Everything you need to know about baking your own no-turn, microbe-rich compost cake.
We learnt this aerobic Compost Cake from Kay Baxter in New Zealand. It is designed to produce high humus content compost that will provide the soil with minerals needed to make nutrient-rich, healthy plants. This cake requires no turning and produces humus-rich compost in six months. We make our Compost Cakes from spring to autumn; we always make our final fling of cakes when we dig up our dahlias and clear much of the garden in October (always leaving seed heads for birds and insects).
These autumn cakes ‘bake’ throughout the winter, ready for digging into the garden in the spring. For those of you who have got used to the bay-system of making compost these cakes will initially feel frustrating but you can still use your bays as storage as they are brilliant for organising your different ingredients like a pantry–or you can layer up your compost within your bays along the lines below. We recommend you gather ingredients over time, storing them in piles or in your compost bays. Note if you have to store nitrogen for any length of time always layer carbon in between to stop it putrefying (we often keep a pile of leaves, a bail of straw or some young woodchip to layer with grass clippings). On the day of making your cake try to gather some fresh greens as these help to activate the pile.
INGREDIENTS:
CARBON
Carbon sources are essentially mature plant materials that have gone to seed and become brown eg straw, old hay, brown leaves, stalks, seed heads as well as plain brown cardboard. Artichokes, Jerusalem artichokes, lupins, oats, quinoa, millet and amaranth are all sources of carbon which we often grow in rows to feed our Compost Cakes. Young woodchips (from twigs or small branches) or hedgerow clippings are good sources of carbon. Often referred to as Ramial Chipped Wood–these woodchips are from branches which are less than 7cm in diameter. Old woodchips cannot be used in the cake as their higher lignin levels make them too difficult to break down over 6 months.
NITROGEN
Nitrogen sources are leafy, green, immature plant materials that are actively growing eg grass cuttings, fresh weeds and garden clippings as well as organic animal manure. Animal manure is not essential–but organic poultry manure adds high levels of phosphate and even one organic cow pat diluted in a bucket of water will be highly beneficial poured over your pile, bringing valuable minerals and microbes. Fresh weeds are a good source of nitrogen but we would not advise putting in perennials weeds (eg ground elder, bindweed or seeding dock) as the pile may not become hot enough to kill them. We grow comfrey (Bocking 14), valerian, yarrow and nettles in the garden to layer in our Compost Cakes as they are nitrogen and mineral-rich. If you have food waste we recommend treating it with Bokashi before it goes into your Compost Cake.
CLAY
A tiny percentage of clay helps hold the minerals in the pile whilst the humus is forming. It is best if this is dried out and fine before adding to the pile. We often spread it out on a hard surface, let it dry in the sun and then break it up. Alternatively we mix clay with water in a bucket, stir it and form a clay slurry to use in layers in the Compost Cake (ideally we suggest two buckets of clay slurry per cake). We use clay because it acts as a home for the microbes and as a pantry for the food and moisture needed by the microbes. Clay is made up of very fine particles so that the combined surface area of all the particles in a peanut-sized clod equals the area of a tennis court. The particles also carry a negative charge so each one is capable of forming bonds with the positively charged particles (ions) of the essential plant nutrients. The expansive surface area and attraction of clay particles mean that both water and nutrients are attracted to it, acting as a pantry of food for the microbes. If you do not have access to any clay in your garden it is possible to buy bentonite clay.
GOOD LOAMY SOIL AND OLD COMPOST
The existing micro-organisms in good loamy soil and old compost help to start the decomposition process of the pile. The soil helps to hold water, prevents the temperature spiking and holds minerals while the humus is forming.
WATER
Ideally use water from a borehole to avoid chlorine as it harms the microbes. Alternatively attach a simple carbon filter to your hose or leave buckets of tap water outside for a few hours for the chlorine to evaporate before making your cake.
BIOCHAR
Biochar is a high-carbon form of charcoal that is produced by heating organic matter at an extremely high temperature and in the absence of oxygen. It adds a highly stable form of carbon and holds onto minerals and moisture.
OPTIONAL ADDITIONS
There are other optional additions that we often add to our Compost Cakes in thin layers:
Climate Compost - inoculates the cake with beneficial microbes to aid decomposition and the formation of humus.
Seaweed–is rich in minor minerals (including calcium, potassium, magnesium, and iron). Ensure it is from an ecologically
sustainable source.
EQUIPMENT
6–8 hazel stakes to form the cake
Cover for rain - we use Toptex or thatch with old straw bails
Ideally rainwater or filtered water
Hose or watering cans
MAKING THE CAKE
The optimal size is 1.6m diameter and 1.2m high. This allows the cake to maintain temperature and a good structure aiding the creation of humus. A 60:1 carbon:nitrogen ratio promotes the development of humus and a diverse range of microbial life, without allowing the cake to overheat. To achieve this in volume terms we use a ratio of approximately 3 parts carbon, to a 1/2 part of greens and a 1/4 part soil/old compost. We build these up in repeated layers.
Place bamboo stakes in a circle of 1.6m diameter, forking the base ground to aerate it.
Start the layering process, making sure the first carbon layer of the cake has structural carbon to maintain airflow, for example, artichoke, cardoon, corn or sunflower stems.
Build up the cake with a 12cm layer of carbon (the sponge), followed by a 2cm layer of nitrogen (the jam) and a 1cm layer of mixed old compost and clay (the cream).
It is important to water each layer as you go. Each layer should be as moist as a wrung out sponge so that a drop or two of water falls when squeezing the material in your hands. Too much moisture means less air flow and anaerobic conditions, not enough makes decomposition difficult.
Continue until the cake is approximately 1.2m high. Cover it with a 15cm thick layer of straw or a layer of Toptex so that the top of the heap forms a mound to allow water to run off. Place a thermometer in the central-top third of the pile to monitor the temperature.
We always finish our cake with a flower.
COMPOST PREPARATIONS
We also like to use biodynamic preparations in our Compost Cakes: Yarrow 502, Chamomile 503, Nettle 504, Oak bark 505 and Dandelion 506 and the liquid Valerian 507, but this is not essential.
When you have finished your Compost Cake make five little balls of clay (you can buy bentonite clay or you may have an area of clay in your garden - you can often find this under your topsoil). Push your finger into one of them and put a pinch of one of the (non-liquid) compost preparations inside it. Repeat for the other 5 preparations. Place these balls spaced out into five holes on the top of your Compost Cake.
Pour half the liquid preparation Valerian 507 into a hole in the middle of the cake and then spray the other half over it to mobilise the phosphate-activating bacteria in the pile.
Stage 1 – DECOMPOSITION
The heap will be damp, the colour light brown/green and the aroma musty. The cake may heat to 57/60°C in the first two weeks but will then go down to ambient temperature over the next few weeks. However do not be alarmed if it does not go over 50°C.
Cakes that stay low in temperature hold more carbon (rather than sending it up into the atmosphere in the form of CO2). Sometimes we make our cakes with vertical perforated pipes to release excess heat as ideally you do not want the temperature to rise over 65°C as this harms the good microbes. Unlike our hot composting windrows we cannot guarantee that the cakes are weed-free but they will be full of beneficial microbial life.
Stage 2 – HUMUS-BUILDING
The temperature and moisture levels will lower and the colour will turn dark brown with an earthy smell. When the compost is crumbly (after approximately 6 months) and you can see life crawling around in it, this is when your compost will be close to maturity. We dig out the material from the middle and use the outside material to form our next Compost Cake.
If you want to see this in action watch Kay Baxter’s course Growing Great Compost or our Create Academy Soil Course.
HOW TO TEST YOUR COMPOST
Whilst we carry out a variety of laboratory tests on our Climate Compost, there are simple visual assessments you can carry out at home to judge if your compost is fully digested and mature. Mature compost should be a rich warm brown colour. Grey or black tones indicate a lack of oxygen (insufficient aeration) and that the organic matter has putrefied. There should be no visible plant fibres (i.e. identifiable organic matter). It should feel crumbly, cool, and slightly sticky (but not wet). It should not feel gritty. The ideal moisture content should be between 50-60%. This can be measured simply by dehydrating a sample of compost in an oven (at a low temperature) and comparing the mass before and after. A mature compost should be within 5°C of the ambient temperature of the surroundings. If the temperature is higher than this, it suggests that the microbes are still ‘working’, so more time is needed for the compost to become fully mature.