Vicia fabia + N = mmm, not quite sure

Looking at my wind-ravaged field beans today, planted as green manure, got me thinking about nitrogen. When I last had a garden I lived in the country and had access to as much farmyard manure as I wanted. I, and I think most gardeners then and many now, regarded an annual load of manure as essential to maintaining fertility on the vegetable plot. But I realise now that that fertility came very easily and no-one thought of introducing nutrients, particularly nitrogen, in other ways. Yes, there was the compost heap but in a way it was looked as a way of disposing of material rather than a valuable resource in its own right. Even dedicated organic growers like myself, while totally committed to not using pesticides and artificials, were perhaps better at avoiding certain practices as opposed to actively pursuing strategies to build soil fertility.

Now that I am gardening in an urban allotment, farmyard manure is difficult to source and the logistics and cost of carting to the site make one question its necessity. But if you start looking at alternatives such as green manures, questions arise. For example, just how much nitrogen can be fixed by a leguminous crop? Of course, many variables affect that figure – variety, existing soil characteristics, weather and so on. For a crop such as field beans estimates vary widely from 40 to 300 kg/hectare (37-275 lbs/acre). On these figures my raised bed, which has an area of 7 sq. mtrs would generate a net nitrogen gain of .03 to .21kg (.06-.46 lb).

Of course, as each leguminous crop is associated with a specific symbiotic nitrogen-fixing bacterium, commercial growers inoculate theirs with the appropriate organism to ensure that there is a happy productive partnership (mine had to look for chance compatible partners); and estimates of nitrogen fixation will be based on more substantial growth than mine on this showing. But ignoring these minor points for the moment, how much manure would I have to apply to supply the same amount of nitrogen? Again, wide variations in estimated nitrogen content, specifically available nitrogen content, make this a very rough exercise but assuming available nitrogen at 2% would require about 10kg of farmyard manure on the bed to give nitrogen equal to the high end of the range – at about 3lb/sq. mtr. which seems roughly comparable to what we used to dig in. Non-organic farmers using artificial fertilizer in liquid or granule form spread at the equivalent of about 150lbs of nitrogen per acre.

Although I may have nitrogen levels generated by blind-date symbionts and small plants, on the other hand the farmyard manure is much more susceptible to nitrogen loss through leaching and gaseous ammonia loss; the nitrogen in my nodules is nicely protected and in plant-available form so crude figures have to be balanced by such considerations.

None of this adds up to science; it’s more just a musing. But it is a start in thinking that I will set out to maintain fertility in the allotment without the help of animal manure. I’m not a vegetarian but with manure difficult to get and then possibly tainted by vermicides, the herbicide Aminopyralid, or antibiotics, why not find other ways? To add to green manures, I still have comfrey, compost teas, foliar feeding and urine to add to the mix, so to speak, and of which more later.