Sunday, 12 February 2012

Improving the soil

Undoubtedly, soil preparation and improvement is a major part of success in the vegetable garden. In one of my lectures I said that it took about 1000 years to produce 20mm of soil.  As I didn't qualify this, I got several gardening student questioning me afterwards.  By 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.

What I should have said in the lecture was that it takes about 1000 years to produce 20mm of soil naturally.  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 over night but there are things that gardeners can do to improve the soil for cultivation of vegetables and flowers.

One of the more insipid practices is building raised beds and then filling them with commercial bagged "compost".  This material is neither compost nor soil.  It is usually made from sterile materials such as peat,  perlite and vermiculite.  The reason that it grows vegetables relatively well is because there are few pests in the growing medium.  The problem is; there is few of anything living in the growing medium.

This is problematic because there is nothing in the soil to recycle nutrients -there is little bacteria, fungi or invertebrate life.  Without this background base level, living fraction of the soil, fertility cannot be maintained. There is little to prevent the leaching of nutrients out of the soil and if a no dig system is used as well then there is no turning up of nutrients from lower in the soil profile.  It is saprophytic organisms that change dead organic matter into substances that plants can use.  We call these chemicals plant soil nutrients.  They are listed as nitrogen, phosphorus, potassium etc., although they are never taken into the plant as elements.  You can bypass the saprophytes by adding just the inorganic chemicals themselves but this leads to two major problems and many minor ones.

In order for the plant to take in these nutrients they must be soluble in water.  This means that they are relatively easily leached from the soil.  Inorganic fertilisers have been indiscriminately applied to soils and disrupted many ecosystems due to leaching into water courses and the sea.  Secondly, they remove the need for addition of organic matter to the soil which leads to depletion of  saprophytic organisms and inhibits nutrient sustainability.  Adding organic matter, where nutrients are locked into molecules that need to 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.

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.

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.  Science is still trying to uncover the complexities of this soil interaction that relies on the addition of dead organic matter.

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 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.

So what dead organic matter can be added to the soil?  The word organic in this context means that which was once alive. (In chemistry it refers to any molecule with carbon in it)  You can use any organic matter to produce good soil.  There are many lists circulating around the gardening forums.  I don't know whether they are accurate or not in their N:P:K ratios but that is beside the point.  They give a list of things that you can add to the soil or compost heap and that will decompose to give plants nutrients.

For example:

I would suggest that adding any dead organic matter to soil will benefit it.  To support this assertion, I would site the evidence that ancient human settlements can be identified by high phosphate and charcoal levels in the soil.  This leads to 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.

There is some organic matter that is more equal than others, though.  Seaweed contains a lot of nutrients - particularly potassium and is a valuable amendment to soil.  We send so much of our nutrients down the sewers and eventually into the sea that using seaweed seems to close the cycle so that these nutrients can be returned to the soil.  Care must be taken not to add too much sodium chloride though.

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, as it becomes available, as  I am then the cycle of decomposition and growth develops into 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.

However, it may well be worth composting organic matter to allow saprophytes to break open the cells and decompose the large organic molecules before adding it to the soil.

The more that I compost the greater my despondency  grows.  We import so much organic matter from other parts of the world, sucking out their nutrients most of which we dispose of down sewers and on land fill sites.  Even composted they are a testament to our exploitation of other ecosystems.  This cannot be sustainable.

One of the major inputs of nutrients into soil must be bird droppings. You can judge the amount of bird droppings that cover the land if you look at house roofs.  They are usually covered in little circles of algae where a bird dropping has landed.   As this has been happening for millions of years there have got to be soil organisms that exploit this nutrient throughout the world.  Many soil organisms like bacteria and fungi produce enzymes that they secrete into the soil that break down organic substances so that they can be absorbed through their cell walls.  These enzymes themselves will be broken down but until they are they will continue to work; providing nutrients for other organisms within the soil.  I would suggest that this is why chicken manure breaks down so quickly and produces such good results when growing vegetables.  Similarly other animals have been weeing over the soil for similar amounts of time producing a situation where the enzyme urease is ubiquitous in the soil.  The enzyme's interaction with clay and humus means that it is relatively long lived in the soil and is a good source of ammonium ions.    Urine breaks down very quickly in the soil and should be a major amendment rather than just flushed into sewers.

One of the major constituents of soil is the mineral content that is derived from weathered base rocks.  It is also the major source of naturally acquired phosphate and potassium nutrients.  I would suggest that cultivation of the soil is another way of weathering the soil.  Continual turning of the soil leads to break up of stone due to frictional forces.  Grinding in this way allows the production of rock dust with large surface area that is more susceptible to attack by humic acids. Release of nutrients in this way will increase the fertility of the soil.    Does this mean that we should add rock dust to soils to enhance this process?  I don't know but I would like to check it out.

To all those that think that adding undecomposed organic matter to soil will lower the nitrogen level in the soil; what about green manures?  A green manure is one that grows quickly and then when dug into the soil will decompose quickly. The speed of decomposition is really the key because nitrogen will be released by dying micro organisms after decomposition.  I just think that any dead organic matter that will decompose quickly could be dug in in a similar way.  Also there are a wide range of plants that can be used as green manure with different ones giving potentially giving a range of benefits.  I am starting to wonder if a more effective and cheaper choice would be to allow a covering of annual and ephemeral weeds and then digging in before they seed.  Yes, something we have all been doing for years.  I can understand the arguments against this.  Ephemerals, like groundsel (Senecio vulgaris ) are quick to flower and seed even in the winter and can easily overwhelm a vegetable bed.  However, if a careful watch is taken of the development of these weeds and you have time to dig them in before they seed then they will make as good a green manure as annual meadow grass.

Green manures can also add nitrogen to the soil.  If they are members of the legume family then they are probably associated with nitrogen fixing bacteria which takes nitrogen from the air and uses it to produce proteins within the bacteria cell allowing excess to be passed to the plant.  When the whole plant is dug into the soil it will decompose releasing this nitrogen in a form that other plants can use.  This is something that I do with cultivated peas, sweet peas and beans as part of a rotation scheme.  I would suggest that addition of nitrogen using both cultivated legumes and such things as winter tares and clover are an ideal way to introduce a slow acting and sustainable nitrogen source to the soil.   

An intimate knowledge of soil structure and texture is not really required in order to be a good grower.  All you need is a bit of common sense.  The soil texture is the percentage of sand, silt and clay in the soil and this cannot be changed easily.  The soil structure is how these particles are arranged into aggregates. Both humus and calcium are implicit in the formation of sol aggregates.  This demonstrates how organic matter, as it breaks down, affects the friabability of the soil.  Calcium helps to flocculate - clump together-  clay minerals in the soil and allow the soil to become more open.  I have a soil with a high percentage of sand which produces a very open soil prone to drought and leaching.  I add a lot of organic matter to ameliorate both of these problems.

Around soil particles is usually a film of water containing humus in one form or another.  Although there is evidence that this fraction of the soil is relatively long lasting, addition of amendments such as comfrey, nettle and sweet cicely liquid together with worm bin liquid cannot but add to the soil humus content.  These liquids seem to be broken down relatively quickly and nutrients locked up in them made available to plants within days or weeks of application.   One application of these liquid manure lasted a whole season on the roots bed.  I just watered a relatively weak solution/suspension of comfrey along the seed drill before sowing seed.   I must admit that I use this liquid manure mix all the time on the allotment and it seems particularly good for tomatoes Lycopersicum esculentum and sweet peas Lathyrus odoratus

I have been experimenting with the use of charcoal as a soil amendment with some success.  Although I was sceptical initially because charcoal had been reported as reducing the fertility of the soil, I did start applying it  in planting holes of vegetable plants.

My reasoning was that charcoal reduced soil fertility by adsorbing or absorbing plant nutrients.  Charcoal has a capacity to do this with a wide range of compounds.  If charcoal had this characteristic when added to soil, why not provide the nutrients before adding the charcoal to the soil.  I decided to submerge the charcoal in a dustbin of comfrey liquid manure and let it marinade for as long as possible before adding to the soil.  Thus the charcoal will become saturated with nutrients and not remove them from the soil.  However, when plants, bacteria or fungi remove nutrients, as is their want, nutrient will defuse from the charcoal into a depleted soil. In addition, when excess nutrients are added they may be taken up by the charcoal preventing leaching and providing the soil with a buffering effect.  With indications that charcoal is a particularly long lasting amendment to soil this buffering could produce a sustainable nutrient system that enhances soil fertility greatly.

After experimenting with different sized lumps of charcoal, it would seem that the very fine dust like particles are the most effective in providing nutrients to the soil.  Rather than crushing  lump charcoal as I have been doing up until now, I am going to use the Takesumi bamboo charcoal which is very fine and has an enormous  surface area for adsorption and absorption.

There is also some evidence that charcoal produces a protective micro habitat that could be exploited by bacteria and fungi.  If mychorrhizal fungi is added to the charcoal before it is used in the planting holes then this might help the mychorrhiza to establish and give it a source of nutrients that can be transported to the roots of vegetables.  Now further evidence suggest that EMs (essential micro organisms) which consist mainly of bacteria can also be used to enhance the growth of plants.  Remarkably, it is now being promoted commercially...

The more I sieve the soil the more I am convinced that sieving is a good choice.  Mixing the soil to make a more homologous blend of material equally accessible to all plants seems to be an eminently sensible thing to do.  Having pockets of high nutrient concentration leads to plant damage because water is drawn out of the plant root and into the soil.  High nutrient salt concentrations are bad for the plant.  Isn't this another argument for the use of organic manures that decompose slowly; limiting the concentration of soil water nutrient salts?  Sieving just adds to the diluting effect; spreading and mixing organic manures throughout the soil.

The worry I have about using the lasagne method in raised beds is that 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 very little.  Also, it seems like an awful lot of effort to achieve what a are dubious results.  Such careful separation of vegetable bed from the natural soil seems perverse to me.  To produce a viable soil with this method particularly if it is no dig you would need worms to mix the various layers of the lasagne.  Most of the procedures start with an impenetrable layer of plastic, cardboard and newspaper.  Only the most persistent of worms would get through that.

While I decry this layering method of gardening there are other procedures that seem to be a little more reasonable if done properly.  Huglekulture is where logs, branches and brushwood are used to make a raised bed.  I would suggest that these should be buried away from the top 300mm of the soil profile.  Trenching and bastard digging allow very high carbon content material to be added to the soil with some success.  I like big deep trenches because you can get rid of a lot of unwanted organic material.  It also means that you can get rid of some of the more pernicious of the weeds.  While Elymus repens and Urtica Dioica can be disposed of in this way, mares tail Equisetum arvense and bindweed Convolvulus arvensis must be put into the worm bin because they will survive burial.

The logs, branches and brushwood rots down to a very friable compost after two or three years and this can be incorporated into the top 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.

Rather than burying high carbon material, it can be shredded and put on top of the soil as a mulch.  I am not sure whether mulches improve the soil but they do seem to retain moisture. I think that they cause more problems than they cure.  My major concern is the way they attract slugs and snails.  I would not put a mulch onto a vegetable bed until the plants are very mature.  I would rather be able to get between my plants with a hoe.

Another way of making an open friable soil particularly when there is a high proportion of clay is to use gravel.  I have used it several times and in conjunction with compost and manure it works very well.

So do we need to dig or not?  I would say that in order to mix the soil and make it homogeneous digging is unavoidable.  It can be used to incorporate organic matter into the soil and maintain a sustainable fertility.   Digging maintains the soil structure and allows free drainage.  It buries weeds and helps to prepare a good tilth for seed sowing.  And what is more; even the very most enthusiastic no digger will dig to harvest roots and potatoes. They will also dig in green manure.   In order to avoid killing worms, I avoid digging where I do not need to, however I am not going to be religious about it and get all guilty when I do dig.  I just do what works for me...

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