Sunday, 20 November 2011

Growing medium, compost or just plain soil?

I had an interesting conversation at the weekend, which I am still mulling over.

Most of the commercial seed and potting 'composts' sold nowadays do not have compost in them.  Most of them are peat mixes with such things as perlite, vermiculite, sand and wetting agents like celcote.

Not much compost in them.  So should they be called composts?  I don't think so.

These are growing mediums.    They are sterile, open and water retaining so what is the problem.  Although we might be focusing on growing and gardening, it does not negate consideration of our actions wider implications.  One consideration is the destruction of  peatland habitat by peat extraction.  Maybe it is better to avoid using peat if possible.

Garden composts use sustainable materials and need less transport than growing media.  It is also more fun to make  your  own compost as  we did in the olden days.

I have a very large growing area and covering this with bought growing media is just not viable.  The cost would be astronomical.  Using the two compost bins, I can generate quite a lot of compost and dig it into several of the  beds.  I supplement this with liquid comfrey; a little chicken manure; leaves; blood fish and bone and the free horse manure that is delivered to the allotments.

This has produced some fairly good vegetables for over thirty years now. 
Emptied compost heaps 

Compost heaps filled again

Compost heap growing potatoes and pumpkins.  

Compost nearly ready to go onto the allotment.

This compost  is ready to be used.

Sieved compost being added to the top soil.

I used to make seed and potting composts at the Glasshouse Crops Research Institute.  This was before they developed their peat based growing mediums.

In the olden days soil or old turf was sterilised in large steam boxes. And they were big boxes.  My first job in the glasshouses was to fill and empty the boxes.  The soil was emptied sieved and heaped into a conical pile on the floor of a large barn after sterilisation.  It being the olden days and not knowing any better, peat and inorganic fertilisers were added and mixed  in by shovelling soil from the bottom of the pile to the top. And they were big piles.

When we had done this about ten times the soil was passed through a shredder which threw the soil into a concrete bay.  

Not too sure whether this was compost either.

I grew some really good tomatoes using just the sieved compost from Fred's Mega Compost:

Maybe this is the way to go?  However, I need a steam sterilizer.

Having always gardened using the original soil, I find it very difficult to accept that developing a vegetable garden with growing medium is the right way to go.  I want to garden with the least cost both  financially and environmentally.  I don't want to spend lots of money on something that I can produce for nothing.  Covering my allotment with commercial growing media  is just not a feasible option; it is just too big.

There are another three beds beyond the sweet pea canes
Although there are many types of different growing media and composts, it seems ridiculous to ignore the most valuable resource which an allotment or garden has to offer.   A good well made soil.  

 Good soil  with lots of  well rotted organic matter

Soil planted with green manure.  

Undoubtedly, soil preparation and improvement is a major part of success in the vegetable garden. It takes nature about 1000 years to produce 20 mm of soil .  However, adding dead organic matter to the soil and artificially weathering it by digging or rotavating, we can of course increase the amount of top soil available for cultivation quite considerably.

Anthropogenic soil improvement by adding amendments will change the soil relatively quickly, although my soil is only just becoming beautifully friable and highly productive after thirty years of adding organic matter.  I would never suggest that pedogenesis is an easy or quick process that can happen overnight but there are things that gardeners can do to improve the soil for cultivation of vegetables and flowers.

It is difficult to think in three dimensions when considering the environments in which plants live.  The environment of the root is very different to that of the plant's aerial organs.  It is easy to observe the parts of the plant that are above the surface of the soil but underground organs are more of a mystery.  This environment can be studied and analysed for its structure, texture and nutrient content but this does not begin to picture the whole amazing dynamic that is called the soil. 
The soil is an amazing environment that is inhabited by vast numbers of organisms.  In order to survive they must interact with each other competing and cooperating in a dance that allows reproduction to be ensured and the continuation of the species. 
While foraging for water and nutrients, the roots of plants must find methods of protecting themselves from pathogens whilst interacting with symbiotic and helpful organisms.  Although the majority of the soil has a sparse population of microorganisms, there is much activity around the roots of plants and wherever dead organic matter can be found.  Thus a homologous soil with organic matter evenly distributed throughout its three dimensional space will be one with the most active organic life and the best environment for plant roots. 
The natural soil profile is one of layers; an organic layer, a top soil layer, a subsoil layer and the bedrock from which the soil is formed.  It relies on soil organisms to mix the constituents and this is probably why it takes over 1000 years to produce 20 mm of good fertile soil.   However , this is not the totality of types of soil profile and where soil has been mixed as in river silt or settled from dust filled air from a volcano the fertility and organic content can be fairly high throughout the profile. Digging and other forms of cultivation just speed things up a little.  There is no virtue in layering the soil and waiting for nature to mix it for you.  It will take 1000 years...

The modern fashion for using raised beds to grow vegetables can be seen as a method of avoiding digging.  The no dig system seems to have been developed, like so many other gardening techniques by trial and error.  Afterwards  the amateurs that developed it become evangelical and the gullible see it as some magical method;  following the method’s recipes and instructions as if there were no other ways of cultivating the ground in a sensible way.  It’s just multiple layers of this and that built up within planked containers to make higher than ground level growing areas.  And lots and lots of paths.  
The raised bed method developed from la culture Maraîchère; the French intensive hot bed method which used horse manure decomposition to create heat and allow crops to be grown throughout the year on raised beds.   Frames and cloches were used to entrap the heat and enable vegetables to be grown even in very cold weather.  The Victorian gardeners copied this but also started to use tan as an alternative. 
I do not think that there is anything magical in the materials that are used to develop these raised beds.  Indeed, filling a raised bed with commercial multipurpose growing media or ordinary garden soil seems to get good results.  Mainly because both materials are either well sieved and mixed or become well mixed during the raised bed construction.    Adding imported sterile commercial growing media that has been mixed and sieved is just the same as digging over your garden but without the effort and with lots of costs both financially and environmentally.  Also this material is neither compost nor soil. The sterility of the growing medium prevents the recycling of nutrients because there are no bacteria, fungi or invertebrate life.  Without this living fraction of the soil, fertility cannot be maintained because there is little to prevent the leaching of nutrients and there is no turning up of nutrients from lower in the soil profile. 

Without intervention, organic matter will accumulate on surface of soil.  Worms incorporate this into the soil’s first few layers. However, they do this relatively slowly so those that do not dig their vegetable plots may experience a lowering of fertility over the years even though they have added organic matter to the surface of the soil.  

Adding organic matter, where nutrients are locked into molecules that can be broken down by saprophytes, produces a long term sustainable process of soil fertility improvement.  It is a slow acting decomposition which does not saturate the soil with leachable nutrient.  What goes in the soil stays in the soil unless it is taken up by plants.

Most gardeners import animal manures, shredded woody material, lawn mowings and leaves to their growing areas.  Although this is a necessity, the environmental and financial cost of these amendments is one of the considerations that has to be taken into account when developing a growing area. 

There are some methods where you start with a layer of news paper to suppress weeds and then cover with compost or other organic material such as straw or hay in layers with some addition of animal manure.  The layers are built up until the correct height is obtained and a surface layer of compost or soil tops it off.  Certainly it is a very intensive organic matter form of gardening. 
However, there seems to be little mixing of layers.  This will give relatively high concentrations of nutrients in some parts of the profile, while others have little.  Such efforts to separate the raised bed from the native soil seem perverse.  To produce a viable soil with this method, particularly if it is no dig,  would need worms  to  mix the various layers of the lasagna.   Most raised bed systems start with thick impenetrable layers of weed suppressant material.  Only the most persistent worms would get through that. 
It is a quick and relatively easy way of cultivating an area of weed infested ground because weeds are suppressed by the newspaper in the raised bed and paths are covered with weed suppressing membrane.  However, choose your weeds well because if you have Calystegia sepium or Equisetum arvense, you will not get rid of them this way.  It is always difficult to eradicate these weed, but digging them out does help to reduce their persistence. 
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 the improved soil in the planting hole and not venturing out into the surrounding soil. 
Sterile commercial growing media (composts) work so well because nutrients, air and moisture are evenly distributed through the sieved and carefully mixed compost.  They are engineered to produce the best results or they would not be able to sell them.
Permanent raised beds need high inputs of water, fertiliser and organic matter to maintain their fertility, just like ordinary garden soil.  The manufacture and transporting of both the raw materials and the final compost product will be dependent on oil.  Also, having to raise everything up to the raised areas makes them time consuming; needing a lot of attention to produce good yields from crowded plants. 

Fungi send out a gossamer candy floss of delicate hyphae that touch all the life in the soil recycling nutrients in an intimate intertwined association that involves most plants.  These are the mychorrhizal fungi. 
Bacteria also form symbiotic associations with the roots of plants allowing nitrogen fixed from the air to be passed to the plants.  Other bacteria fix atmospheric nitrogen and only pass it to the soil when they die or are consumed by some other part of the soil fauna and flora. Should these vital components of soil be disturbed by digging.  

There is some debate that says that the soil is rarely disturbed in nature and has not evolved to cope with being cultivated to the extent it is on allotment gardens. 
I would rather work with native soil which is not sterile and contains high levels of soil organisms such as nitrogen fixing bacteria and mychorrhizal fungi.  Digging is supposed to damage these organisms but they are microscopic and great effort would be needed to damage such small structures. 
Digging does damage some of the larger organisms – particularly the worms and this does need to be taken into account.  However if homemade  compost  is being used to  increase the organic content of the soil then a large population of soil organisms are introduced particularly if it is used as a mulch on top of the dug earth. 

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.

You are going to have to weed regardless of any strategy that you adopt;  you just have to get over it. Gardening is hard work - but very rewarding.

Mulches and green manures enable the beds to reach high levels of fertility and this can be maintained over many years.  Leaching is reduced and chelating humin, fulvic and humic acids will form complexes  that will entrap minerals in the soil structure.  

Going up or going down does not matter.  Deepening and increasing the fertility of the layer of top soil can be achieved by both.  I have raised my whole allotment about 600 mm above the original ground level and this does seem to improve the drainage.  However I have dug down quite deeply too and broken up the subsoil to quite a depth, adding organic matter, and this might be the main factor in improving the drainage.  Large amounts of organic matter in the form of logs, branches, woody shreddings and leaves have been added to the subsoil in a kind of trench Hugelkulture which might also improve the drainage of the allotment soil. 
Cupressocyparis laylandii shreddings

Mostly Acer pseudoplatinus leaves
Quercus  robur brushwood

Quercus robur branches

Good old farmyard manure.
In my experience adding large amounts of organic matter does seem to improve the fertility of the soil.  It could be conjectured that this is part due to the provision of carbon for free living nitrogen fixing bacteria such as Azotobacteraceae.    These bacteria are fairly ubiquitous and once they have a carbon source will multiply rapidly whether the organic matter is on the surface or mixed into the top soil.  It is hard to believe that digging would severely deplete the numbers of bacteria in the soil especially if it is combined with adding organic matter.  Hugelkulture, where logs, branches and brushwood are used to make raised beds should really be buried away from the top 300 mm.  of the soil profile.  Trenching and bastard digging allow very high carbon content material to be added to the soil with some success. 
Deep trenches will dispose of large amounts of unwanted organic material.  It can  also be a repository for more pernicious weeds.  While Elymus repens and Urtica dioica can be buried, mare’s tail Equisetum arvense and bind weed Calystegia sepium must be put into the worm bin because they will survive burial even at this depth. 
Logs, branches, brushwood and shredded woody material rots down to a very friable compost after two or three years and this can be incorporated into the tops soil.  While it is rotting down it is forming a sponge like layer that allows water to pass through for drainage; keeping some water as a reservoir for drier periods. 
Tanner’s bark was used in hot beds during Victorian times because it warmed up just like fresh farmyard manure. It  was used in tanning leather.  Mostly oak bark was soaked in hot water to remove tannin and afterwards discarded or given to gardeners.   This woody material was preferred to manure because it kept its heat for up to six months.  Putting shredded woody material under the soil may well have a warming effect too. 
The alternative to burying high carbon material could be to shred it and put on top of the soil as mulch.  It will help to retain top soil moisture and suppress weed seed germination.   However, mulches do attract slugs and snails and may be of more use on mature plants that are not so attractive to these voracious molluscs.  I do not use mulches on vegetable beds until the plants are very mature.  Hoeing is just as good when the plants are immature.  Putting large amounts of decomposing organic matter on the surface of the soil may deplete the nitrogen content of a couple of centimeters of top soil.   

Remember adding carbon reduces nitrogen; adding nitrogen reduces carbon and adding air reduces both.  

Having large reservoirs of carbon deep in the soil may increase the population of beneficial carbon eating microbes like mychorrhizal fungi.  Symbiotic connections could link plants to the decomposing plant material deep in the soil through fungi mycelium.   In order to protect these fungi and other essential microorganisms inoculated charcoal could be added to provide both a protective habitat and  a source of nutrients.   This is what the native South Americans did for thousands of years.  It is the soil called terra preta. 

It is understandable that the repeated addition of inorganic fertilisers would reduce the number of soil organisms by reducing the amount of carbon available to the heterotrophic soil fauna.  However, there are few gardeners that will use inorganic fertilisers to the extent they were used in the past. 
The adding of dead organic matter to the soil also has a number of other benefits that add to the fertility of the soil.  Plants respire as well as photosynthesise.  This means that they take in oxygen as well as carbon dioxide.  As the roots are below the soil they do not have access to light and cannot photosynthesise.  Yet they do respire and need a supply of oxygen from the soil.  Dead organic matter can increase the amount of oxygen that can penetrate the soil by providing air spaces and keeping the soil "open".  In a similar way organic matter can provide a route through the soil for water increasing drainage to avoid water-logging.  As organic matter absorbs water it also provides a reservoir that buffers water-logging and drought. Organic matter and clay are the hooks that enable nutrients to remain in the soil and available to plant roots and mychorrhizal hyphae.  In order to get the maximum benefit from the organic matter added to the soil it should be distributed evenly throughout the profile by digging. 

Mychorrhizal saprophytic fungi form symbiotic associations with plant roots which allows nutrients that are produced by fungal breakdown of dead organic matter to be transported to the plant.  There is some suggestion that these networks of fungal hyphae connect plants with each  other allowing the flow of nutrients and photosynthesis products to move to plants that are compromised because they are in shady or nutrient poor environments.  

I would suggest that my soil is more productive now than it was when I took the allotment on over thirty years ago. A lot of nutrients have been taken off the allotment in the form of vegetables, eaten and disposed of down the sewage system. The reason why it is even more fertile now is due to the addition by digging of dead organic matter in its many forms.  It would never have occurred to me to use growing medium to improve my top soil when I started gardening. Indeed I would have avoided peat because I saw it as an acid medium.    A good load of cow muck was about all that was needed.  I doubt if I could have increased the fertility as much by adding peat based growing mediums even though they are infused with inorganic fertilisers.

You can use any organic matter to produce good soil.  The word organic, in this context, means that which was once alive. (In chemistry it refers to any molecule containing carbon chains.)  There are many lists circulating around the gardening forums.  Regardless of their NPK ratio accuracy, they give a list of things that you can add to the soil or compost heap  that will decompose to give plants nutrients.

Adding any dead organic matter to soil will benefit it.  There is evidence that ancient human settlements can be identified by high phosphate and charcoal levels in the soil.  This leads to a lush growth of plants that has been maintained over many centuries.  I doubt if ancient human civilisations were as selective of the organic matter they buried as modern man is. The self sustaining properties of Terra Preta soils are probably due to an indiscriminate addition of dead organic matter together with charcoal.

Seaweed contains a lot of nutrients - particularly potassium and is a valuable amendment to soil.   So much of our nutrients are sent down the sewers and eventually into the sea. Using seaweed seems to close the cycle so that these nutrients can be returned to the soil.  

There is a suggestion that adding undecomposed organic material could be detrimental to the soil.  This is because micro organisms need nitrogen and forage for this in the soil when they are decomposing organic matter.  Well what goes around comes around.  These organisms will die themselves and decompose in the soil and provide nutrients.  If you are continually adding organic matter into your soil,  then the cycle of decomposition and growth develops a dynamic equilibrium where the level of nutrient in the soil matches the amount being used by living things preventing leaching and locking the nutrients into a sustainable cycle.

If the soil is very compacted then it will need to be double dug to improve the drainage and begin to increase the depth of the top soil. 
Raised beds mean that you don’t have to step onto the bed to cultivate it having access from the many paths around the beds.  The soil is never compacted and is always aerated and well drained. 
Plants need water which they obtain from the soil through their roots.  Roots also obtain dissolved nutrients from the soil and they need energy to do this.  To obtain energy they need to respire using oxygen from the air.  In other words plants need both air and water in the soil.  The more fibrous the structure of the soil the better the relationship between these soil constituents.  Walking on the soil will squash out some of the air.  Walking across wet soil will squash out some of the air and fill the pores with water muddying the soil. 

However, needs must and sometimes you just have to work the soil in wet weather.  Going over the ground that you have walked on with a fork restores the structure and allows air to reenter the ground.  Having said this, common sense says trying to work muddied ground when it is pouring with rain is pointless.  

Soil compaction by the rain is more of a problem in the vegetable garden although taking a hoe through the surface will usually be good enough to allow air to enter the soil. 

When making seed beds you should consolidate the soil by shuffle walking over it to break down the clods of soil and produce a fine tilth.  

The soil can also be compacted by animals like rats, badgers, foxes and the like.  Let’s be honest here, dinosaurs trampled the earth in the past and soils survived.  

The structure of the soil is much more dependent on its organic and mineral content than whether it is walked on.  I have read somewhere that if you get the calcium and magnesium content of the soil just right you can park your car on the soil and it will still be friable.  I would like to try this out before I recommend it – and that will never happen. 

Seedbeds need to be consolidated to conserve water and to ensure good seed soil contact for optimum germination.  This is why you sometimes see farmers using Cambridge rollers to consolidate their land.   I was always taught to shuffle over the soil to consolidate it and then rake it carefully to make a good seed bed.   It would take a very large weight or constant use to squeeze out all the air from soil.  Keeping your soil too fluffy just leads to problems with irrigation.   In any case, plants will grow through concrete – how compacted is that?


  1. I agree, although I don't compost my flower beds, relying on mulch alone. I have been the victim of a too-hot compost and an amateur overeagerness in the past, caveat: allow compost to mature before using it!

    1. Hi Andy, just been reading and writing some more about compost and this is really the classic way of composting. Leaving it to mature after heating up makes a much better compost and allows soil animals like earthworms to move back into it and make it even more valuable.