I was asked recently “how much compost do you need for a plot?” A simple enough question but the answer is a little more complicated. My first thought was to answer “As much as you can get” but I think I can answer the question a little better.
First let’s look at what we mean by compost.
Obviously compost in this context is the result of rotting down the waste green materials from the garden either by hot composting or slow cold composting but for the purposes of this discussion I’m also including well-rotted manure, most often horse or cow manure mixed with straw or wood shavings.
To me, the best compost is a mixture of both animal manure and green waste with carbon rich materials such as wood chippings, shredded cardboard or paper etc.
Calculating How Much Compost is Needed
To calculate your compost / manure requirements you’ll need to know the area of your growing space and how much compost you want to place on it. Traditionally a plot was divided into thirds; one third limed, one manured and / or composted and one third untouched.
However, nowadays most of the organic, no-dig market-gardeners work on adding compost to a depth of 2 in. or 50 mm on their growing beds annually. That gives me a good starting point. So, let’s say you’ve got a plot with 8 raised beds – each 10 ft. long by 4 ft. You won’t need to add compost to the paths so they can be ignored.
Now it’s far easier to work this out if everything is in the same measurements. I’ll run through in Imperial and then repeat the calculation in Metric.
Calculating Compost Requirement – Imperial
10 ft. = 120 ins. And 4 ft. = 48 ins. That gives us an area for 1 bed of 120 x 48 = 5,760 sq. ins. Times 2 ins. Deep = 5,760 x 2 = 11,520 cubic inches. Finally times 8 beds = 92,160 cubic inches in total.
Usually you would measure home-made compost in cubic yards or cubic feet. There are 46,656 cubic inches in a cubic yard (36 x 36 x 36 ins.) So we divide our 92,160 by 46,656 to get 1.98 cubic yards. Call it 2 cubic yards.
Calculating Compost Requirement – Metric
The same calculation in metric measurements would be 3 M x 1.2 M = 300 cm x 120 cm giving an area of 36,000 sq. cm Times 5 cm depth gives us 180,000 cubic cm and times 8 beds gives us 1,440,000 cubic cm in total. The next bit is easy as there are 1 million cubic centimetres in a cubic metre so just move the decimal point to give us 1.44 cubic metres. Call it 1.5 cubic metres.
There’s a small difference between the metric and imperial results because 10 feet is actually 304 cm etc. Since you’ll never spread your compost exactly to 50 mm or 2 in. depth, a rough result is good enough.
Incidentally, 1 cubic metre = 1,000 litres so if you were buying compost in litres you’d need 1.44 x 1,000 = 1,440 litres which is about 20 standard 70 litre bags of bought-in multi-purpose compost.
Producing the Compost
As you know, when you compost your green waste the volume reduces. As a rule of thumb, the ratio is 5:1 and that’s with shredded waste which is more dense and takes up less room than un-shredded waste. You can double that ratio for un-shredded waste; i.e. 10:1.
You’ll need at least 5 cubic yards of shredded green waste to make 1 cubic yard of compost. Manure doesn’t tend to reduce much as it’s already nearly compost having been processed through the animal’s gut.
Market-gardeners tend to buy in their compost and in Crewe we were lucky enough to be able to get large skip loads of compost, about 12 cubic yards, delivered to the plot for a nominal £40. The addition of a number of loads improved the heavy clay soil on my plot out of all recognition.
Producing that large volume of compost yourself is hardly practicable just from your own green waste. What you should do is seek out as much manure as you can get and if your neighbours throw away their green waste, offer to dispose of it for them!