Digging into Sonoma and Napa County Soils and Rocks
Exploring the Foundation: Rocks, Soil, and Land Management in Sonoma County
Imagine walking through a landscape where every rock, every patch of soil, and every water way tells a story of millions of years. In Sonoma County, our outdoor spaces are more than just beautiful—they are a direct connection to the earth’s history and the lifeblood of our environment. Whether you're planning a new garden, managing a vineyard, or hiking out in nature, understanding the geological resources around us is key to creating sustainable and thriving landscapes.
I’m always looking for ways to learn more about the landscapes that define Sonoma County, understanding the very ground beneath my feet is essential, both for my work and hobbies. This week we’ll learn more about the region’s rocks, soil, and land management practices. Let’s dig a little deeper into what makes our local landscapes so unique.
Loose soil in Glen Ellen CA photo by Laura Bassett
Unearthing Mineral Wealth: Sonoma County’s Geological Treasures
Napa Wall Rock wall with Virginia Creeper (Parthenocissus quinquefolia) and Agave americana. Photo by Laura Bassett
In a world where the right materials can make or break a project, knowing what lies beneath the surface is crucial. My first job here in Sonoma was working for a local landscape installer. One of the best experiences I had was getting to help with bid projects. My mentor always had a disclaimer “should unforeseen problems arise after the work has started” which included unforeseen subsurface issues.
Deep hole dug on Healdsburg project by pool installer to find solid ground. Photo by Laura Bassett
In the decade and half I’ve been working in the area, I see why he added this. I’ve had projects where hitting large rocks or roots required major redesigns. One where a large tree had been removed years before but when we started installing it took days to dig up and remove the roots. On another one, we had the opposite problem, the top soil was too deep. The pool company had to excavate an extra 10 feet down before finding suitable rock to tie a pool into so it wouldn’t slide down the hill. There’s way to know what you’ll find until you start digging.
According to a recent report from the Napa County Watershed Information and Conservation Council, Sonoma and Napa Counties are a veritable treasure trove of mineral resources. These geological formations are more than just rocks; they are the backbone of our construction and landscaping industries. From the sturdy basalt that forms the foundation of our roads to the precious stones that add character to our garden paths, these minerals are the unsung heroes of our landscapes.
Blue shale used to mulch around exposed roots. Photo by Laura Bassett
Here’s a closer look at the types of rock found in our area, along with where they can be found:
Basalt: Found primarily in the Sonoma Volcanics, basalt is common in the areas around Glen Ellen and Kenwood. This dark, dense volcanic rock is widely used for building and road construction throughout the county.
Sandstone: This sedimentary rock is abundant in the Franciscan Complex, particularly in areas around Petaluma and Santa Rosa. Sandstone is often used for decorative purposes, including garden paths and retaining walls.
Gravel: Gravel deposits are widespread throughout Sonoma County, particularly in riverbeds and alluvial plains like those in the Russian River Valley. This mix of rock fragments provides essential drainage and support in landscaping projects.
Shale: Shale is prevalent in the Franciscan Complex as well, especially in the coastal ranges west of Highway 101. This fine-grained sedimentary rock can be used in various applications, from building materials to garden accents.
Quartz: Found scattered throughout the county, quartz is most commonly associated with the metamorphic rocks in the Mayacamas Mountains. This hard, crystalline mineral adds a unique sparkle to pathways and garden features.
Sandstone flagstone path with creeping Thyme (Thymus sp.) and Alpine Strawberry (Fragaria chiloensis). Photo by Laura Bassett
Explore these locations on the map to see where these minerals are found in Sonoma County.
This report doesn’t just skim the surface but delves into the economic significance of these materials, making it a must-read for anyone involved in construction or land management. It’s also great for us designers and homeowners who want to use local materials.
B R Cohn Vineyard Hill in Spring with Mustard Cover Crop. Photo by Laura Bassett
Cultivating Life: Best Practices for Soil Health in Sonoma County
Laura's cat Sundance hiking in the vineyards cover crops. Photo by Laura Bassett
Soil isn’t just dirt—it’s a living, breathing ecosystem that requires careful stewardship. Growing up as Waldorf kid, many hours of my childhood were spent double digging beds. Which is one of the most intensive methods of preparing soil for planting. At home my mom and I tried the lasagna method, layering materials up over the bare dirt to make a garden. Now as a busy professional I’ve been seeing a lot about no-till methods. Can this low labor way of feeding the soil actually work? Let’s hear from the experts.
The Sonoma County Agricultural Preservation and Open Space District reminds us that maintaining soil health is fundamental to sustaining our local environment. Their latest discussion on soil management techniques highlights practices that not only protect the soil but also enhance its productivity. Here are some key techniques and findings:
Minimal Tillage: Reducing tillage helps preserve the soil structure, prevents erosion, and maintains organic matter levels. The article notes that minimal tillage can increase organic matter in the soil by up to 20% over five years. This practice also improves water retention by up to 30%, significantly reducing the need for irrigation in Sonoma County's dry climate.
Use of Cover Crops: Planting cover crops, such as legumes or grasses, between growing seasons helps to enrich the soil with nutrients, reduce weed growth, and prevent erosion. According to the findings, using cover crops can reduce soil erosion by as much as 40% and increase nitrogen levels in the soil by up to 50%, which is crucial for maintaining soil fertility and promoting biodiversity.
Compost Application: Regularly adding compost improves soil structure, increases organic matter, and enhances microbial activity. The article highlights that applying compost can boost soil organic matter by up to 5% per year and reduce the need for chemical fertilizers by 25%, leading to healthier crops and gardens.
These methods aren’t just about keeping the earth in place; they’re about nurturing the land so it can continue to produce the vibrant gardens and bountiful harvests that Sonoma County is known for. Whether you’re a farmer, a gardener, or just someone who loves to watch things grow, these best practices offer invaluable insights into keeping our soil—and our landscapes—healthy and productive.
We use this method with our clients, every project we install starts out with good soil mixed into the native soil. Then we top it off with mulch to retain soil moisture. However in the past few years what we’ve started doing is adding a couple inches of compost before the mulch to layer nutrients back into the soil. This is refreshed every couple of years, adding organic mater into mature landscapes and retaining more moisture within. No digging required!
Local Soil on Moon Mountain Rd Sonoma CA. Photo by Laura Bassett
Mapping the Landscape: Understanding Sonoma County’s Soil and Vegetation
Soil profile from an installation complete with tool marks. Photo by Laura Bassett
We’ve built a sandwich so far, we’ve talked about the topsoil or the surface. We’ve delved into the depths with the stone and geology. Now let’s get into the good stuff, the types of soils we have here in Sonoma County, and how it ties into the plant communities and what will do best growing there.
One of the things I love about working here in Sonoma County is we have such diverse microclimates and unique mixes of soil types. I’ve worked on the steep hillsides of the Mayacamas, the alluvial deposits of Carneros and everything in-between. We’ve had projects where there are seasonal wetlands or ones where it’s too well drained. I’m going to share this great resource and some personal stories with a few of the soil types you might find in your own backyard.
Certain plants thrive in one area but not in another, the answer might lie in the soil. The Sonoma County Vegetation and Habitat Mapping Program has been hard at work creating detailed 'Soil-Veg' maps that reveal the intricate relationship between soil types and vegetation. These maps are more than just tools; they’re guides to understanding how to best manage our land. Here are some key soil types in Sonoma County and the vegetation they support:
Goulding Soil from Pride Wines
Goldridge Sandy Loam: Found primarily in the western parts of Sonoma County, particularly in the Sebastopol area, this well-drained soil supports a variety of crops, including vineyards, apple orchards, and certain types of oak trees. It’s ideal for agriculture, particularly where deep-rooted plants are involved.
Goulding Clay Loam: Predominantly located in the rolling hills near Santa Rosa, this clay-rich soil supports native grasslands and chaparral vegetation. The dense structure of Goulding Clay Loam makes it excellent for retaining moisture, which is beneficial for drought-tolerant plants.
Laughlin Loam: Common in the Russian River Valley, this loamy soil is well-suited to support mixed evergreen forests and coastal redwoods. Its ability to hold water while still draining well makes it perfect for supporting the lush, green vegetation found in this region.
These detailed soil-vegetation connections help landowners and landscapers make more informed decisions about conservation and land use. Understanding which plants thrive in which soils allows for more effective and sustainable landscaping practices, ensuring that every garden, park, and open space in Sonoma County is as thriving and sustainable as possible.
Xero-scape front yard in Sonoma CA. Photo by Laura Bassett
Takeaways from under our feet
These recent articles shed light on the critical elements of Sonoma County’s natural landscape, from the rocks that form our foundations to the soil that sustains life. Whether you’re designing a new garden, managing vineyards, or simply curious about the land you live on, these resources can help you make more informed decisions.
My personal mission is to always be learning more about the world around me. Thank you for joining me on this journey of continued education. Who knows where the path will lead next week!
