Monthly Archives: February 2017

Hot Composting of Vegetable and Garden Waste

Gareth Goode – on behalf of the Transition in Kings Food Group

Last year I decided to attend a weekend-long composting course.  I’d never, in 4 years of composting at my allotment, achieved a hot compost pile.  You hear about people making compost within 6 months, in time for the next growing season.  I didn’t really understand what you needed to do to achieve this.  Well, now I do – and I want to grow abundant and vibrant veg for myself, and at Rectory Farm.  So, I’d like to share with you what I’ve learnt about composting, ready for the next growing season.

Every household should be composting their waste to help reduce transport costs and CO², and to save the financial cost of buying compost from the garden centre.  I once read a short article and it basically said ‘you prune your garden and put the waste in your green bin; you pay for it to be taken away, then, in the summer, you go to the garden centre and (at great cost) buy it back again’.  What a crazy world we have made!

I hope this article will encourage you to question how you treat your waste and to turn it into a valuable, and rich, garden material that your plants/vegetables will love.  Perhaps this will make the idea of composting worth ‘having a go’ or help you to improve it, if you’re doing it already. Transition in Kings food group would welcome you to join in nurturing the vegetables we grow at Rectory Farm, Gade Valley Close, which can be seen opposite Taylors Tools.

Whatever you have available to compost, whether it is kitchen/garden waste, vegetable waste on a farm level (like at Rectory Farm) or animal manure on a farm, the principle is the same.  The first thing to know is simple, that a composting pile is a living thing and that almost every living thing needs oxygen and water.  The second important factor is the carbon to nitrogen ratio (known as the C:N ratio).  Creating a stable environment with food, oxygen and water, for life to thrive, is what’s needed to enable the bacteria, fungus, and actinomycete to work efficiently.

C:N ratio

Carbon to nitrogen ratio can be quite tricky, especially when composting non-kitchen/farm waste. What you’re looking for is 30:1 C:N ratio.  Almost everything that goes into the compost piles has a mixture of carbon and nitrogen.  However, if you are composting kitchen waste, then generally it’s the right mix.  Carbon is known as ‘the browns’.  These are the woody materials, like branches of trees and plants.  Nitrogen is known as ‘the greens’: these are the leafy materials, like leaves, soft flower stalks and grass clippings.  See the table here for more information.

cn ratio


If you can squeeze more than a few drops out of a handful of compost, then it’s too wet.  Too much or too little water will also change the temperature and could either cook the pile by getting too hot (>70°C), or cool the pile by allowing in too much rain.  When the pile gets hot and the decomposition works quickly, the pile will dry out and will need rehydrating.  Too much moisture will cause the oxygen to be pushed out, under the weight, and cause it to go from aerobic to anaerobic digestion.

Anaerobic digestion is when the pile smells putrid.  A prime example is the compost bin in your kitchen.  When it is full, it is heavy and smells awful.  This type of bacteria does not need oxygen; they work much more slowly and at a lower temperature.  They produce a foul smell like rotting eggs; they lock up nitrogen and make it unavailable to plants.  However, aerobic digestion is favourable, as the by-product of the microbes (bacteria, actinomycetes & fungi) creates heat. Therefore, aerobic digestion produces heat and that sweet, earthy smell.


So, let’s touch on layering, Layering is the long-term, cold-composting method.  When someone tells you to make your compost pile, and then to sit back with a glass of wine and let it work itself, well, unfortunately it’s not that simple.  This will likely result in having anaerobic bacteria and locked up nitrogen, and may introduce unwanted pathogens into your garden.  It may not smell any more, as the decomposition process will be over by the time it looks like compost.  It might even help your soil structure, but when growing vegetables, it is important to get the nutrients in the soil and in Hertfordshire, we really need the nitrogen.

With a free-standing pile (see picture), you can monitor the process more easily.  Turning is done ‘on the spot’, to get air into the pile, if it’s too compact and lacking in oxygen, or if it needs opening to rehydrate.  So, a little intervention is required; with a few hours input a year, you can make earthy-smelling compost, rich in humus, aerobic bacteria and fungus.  This can be ready for when you need it, before the next season.

By making a hot pile, the process is quicker and it activates actinomycetes which form the humus.  Also, more waste can be composted, eg citrus, thicker woody garden prunings and tough leafy waste.


What it will look like and when to take action.

So, during the process you will notice a few changes.  The top sagging indicates that decomposition is taking place, so check inside that it’s not too hot or dry, as this would kill the microbes.  The fungus will look like white dots on the wet woody material, towards the end of the decomposition process.  The actinomycetes look like small cobwebs around the dark material.

If the pile is too wet, then you’ll need to dry it out. This can be done by adding dry wood chips (small pieces about ½” diameter), and it’s good to have these available to hand in case you need them.  Although wood chips are high in carbon, it won’t be available to the pile at this size.  Sawdust, on the other hand, is very high in carbon, about 500:1, so use it sparingly.  Straw can be used, but be aware of the increased carbon.  Cover with something waterproof and ideally breathable.  At Rectory Farm we have some proper compost covers available, at cost price, from £25.  Come and see us from 10-12am Thursdays and Sundays.

If you have the right mix, then your compost will generate heat, this is the activity of the microbes hard at work.  So, your heap should get up to 60°C.  If it gets too hot (>70°C), then there is too much carbon and action is required to cool it down, ie break it apart and remix it.  The heat will also have reduced the moisture content of ideally 60%, and more water will be required.  You’ll have to spray the inside of the heap and mix together.  If it’s too cool, then it’s lacking in carbon and you’ll need to add something like straw (straw has a ratio of about 70:1).