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Musings on fertilizers

A guest at the Abby Aldrich Rockefeller Garden recently exclaimed, “What are you feeding these plants? They're all so huge and healthy!” This is a common question at our three gardens.

This past season, Erin Dilworth, Border Gardener at the Abby Garden may have answered that question with, “It’s thanks to a mixture of compost in the fall, an organic granular fertilizer in the spring before planting, then a foliar feed weekly with a fish fertilizer on all the annuals.”

I am starting to think that the answer to that question may become even more complex in the future as we learn more about growing healthy plants.

Our annuals, perennials, shrubs, and trees develop roots primarily within the top 12 inches of the soil, usually just the top four inches. These roots not only anchor the plant, but they will do their best to extract water and nutrients from the soil using a complex array of chemicals and microorganisms. This all happens naturally if those chemicals and microorganisms are present.

Well over ten years ago, we made the shift here at the Land & Garden Preserve from using chemical fertilizers to using organic soil amendments and liquid fertilizers. Is this enough? Do we need to take one more step?

In a built garden, new or old, it is not difficult to find soils that have been altered from the use of soil amendments of the chemical or organic nature, from construction, or from tilling, the breaking up of the soil. All our gardens have had soil brought in to create the garden beds that you see today.

The Asticou Azalea Garden had a sandy loam topsoil spread across its site which originally was swampy ground thick with alders. Thuya Garden, up on the side of Eliot Mountain, had truck loads of loam brought in. It was piled onsite and then screened before being spread over what is lawns and garden beds today. At the Abby Aldrich Rockefeller Garden, soil was also brought in as the walls were being built in the late 1920s. This is not surprising, since little if any topsoil can be found in the conifer-thick woods outside the garden walls.

In addition to soils being brought in, plants have been grown in those soils for decades. The talented garden staff used the best-known practices over the years to ensure that there were beautiful gardens for guests to enjoy, just as we still strive to do today. The staff continue to customize their feeding strategies based on each of the garden’s soil condition, the plants grown, the weather, and issues they see occurring.

The past few years have taught me that even though these three gardens are separated by just a few miles, their soils and the maintenance practices that have been conducted over the years could not be more different. What may work wonderfully in one garden, is not the solution in another of our gardens. If this is true in our gardens, this is going to be true from our garden to your garden.

I have started to hear that soil microbial health is super important to growing healthy happy plants. Most soils that have been gardened in for years, for various reasons, do not support high microbial health. As a gardener, I should know better, right? Work to really understand the complexity of soils in an interdisciplinary way is relatively young. “Most gardeners are stuck in traditional horticultural land, a place where a blend of old wives’ tales, anecdotal science, and slick commercial pitches designed to sell products dictates our seasonal activities,” write Jeff Lowenfels and Wayne Lewis in their book, The Organic Gardener’s Guide to the Soil Food Web.

“We looked for answers, and soon realized that while we were out spreading fertilizer and rototilling our garden beds by rote, an ever-growing group of scientists around the world had been making discovery after discovery that put these practices into question. Slowly, their findings about what goes on in the soil are being applied to commercial agriculture, silviculture, and viniculture. It is time that we applied this science to things we grow in our home yards and gardens.” I want to read more!

Maybe the best thing we can do as gardeners in the future when asked about how we feed our plants to make them look amazing is shift the dialog. Instead of sharing the fertilizers we use, we can share what we are learning and doing to improve our soil health.

Rick LeDuc, Thuya Garden Manager, has begun to have similar conversations around their shift in pest management strategies at Thuya. He told me about a conversation he had with a guest last season, “A visitor we had who had trouble believing that we did not have any serious insect pest problems - aphids, thrips, whatever. He thought we had to be spraying. It was fun to explain how by being organic and having a lot of plant diversity, the predatory and parasitic insects were doing most of the work for us.”

So, what can we do more of? What can you start to do?

We often have our soil tested.

We are lucky here in Maine that the University of Maine offers soil testing for a small fee through the Analytical Lab and Maine Soil Testing Service. Each state can be a little different, you can check your state’s agriculture colleges to see if they offer soil testing. If not, there are other options. I used a privately owned soil testing company when I lived in Indiana.

What is the soil texture like? Can I easily dig in it or is it hard? How long does it stay wet after I water, or it rains? Are there insects in my soil? How much microbial life is in my soil? A microbial test is available as part of the soil test from the University of Maine soil lab.

You may have learned from talking to a gardener at Thuya Garden that they may apply leaf mold to their garden beds in the fall, top dress the beds in the spring with an organic slow-release fertilizer, apply a horticultural grade alfalfa meal just before planting annuals, and make small batches of compost tea that they apply to the annuals in August.

Wendy Dolliver, Head Gardner at Thuya, may have told you that they do this because of the results from soil tests and from consulting with a soil scientist. This information has altered their practices based on what was discovered.

Learn about what the plant wants.

What kind of light does the plant prefer? Is it a heavy feeder? Early in my career I referenced books often to learn these things, now I might find myself often doing an internet search instead. I pulled off my bookshelf one of my favorite references for annual plants, Annuals with Style by Michael Ruggiero and Tom Christopher and sure enough they talk about why annual plants are heavy feeders. Since this one was published in 2000, I would now need to reference a more recent book or article for actual amendments because in the 2000s it was still the norm to use chemical fertilizers. It shows how much practices have changed since I was in school. Regardless, books focused on specific plant groups or genera are still very helpful when it comes to learning what kind of growing conditions plants prefer. Look for reference books on perennials, annuals, shrubs, and trees at your local library or favorite bookstore.

What pH is ideal for my plant? At the Asticou Azalea Garden, Garden Manager Mary Roper was inspired by an article she read through Maine Organic Farmers and Gardeners Association ( on ramial wood. She learned that the fine tips of trees and shrubs are the most nutritional part of the plant. This material was generated annually at the garden from their prune practices. Why not turn this into food for the very plants they are pruning? This compost that they make, woods compost, is half a point lower in pH than traditional compost, traditional compost typically runs more alkaline in pH. This is perfect for the acid-loving rhododendron and azalea collection. The staff add leaves and pine needles along with a half inch or less (pinky finger width) of branches from pruned pines and rhododendrons. At the end of each season the piled material is shredded and used the following year as an amendment to the soil around newly planted rhododendrons and azaleas in the garden.

Rarely is a fertilizer other than some holly-tone (because it is an acidic fertilizer) used in this garden primarily because of the need to keep the stream and pond system healthy. Goat manure is the go-to for feeding newly planted non-acidic loving shrubs and trees. Shredded leaves are applied each year to the prized perennials in the garden as a two-inch top dress after the ground is frozen.

Try something different and be patient!

What is the harm in trying something new? Just remember that change takes time. You can do all the research ahead of time, make observations during the growing season, and adapt accordingly. That is what we do!

Each year, we learn as we go. Check out the University of Maine extension website. It has many useful articles and videos.

Rodney Eason recommends James Urban’s Up By Roots: Healthy Soils and Trees in the Built Environment as an interesting book to help you grow your knowledge of soils.

This winter, I am looking forward to reading more from the book, Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web by Jeff Lowenfels and Wayne Lewis. I hope you are also inspired to keep learning about your soils.

Cassie Banning, Director of Farm & Gardens


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