The Rambling Rose: Walks with a Master Gardener

Soil Compaction, with Theresa Lyngso

By Susan Kornfeld

Some weeks ago I watched a short video from the Washington Post about our country’s thirty-year effort to find evidence of life on Mars. We’ve all seen the pictures sent back through space: dusty red dirt, surface littered with rocks, runnels and canyons indicating the presence of water in some distant past. And nary a microbe to be found. What a contrast to our teeming Earth where tens of thousands of species can be found in a tiny pile of soil.

Mars landscape [Photo by NASA/JPL-Caltech/MSSS]

“It’s life that makes the difference between dirt and soil,” Theresa Lyngso remarked on our recent walk in Half Moon Bay. President of family-owned Lyngso Garden Materials, Theresa knows a lot about soil. Her presentations on “Living Soil” are always in demand. After attending one a bit over a year ago, I gained a much deeper appreciation for what goes on unseen below the surface of the earth. And so I asked Theresa, a UC Master Gardener since 2009, if we couldn’t take a ramble and talk about soil.

“It’s life that makes the difference between dirt and soil.”

What we were focusing on during our June walk was the difference between compacted soil and soil with good structure. I’d heard Theresa claim that a lot of plant problems come from compaction and I wanted to see how that was reflected in actual gardens and fields. After all, I pointed out, lots of trees and foundation plantings seem to do all right despite surrounding soil that seems hard as a rock. It can look that way, Theresa agreed, but under the surface the process of decline may already be underway. It isn’t always readily visible.

“It’s the new plantings that tell the real story,” she said. “They just won’t thrive.”

Theresa and I met at the Moonside Bakery and Café in downtown Half Moon Bay, then wandered a bit enjoying the mix of Mediterranean and Victorian homes and office conversions before heading up Miramontes street towards Our Lady of the Pillar Cemetery. We had only gone a block or two before we were drawn as if by a magnet to a house on the corner of Johnston Street. It is one of those charming old bungalows that respond so well to any gardening effort and this one had recently received a lot of love. It was not just the low-walled Mediterranean garden where plants thrived, but the owner had turned the sidewalk verge into a place of beauty, too.

A rich investment in compost and mulch pays off with healthy soil and thriving plants [photo by Susan Kornfeld]

Theresa admired the thick micro-bark mulch. It’s not a coincidence, she said, that these young plantings are so full and healthy. She reached down to feel the soil beneath the mulch and had me put my hand into it, too. It was warm, crumbly, and soft. I was bent over with my fingers in the earth when the owner came out. She smiled. And no, she didn’t mind at all a couple of gardeners dabbing about. It was historically the butcher’s house, she told us — “and the first time I saw it I knew it was the house I wanted.” We chatted gardens in general and then turned the discussion to soil. Her standing instructions when her garden was installed, she said, were to spare no expense on compost and mulch. “Her investment will pay off,” Theresa said as we turned away. “It’s not only beautiful, adding value to the property, but people will stay off it. That’s going to give the soil a chance to develop good structure.” The climate-appropriate Mediterranean plants plus healthy soil means that this corner of Half Moon Bay will delight passers-by for years to come.

As we continued up the street the plantings fell away until the verge was cracked and bare. This soil has a lot of clay in it, Theresa said, and without leaf litter, mulch, or even weeds, it is particularly vulnerable to the elements. It’s a downward spiral. “The wind, or power-blowers, sweeps the leaves away and the sun just beats on the bare soil, pulling the moisture out.” Rain is hard on bare soil, too, she went on. Without something to intercept and soften it, rain just pounds down and and compacts the soil to the point where the water has a hard time moving through the soil. It either remains, a barrier of water between soil and air, or else runs off into the gutters eroding the soil as it goes. “You simply don’t want bare soil in the garden,” Theresa concluded.

“The wind, or power-blowers, sweeps the leaves away and the sun just beats on the bare soil, pulling the moisture out.”

I looked for plant life and saw only a struggling nandina — a hardy plant that should thrive in this location. Next to it a Western sword fern was struggling for life and failing.

Nandina, sword fern struggling to survive in bare, cracked, compacted soil [photo by Susan Kornfeld]

I tried in vain to poke my finger and then a stick into the ground but with no success. How, I wondered, could roots move through that ground? Theresa explained that, with the help of soil microbes, roots can transform compacted soil into soil that has good structure. A layer of compost and a cover of mulch will begin the transformation from compaction to open soil structures where air and water can move through the soil.

“If you look at forests,” Theresa continued, “you’ll see the ground covered by tree litter and duff. Trees evolved to care for themselves and not just through fallen leaves, bark, and twigs. I’ve spent lots of time watching pine trees, for example. Their layers of branches and needles very gently direct the rain right to the root zone.” Rain through pines, rain pattering on leaf litter. It’s a lovely sound. I remember as a child spending weeks at Friendly Pines Camp in the conifer forests of central Arizona. Summer rain often came at night where from my top bunk I could hear it on the roof and then that soft pattering as it hit the ground. I would drift off to sleep to the sound and to the tangy scent of fresh pine.

I was abruptly jerked back from my reverie as the sidewalk ended. From here on, the verge itself had to bear all foot traffic and it became abundantly clear that without plantings or some sort of cover, bare ground can do quite well mimicking concrete.

A patch of bare soil, compacted further by foot traffic [photo by Susan Kornfeld]

Not too far ahead there were some very old jade plants between a garden wall and the sidewalk. They were a raggedy bunch and had only a few branches, but still they were a welcome sight. Around them were patchy grasses, clovers, and other miscellaneous weeds.

…without plantings or some sort of cover, bare ground can do quite well mimicking concrete.

“They’re doing their part,” Theresa said. “Those weeds are sending down lots of roots; the clovers are fixing nitrogen, and those thistle taproots are opening up the soil. Even the weedy grasses contribute a good amount of carbon to the soil.” They are the pioneers, she went on. The first wave of transformation. Transform on, soldier plants, I thought with new respect. Prepare the way for successors who will have energy to spare for a bee-pleasing show of flowers.

Many weeds send long taproots into soil, helping to open up compacted earth and begin its transformation back to living soil [photo by Susan Kornfeld]

“But what if you want to hurry up and replace the weedy stuff with something more desirable?” I asked. “Would you have to dig up and replace the fledgling soil?”

“You don’t need to,” Theresa said. “Plus, you don’t want to remove the native soil or the biology.” The quickest and most direct way to prepare a good planting bed here, she told me, would be to put down a half inch of compost and about three inches of mulch. The transformation will happen over time and with the help of winter rain. It’s not that the compost has everything in it all ready to go, she said. It’s that the microbial life in the compost attracts other organisms. “It’s a food web. Compost feeds the life in the soil — and adds life, too. Earthworms come for the microbes, and they play a big role in developing a soil structure that allows plant roots to grow and spread and encourages other larger life forms, insects and such, that further open it up.”

We discussed whether or not to use cardboard sheeting or if just thick mulch would suffice to protect the soil and starve out the weeds and grasses. Theresa thought that thick enough mulch would suffice for most situations, but that for large areas with established weeds, overlapping layers of cardboard or newspaper would be best. Fall is the best time for this, as the ground needs to be moist enough to welcome living organisms that build the soil.

“It’s a food web. Compost feeds the life in the soil — and adds life, too.”

If you want to start such a project in the summer, Theresa added, you start with a thorough watering, add a half inch of compost, layer in the cardboard or overlapping newspaper layers, add another half inch of compost, and then two to three inches of mulch. Water each layer as applied. “The weeds will be decomposing beneath the layers,” she said, “although some crabgrass might remain.” Like cockroaches, I thought. They are always what remain. Indefatigable and supremely adapted.

It was a beautiful day for a walk, one of those too-infrequent coastal days with the lightest of breezes and bright white clouds that left room for the sun. It was the sort of day for looking up rather than looking down, yet every hundred feet or so Theresa would stop and put her hands in the soil. Sometimes she could get only a fingernail deep, sometimes not even that. But at the edge of one rather neglected garden the soil easily accepted her whole hand. A large big-leaf maple tree provided shade, and the property owners had let the leaves lie where they fell, creating a soft layer of natural mulch. Beyond the fence was a yard completely bare of vegetation. The soil was churned up, holes everywhere. Gophers, we surmised, and a dog digging after them. Remove dog and gopher, add some compost, and the ground would be ready to plant.

Fallen leaves left on the ground make an excellent natural mulch that will protect and feed the soil underneath [photo by Susan Kornfeld]

Turning up to the cemetery, we crossed the little bridge over Arroyo Leon and paused in a clearing bounded by tall eucalyptus. Theresa admired the soil, soft and giving, overlain with at least an inch of eucalyptus leaves. She agreed that many people consider eucalyptus a poor source of mulch, but added that she hadn’t had a problem with it. She lifted a clump of soil: “See, this is full of little roots and fungal hyphae — all good signs.” She leaned over and inhaled deeply. “And it even smells good.” I took a whiff myself. It did smell good, earthy like forest.

Farther along, though, she had me feel down into the earth. “See here — there’s compaction just two or three inches down.” Theresa speculated that this part of the clearing might be used for parking once in a while, either for burials or for heavy equipment. Between such events, the trees would continue to feed the soil, building up natural mulch that would begin to structure the soil until the next round of parking compacted it down again.

A blanket of natural mulch conceals a compacted layer of soil two to three inches below, possibly a result of parked cars on this area [photo by Susan Kornfeld]

“It’s very sensory,” she told me. “I sensed the change from one section to the next. Your feet can feel the soil quality. Your hands can feel it. You can see the texture of good soil — or the pavement quality of hard-packed dirt. And there is that wonderful smell!” I was reminded of how often I focus on what I see above ground: leaves canting to the sun, bees buzzing and nuzzling in flowers, grasses rippling in the sea breeze. As a new Master Gardener I’ve learned — as I was learning from Theresa — to think about the teeming complexities below ground.

We strolled about the cemetery, named for the shrine Nuestra Senora del Pillar in Zaragoza Spain, lingering over one grave or another. Some of the oldest sites had fresh flowers; others had only plastic bouquets whose color had faded years before.

“It’s very sensory,” she told me. “I sensed the change from one section to the next. Your feet can feel the soil quality. Your hands can feel it.”

As we wandered down the hill from the cemetery, we stopped beneath a sprawling Monterey pine. Theresa reached down and plucked a soil sample as easily as picking a daisy. Rich and alive, it was the nicest sample we had seen. Decomposing pine needles blended into crumbly dark soil riddled with tiny white roots and even tinier threads of fungal hyphae. The pine needles had been there undisturbed for a long time, Theresa said.

Years of undisturbed rest under a covering of pine needles results in rich, living soil [photo by Susan Kornfeld]

I asked her about cultivation practices such as turning over the soil. “The only time you should rototill is to begin a rehabilitation process, she told me. “You can till it once to break up the compaction and allow air and water to penetrate. Then work in compost to add life and food. Regular rototilling breaks down organic matter. The oxygen it lets in ignites the bacteria. They just start chowing down so that organic stuff isn’t allowed to gently become good, nutritious humus. Plus,” she continued, “if you keep turning the soil you keep breaking up structures that might have allowed the next wave of beneficial life, such as earthworms. You end up with fine, structureless dirt subject to compaction. Then you have to till again and need to start adding fertilizer and worrying about plant disease and pests.”

She repeated what was becoming the mantra of the excursion: add compost and then mulch. A half inch of compost should be applied each fall, especially around plants like roses that have a lot of needs. The interstitial zone where top-dressings such as compost and mulch meet the soil is critical. That’s where the root-microbe action begins. “We’re feeding the soil when we add the compost and the mulch, doing our part to nurture a living soil. When the food web is fully engaged, the plants are stronger. They’re getting exactly what they need when they need it.”

The only time you should rototill is to begin a rehabilitation process.

Then Theresa said something I’ve been thinking about ever since. “The plants actually communicate with the microbes. Root exudates call in the ones they want in their zone.” Specific chemical compounds, the exudates, attract specific microorganisms. They come to feed and, in return, help promote plant health and growth. Textbook symbiosis.

At the time, however, I was worried about more mundane things such as having to rake off the mulch each fall to put on the compost. Theresa quickly assured me that new compost can be put down on top of it — but then add more mulch!

We were, by this time, back on Miramontes leaning over a split-rail fence admiring some roses despite their spotted and rusty leaves. You get good at that living on the coast. “I don’t think it’s been extensively studied, “ Theresa said. “But I feel, as do many gardeners, that spraying with compost tea early in the season gives an extra boost to plants prone to rusts and fungal infections.” Such tea contains important nutrients, but perhaps more importantly it introduces beneficial microbes to boost (or create) and diversify microbial populations.

We picked up a clump of soil and it smelled like summer.

The rose garden in San Mateo’s Central Park, Theresa told me later, has gone organic and is using well-made actively aerated compost tea, quality compost, and mulch.

Theresa and I continued discussing needy plants we had loved as we walked back down Miramontes. We stopped by the bridge again to admire the steep stream banks covered in nasturtium and shaded by maples, pines, and eucalyptus. No need to feed or mulch those cheery yellow and orange flowers. With their own leaf droppings, plus those from the trees, they were thriving among some of the best natural compost and mulch available. We picked up a clump of soil and it smelled like summer.

Soil along the shaded and nasturtium-covered bank of a stream shows excellent structure [photo by Susan Kornfeld]

Our last stop was in front of Oddyssea, the outdoor garden and craft products establishment on the corner of Main and Miramontes. The garden plantings are healthy and attractive and the split-rail fence is a nice place to enjoy them and watch the children make little art projects on the big tables. Theresa drew my attention to the ground (again). While Oddyssea had put in quite a bit of mulch, just beneath the fence were a couple of bare dry places where people like us have stood, one foot on the lower fence railing and the other on the ground. “That shows what foot traffic can do,” Theresa said. I wondered about all the to-ing and fro-ing I do in my own garden, and Theresa suggested stepping stones or mulched pathways. Good enough — I am a sucker for stepping stones.

Foot traffic can cause mulch to be displaced, resulting in bare compacted soil [photo by Susan Kornfeld]

We ended the walk back at the café. Theresa wanted to finish our discussion by addressing leaf blowers. As a professional gardener I hate them — I spend a lot of time picking out masses of blown leaves from plant centers or dealing with depleted soils. Even though many leaf-blowing services do their best to be careful, compost and light-weight mulches get blown away. So I was a very receptive audience to her critique.

Theresa: “There is no place on soil, ever, for blowers. Never ever. Even if you try to be gentle, you’re still disturbing the soil and blowing away the organic matter, leaves, and other light detritus. That organic matter resting on top of the soil is decomposable matter. If you must remove leaves, use a rake. It may not seem like it, but over time, blowing does indeed compact soil.

“There are lots of wonderful landscapes in the Bay Area, but if you look at the sidewalk strips in residential neighborhoods you sometimes think you’re looking at concrete and asphalt, not soil. It’s heavily compacted and that is due to leaf-blowing. Water can’t penetrate the soil so it just pools on the surface until it evaporates. That’s not healthy for trees. Their roots need oxygen, but if you have a layer of water on the soil they’re not going to get any.

Leaf blowers are more than just noisy — they contribute to soil compaction [Photo by Anthony Appleyard, Wikimedia Commons]

“We have to respect the plant communities. If we don’t, they will go into decline. I’ve watched it happen to some magnificent oak stands. I think the problem lies in the root zone — not just from construction work compaction but from week-after-week disturbance of the root zone. It’s not just from the blowers, in fairness, but from overwatering, too.

“If you are fortunate enough to have a large tree in your garden, get to know the needs of that tree and do your best to respect and provide what it needs.

“There is no place on soil, ever, for blowers. Never ever.”

At the very least, respect the area at the crown of the tree, where it enters the ground. Don’t build up soil or mulch in this area. We’ve lost many of our heritage oaks due to lack of understanding their needs.

“But back to blowing: You want the soil to breathe. The blower works against that goal. I’d say the only use for a blower would be on hardscape: sidewalks, driveways, streets. But then blow the leaves into a pile. Collect it and spread it around the plants — or make compost out of it. Don’t, as too many services do, just blow it into the hedge or shrubs where they choke the plant.”

I started thinking about an army of blowers. I started thinking about Mars. I resolved to add some sort of mulch to some sort of bare earth every day.

Theresa’s Tips

  • Check for compaction: standing water; dense, hard-to-dig soil; stunted and stressed plants.
  • Loosen compaction and amend depleted soils by adding compost and two to three inches of mulch on top.
  • Add a quarter inch layer of compost on lawns and groundcover in fall and spring to provide food and life — and to encourage deep root growth.
  • Don’t blow.
  • If you can, allow clippings and leaves to stay on the ground.
  • Avoid working the soil, especially clay soil, when it is very wet.
  • Designate pathways in the garden.
  • Keep equipment traffic to a minimum.
  • Use rototilling only as a one-time rehabilitation tool for severely compacted soil; do not rototill around tree and shrub roots.
  • Never build up compost and mulch against the base of your plants; respect the crown and its need for good air circulation.
  • Use layers to create planting beds out of compacted and weedy ground: compost, cardboard, compost, mulch.

About Theresa Lyngso

Theresa Lyngso is President of Lyngso Garden Materials, a 63-year family-run business in San Carlos. Her interest in soil started out growing up in the oak woodlands of Belmont. She is a Master Gardener and a Master Composter and knows what a difference living soil can make in a garden.

About Susan Kornfeld

The Rambling Rose, known to her gardening clients as The Cottage Gardener, Susan has been gardening professionally in San Mateo County since 2012 after a five-year stint in New Zealand. She just earned certification as a UC Master Gardener and is thrilled to be a master of anything.

UCCE Master Gardeners

Do you have a gardening question or problem? The UCCE Master Gardeners can help! Contact the HelpLine — our UC-certified Master Gardener volunteers are standing by to provide you with research-based home horticulture advice. Email us your question and details anytime, along with close-up photos. Alternately, you can leave us a voicemail, stop by in person, or ask your question via social media @SFBayGardeners. Don’t forget to provide your contact information so we can get back in touch with you.

Mon & Thu, 9am-4pm (closed holidays)
(650) 276–7430 (voicemail only)

1500 Purisima Creek Road
Half Moon Bay, CA, 94019