Saturday, 5 December 2015

More myths and legends: misconceptions in gardening

I have seen people take off weeds together with top soil, bag it up and take it to the local municipal waste disposal site.  They dig but do not add anything to the soil and plant and sow into what I would imagine is very unpromising soil.  However, they seem to get a really good crop of vegetables.  And this continues over several years. 

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.
-everything helps
I have also seen the misconception that all of the nitrogen produced by nitrogen fixing bacteria passes into the seeds of legumes.  This would wrongly suggest that digging in the roots and stems of say runner beans would not necessarily introduce nitrogen into the soil. 

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 plantThese 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.
Some books say just dig in the roots.  The roots contain Rhizobium filled nodules.  The usefulness of nitrogen, as a basic building block of proteins, would mean that it would be transported away from the nodules as quickly as possible.  Thus there would be no more nitrogen in the nodules than there would be in the rest of the plant.  The increase in fixed nitrogen is distributed as protein throughout the plant.  The farmer does not mow off the leaves of clover and just plough in the roots.

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.


  1. I agree that our British soils are very resilient to the tortures that many non gardeners submit them to but not that there is sufficient nutrient replacement on most from weathering of rock. Perhaps from crushed granite but a normal soil….
    Not so sure about the laburnum either although I do agree about the stupidity of laburnum removal in school gardens - what has our society come to?
    I had a client who put out the children's pet rabbits on the lawn in a bottomless cage to play. Unfortunately laburnum seed lay on the grass. Sadly the rabbits died
    As to the fate of the nitrogen from bacterial fixation. What confusion you have uncovered. I cannot see that it matters whether it is recycled from the decayed protein of the plant whether or not it is left on the surface or dug in. Also the whole plant should be recycled. The only nitrogen that might be lost is the actual mineral nitrate when leaching occurs. I find your strategy of letting soluble nitrate wash down the slope very interesting - even imaginative!

  2. Hi Roger,
    Thanks for the comment. I have to confess that the strategy of nutrients washing down a slope is not mine. It is one of the strategies that permaculture suggests. Permaculture is not a hippy, new age, air brained idea.

    Permaculture is based on some very solid foundations and a lot of science.

    I have found it very easy to incorporate their basic principals into my own gardening without any effort at all. I like its self sustaining methods and its inclusive nature. If it works then do it more.

    I will be using permaculture in my new flower garden planting legumes like wisteria at the top of slope. There is not much slope in the new garden but I am told it still works. I would love to try all the different techniques in a small holding of my own but I don't think that that will happen.
    All the best,

    1. Right the rabbits. There is a correlation between the rabbits and the laburnum seeds - the were both in the same place. However, that does not mean there is cause and effect. Were the rabbits ill beforehand? What were the living conditions like? Were they looked after properly? How long had they had the rabbits? Where they experienced in rabbit husbandry? Did they have enough water and food? Were they recently bought and had an infection from other rabbits they had been housed with? Were there any other poisonous plants around? Was it a particularly cold or warm? Was there any other evidence of infection? Were there any sweet peas growing nearby?
      To assume that the cause of their death was the effect of eating the laburnum seeds may be a little premature.