen is if you dig the ground in the autumn, breaking up the soil containing the nodules. Particularly if it is warm and wet in the autumn (which makes the organic matter break down and the nitrogen be converted to nitrate), followed by a wet winter, you lose the nitrogen through nitrate leaching. Then you could lose most of the benefit. In contrast on a heavy soil which is left undug, and cools down quickly in the autumn and you experience a dry winter, you will keep most of the nitrogen in the nodules for the following season.

Having said all of that, the residues left after peas and beans are actually quite small in the grand scheme of things: 2 g of nitrogen per square metre is about 0.1 oz per square yard, equivalent to about 0.4 oz of sulphate of ammonia per square yard. Compare that to the amount of nitrogen recommended for a crop with ‘medium’ requirements for nitrogen like potatoes, lettuce, onions of about 100 oz per square yard as sulphate of ammonia !

So the conclusion is the peas and beans probably don’t supply much nitrogen to the following crop; their benefit is mainly that they can themselves be grown without addition of nitrogen fertilizer. But that doesn’t mean you should ignore their other benefits in terms of providing a break in the rotation so benefitting the following crop, and also adding organic matter to the soil (as long as you don’t madly dig the soil over in the autumn, and again in the spring).