No dig, single dig, double dig, triple dig?

Gardeners do not go out of their way to make work for themselves and digging is hard work.  So why do I still do it?

It is said in various publications that it takes considerable time, maybe 500 years, for just a centimeter of soil to be formed naturally.

Well, I haven’t the time to wait.

My allotment soil.
Growing very healthy grazing rye and tares.


I must admit that my evidence in favour of digging is anecdotal mainly because I have found little scientific evidence one way or other. However, when allotments are first taken on they are usually in a terrible state and need clearing by digging.


I have taken down the 6 foot net frame and triple dug the soil.

Before I took over the allotment.
I challenge any no dig advocate to turn an allotment round that has been inundated with mare's tail, bindweed and couch grass and produce higher yields than a double dug allotment  in the same condition.  
Couch Grass, nettles, bindweed and mare's tail. 

And I would give them as many years as they want to do it.  However, I would expect the double dug allotment to produce good yields in the same season as it was dug.  Now that is an allotment challenge BBC 2.



I put a greenhouse where the net frame was.
The only way that I could clear the ground was to dig out the weed rhizomes by triple digging.

Cleared the top beds in December 2014.


I think that it is impractical to suggest that gardeners could avoid digging because we have to dig when cropping such things as potatoes, carrots, parsnips and other vegetables.

Digging out potatoes

Also, adding carbon to the soil in the form of manure, compost, charcoal etc. will also involve digging.

Organic matter - manor from heaven... All buried during
triple digging

Muck and shreddings. All to be buried.  


Furthermore, when you grow green manures you have to dig them in when you need the land for crops.

Green manure

(Having said this, you can cover the green manure with cardboard and black plastic then remove the coverings when the green manure has died back to allow mulching.)

We must take into account that we may well be destroying a community of soil invertibrates that contribute greatly to the fertility of the soil and try not to dig, especially when a quick fork over, mulching or hoeing would suffice.


Many modern, vegetable garden techniques are based on those of British, Victorian, walled kitchen gardens where gardeners developed high level skills to grow exotic plants in a difficult, temperate climate. They had to provide high quality year round vegetables and fruit for the house. As labour was relatively cheap, the Victorian gardeners dug their soil deeply incorporating large amounts of organic matter, mainly in the form of horse manure.  In the BBC’s “The Victorian Garden” Peter Thoday dug down to expose over a metre of good, friable top soil in a walled garden, whereas there was a mere 30 centimeters under pasture outside the walls.


Product Details


Double and triple digging seemed to work in an English climate and produced high yields of vegetables and fruit.  This does not mean that it is a generic technique that will work anywhere in the world.
 
It will not. 

I am lucky living in England with a climate that has cool summers and relatively mild winters.  I can use the techniques of the Victorian walled gardeners without worrying about my soil life or structure.   You don’t have to double and triple dig like the Victorian gardeners but it is a great way of deepening your top soil; enhancing drainage; preventing leaching; increasing the soil’s organic matter which also increases the diversity of the living things in the soil; removing deep growing pernicious weed rhizomes; and allowing you to mix organic matter into the topsoil to make nutrients available to plants wherever they are planted.  It grows big pumpkins too.

2014 pumpkins Dills Atlantic Giant.



A homogeneous mixed soil with a lowish bulk density and a raised porosity produced with additions of organic matter and grit seems to produce the greatest harvests of annual crops.

Various additions to the soil should be really well mixed by sieving. Sieving also produces a very friable soil. There is now a good mixture of manure, top soil and compost fairly evenly mixed throughout the soil profile of my allotment to a depth of about 600 mm. Homogeneity of the soil mixture means that there are no high concentrations of manure in some places while other parts of the soil profile have little or none. There seems to be more benefit in producing a homogeneous soil, where all plants have equal access to added nutrients and organic matter, than having a patchwork of high and low concentrations of nutrient. Manure added in the top 300mm of soil is most readily taken up by plant roots so this is where the manure should be mixed in. However, I am trying to encourage mychorrhizal symbiosis and the fungi will be able to access nutrients below the 300mm depth and be able to transport them to plant roots.

Digging can be done in winter when there seems to be a drop in the populations of soil invertebrates.  Furthermore, herbaceous weeds like bindweed die back in winter so that it is easier to remove their underground rhizomes.

Commercial growing media is mixed by sieving to produce a homogeneous mixture. We do not grow in layered ‘composts’.  We do not need to grow in layered soil. A good garden soil would be one that is homogeneous.  The fertility and organic content should be evenly distributed throughout the profile where it will be equally available to plant roots wherever they are.  This will prevent the roots from bunching around improved soil in the planting hole and not venturing out into surrounding soil. 


Triple digging with a four foot trench.
January 2015. Topsoil on the left and subsoil
on the right. I try to keep the top and subsoil
separate remembering to replace the subsoil first.
The good thing about double and triple digging is that you can bury a lot of organic matter and mix it in to make very friable subsoil.    I use a kind of Hugelkultur adding logs or processed wood together with woody chippings to the bottom of a triple dug trench mixing them into the clay like subsoil with a garden fork. 

Shredded brushwood added to the subsoil.

As the wood slowly rots it forms a sponge like structure that absorbs water when the ground is saturated but will slowly release it when the soil starts to dry.  So the hugelkultur is a water buffering system that will provide water to plants when there is little rainfall.  Adding the woody chippings increases the porosity of the subsoil and reduces the bulk density.  This allows excess water to drain away and also allows deep roots of plants to penetrate into the subsoil to obtain water and nutrients they would otherwise be unable to reach. 
Soil put back in the trench.  
Although the scientific research is confusing, it is said that adding woody material to soil will lock up nitrogen, an essential plant nutrient.  In order to decompose wood, fungi and bacteria need nitrogen to make the protein of their bodies.  Nitrogen is scavenged from the wood to start with but this is either quickly exploited or is locked away within the structure of the wood.  The fungi and bacteria decomposers then turn to nitrogen in the soil and lock it into their bodies depleting the soil of nitrogen.  This nitrogen is released to the soil again when the decomposers die and is not locked away till the end of time. 

The problem is the solution; if we add woody chippings to the subsoil the nitrogen that is available to decomposers has been leached from the top soil.  As the nitrogen passes through the woody chippings it is captured by the decomposers and incorporated into their bodies.  The woody chippings in the subsoil become a trap for nutrients preventing them from being washed out of the soil and leached away. Any nutrient leaching from mulches, organic fertilisers and manure added to the surface will be captured by fungi and bacteria decomposing the wood and will be recycled into the top soil by deep digging.

Decomposition of wood is very slow.  Locking up carbon in the soil making it a carbon sink.

There is also a suggestion that ploughing and digging increase the decomposition of carbon in the soil adding to the carbon dioxide and methane load of the atmosphere. This is probably correct. After the last ice age vast forests covered Britain for thousands of years producing copious amounts of organic matter. The soil should be saturated with it, however there is little sign of ancient carbon in my allotment soil so where has it all gone.  Farming and gardening increase the amount of air in the soil encouraging aerobic microorganisms that will decompose organic matter relatively quickly. Decomposition produces carbon dioxide and this passes into the atmosphere.  So the forests have 'evaporated' into the air.

I cannot believe that we can separate aerobic and anaerobic decomposition of dead organic matter in natural systems.  Anaerobic conditions will form in any decomposing organic matter particularly in compost heaps due to rapid respiration and production of carbon dioxide.  Compaction and excess water will also lead to anaerobic conditions - even in the best of compost heaps and there are not too many of those.    You would have to make superhuman efforts to produce a compost heap that is populated entirely with aerobic bacteria.  In  anaerobic decomposition process, some carbon is converted to methane CH4 or other small molecule carbon compounds.  Methane itself is a molecule that is used by organisms to give them a source of carbon for both catabolism and anabolism leading eventually to carbon dioxide the final gas of respiration.

Unfortunately some of these processes involve the production of hydrogen sulphide and other sulphur compounds, which have very characteristic smells. An archaeon is implicated in the breakdown of methane by sulphate reducing bacteria, and leads to the characteristic smell of hydrogen sulphide or rotten eggs that is sometimes associated with the breakdown of  organic matter. Methane is an odourless gas - in other words it does not smell.  

Regardless of almost everything, when something decays the process is catalyzed by microorganisms and the formula is:
     CxHy+(x+y/4)O2=xCO2+(y/2)H2O
Of course the process of decay is a lot more complicated than this and the contribution of microorganisms like archaea is not fully understood at the moment.

However, the final product of composting, everything else considered, is carbon dioxide and water.  A proportion of the carbon will be diverted for a time into the bodies of bacteria, fungi, plants and animals but eventually, lastly and finally we all end up as carbon dioxide and water – with a relatively small amount of iron, calcium, phosphorus etc. 

This is exactly the opposite of photosynthesis where carbon dioxide and water are combined with the help of sunlight energy to produce carbohydrates. 

There are several pathways for the carbon to go depending on the microorganisms that it encounters.  If anoxic conditions occur – and they very often do, then methanogenic bacteria will use organic molecules to produce methane.  They are strictly obligate anaerobes, which are poisoned by the presence of oxygen levels as low as 0.18 mg/L of molecular oxygen. 

2C organic+2H2O=CO2+CH4

Composting, it is suggested, when done well constitutes an aerobic environment where methanogenic bacteria will not be able to live. Let me tell you a secret, methanogenic bacteria are ubiquitous.  They are everywhere and what is more; anoxic environments are more common than you expect.  Obligate anaerobic bacteria can happily live in the plaque on your teeth.  Micro environments in compost heaps will be anoxic.  Methane is produced. Get over it.

So why are we not overwhelmed by methane?  Why is there so little in the atmosphere?  

Possibly because there are another set of bacteria called methanotrophs or methanophiles that are able to use methane as a source of both carbon and energy.  What is more they can grow aerobically like Methylococcus capsulatus or anaerobically, which means that regardless of where the methane is formed (in the compost heap or deep in the ground after I have buried logs and brushwood) these bacteria can metabolize methane incorporating the carbon into their bodies or producing carbon dioxide and water in energy production.  

Admittedly, some like Methylomirabilis oxyfera reduce nitrate to nitrogen with the help of other microbes and so contribute to nitrogen loss from the soil and this might be a consideration that we need to think about. As these microorganisms are ubiquitous and already present in the soil, we are just increasing their population.   This will also increase the population and diversity of heterotrophs which feed on them.  I would suggest that regardless of the disadvantage of some nitrogen loss, increasing the diversity of microorganisms can only strengthen the food web within the soil.  

The difference between the relatively aerobic conditions of a compost heap (I still maintain that even the best of compost heaps are anoxic in many places) and the relatively anoxic conditions of buried vegetation is the speed at which methane can be metabolized.  As methane use by methanotrophs is a relatively slow process in anoxic environments there could be a buildup of methane but my contention is that I bury comparatively small amounts of organic matter and that the soil methanotrophs can deal with it fairly easily capturing some of the carbon to form their bodies and converting the rest to carbon dioxide.  Burying the organic matter deeply means that any methane produced has to pass through a large volume of soil before reaching the surface and this increases the chance of it encountering bacteria that can deal with it.

In land fill sites there is a vast amount more organic matter compared to my allotment.  This has been demonstrated to overwhelm the methanotrophs and the production of methane is very evident. 

So I argue that burying organic matter will produce no more methane than a compost heap would. If some of the carbon buried in whatever form is prevented from quickly decaying, possibly it could become a carbon sink. There is some evidence of carbon staying in the soil for considerable amounts of time. 

Remember, burning just bypasses all of these processes and goes straight to the greenhouse gasses of water vapour and carbon dioxide without the opportunity for carbon capture within the bodies of soil organisms.

Most organic or permaculture systems make topsoil from the top by adding mulches.  With hugelkultur you are making topsoil from the bottom.  I do both - making soil both from the top and the bottom.  The idea for my allotment garden is to increase the deep, productive volume of the soil while also improving the drainage.  I want the ground to be damp throughout the growing season and to allow excess water to drain away during the wettest months.  Both Hugelkultur and mulching seem to do this very effectively.  Also the slow decomposition of big pieces of wood be they processed or logs, releases nutrients locked up by trees and allow it to be recycled over very long periods of time.

Furthermore, increasing the level of oxygen in the soil by digging will reduce methane production by allowing aerobic decomposition in the same way as turning a compost heap does.  Adding carbon reduces available soil nitrogen.  Adding nitrogen reduces soil carbon.  Adding air reduces both carbon and nitrogen because it increases the population of aerobic microorganisms that quickly decompose buried organic matter incorporating it into their bodies.  We can see this in the decomposition of fen land peat in East Anglia where the depth of peat has been reduced considerably through draining and allowing air to penetrate deeply where water previously occupied the soil pores.  Roots need oxygen to respire. Oxygen in the soil will rise and fall with water levels which draws in oxygen as its level falls and expels carbon dioxide into air when it rises.  Adding copious amounts of organic matter and increasing the porosity of the soil by deep digging allows this ebb and flow of oxygen to the roots to occur much more efficiently.   

If you look at the video "A farm for the future":

http://transitionstourbridge.co.uk/2010/01/05/film-a-farm-for-the-future/

there is some anecdotal evidence that cultivation seems to lower the number of beneficial micro organisms in the soil.

However, if this is true for soils why is it so different for compost, which we are supposed to turn every two weeks.  If turning compost heaps does not reduce the numbers of microorganisms then how is turning the soil different, especially as this is usually done only once a season?  Also, adding air to compost means that aerobic microorganisms can grow and replicate and this is seen as a good thing. So if we turn the soil adding air why does this not increase the population of 'good' microorganisms?

You can't have it both ways surely?

Digging does not sterilize the soil.

Most microorganisms are found in the rhizosphere around the roots of plants.  This is the area with the highest organic mater content because plants are always secreting  molecules into the soil which can be used by microorganisms.  Except where there is buried organic matter, the rest of the soil has a relatively low population and diversity of microorganisms.  Although I agree that digging reduces the population of beneficial soil organisms, digging in lots of organic matter will help to increase the the numbers again.

Even though they say that mychorrhizal fungi are ubiquitous in undisturbed environments, I always add commercial mychorrhizal fungi when planting or sowing seed.  Digging, I would suggest, does damage fungal hyphae and reduce their numbers in garden soil.

Digging in compost buries most beneficial microorganisms rather than leaving them on the surface where they can contribute to the soil's diversity.  However, using both digging then mulching with well rotted compost should give us the benefit of both.

It is said that digging is not a 'natural' process. It would not happen without the intervention of humans. Boars, pigs, chickens (chicken tractors), badgers, rabbits, rats and mice all turn  over the soil to greater or lesser extent.  Moles turn over large areas of soil as they make their way through the soil.  

Walking on the soil to dig leads to compaction and loss of soil structure.  While I agree that soil compaction should be avoided so that plant roots are not impeded and can penetrate throughout the soil profile,  plants root hairs need an intimate contact with soil particles for ion exchange and water absorption particularly in germination.  That is why Harry Dodson 'consolidates' the soil in the BBC's “The Victorian Garden” (2006).

A gardening philosophy that suggests that soil should not be walked on because it destroys its structure just does not ring true.  I was watching "The Victorian Kitchen Garden" for May on YouTube and the gardeners there were scuffing the ground along the drill lines to consolidate it before raking and making a seed bed.  This is what I was taught to do when I was a lad. It breaks down the large lumps of soil and allows you to make a really good tilth with the rake.

When I was learning how to grow from my grandfather and father they always firmed the soil in seed beds by treading along the rows and raking afterwards.  In this way a really fine tilth could be made fairly quickly.  I still tread along my seed lines to firm the soil before a final rake and taking out the seed drills.  This technique does not seem to prevent the seeds from germinating.

I tread between the rows to hoe and cultivate along the plants.  Removing weeds and scuffing up the soil to slow evaporation seems to be much more important in producing good crops than avoiding compaction.

There are times that I avoid walking on the soil.  When the soil is very wet it can be easily compacted and will form an impenetrable cap on the top soil.  Otherwise I am constantly walking over the soil.  I do usually scuff it up afterwards but that is just to make the soil look presentable.

As far as compaction is concerned - its a good job we don't have dinosaurs any more...

Some commentators say that digging will bring dormant seeds to the surface of the soil; their dormancy will be broken and germination will ensue.  I have dug and not dug and regardless have about the same amount of weeds whatever  I do.  The survival strategy of ephemeral plants is to quickly colonise bare ground by various dispersal mechanisms.  Senecio  vulgaris and Taraxacum officinalis  both have very effective clypsela which are wind blown particularly onto my allotment and I am having to clear them off continuously.  Weed seeds are ubiquitous throughout the soil profile and you will get seed germinating whether you cover with mulches, cover with weed suppressant membrane or concrete.
 
Putting mulches on the soil surface replicates nature's soil formation and probably takes as much time.  Having said this, I still add organic matter to the surface of soils or dig it into the top 15 cm.  In this way, I build soil from the top and allow soil animals to slowly – very slowly - mix it with the top soil. 

Adding organic mulches increases the diversity of fungi, bacteria and invertebrates within the top 15 cm of the soil and increases productivity probably because the microorganisms in compost offers competition to pathogenic soil organisms. However, mulches also attracts pests like slugs and snails.

Mulches help to prevent capping by preventing the rain from hitting the soil surface.  Capping is when the soil is compacted so that water runs off its surface rather than soaking in.  This job can also be performed by a cover crop like grazing rye or tares. 

I single, double and triple dig to build topsoil from the bottom, however I also add compost and mulches to the surface.  The two methods are not mutually exclusive.


2 comments:

  1. what kind of wood chippings do you use or recommend please?

    ReplyDelete
    Replies
    1. Hi pendipidy,
      When I put wood chippings into the subsoil, it does not really matter the size or the origin of the chippings. I bury lots of different kinds of organic matter into the subsoil from cardboard to cotton. It will all decompose and contribute to the soil organic matter.

      If I am mulching with wood chippings then I would rather use broad leaved chippings than conifer, however beggars can't be choosers so I use whatever I can get hold off.

      I do not add chippings to topsoil unless it has been composted for at least a year and even then I add a high nitrogen manure such as chicken or pigeon to make sure there is no nitrogen depletion.

      Delete