Vegetable Mould - Taking Organic Matter to a Totally New Level.

Regardless of soil type, texture, structure pH, and the amount of available nutrient, the one thing that makes the most sustainable impact on crop yields is vegetable mould.  Vegetable mould is the friable, sweet smelling, life rich, blackish material made by kitchen garden compost heaps.  

     I am not going to pretend that making good vegetable mould is anything but hard work.  I certainly don't find vegetable mould making a spiritual experience.  However, being pleasantly worn out, in the fresh air, with the Sun on your face, listening to the hum of nature, on a warm afternoon is ample reward for time in the composting area.

     When I was a toddler, I was sent down the garden to put kitchen scraps onto the vegetable mould windrow and I am still doing this, seventy growing seasons later.  Both my father and grandfather were enthusiastic composters putting copious amounts of vegetable mould onto the growing plats and digging it in.  They were greatly influenced by Albert Howard's promotion of the Indore method of composting.  Howard qualified as a botanist and worked as an agricultural educator throughout the world.  His methods of composting were made popular during the dig for victory campaign during the second world war.  

     The Indore method is one of the most recognised by British gardeners and involved composting green residues with manure.  With the tragic loss of so many horses during the wars and the relentless onslaught of mechanical devices, stable manure became scarce and much less was used in composting.

     In my experience only mixed plant residues were used and put into a bay made from corrugated iron sheets.  One spit of soil was dug out of the base and the top soil was kept neatly to one side to be added as the heap grew.  Brushwood prunings were put at the bottom of the pit to "aid drainage".  Whether this practice had any effect on heap drainage is beyond me.  I continued adding brushwood to the base of heaps for many seasons until I finally decided that it was not improving the vegetable mould.  Cutting brushwood into <10cm. pieces and mixing with other residues is much more likely to improve the drainage of the heap than having a layer of brushwood at the base of the windrow.  Large pieces of brushwood do not make turning easier.  If the heap is sited at the top of slope, drainage should never be a problem. 

     Weeds, prunings, leaves, kitchen peelings, crop residues and lawn mowings were added in no particular order.  After each foot of litter, a layer of soil was added and dafter three applications of soil, a little lime.  

     Nowadays most allotmenteers build a pile of residues at the back of their allotments and do little else until removing the confusion of undecomposed residues on the top of the heap to reveal the disappointingly small layer of vegetable mould below. 

     Although the festering pile of rubbish at the back of the allotment is not recommended, even the least maintained of heaps provides some vegetable mould that is a valuable addition to plat soil. 

 

The compost heap that my friend Fred inherited was at least fourteen feet tall and was slowly enveloping his shed.  It was a heap that was neglected and forlorn.  As I dug it out for Fred, I found various plastic trays, tubs, pots, netting, and other broken, miscellaneous garden paraphernalia.  The heap remained uncared for for twenty one years.  The sides and top were covered in a mass of Elymus repens and Calystegia sepium stolons that needed to be sieved out before the friable, life rich soil could be reached.

The Bread Tray Sieve

Luckily, I found a bread tray buried deep in the heap that had a one-inch mesh that could be used to sieve out the couch grass and bindweed.  A buried treasure that fitted neatly over the wheelbarrow. 

This vegetable mould windrow had never had its carbon to nitrogen ratio worked out, been turned, layered, or otherwise mollycoddled, yet it was as good as any carefully crafted vegetable mould I could make. 

It is amazing what twenty-one years can achieve.

The mega vegetable mould windrow was teeming with various invertebrates like worms, millipedes, centipedes, spiders, mites, woodlice, and flies. Their burrowing and churning played a crucial role in maintaining oxygen-rich air throughout the heap.

I Can’t Wait.

It takes hundreds of years for worms to make a centimetre of top soil and I can’t wait.  Twenty one years of continuous additions of residue can achieve remarkable results.  However, so much vegetable mould is languishing in a heap doing nothing.  The purpose of composting is to speed up decomposition so that vegetable mould can be cycled quickly.  By managing the moisture content, aeration, particle size, residue sources, mixing and temperature I can add a reasonable sized heap of vegetable mould to the growing plats every three or four weeks. 

The Berkely Composting regime.

The University of California developed Alan Chadwick’s biodynamic intensive method of composting.  The Berkely method relies on regular turning, highish temperatures and keeping the heap very moist.  The turnover of hot vegetable mould is remarkably fast, and nutrient elements are not subject to excessive leaching or outgassing. Continuous addition of residues and consolidation are avoided speeding residue breakdown.

The Heap Is Made in One Go

The heap is made in one go and once it is constructed nothing more is added.  All residues have the same time to rot down and there is no confusion of undecomposed residues covering the top layer of the windrow.  To prevent the temptation of further residues being added to the heap, new material is stored, nearby, in dalek bins so that ingredients are to hand to make the next batch of vegetable mould. 

Residues from the dalek bins together with litter from around the allotment (path sweepings, lawn mowings, hedge clippings, comfrey leaves, sweet cicely leaves, nettles and other weeds) are teased out to release any consolidation and added to the new windrow – not necessarily in any particular order. 

The Berkely Method of Regular Turning.

Once the heap has been made, thoroughly wetted and mixed, it is left for four days to enable microbe numbers to multiply and for other organisms to migrate from air and soil.  To get heat into the heap there must be life.  You can’t sterilise vegetable mould by turning it. 

After four days the heap is turned every two days. 

There are some that would say that this is excessive.  I suspect that even more would say that turning once a year was excessive.  However, the more that the heap is turned the quicker vegetable mould is formed and the sooner it can be added to the plats.  Turning the vegetable mould windrow keeps you warm in winter… It keeps you warm in the summer too if you live in Britain.

Slumping and Consolidation

Vegetable mould windrows shrink over the composting period.  Microbes use a massive amount of carbon from organic molecules like cellulose and lignin mineralising them to carbon dioxide and water vapour leaving residues limp and floppy.  After following the two day turning regime you might wish that the heap would loose weight and volume a lot more to save your poor back. 

The physical strength of residues is undermined as carbon is stripped away during catabolism and the loss in turgidity causes slumping and consolidation.  Air filled pores are critical in moving oxygen around the vegetable mould by both diffusion and convection.  Air and water tend to have preferential routes through the vegetable mould matrix producing anaerobic and dry volumes.  Regular turning reduces consolidation by loosening residues and introducing new macro and micro pores.  A good network of pores helps to create an even flow of air and water throughout the whole volume of the vegetable mould windrow. 

Turning does introduce fresh air and allows gas exchange, but this is short lived lasting just a few hours.  It is the diffusion and convection currents of gases through pores that introduces oxygen into the core of the heap and allows excess carbon dioxide to escape. 

Pores can be managed by the type of residues added and the number of times the heap is turned.  Residues like ramail keep their frame strength for longer than other material while being small enough to be easily turned. 

New surfaces are exposed by turning, invertebrate churn and larger animal burrowing and are quickly colonised by microbe biofilms. Decomposers mainly colonise surfaces so the more exposed surfaces the more decomposition.  Fragmentation by invertebrates grazing constantly generates small pieces with large surfaces.  Turning relatively evenly mixes these newly exposed surfaces with saprophytic fungi and bacterial biofilm infected particles enabling rapid migration onto pristine surfaces.  The intimate gossamer growth of fungal hyphae can extend its reach between particles finding new surfaces with little effort. 

Size of Particles.

Large residue pieces make big voids that threaten to quickly dry the vegetable mould.  Brushwood is a poor matrix for decomposition having a hydrophobic nature and large dry air spaces.  Cutting up longs seriously reduces voids enabling easier movement of organisms from one piece to another.  With smaller spaces, films of water covering pieces are less likely to evaporate and air pores are saturated with water vapour.  Small pieces of woody twig, stem or branch reduces confusion making turning much easier.  Ramail mixed with lawn mowings heats the windrow surprisingly quickly.

Longs need to be cut up or shredded.  Small pieces are easier to wet and keep damp.  As top fruit trees and soft fruit bushes are pruned the trimmings can be cut into small <10cm. pieces that wet more easily and break down quickly.  The size of residue pieces regulates the surface exposed to microbial attack.  Small pieces can be eaten by invertebrates and processed through their guts.  The smaller the particles the quicker the decomposition.  Although long fragments of residue should be cut into <10cm fragments when the heap is made, chopping up longish pieces should continue during turning, teasing and stirring. 

Breaking up residues by turning seriously reduces large voids within the vegetable mould matrix and with smaller pores the films of water covering every particle are less likely to evaporate excessively and air gaps are saturated with water vapour.  Short dense woody pieces added to the mould reduces soil evaporation and mass flow of groundwater. 

Shredding, cutting up, mashing, breaking and chopping brushwood prunings still leaves voids but these can be filled with finer material like grass mowings, weeds and vegetable residues as the heap is turned.  Small pieces of woody twig, stem or branch reduces confusion in the heap considerably and makes turning much easier. 

To attract beneficial invertebrates the range of vegetable mould thickness should reflect those found on woodland floors.  Although a lot of woodland floor natural vegetable mould is usually less than 10cm. thicker pieces are included in the mixture creating habitats.   Longer thicker pieces that last longer in the vegetable mould windrow play an important role in evaporation reduction slowing the mass flow of ground water and attracting different rarer woodland floor organisms. 

Larger denser pieces of vegetable mould provide habitat for less common microbes, invertebrates and fungi.  As denser small pieces of residue build up in plat soil so too do the numbers and diversity of forest floor decomposers.  To attract beneficial invertebrates, the range of vegetable mould thicknesses should reflect those found on woodland floors.  Thicker denser pieces are more resilient and make longer lasting habitats attracting different rarer woodland floor organisms. 

Mixing

Turning the sides and top into the center of the heap ensures all the residues contribute to decomposition.  Evaporation causes the outer layers to lose water and become too dry for efficient composting.  Some effort is needed to mix undecomposed residues with the cool, dry, oxygen rich outer layers of the heap with residues from the hot, steaming, carbon dioxide rich core helping to achieve a more uniform distribution of residues and temperature throughout the heap.

However, I just go for it not worrying too much where individual fork loads end up.  The residues are being folded in, mixed and broken up regardless of where they originate.

Turing allows assessment of the moisture content of the heap.  Adding copious amounts of water as you turn the residues maintains rapid decomposition.  There must be a substantial amount of water throughout the volume of the heap for hot composting to continue apace.  Microbes feed, move, communicate, grow and develop consortia in water films covering vegetable mould pieces.  Fungi can survive dryish vegetable mould by translocating water from wet volumes of the heap; however, they thrive in moist mould.

Moving the core to the outside cools the heap and this is the only way of controlling the temperature.  Spraying the windrow with water may initially reduce the temperature but increased microbe activity soon increases the heaps temperature far beyond its original warmth.  Barn fires are often started by wet hay or straw decomposing. 

Microbes and invertebrates are moved around the heap when it is turned creating a more even spread of organisms throughout the vegetable mould windrow quickly introducing them to new surfaces.  An even distribution of microbes throughout the heap gives a quicker and more thorough decomposition. 

Bay Partitions.

Why have a bay partition that you have to heft heavy vegetable mould over? If there are no pallet barriers in the composting area, it is so much easier to turn.  Building one large bay makes turning easier because the mould does not have to be hoiked over a barrier.     

What To Use for The Base of The Heap

It is difficult to know where the bottom of the heap is when vegetable mould windrows are built on soil.  I annoy myself digging holes in the soil as I turn or excavate the mould not noticing where vegetable mould ends, and soil begins.    Cardboard can be used to separate soil from residues, but card rots away surprisingly quickly, regardless of its 378:1 carbon to nitrogen ratio.  The rapid decomposition of card leaves the soil vulnerable to my enthusiasm.  Copious amounts of sticky tape cover corrugated cardboard boxes most having absolutely no functional use.  As I am not fastidious in initially removing sticky tape and it does not rot down in the heap, I have the chore of removing long strings of tape from the vegetable mould.

Next, I tried using old woollen carpets but they have an irritating habit of rucking up making turning difficult.  They last longer than cardboard but eventually decompose.

Tarpaulin bases make turning easier but prevent microbes and invertebrates reaching the residues quickly.  They are made of plastics and do not allow free drainage. 

I now use a broken concrete slab base, crazy paved style, covering the soil but not concreted in.  They are higgledy piggledy with large gaps between to allow access for microbes and invertebrates. 

Matting.

If a wide variety of residues are added to the vegetable mould windrow, then there is less danger of materials matting together.  Matting slows decomposition by excluding oxygen and retaining too much water. 

Invertebrate Churn.

Fragmentation by invertebrate grazing constantly generates small pieces.  When a new face of organic matter is exposed by turning, invertebrate churn or animal burrowing it is quickly colonised by microorganisms.

Heat In The Heap.

If the vegetable mould warms up it is probably large enough, has enough water and has been regularly turned.  To get heat into the mould there must be life.  It is very unlikely that there are separate consortia of mesophilic and thermodynamic microbes.  A diversity of microbes adapted to a wide range of temperatures is much more likely.  The heat tolerance of ordinary bacteria, archaea and fungi must change over the composting period.  Archaea and bacteria have the biofilm matrix to protect them from high temperatures.   Due to the changes in temperature, composition and structure of the heap there may be some kind of succession over the duration of composting but the succession is more to do with genetic and structural change of microorganisms within the biofilm consortia enabling common or garden microbes to tolerate highish temperatures.  There also may be a secretion of a more heat tolerant biofilm matrix to protect consortia in the biofilm.  

High temperatures do not winkle out pathogens.  Heat is nonselective and kills all life by denaturing proteins.  I do not want my vegetable mould windrows to have a temperature higher than 40°C.   40°C does not kill off many pathogens – other things do that but 40°C temperatures accelerate decomposition.  Temperatures over 40°C kill the very organisms needed to decompose residues.  I want to maintain and increase my beneficial saprophytic and plant growth promoting microbes by keeping the mould at or below 40°C by regular turning.  Long lasting high temperatures were used by Victorian gardeners to keep hot beds warm during the coldest seasons.  Microbe activity can be sustained during winter.  My vegetable mould was 55°C on December 17^th because I did not have the time or inclination to turn it. 

While there is adaptation to high temperatures, we are not talking about mid Atlantic hydrothermal vents or boiling volcanic sulphur pools.  Whilst I would not put it past thermophilic microbes to travel around the world in the air fog of microbes reaching my little vegetable mould windrow, an evolution of specialist thermophilic bacteria and archaea in a temperate maritime climate would be puzzling.  Bacterial biofilms adapt to the changes in temperature so they can remain active even at highish temperatures.  Microbes have life cycles and their preference for warmth or coolness is less likely to cause fluctuations in specie numbers that the weather, the season, the availability of particular residues or the availability of other specific microbes. 

Insulators

After the heap has been turned, coverings can be put over the vegetable mould windrows.  The heap does not have to be completely covered.  You need to allow oxygen rich air to enter the through the base of the heap and carbon dioxide rich air to leave the top.  Old cotton or woollen rags are useful insulators for covering the heap.  Tarpaulins are good coverings for heaps but they are plastic. 

Journey to the heap.

Root hairs, fine roots and root tips are covered in actively recruited plant growth promoting microbes feeding on biomolecules generated by plants during photosynthesis.  Exudates, sheared off cells and fine root dieback provide a valuable, long term food source encouraging archaea, bacteria and fungi to congregate and multiply in the intimate couple of millimetres around roots. 

The interaction between microbes and roots is not a one-way process.  Apart from breaking down organic matte and cycling nutrients, bacteria make auxins, cytokinins, gibberellins, abscinic acid and such like that can increase crop growth by up to 30%. Microbes also secrete beneficial siderophores that enable iron and other micronutrients to be absorbed by crops.  The motley associations of bacterial consortia around, on and in plants do not produce the same medley of secretions at identical times or in uniform amounts.  With a wide diversity of plant growth promoting rhizobacteria there is more potential for an even distribution of growth promoting biomolecules over the growing season. 

Although my vegetable mould windrow comes a close second, the greatest diversity of life on Earth is in the soil and the rhizosphere is by far the most dynamic.  When unwanted plants are pulled from the earth, much soil remains on the roots even when the plants are shaken vigorously.  The contribution of diverse microbes from around the roots of residues and weeds ensures there is always some valuable consortia of bacteria thriving and multiplying in the vegetable mould windrow.  The descendants of these beneficial microbes are returned to the growing plats when vegetable mould is spread over the soil. 

Every plant has its own life cycle and root exudates wax and wane during the growing season altering the diversity and numbers of plant growth promoting rhizobacteria.  Adding the diversity of weeds even a small allotment generates, provides the vegetable mould windrow with multifarious accompanying microbes.

Ephemeral and annual plants provide a fleeting source of food for microbes, while biennials and perennials provide greater stability of exudates.  Cut into <10cm. lengths I often add roots of Rubus fruiticosus (bramble), Rosa arvensis (field rose) and Rubus idea (raspberry) in the hope of gaining novel consortia of microbes. 

Each time a new vegetable mould windrow is constructed many endophytes and foliage microbes piggyback in and on the residues.  Plants may be infected by several mycorrhizal fungi, numerous nitrogen fixing bacteria, a myriad of root surface bacterial biofilms and various saprophytic fungi.  Many symbiotic  and endophytic organisms find survival without a living host challenging and have to resort to producing propagules.  Nitrogen fixing Rhizobium bacteroids living symbiotically in the root nodules of Fabacea (legume) plants produce free living platonic cells.  These cells no longer fix nitrogen but contribute to the breakdown of residues in vegetable mould windrow biofilms. 

Mycorrhizal fungi

Mycorrhizal fungi gain their sustenance mainly from their host plants and cannot break down residues to produce biomolecules they can feed on.  Mycorrhizal fungi propagules remain dormant in the heap until vegetable mould is cycled onto growing plats where spores encounter

Bacteria in the soil.

I am not impressed by the number of bacteria in a teaspoon of soil.  It depends on where you look.  The bulk soil millimetres from roots is a microbial desert except for huddles of microbes around scant fragments of organic matter.  Although increasing the soil organic matter by adding mulches of vegetable mould boosts the bulk soil microbe numbers, I really want plant growth promoting microbes concentrated around the roots of crops.  The bulk soil may loose out when it comes to numbers, but it wins hands down when it comes to diversity.  Building the vegetable mould on soil – or with access to soil attracts a great variety of these saprophytic microbes enhancing decomposition. 

There are some remarkably hostile microbes in vegetable mould and soil.  However, their numbers are so insignificant that they do not pose much of a threat.  It is only when these species reach a tipping point of high numbers that quorum sensing kicks in and they cause a problem.  Always wear gloves when gardening and keep a bar of soap in the shed first aid box.  Wash cuts immediately with tap water, drying them with a clean paper towel finally covering the wound with a plaster.  (Put the soiled paper towel on the compost heap.  Blood cells and plasma contain protein and protein contains nitrogen.)

In nature bacteria and archaea are always found in consortia.  Biofilm consortia are highly diverse, multispecies associations continually adapting and cooperating to organise decomposition.  It is not until the establishment of mature consortia that residues warm and decomposition continues apace.

Biofilms

The formation of biofilm consortia begins with the permanent attachment of unicellular colonising bacteria to a surface.  Once bacteria, like Bacillus subtalis are tightly attached to residue surfaces, their structure and genetics change so that they can work synergetically with other species attracted to the biofilm. 

The basic structure of the biofilm looks like a forest of rubbery towers built from bacterial secretions of mucus, enzymes, nucleic acid and debris.  Cellulose microfibers reinforce the attachment to residue pieces.  The polysaccharide mucus matrix is punctuated by water pores so that oxygen and nutrients can diffuse into the film and unwanted nutrient elements excreted. 

Biofilms provide a protected and stable microenvironment where microbes are safeguarded from changes in temperature, pH and antibiotics.  Bacterial grazers like slime moulds and nematodes are less likely to prey on biofilms than free living platonic bacteria. 

As the biofilm grows it secretes biomolecules that attract an influx of unicellular bacteria.  Biofilm recruitment of new bacterial cells is very sensitive to the watery biochemical environment.  The use of xenobiotic pesticides and herbicides hinders the important messenger molecules causing the absence of critical organisms.  Only when the messenger molecules are able to diffuse without the masking effect of xenobiotic chemicals can co-option and critical stages of biofilm development take place.  While the overarching numbers of microbes are said to remain the same before and after xenobiotic chemical application the diversity of biofilm consortia and the prominence of species change most being severely depleted.