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 as far as I could see.  Cutting brushwood into 

     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.  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. 

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. 

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.  Fragmentation by invertebrates grazing constantly generates small pieces with large surfaces.  Turning 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.  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 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. 

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.

Mixing

Some effort is needed to place undecomposed residues from the cool, dry, oxygen rich outer layers of the heap with the decomposed residues from the hot, wet, carbon dioxide rich core of the heap helping to achieve a more uniform temperature throughout the heap for longer.  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. Turning the sides and top into the center of the heap ensures all the residues contribute to temperature rise.  

Turing allows assessment of the moisture content of the heap.  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.  

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 it quickly returns to its previous temperature – or higher after a few hours.  Barn fires are often started by wet hay or straw decomposing.