"What makes soil good?"

“Do you have tips to share on general soil health (how to identify, how to reinvigorate, does soil ever ‘go bad’?)?”

—Amanda

I’m embarrassed to admit that it took me over a year to figure out all the ways that compost is connected to dirt, and to understand how it can be used to build and sustain the health of my soil. Now, it’s all that I think about.

Soil, like compost, can give you a lot of information through simple observation. What color is it? What texture? What’s living in it? How many of them? The answers to these questions will provide you clues as to the current condition of your soil, which you can then apply to your soil goals. Generally, people growing stuff in their yards want to look for soil that is dark, crumbly, and fragrant. It should also show signs of life: worms, beetles, and other insects. That means there’s a thriving microbial population present, which means the soil is active and healthy. If your soil is gray, dry, and insect-free—that means you’ll have some work to do, in order to bring it back to life. (Spoiler: this work involves compost.)

All that said, “good” soil is a dynamic concept. Different types of soil are “good” for different types of plants and needs. What’s growing in your soil? Is it dominated by one plant, or are there many different kinds? What do they look like? If your yard is colonized entirely by one species of weedy-looking plant or flower, that probably means you have low-nutrient soil. These types of plants are often referred to as “pioneer” species because they’re first to spring up in recently disturbed soils (like after a fire), where nutrients are much less abundant. Are their signs of disease? Are they vigorous and upright? Are there bees and butterflies around? These are all great questions to ask on your road to soil discovery.

When it comes to properly digging (no pun intended) into this particular reader query, though, I’m simply not a qualified expert. For that, I’ll turn to Andie Marsh, a real deal soil scientist in Austin, Texas who founded and runs her own lab. Andie loves microbes even more than I do, and definitely knows a lot more about how they work, so I’m turning this week’s edition over to her for a quick soil primer.

Hi Andie, can you talk to me a little about how you define “good” soil?

Depending on the type of vegetation the definition of soil health will change but, generally speaking, I define soil health by both its structure and its ability to sustain microbial life below-ground. These two qualities go hand-in-hand as bustling microbial life begets beneficial soil structure—the kind that filters, absorbs, and stores water!

Even with our best intentions, I find soil can get “sleepy” in containers and abused in plots if we’re not thoughtful about it, though.

What about in yards or fields, where you might want to grow vegetables?

Contrary to the belief brought on by conventional farming methods, the soil beneath our feet does not have a finite amount of fertility. In fact, I’d say it has an infinite amount! But it does rely on nutrient cycling to maintain access to that fertility.

Studies have shown that mineral particles —our clay, sand, and silt particles—all hold pretty similar potential in terms of nutrients (Sparks 2003). So when we experience nutrient deficiencies in our plants, it’s not because the nutrients aren’t there in the soil, it’s that they’re not plant-available. Microorganisms are the MVPs that facilitate the transition of mineral nutrients into water-soluble, plant-available nutrients.

Where does compost play a role in the nutrient cycle?

When we harvest plants without returning organic matter to the soil, we are effectively breaking the cycle. We are taking food, partners, and habitat away from the microbial populations that churn the wheel of soil building and nutrient cycling. 

Organic matter, i.e. well-made compost, can provide food and habitat to a soil system that’s been harvested, which is excellent! But it’s important to note that organic matter alone can’t fill the role of “partner” or protector like plants and their roots can. So, it’s important to find ways to keep roots and living plants growing as much as possible pre- and post-crop harvest and to never leave the soil bare, even in dormant times (i.e. mulch!)

That’s fascinating! It makes me think about how we care for houseplants, though. We tend to “set and forget” with indoor plants. With the exception of regular watering, we don’t do much to care for their soil.

Totally. Containers can limit the ability of plants to find what they need in the rootzone, considering how there’s a significant loss of introduction of new rootzone organisms when the soil is contained. Outdoors, you’ve got a general migration of spores and critters able to move from here to there. Less so with a container, but fear not—we can steward nonetheless! Inoculating regularly with well-made compost and compost liquids is a key part of maintaining healthy soils, even in house plants.

Something I see happen a lot with containers of various types, from large raised beds to boston fern hanging pots, whether you’re hand watering or on irrigation, is that moisture tends to follow the same path over and over. So, the whole soil system often isn’t maintaining enough moisture to support microbial activity across the whole volume of the container, which limits the plants’ access to the microbial partners and soluble nutrients they need. I think of this when I make a pour-over coffee. If I just left the spout in one spot I’d get a fraction of the delicious coffee extracted compared to going in the hypnotic spiral pattern, wetting all the grounds. 

Watering houseplants can also create compaction.

Right, moisture is related to compaction. The process of water evaporating from soil tends to bring smaller particles up through the soil profile over time, which contributes to compaction layers and crusts. Secondly, the more uniform the moisture, the more widespread the root system, which encourages better soil structure (as plant roots feed microbial populations, microbes build soil structure).

Finding ways to regularly water areas away from the stem is also important, as they are equally important to your plant (just because we CAN grow a plant by exclusively watering its base doesn’t mean we should). When you prioritize watering more uniformly, you may find you can actually water far less frequently.

On a related note, containers are inherently more exposed than the soil below our feet. For instance, we may find that a plant in a pot is more sensitive to temperatures than the same plant in the ground. Moisture is QUICK to evaporate out of a container and, unlike the ground, there are less reserves available to your plants, so definitely protect the surface of container plants and in-ground plants alike. Also, know that if any side of a container is exposed to direct sunlight that’s an added stress to account for in our plant management—just monitor it. I’ve found I simply can’t grow certain plants in certain containers because the rootzone gets too hot.

What can the average plant-owner do to treat their plants a little better?

In theory, if you’re amending contained soils regularly with good compost, teas, and mulch, then you may never have to take rejuvenation efforts. Of course, none of us are that perfect, so just consider the concept a north star and remember, these are natural systems and they’re resilient!  

I have lots of pots laying around from transplants that don’t have anything growing in them. To maintain some activity in the pot I actually water them when I water my plants (at the risk of looking like I have imaginary plant friends). I try to keep them mulched, too. When I’m ready to put a plant in an available pot, I test to see if the soil is hydrophobic by watering the pot evenly as I have been, but this time I remove the mulch and dump the soil onto a tarp to see if it’s moist all the way through. It almost always looks lackluster (gray or ashy in places, some compaction, etc.) At this point, I’ll mix in some compost or quality soil conditioner (I like Happy Frog’s soil conditioner). I’m loosey-goosey with the ratio, whatever I have on hand will have to do, but I’m hoping for 1:1. While the mixture’s still on the tarp I spray it with some water and organic fertilizer or tea to wet it. I’m trying to get that “wet wrung-out sponge” vibe. Then I give it one more soak after planting and mulching! It’s satisfying to reuse potting soil. Even if it looks really dry and tired, I still value it as a substrate it just needs some inoculation of the good stuff—organic matter and soil life.

A quick note on potting soils that are actually really wet, though:

Sometimes this happens when there’s a bag of soil I’ve been meaning to use, but it gets left outside and rained on and god forbid there are multiple bags stacked one on top of the other. The problem here is oversaturation and a lack of air in the soil. I wouldn’t want to immediately use this as it likely has detrimental organisms growing in it by now (especially if it stinks!) Ideally, I’d compost it. Less ideally, I’d break out the tarp again and spread it and let it dry pretty good. Then, I’d mix it with a well-made compost and follow the process outlined above!

I like that we’ve circled back to compost.

Haha, yeah. Well, to answer the initial reader question more directly:

Does soil ever “go bad”?

When a soil system is just doin’ its natural thing, left undisturbed, with living vegetation, it has what it takes to be resilient, and move in and out of periods of dormancy. 

I see a soil that’s “gone bad” or “gotten sleepy” as one that we’ve deprived—through our management methods of microbial food and/or habitat. This puts stress on beneficial species, and they eventually die or go into a dormant state. It’s not until we create the conditions for them to thrive again, that we can turn things around for our soils and plants!

If we’re harvesting, we need to consider how we’re returning organic matter to the soil and protecting it when the crop isn’t growing. And if the soil is in a container and either too dry or too moist for a long period of time, we’ll want to take some measures to nurture it back to a state that can support microbial activity.

Amazing. Thank you!

I hope this was helpful! Thank you for having me here!

On a closing note, Andie kindly offered to biologically assess each of my compost piles here in Los Angeles. I’m proud to say that she found them teeming with life: bacteria, fungi, nematodes, and microarthropods. Pretty neat. Anyway, I recommend you find her on Instagram and Substack if you’re tickled by learning about soil and want to know more.

More soon!

Love, Cass