The Medium Is the Message: On Potting Mix, Mulch, and Fertilizer

Growing plants in pots is sort of like raising fish in an aquarium. You’re trying to recreate the conditions that an organism enjoys in the wild. But if you plop a tropical fish into a tank of seawater, without the oxygen or nutrients or temperature that the natural ocean environment provides, that fish won’t live very long. Like the ocean, the earth’s topsoil contains a complex ecosystem of bacteria, fungi, invertebrates, nutrients, and distribution of water and oxygen that won’t survive the confines of a container. That’s why you can’t plop a plant into a pot filled with regular outside dirt.

Just like you need oxygen pumps and various additives for a healthy aquarium, healthy planters require certain accoutrements. Thankfully, it’s much simpler and cheaper to create healthy conditions for plants than for fish (I remember my childhood aquarium: So. Many. Dead. Fish. Sorry, mom and dad!).

I know the world of planting mediums and fertilizers can be daunting; I’ve been there. Sometimes I am still there! But after several years of trial and error, I’ve landed on a short list of “ingredients” for my pots that I mix and match depending on the plant.

Non-Dirt Dirt

Two general rules of thumb: 1) Anything labeled “potting mix” should theoretically provide most plants with enough drainage and nutrients to survive in a pot; 2) This is up for debate amongst those more science-y than me, but I stick with the organic stuff and avoid Miracle Gro and its ilk. From an environmental and quality standpoint, it seems like the safer bet.

Organic potting mix will contain natural ingredients that provide nutrients, water retention, and drainage. For cacti, succulents, some Mediterranean plants, and some California natives, it might provide too much of the first two. For these, I opt for a cactus potting mix, which typically contains lower nutrients and extra materials that drain well, like perlite, gravel, or sand.

Honestly, if you just buy a bag of each, you’re basically set. Use regular potting mix for hungrier, thirstier plants, cactus mix for the tough guys, and combine them for those somewhere in between. 

Optional Add-ons

If you want to finetune your medium to suit your plants’ needs, I have a modest list of recommendations.

First, I add a high-quality organic compost to most of my pots. I’m not set up to make compost in my apartment (though people do it!), and I shell out about $25 per bag, but a little goes a long way in a small balcony garden. I’ve probably gone through two whole bags in the past three years.

This stuff adds living goodness to my planters. It’s great for tomatoes, roses, and other hungry flowers and veggies. For drainage-loving bulbs and natives, I simply add a few scoops of compost to straight cactus mix. I sometimes add a few handfuls to older pots, working it into the surface to replenish the nutrients.

I also recommend amending your mix with perlite or vermiculite, depending on what you’re growing. Both natural materials, perlite helps with drainage and is a super lightweight alternative to gravel or sand ­— cuttings and California natives love it. Vermiculite helps retain water, so it’s great for seedlings and plants that don’t like to dry out. You can find both in small bags, which is nice if, like me, you have limited storage.

Mulch

Mulching is the act of blanketing the surface of the soil with any material — be it wood chips, compost, leaves, straw, gravel, or what have you — for the purpose of retaining moisture, improving the soil, and controlling weeds. 

Though it’s not always necessary, mulching can benefit container-grown plants. On my balcony, I use two types of mulch: usually straw, and then gravel or grit for my cacti, succulents, and Mediterranean herbs and trees. I find they both help regulate temperature in the summer heat and winter cold and allow me to water less frequently.

The Dreaded Fertilizer

The topic of fertilizer probably deserves its own post, but I am unqualified to write one. To understand the scientific aspect of fertilizer, the NPK numbers and all that, I suggest reading a good explainer written by a real horticulturalist.

I do know that you don’t need a science degree to feed your plants properly. Although I learned how to fertilize in a rather inefficient, experimental way, you can skip some of that by following these basic tips:

• Take a cautious approach, erring on the light-handed side when applying fertilizer, and you and your plants will probably be okay.
• Use the right fertilizer for the right plant. If the NPK numbers puzzle you, you’re not alone, but thankfully fertilizer labels tell you what they are for. They might say “all-purpose” or “citrus and fruit tree” or “roses and flowers” or “tomatoes and vegetables” or “orchid food.” The variety of plants you have will determine how many different types of fertilizer you need.
• At the very least, buy one all-purpose fertilizer in liquid or water-soluble form and another in slow-release granules or powder. Add the second to your planting medium when potting, and the first over the course of the growing season to give plants a quick boost.
• This is a personal choice, but as with my potting mix, I go for the organic stuff. I figure the world doesn’t need more toxic runoff, even in miniscule quantities. 
• Don’t feed a sick, struggling plant. The problem may be lack of nutrients, but it’s more likely to be disease or an under- or overwatering issue. Think of fertilizer as a treat for healthy plants rather than a cure for sick ones.
• Fertilize plants when they need it: at the beginning and throughout their most vigorous growing season. As their season slows, stop fertilizing. Many plants benefit from a period of dormancy, and too much feeding can cause a growth spurt just when they’re trying to lay down to rest for the winter.
• When in doubt about when and how much to fertilize specific plants, look it up!

Ready to Plant!

One you’ve got your pots and your medium sorted, you are ready to plant! If you need help sourcing pots, I have a post for that, and if you need tools, I have one for that, too. When it’s time to re-pot, I have a step-by-step guide. To help you choose plants, I have short lists of suggestions for sun and for shade, plus California natives for the locals out there.

Don’t feel overwhelmed by those mountains of bagged soil and aisles of plant food next time you’re at the nursery. As a small-scale container gardener, you only need a few essential ingredients to keep your plants happy — lucky you!