Composting My Urban Container Garden

(Philadelphia)

Hi! I Live in the city and am lucky enough to have a good sized outdoor deck that supports all manners of vegetables in containers during the summer.

At the end of the summer I have a hard time figuring out what to do with all the spent plants and potting soil, because it seems such a waste to throw that volume of organic matter in the trash. To try to fix this problem, I recently invested in a great 38 gallon compost tumbler.

My question is: can I put the old potting soil in my compost tumbler along with the plants and other compostable waste? Or will that kill the compost reactions? Thank you!

Your Answer

So great that you have a food garden on your deck. And now a compost tumbler. Next thing you'll be giving workshops on the urban apartment homestead.

The short answer is that yes you can add soil to your compost but we should really look a little deeper here.

Is Your Soil Spent?

Way back in the 1700's a Belgian scientist by the name of Jan Baptista van Helmont made this experiment. He wanted to know what nutrients plants get from soil.

He filled a container with exactly 200 pounds of soil that he had dried very thoroughly. He then watered the soil and planted a five pound willow seedling. He covered the container with a perforated sheet of metal to prevent anything but water getting into the container.

Five years later the willow weighed 169 pounds for a weight of 164 pounds. He then turned out the soil, again drying and weighing it.

Guess how much it weighed. It weighed 199 pounds 14 ounces. In other words the soil only provided 2 ounces of the nutrition...

It's a little more complex than that but the fact is that green plants get most of their nutrition from the elements in the air and water. They use photosynthesis to take the energy from sunlight along with CO2 and H2O to make the carbon compounds that grow the plants and just as importantly feed the microbes in the soil that manage to do things like make the important trace minerals available and to fix the nitrogen in the air into a plant available form.

So... the question might be put thus... Is the soil spent?

What I would suggest is that you leave a good portion of the soil in your pots over the winter. That is what I do. You can add some of the soil to your compost tumbler. It will help balance out the kitchen scraps you are also composting in the tumbler.

In the Spring when you want to get your containers fired up again just topdress them with some of your compost and carry on. Much less work don't you think?