Spoiler alert: there is actually no agreement on this. Everyone is happy to identify soil as that good stuff in the garden having organic matter, minerals, nutrients, microorganisms, mycorrhizal fungi, assorted worms, insects, and so on in it; everything that plants need to be happy. And that is what plant scientists call it, but lots of other people use “dirt” and “soil” interchangeably. Some define dirt as “soil out of place”, so the stuff you track into the house after gardening would instantly go from soil to dirt. Others think that as soon as any digging occurs and the natural layers that developed in the soil are destroyed, it changes into dirt. Still others define dirt as what you have left if all the organic and living stuff is removed from the soil, although I’m not sure how this is supposed to happen. Very puzzling.
So, what do I think? There are lots of things that constitute what is commonly thought of as dirt: dust bunnies under the bed, coffee spills on the carpet, fingerprints on the bannister, bird droppings on the patio. When soil gets into places where it’s not wanted, many people call it dirt, just like the grass growing in a flowerbed rather than a lawn suddenly becomes a weed. Household dirt is pretty simple, but what makes up the ground is not, and lumping the two together by using the same word often ignores the difference. So regardless of the long history of people going outside “to dig in the dirt”, as far as what my plants grow in, I always call it soil; soil is so much more than the word “dirt” signifies.
Some soil, of course, is better than others. Too much sand and it dries out too quickly, shriveling roots and microorganism; too much clay and it can stay soggy and drown roots and those aforementioned critters, and dry hard as a rock. A good balance of sand, silt, and clay produces the loamy soil which every gardener desires-it drains at a reasonable pace and provides sufficient air space for roots and those little creatures to “breathe”.
But a good balance of sand, silt, and clay is not enough; the soil also needs to provide the nutrients plants use to build their bodies. They need phosphorus, potassium, nitrogen, calcium, sulfur, magnesium, and a host of other chemicals along with the carbon, oxygen, and hydrogen they get from air. Soil minerals break down over time and release many of these; nutrients are also recycled from decaying organisms when their bodies dissolve into the soil water and the solution is absorbed by roots. The acidity of the soil determines the form in which many of these chemicals are found and therefore how easily the plant can take them up. Although most plants prefer soil on the acidic side (where the majority of the nutrients are in their most soluble form), many do OK in the slightly alkaline soils we garden in.
Living organisms are also crucial to a good soil. The fine filaments of mycorrhizal fungi spread throughout the soil, breaking down organic matter, absorbing nutrient-rich water and transporting it to plants where they merge with the roots and exchange it for energy-rich sugars. Nitrogen-fixing bacteria find homes in root nodules in some species of plants and make a similar trade as they convert atmospheric nitrogen gas into a form that plants can use. Many other small organisms interact with each other and the plants growing in the ground to produce a healthy environment. Worms, beetles, ground squirrels, ants, and other burrowers help to keep the soil open, and they drag stuff down below the surface where it can decompose.
So, how can you keep your soil in fine fettle?
- Water sensibly: too deeply and nutrients will be leached out of reach of the roots; too shallowly and salts may build up in the surface layers.
- Maintain the air spaces in your soil by avoiding walking on it or planting in it when it is wet.
- Improve water and air penetration by breaking up the soil surface a bit if it gets crusted over.
- Dig only as much as is really needed.
- Use fewer herbicides and pesticides (better yet, omit them entirely)—they can mess up the chemical and biological balance in your soil as well as contaminate the ground water.
- Minimize the use of fertilizers. When you use them, be sure to chose the right formulas for your different plants and follow the instructions. Too much can change the soil pH, damage roots, or lead to salt buildup.
- Don’t cover the ground with artificial turf; this adds nothing to the soil and absorbs enough heat to kill the organisms below it. And, unlike plant material, it’s hard to recycle when it starts looking tatty.
- Mulch the soil to help keep it cool and reduce water loss so that roots and soil organisms have a stable, comfortable environment.
- A few times a year, add some amendments such as compost, leaf mulch, aged manure, or peat moss to the soil surface or just lightly dig them in: as they decompose, they will feed the soil critters, release nutrients, absorb water, and help create a spongy texture that makes for a good soil.
- If your plants don’t thrive in spite of your best efforts, a soil testing lab can help determine if there are problems that need fixing. There is contact information for several labs on the Resources page of the Garden Club website.
Whether you call it soil or dirt, that almost magical stuff plants grow in is worth treating well!
The Claremont Garden Club is free and open to everyone, and we’d love to see you at our meetings! You can reach us through our website (www.claremontgardenclub.org) or by sending a note to email@example.com. We are a working group of Sustainable Claremont (www.sustainableclaremont.org).
Demystifying Sustainability is an initiative of Sustainable Claremont (susustainableclaremont.org).