Editor’s note: Containers are an accessible gardening gateway for many urban dwellers. In her new book The Rooftop Growing Guide: How to Transform Your Roof into a Vegetable Garden or Farm, local farmer and educator Annie Novak breaks down different growing media components, helping you determine what potting soil mixture best meets your needs.
The following is an excerpt from her book.
Container gardeners typically use potting soil as their growing media. Potting soils are soilless, instead using base ingredients of peat moss, perlite and vermiculite. Each has characteristics that help with water retention, pore space (for water and airflow), and bulk up volume without adding much weight.
Additionally, potting soils will include a regional range of additives designed to increase moisture-holding capacity and fertility or balance the pH of the mix. These ingredients are listed on the packaging. If you choose to use organic potting mix blends (highly recommended), here are a few good additives to look for and what they do for your plants:
Anything from animals (feathers, blood, manure)
Upcycled from animal processing, these various amendments generally add nitrogen to the mix. An alternative to blood meal and ground feathers is manure; bat and bird guano and chicken and cow manure are the most common. They should all be well-aged for your safety as well as to avoid causing nitrogen burn on your plant’s roots. If making your own mix, wear gloves and a mouth and nose protector when adding these materials.
Anything from the sea with a shell (lobster, oyster)
These products vary by regional availability and are often sourced from the fishing industry. Shells are made of calcium, a necessary micro-nutrient for plants.
Harvested from the outer husk of processed coconuts, coir is a fibrous product touted as a substitute for peat. Both dry are extremely light and compact, and saturated, can hold a good deal of water.
Compost is made from a wide range of materials, from vegetable scraps to sewage sludge. The detail-oriented rooftop gardener can call up the company in question and ask for their processing practices.
Forest loam or humus
Loam and humus are soils taken from land as it is cleared or developed. The terms apply to a crumbly, well-aggregated soil. Descriptors like “forest” can regionally indicate variation on acidity. A forest loam from the conifer-rich Pacific Northwest may very well have a lower pH than humus harvested from the prairie states. If you’re feeling detail-oriented, check the origin of the product or call up the manufacturer and ask for the results of their pH test.
Peat moss is derived from sphagnum moss, a genus of moss of which there are over a hundred species. In a potting mix, peat moss is frequently used as a base product to add volume while contributing little weight of its own. Peat moss can carry a significant amount of water, however, increasing the weight of your potting mix by up to twenty times its original weight. Over time, peat moss can dehydrate past recovery and will need to be amended with additional peat moss and other potting mix materials. Peat moss by its nature is slightly acidic.
Frequently mistaken for fertilizer, the small, white, lightweight particulates of perlite are popped volcanic rock. They add air space to the soil, bulking up its volume while adding little weight. Perlite often migrates to the surface of the soil over time and can blow away. Work it back into the soil and amend as needed.
Vermiculite’s small, flaky, shiny particles are used in potting mix for plants and as a pure product for seed germination. It has the Goldilocks “just right” qualities of both breaking up sticky soils and being absorptive enough to retain and regulate moisture well.
Worm castings, typically harvested from commercial red wriggler vermicomposting operations, are nutrient-rich, pH balanced and crumbly in texture. If you can find a mix with this amendment you will not only have a great soil product but also the happy possibility of introducing worms to your rooftop!
Illustrations by Adriana Gallo.