I don't know how they do this unless they surreptitiously creep in and spread artificial fertiliser around. It may, however, suggest that soil is more resilient than we imagine and can deal with human interference better than we expect.
Reason would suggest that you have to replace what you take from the soil. However, plants take very small amounts of nutrient from the soil. A great deal of which is replaced by the weathering of rock and stone already in the soil.
Also, plants don't want to die. They don't die just to spite us. They will make great efforts and struggle to survive wherever they grow.
This is why I take little credit for the vegetables that grow on my allotment. They do all the growing; I just try to make it as easy for them as possible.
So what is my point? There are some pretty amazing misconceptions out there but most of them are harmless and amusing. Plants grow regardless.
Does it really matter if we put egg shells on our compost?
I don't think so.
Whether they decompose or just fall apart because of weathering does not really impinge on my gardening at all.
However Dowding doesn't seem to understand the role of nitrogen fixing bacteria in legume root nodules. A slightly more important misconception, that does affect my gardening. He quotes Chris Beardshaw on "Gardeners' Question Time." when he says that most of the nitrogen fixed by bacteria in the root nodules passes to the stems, leaves and fruit. I doubt very much whether just 3% of the original fix of nitrogen remains in the roots because I have read that up to 40% does. However, I'll go with the 3% because it is not relevant.
Dowding says that to get an increase in soil nitrogen you have to dig in the stems when they are in full leaf. Well yes. Even though I was keeping a lot of fruit for seed, I still dug in the beans when they were in full leaf. They were in full leaf until almost the end of November. Even when they are caught by the frost, the leaves and stems and consequently the nitrogen are still there and can be dug in blackened. I don't see the problem. Nitrogen does not flaunt the law of conservation of matter. It does not disappear. If all the leaves fall from the stems onto the soil surface they will add their nitrogen to the soil eventually. Even if you leave the plants till the end of the winter and they are brown and crinkly, they will still contain some fixed nitrogen and can contribute to adding extra nitrogen to the soil.
The problem for others is that the root or the root nodules are seen as the only source of nitrogen that beans plants contribute. I hope to explain why this is a misconception.
I questioned this on one website and they said that they were trying to simplify nitrogen fixation for general gardeners. If we are going to try to overcome misconceptions then we have to tell people exactly what is happening based on evidence and data. And anyway who decides whether something is difficult or easy? Is there a scale of difficultness. If so what are the units?
|All these bean plants have been dug into the|
soil to add a small amount of fixed nitrogen.
It is true that a lot of nitrogen passes into bean seeds (and this is where vegetarians get some of their protein from.) Indeed all plants pass nitrogen, as part of protein, into their seeds. (That's why vegetarians eat lots of nuts. )
Why am I talking about proteins rather than nitrogen? The element nitrogen is incorporated as part of protein molecules almost immediately it has been fixed by nodule bacteria like Rhizobium leguminosarum or Rhizobium gallicum and passes into the plant. These proteins are then used throughout the plant for a vast number of different jobs. So to say that all or even most fixed nitrogen is passed to the seeds or stays in the root nodules seems hard to believe. Even if only a small amount of nitrogen is left as protein in the main structure of the plant, you are still getting an increase of nitrogen that can be incorporated into the soil if you dig in the bean plants at the end of the season.
If you cut the tops off beans and burn them then the nitrogen will be lost to the atmosphere as nitrogen oxide gases. At the very least the tops of beans and peas should be composted for their fixed nitrogen.
However, why not just dig the whole plant in?
|Tall climbing pea plants have been dug into the |
soil to add nitrogen.
Everything helps, so dig in the whole legume plant. Why wouldn't you? Maybe because the stems are a little woody and this might lead to nitrogen immobilisation? Possibly, but the jury is definitely out about how much woody chippings cause immobilisation so a few bean tops, which in my anecdotal experience rot down relatively quickly, are not going to cause a problem. Also legume green manures, such as vetch, tare, fenugreek and field bean, are dug into the soil, tops and all. So why not beans and peas?
|Green manure of tares, clover and rye grass|
Once the legume plant dies, the Rhizobium bacteria pass back into the soil where they can survive but do not fix nitrogen. The nitrogen contribution that bean roots make comes from decomposition of the proteins that make them up.
|Sweet peas were used as a break crop and |
dug in as a legume nitrogen fixer.
There are other free living nitrogen fixing bacteria but they are not thought to make a very significant contribution to total soil nitrogen.
A misconception I had was that legumes could only introduce nitrogen into the soil when they were dug in. However, as with other plants, the turnover of their roots is quite significant. They are loosing material from their root tips continually and root hairs only have a very short life. This is why lawns with clover growing in them look much healthier and a darker green than those that don't. Indeed, Abercrombe suggests that during the eighteenth century the lawns of the super rich Victorians were sown with a grass clover mix.
The nitrogen is coming from the death and decomposition of proteins in roots, root exudates and sheared off cells. Not necessarily from root nodules which only contain bacteria. The only way that root nodules would contribute to soil nitrogen is if they were seared off or died and the protein within them decomposed. They are not little bags of nitrogen as Dowding seems to suggest.
Dowding's objection to green manures really stems from the fact that most gardeners will dig them in and he doesn't like digging. Well actually you don't necessarily need to dig them in or compost the tops as he suggests. If you lay cardboard and chippings over them, maybe tarpaulins or black plastic or even as a last resort carpets, green manure will decompose under the soil covers but above the soil. The soil does not have to be disturbed and you get the benefits of the nitrogen fixation and added organic matter.
So this is why I have planted legumes Laburnum anagyroides and Lupinus x Russel Hybrids at the top of the allotment slope. I am hoping that any fixed nitrogen that is produced by the decomposition of root proteins will be taken by mass flow of water through the allotment soil thus making it available for the vegetables down slope.
Which takes us to another misconception: the danger of Laburnum spp.. They are not that poisonous and are no more poisonous than sweet peas, cherry laurel, pyracantha, and even French beans ( Phaseolus vulgaris). A hysteria about laburnum in the UK means that they have been rooted out of all UK school gardens. Have a look at the Youtube film:
I grow laburnum, sweet pea and lupin on the allotment as nitrogen fixers. I just don't eat them.