Gardening Nature’s Way: Digging Harvard’s ice-age dirt – Harvard Press


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When I moved to Harvard and started gardening, I noticed immediately that the soil smells, feels, and looks different from the Virginia clays I had grown up with, and this made me wonder why. I also noticed far fewer earthworms, and, at first, I thought this was a clear sign of unhealthy soil. Soon I learned that the soils here in Massachusetts, as many readers know, were created by retreating glaciers approximately 11,000 years ago. Areas that have been under glaciers in the U.S. don’t have native earthworms.

Glacial till, which forms the basis of much of the soil in Harvard, is by definition unstratified and formed of soil particles and rocks jumbled together because it was plunked down by the glacier rather than wind or a body of water. Depending on the terrain, Harvard’s glacial till either has weathered into coarse loam that is good for agriculture or, on steeper slopes, eroded into thinner soils with shallow depth to bedrock.

‘Not prime farmland’

Thus, the two most common types of soil locally are the agriculturally rich Paxton (also the official state soil of Massachusetts) and Chatfield-Hollis, which is very shallow bedrock found on slopes, diplomatically categorized by the U.S. Department of Agriculture as “not prime farmland.” In addition, varied wetlands around Harvard are characterized by lots of organic plant material, sediments, and anaerobic soil. They are legally protected, so should be appreciated but remain free to do their own thing.

The terrain found in your landscape tells you a lot about the kind of soil you have to work with. Several rock outcroppings scattered about my yard provided a clue. I searched via the online interactive soil survey hosted by the USDA’s Natural Resources Conservation Service (NRCS) and found that I am sitting on a Chatfield-Hollis rock outcrop complex. This means I have thin rocky soil, on the sandy side, and it dries out pretty quickly. This information, along with observing the effects of last summer’s drought, convinced me I need plants that thrive in dry conditions. To find the right plants, I took a class on meadows offered by the Native Plant Trust, and then searched the online catalogs of the Maine Wild Seed Project, as well as the inventory of a native grower based in Wendell.

Another way to learn about your soil is to look at what grows there naturally. White pines and oaks grow in thin, rocky, acidic soils, and if your property is bounded by natural stands of these, it hints at your broader soil conditions. In a portion of my yard near a rock outcropping, I noticed widespread ant complexes; ants favor sandy and dry soil. In between the little anthills, the grass was thin and crowded out by weeds. On the edge of the ant complexes, violets (beloved by fritillary caterpillars) and pink dianthus were mixed liberally into the lawn. Ants, it turns out, like to eat a fleshy exterior part of the seed and then deposit the rest in their trash heaps on the edge of their networks, which perfectly explains the patterns of flowers I found woven into the lower part of my yard.

Since the ants were already working for me as seed dispersers, I decided to turn the anthill portion of my lawn into a meadow fringe with native wildflowers and prairie grasses (without disturbing the ants too much). Later in the summer, I discovered volunteer patches of little bluestem (seeded perhaps by birds or wind from a neighbor’s meadow) and Solidago puberula, downy goldenrod that favors sandy, dry soil. These plants indicated that I had chosen a good area for a meadow filled with drought-tolerant grasses and flowers including, quite fortuitously, little bluestem.

Plants don’t just react to the soil; they also make the soil, in conjunction with the soil’s microorganisms. Protozoa, fungi, algae, bacteria, and little arthropods do a lot of the work to make Earth’s skin produce life by cycling nutrients (such as nitrogen), water, and air through the soil. Organic plant and animal materials are decomposed with the help of soil organisms, thus recycling nutrients for new plant growth. A healthy soil food web filters the water that cycles through the soil by removing chemical impurities (this is why you can drink your well water), improving soil structure, and fighting disease-causing fungi and bacteria.

Microbiologist and former professor Elaine Ingham has characterized the complex interactions among these organisms, the soil, and plants as the “soil food web” to highlight how important the processes going on below ground are to supporting life above it. USDA programs on soil health, built upon data collected about soil degradation and erosion in much of the United States across the 20th century, have come to embrace this dynamic view: Elaine Ingham wrote the “Soil Biology Primer” for the NRCS, and the soil food web is explained in detail on its website.

Tips for better soil

What can we do as backyard gardeners to help sustain and nurture our soil? After all, many of us live on home sites that were built by removing all the topsoil (the good part) and bringing in fill consisting of subsoil. Grass lawns are also typically managed in a way that doesn’t support soil health. Even those of us blessed with good soil need to manage it to prevent erosion and loss of nutrients.

Add compost. Adding organic material introduces healthy soil microorganisms and improves the structure of the soil, allowing it to hold the appropriate mix of air and water for root development. Compost also prevents erosion by holding onto water better than soil particles like sand or silt.
Do not till. Tilling disrupts the topsoil and exposes it to air and rain, often killing the good microorganisms, especially the beneficial fungal threads that form around root systems. Tilled soil is likely to erode and get compacted (a condition to avoid because your plants’ roots won’t get enough air to thrive). Nutrients leach out, and more fertilizer inputs are required.
Avoid overwatering. Overwatering is not helpful, especially in soils that are already compacted. It may lead to salinization of the soil. Do not let water pool on top of the soil when you do water—you may starve your plants of air and kill off good microorganisms.
Do not allow your soil to lie bare at any time during the season. You can plant a cover crop or use mulch. Bare soil gets eroded, leached, and compacted.
Observe and enjoy your soil. Short of adding lots of chemicals (which may provide a short-term payoff but long-term headache), there is not a lot you can do to improve your soil quickly. Find out what grows well there, and plant accordingly. Over time, the plants’ roots attract good microorganisms, and organic matter gets cycled back into the soil.

In conclusion, do less and enjoy your garden more, in whatever form it takes.

Catherine Warner is an enthusiastic amateur gardener who got started by following her dad around the garden and later worked on an organic farm in college. After a detour as an academic historian, she now stays at home with her son and serves as a member of both the Garden Club of Harvard and the Community Resiliency Working Group.

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