Gardening with Xpress: Replenish the soil so your plants can ‘eat’

Warming greetings to you, March lambs. Our mountain home is abundant with blossoms as the trees and bees awaken and the green ones begin growing. As you get into gardening this year, be sure to reach out with any questions that pop up with the dandelions and violets. Send them to me at gardening@mountainx.com. This month’s questions were all about soil, which feels very appropriate as we begin to engage with the living earth once more after winter’s rest. 

Container confinement

Do I need to change out the soil in my container garden each spring? Or add anything into them?

Gardening in containers is a bit different from gardening in the ground, but many of the same principles apply. Since the soil in raised beds is finite in quantity, plant roots can’t explore beyond it in search of water and nutrients as they can in the ground. In addition, raised-bed soil mixes generally need to be fluffier and more aerated than the soil of the ground. Water, nutrients and plant roots need to be able to move about easily in the small amount of soil that’s contained in even the biggest containers. 

The short answer to your question is no, you don’t need to completely change the soil in your container garden each year, and yes, it’s a good idea to add some amendments to it. Whatever plants you grew in that particular soil last year pulled many of the nutrients into their bodies in order to grow. That means those nutrients will be in shorter supply in the soil mix this year. Additionally, pests and diseases can build up in soil over time, especially in a closed system like a container that probably isn’t teeming with beneficial organisms. 

There are a couple of ways to add good stuff to your used potting soil. One is to simply sprinkle an all-purpose organic fertilizer like Plant-Tone onto the surface of the soil and scratch it in with your fingers, a trowel or hand rake. Another is to remove the soil and add some amendments like compost, worm castings, fertilizer, etc., thoroughly incorporate the additions, then put the soil back into the containers. A third option is to leave the soil and be prepared to “fertigate” your plants with liquid amendments like fish emulsion, Neptune’s Harvest fish and kelp blend, compost tea, and/or urine. (Yes, urine, human pee. You can read more about using urine as a garden fertilizer on my blog post here: avl.mx/cs1.) Of course, you can choose some combination of all three of these approaches, too. 

Another important step to care for soil in container gardens, or any garden, is to employ crop rotation: Don’t plant the same thing in the same place two years in a row. Different crops require different amounts of nutrients and are more susceptible to different kinds of pests and diseases. Moving things around helps reduce plant illness and maximizes the use of fertility in the soil. 

Beans and peas are members of a very special plant family called Fabaceae (some of us think this should be renamed Fabulouseae because of the amazing ability I’m about to describe). All plants in this family — also known as legumes or pulses — have a symbiotic relationship with a type of soil bacteria that can transform atmospheric nitrogen (dinitrogen) into a form of nitrogen that’s usable by plants (ammonia). The legume plants grow nodules on their roots that house this bacteria, and they share some of the sugars they make through photosynthesis with their tiny tenants. In return, the bacteria provide nitrogen for plant growth. This is part of why beans and peas are high in protein, which is a nitrogen-based substance. It’s also why following “heavy feeder” crops that pull a lot of nitrogen from the soil with “nitrogen fixing” legumes is a good move for the soil and your garden’s productivity. 

Even with these soil-care practices, it is a good idea to change or significantly add to your container garden mix every couple of years. It doesn’t have the benefit of billions of micro- and macro-organisms carrying nutrients and other compounds from nearby decomposing organic matter to enrich it over time as the soil in the ground does. 

Picking the best seed-starting soil mix

What’s the best potting mix for starting seeds? Can I make my own? 

The growing medium, or soil, that you choose for growing transplants is very important. An ideal material will be fine and without large particulates, have a balance of water-holding capacity and drainage, and contain some amount of fertility for the plants as they grow and get hungry. Potting soils that are appropriate for container gardening are usually too chunky for seed starting. Similarly, soil dug out of a garden bed isn’t ideal because it’s likely to be both too chunky and too heavy (dense with clay particles).

You may find growing mediums sold specifically as seed-starting mixes. I don’t tend to use these because they’re typically devoid of plant nutrients, and therefore only appropriate for germination and not any growing. If you use a seed-starting mix that doesn’t contain compost, worm castings, or another source of fertility, you’ll need to transplant your babies very quickly into something richer, so they don’t starve. The reason these seed-starting mixes don’t have fertility is that they’re meant for seeds that have very long germination times. If you water soil for many weeks without a seed growing in it, other organisms, like algae, may make use of the nitrogen and actually create a toxic environment for your seed. Low-fertility seed-starting mixes are great for slow-germinating seeds because they avoid this problem, but they’re not ideal for hungry little veggies.  

You can mix your own growing medium using coconut coir or peat moss, compost, vermiculite and perlite, along with amendments, and up to one-quarter sifted garden soil. I usually don’t do this because I’ve found that when I do, it doesn’t work as well and doesn’t actually save me much time or money. Unless you run a nursery or have a very large garden or farm, the cost savings associated with mixing up your own growing medium is pretty minimal. Plus, high-quality premade mediums have been crafted and analyzed for best results and usually contain lots of ingredients to give your transplants just what they need. Two nationally available mediums that we’ve had success with are Daddy Pete’s Sea ‘n’ Farm (which is blended here in North Carolina) and Vermont Compost’s Fort Vee and Fort Light. 

If you have some old container gardening soil that you know isn’t carrying diseases, you could make your own mix by sifting that if it’s chunky and adding worm castings and an all-purpose organic fertilizer, plus other amendments that you can afford such as microbial inoculants and kelp meal. Similarly, if you have trays or pots that you tried to start seeds in and it didn’t work, you can add that soil to a container garden mix. But keep in mind that if it’s been exposed to water and sun for many days or weeks, it’s probably lost most of its nutrition and is now simply a physical substrate rather than food for plants.