Thursday, April 24, 2014

Composting with Clay

Gordon is a food gardener who has made compost for many years.  Recently he decided to experiment with clay in his compost and he believes that it is now significantly better than it used to be.  Here is what he had to say on the subject:

It is very satisfying to be able to say that I have finally discovered how to build durable humus levels in my soil and permanently increase its nutrient and water retaining capacity, using methods I read about in ‘Farmers of Forty Centuries’, written in 1905 by an American agronomist named FH King.

In the book King tells of his travels in the far East studying in great detail the way of life of the large populations there and the agricultural systems which sustained them and which he consistently found to be superior to the lazy and wasteful systems in his own country.

On the flood plains of the major rivers large cities were built and around them networks of navigable canals radiated out across the plains, serving as a water reticulation system for irrigation, to reduce flood damage and as both a transport network and a giant fish and duck farm.
A network of navigable canals across the plains
All the night soil from the cities would be loaded on barges and distributed among the farms of the plain where it would be composted along with crop residues and or green manure crops and clay mud dredged from the canals. Thus the minerals from upstream would be regularly added to the fields and those sent off to market as produce would return as compost ingredients. In this way fertility was maintained at a high level for thousands of years. King was at pains to say that he never came across infestations of flies or foul smelling piles of anaerobic manure. He was impressed by the attention to detail paid by farmers in gathering up everything that could be composted.

His book contains descriptions of many of the labour-saving devices and machinery used in diverse processes, all driven by animals, people, wind or water and built from mostly wood and bamboo, with iron being used only in those applications where it could not be substituted.
Machinery mostly built from wood and bamboo
Inspired by reading King I went on to experiment with clay in composting and the results have only inspired me more.  What blows me out every time is the uncanny affinity the composting critters have for my hitherto lifeless subsoil clay which turns from a faded yellow to a rich brown as the compost matures.

It's a bit like putting up blocks of flats for a billion homeless people, filling the pantry of each apartment with lots of good food and drink, and then letting them go for it. There is a lot of friction and competition as they rush in and start getting themselves sorted out, filling the easily accessed units first and then having too many children and having to struggle to move in to the progressively more difficult zones up narrow winding cul-de-sacs on steep terrain.  Unfortunately for the homeless people there is no sewage system in these insalubrious apartments so they gradually fill up with waste, sticking fast and firm to walls, floors and ceilings as the food in the pantries is consumed.

When I used to make compost without clay I would now liken that to making a big pile of food and a few drinks for a billion homeless people and letting them go for it.  They would get stuck in and rush around looking for the tastiest morsels. Lots of friction and competition, but not very much to drink. If I didn't turn the heap and add more water my homeless people would die of thirst, cooking in their own body heat and cursing me for not having thought my generous gesture through properly.  If I did turn the heap and add water, then off they would go again guzzling away until the heap would only be a fraction of the starting size.  How many tons of this inferior product I have laboured over in 35 years of gardening I shudder to think.  Year on year I would shovel it on or dig it in but it didn't grow especially good veggies and it always disappeared quickly from the soil.

The heaps made with clay, so long as they contain a reasonable amount of coarse material to enable some air movement, do not need to be turned.  The ingredients all get to soak and mix in a thick clay soup before stacking (putting a pile of food and drinks in every pantry) and the heaps seem to stay moist for a very long time. I recently opened up a heap I hadn't touched for three months and it was still moist and generating warmth.   A reasonable compost can be made by simply wetting the materials with clay slurry as the heap is built but remember that only the material which decomposes in association with clay particles is going to become durable humus/clay complex.  A heap built this way will probably need to be turned and re-watered too. Think of the difference between a dish that's been marinated compared to one that's only been sprinkled with a dressing.

Clay is made up of very fine particles so the combined surface area of all the particles in a peanut sized clod might be equal to a tennis court or three Clive Palmer skins or some such mind boggling factoid.  No wonder, then, that it can hold so much water.  Also these particles carry a negative charge so each one is capable of forming bonds with positively charged particles (ions) like many of the essential plant nutrients.  They gradually fill up with waste, sticking fast and firm to walls, floors and ceilings as the food in the pantries is consumed.

The resultant compost is packed with nutrients which are more or less available depending on how complex the chemical bonding with the clay is.  What we have here is humus in close association with clay, a long lasting, water retentive material in which plant roots and soil organisms can find all the nutrition they are looking for. A material which will keep carbon, not only locked up, but also doing a great job for years to come.

I dig my clay out of a bank using a rotary tiller when it's not too dry, hard and dusty or wet, sticky and heavy.  Then I spread it to dry in the sun or under cover if rainy.  When dry I pulverise it on a concrete slab with a sledge hammer wielded like a pestle until the largest crumbs are about 10mm. Then I make a rough measure of it's volume and mix in the appropriate amount of blended minerals. This mixture can be bagged up and stored in a dry place for future use.  

To make compost I put a 100 to 150mm layer of the clay mixture in the bottom of a 200 litre drum, or if it is a big batch, a 1000 litre container, but an old bath tub would be easier to work with and would be well suited to a smaller garden.  
The process becomes more ergonomic and efficient when I use a 1000 litre container
I half fill the drum with water and let the clay soak for a few hours before working the clay into suspension.  Then I start adding the compost ingredients to the drum and mixing them. Very dry water repellant materials are left to marinate in the clay soup until saturated.  Each load of materials in the drum should have some "green" (high nitrogen) and some "brown" materials and a small amount of mature compost to inoculate the brew.  After a mix and a stir to bring up clay from the bottom where there's always a thick settling layer, the material is lifted with a fork and after draining back into the drum for a few seconds the material is added to the heap.  As the build proceeds, more clay and water are added as required.  A whole heap can be made in a day or the process can be ongoing, using materials as they become available, but in the latter case a lid is needed to reduce the odour from the drum.  Shredding ingredients with a mower or a brush cutter fitted with a sharp "weed whacker" or chopping with a sharp spade will make a finer and more homogenous compost which heats up quicker. At the very least, long stalks and large tough leaves and large clumps of grass should be chopped.

In my early experiments I added to the clay soup a blend of minerals and trace elements as developed by Steve Solomon in his book "the Intelligent Gardener" and which he calls Complete Organic Fertiliser.  But then I decided to have my sub soil clay tested for 12 mineral elements and work out a script for a custom blend of minerals that I could add to ensure that the clay I'm using in my compost contains a luxurious abundance of plant nutrients in appropriate proportions.

The vegetables I now grow are healthier and higher yielding and better tasting than ever before and because the clay humus lasts so long I believe that if I keep adding it to my garden it will continue increase its overall fertility.
The finished product, rich like a well seasoned fruit cake
If I can get the organic matter level in the soil up around 15% I will reduce compost applications to just maintain that level.  That amount of quality organic material should provide ample nitrogen just by steadily decomposing.

I will demonstrate this process during a composting demo in my garden on 18 May.  The Food Garden Group newsletter for May will have details of the visit to Gordon's garden.

Many thanks, Gordon, for a very interesting blog post.


  1. Thanks for this, Max. So, in the end, what is the custom blend of minerals that he adds? Does he no longer add any standard COF? It seems that it is a combination of the COF (or his custom blend) and the clay that is improving his compost and subsequent vegetables. That Steve's COF is a mix of synthetic chemical additives still bothers me as he is supporting chemical rather than 100% organic food gardening. It is interesting to compare Steve's approach to Peter Cundall's simpler ways..... and that Peter is a very healthy 86 years old!! Anyway, I will be trying out the clay bath (minus chemical additives) method as I think this alone will be great.

    1. The custom blend Gordon adds is based on a soil test of the clay he uses and which of 12 important elements is missing in his clay. The aim is to make his soil as packed with nutrients as possible. And yes, there is more than one way to achieve this. In the Soil Biology workshop to be held on 31 August Letitia Ware will explain a different approach to the same problem. Her approach may appeal very much to you and it may like to attend for that reason. On the day I will raise the issue of Steve Solomon's approach and hers. One could say that the two are complementary (see blog post 'And now improve your soil biology' on this blog). Cheers, Max

    2. I sent my subsoil clay to the lab along with a sample of some of the first clay compost I made a year ago with the addition of some of the amendment blend that was tailored to my garden soil. I would like to post the lab reports here but may have to do it on the facebook group page.
      It shows how a clay with an exchange capacity of 17 can build a compost with an exchange capacity of 37. The garden soil had an exchange capacity of about 12. The next time I test the garden soil I expect to see a substantial increase in this figure.
      The compost is still way short on Calcium, and excessive amounts of Magnesium and Potassium are present. By using a blend that is tailored to the clay and takes account of the mineral content of the other compost ingredients a compost can be made which is going to be even more condusive to microbial activity and subsequently to plant nutrition. The amendments are simply crushed rocks or are extracted from crushed rocks using acid.
      These processes are relatively tame and harmless compared with natures way of delivering minerals to the Earth's surface by spewing them out of volcanoes in lava flows and clouds of sulphurous gas and choking dust that kills everything for miles around. It is from volcanic rocks thus formed that the most fertile natural soils are created. If you are not lucky enough to live somewhere where the full spectrum of essential plant nutrients are present in the soil then organic gardening practises will not produce fully healthy plants unless you bring in some materials which contain those vital minerals in suitable amounts.
      A strong microbial community in the soil can do a lot to make available minerals which would be otherwise unavailable, locked up in compounds with other minerals, but if those minerals are simply not present they need to be added.
      For this reason the BFA [Biological Farmers Association] permits the use of these substances if they have been shown to be deficient through soil analysis.
      Commercial organic growers would be hard put to grow saleable produce and stay in business without them.
      By adding the amendments via the composting process the minerals have already been metabolised into the web of life before being added to the garden soil and therefore there is no local toxic shock effect when the compost is added to the soil.

  2. That was interesting, thank you! I have always been a very lazy composter, but had achieved remarkable results with digging my lazy-girl's compost into the clay-based soils in my last garden and now I have some idea why.

    I found the history interesting as well. Pre-industrial agricultural systems have much they can teach us.


  3. I too have added clay to my compost, albeit not in as scientific a way. When I dig a hole for a tree or such, I spread the extracted soil to the top of my compost as evenly as possible so it gets distributed into it. Somehow that seemed like a good use of the extra. Now, as Toni mentioned above, it makes more sense. Thanks. Great article!

  4. P.S. Would love to know your facebook group name.

    1. Our group's Facebook page is named Food Gardeners Tasmania. It is for people who live in Tasmania. Cheers