We made it, part IV, the finale of this series on gardening for good bugs. In the course of my research and writing, I have learned so much, and I hope that you have as well. I find it to be grounding (pun intended) to connect bigger picture ecology to what gardeners do, to understand the methods to our madness. I sincerely believe that gardeners can contribute meaningfully to environmental conservation and habitat restoration.
Following an introduction to botany, the next two topics taught by the DC Master Gardener training program are soil and compost. Obviously, soil is the foundation of gardening. Soil is mighty! I attended a presentation yesterday about climate change and vegetable gardening and one of the speakers talked about the roles of plants and soil in capturing carbon dioxide from the atmosphere to fight and reverse climate change. I find this field of research to be really exciting and hopeful.
I hate that urban soil tends to get a bad rap out of fear of contamination. These fears are not unfounded, especially if you're talking about a site with a history of industrial use. For residential sites like a backyard garden, the main concerns are lead and arsenic. This is why soil testing is highly recommended, especially if you will be sharing your harvest with children. Lead contamination mostly comes from our past bad romances with lead paint and leaded gasoline. Arsenic-containing compounds were previously found in pesticides and pressure treated lumber. All of these sources of contamination have been banned by the EPA.
The results of my summer 2014 soil test were that there was no evidence of lead contamination, but there was a measurable amount of arsenic, though not at a level high enough to cause concern. This result reflects what other DC gardeners have found in their own gardens and it's also comparable to soil test results in rural areas, as there is no such thing as untouched pristine wilderness anymore. On the bright side, the nutritional analysis portion of my soil test was superb. I've never used fertilizer, only compost, so it was enormously gratifying to see my efforts validated.
I sympathize with the beginner gardener who looks at their hard clay soil with despair. What the beginner gardener may not know is that clay soil is actually highly nutritious. Clay soil has a high cation exchange capacity: clay particles, which are negatively charged, attract mineral nutrients such as potassium and calcium, which are positively charged. Clay also holds on to water really well, a bit too well at times, as anyone who has ever gotten stuck in the mud after a heavy rain can attest to. All we need to support the soil-dwelling good bugs is any kind of dead and decaying plant material, aka compost.
The above diagram is nearly identical to the one that appears in the chapter on compost in my Master Gardener handbook. To read the diagram, start in the bottom right corner with "organic residues" (aka the contents of your compost bin) and follow the arrows up and out. These organisms are decomposers, performing the important service of consuming dead stuff and making those nutrients available to other living things. In our garden's soil, these bugs make nutrients available to our plants. Of course, this diagram is simplified and does not include all of the living things found in soil and compost, but the most valuable players are highlighted, including organisms that can only be seen under a microscope. This diagram is also zoomed in; the food web extends further to animals that feed on all of these bugs such as toads, turtles, birds, and rodents.
"It's the ciiiiiiiircle of liiiiiiiiife!"
Do we need to mix compost into the soil? Let's look to Nature. After all, no one double-digs the forest. Forest soil is covered by a protective layer of organic matter from fallen leaves and other dead plant material. It breaks down and becomes incorporated into the soil through the activity of bugs (and other small animals) and seasonal cycles. We can follow Nature by top-dressing our gardens' soil with compost and allowing the good bugs to cultivate the soil for us.
I am proud to say that I am a lazy gardener. Who wants to break their back swinging a pick-ax or be dragged behind a medieval torture device if you don't have to? No-till gardening also minimizes our exposure to possible soil contaminants because (1) tilling creates dust, so not tilling means you won't inhale dusty soil contaminants and (2) layering on organic matter helps to sequester soil contaminants, making them inaccessible to our garden plants.
Going back to the good bugs, what do you suppose happens to all those beneficial soil organisms when soil is tilled or double-dug? No-till gardening is our Bug-protective Gardening Practice 4.
For more about compost, check out my post about building a compost bin.