Gardening starts with soil, and soil is a product of complex geological and environmental processes that span centuries and millennia. As gardeners, it’s often helpful to have a basic understanding of geology: it shapes the soil in which outdoor gardens grow. Geology also shapes the evolution and preferred growing conditions of all plants, so even indoor gardeners can benefit from an basic understanding of geology. As with all things gardening, there’s so much to learn and this blog is only a basic introduction. We hope it helps you understand how gardening and geology relate to each other and maybe just maybe sparks an interest in further study. So—and it’s never been more appropriate—let’s dig in!
Three Basic Types of Rocks—And How Rocks Become Soil
While there’s a wide and sometimes bewildering number of names for different rocks and minerals, the overall classifications are much simpler than we thought at first. As defined by geologists, a rock is a naturally occurring solid mass of mineral matter. There are three main types of rocks, with numbers subdivisions within them. Think of these as akin to botanical families which contain many different species, sub-species, and varietals:
· Igneous rocks are formed when lava or magma—liquid mineral material from the depths of the earth-- cool and solidify. About 65% of the planet’s crust is comprised of igneous rocks. This type of rock is divided into two main categories: plutonic or intrusive rocks, which forme when magma cools slowly within the Earth’s crust, and volcanic or extrusive rocks which form when lava reaches the earth’s surface as magma and cools there. Granite and quarts are common examples of plutonic or intrusive rock, while common examples of extrusive or volcanic rock include pumice, basalt, and rhyolite.
· Sedimentary rocks are just that: rocks which form as sediment compacts and solidifies. This sediment may contain a mix of fragments of earlier rocks, minerals, or organic materials. Sandstone is a type of sedimentary rock formed by particles of mineral materials like quartz or feldspar cemented together by other minerals. Limestone is another type of sedimentary rock, formed by the calcified remains of corals, mollusks, or microorganisms.
· Metamorphic rocks are just that: an existing rock that’s been subjected to changes of heat, pressure, or both. These forces create a process called metamorphism, which creates a new kind of rock. Metamorphic rocks may have begun their life as rocks as any of the three rocks types; yes, an existing metamorphic rock can undergo metamorphism! Common examples of metamorphic rocks include jade, slate, and marble.
Each type of rock will become part of the soil above and around it. While rock is not the only component of soil—don’t forget about the organic matter, moisture content, and microorganisms—the mineral content derived from rock is a big part of most soils. So how does rock become soil? There are a few processes in play, with some overlap between them:
· Weathering is the physical wearing down or wearing away of the rock in question. Rock is solid and many rocks are quite strong and enduring, but they all wear away over time when exposed to wind, water, and sun. Small particles break away and become part of the soil.
· Erosion is closely related to but distinct from weathering. Weathering is the physical wearing away of the rock, whereas erosion removes those particles and re-deposits them someplace else. These particles often become part of soil, but they can also reform as a sedimentary rock if circumstances are right.
· Deposition is the end result of erosion: the particles produced by weathering and relocated by erosion accumulate as either soil or sediment.
These may seem like fine distinctions, but it’s important to be precise when talking about these sort of geologic processes. Most of the time, these happen quite slowly and gradually. Soil can take years, centuries, or even millennia to create and understanding the different stages of the process helps us understand the role rocks play in creating natural soils.
How Rock Type Affects Garden Soil
As we read about how different types of rock are formed, it became apparent to us that rocks and soil interact constantly. As it turns out, different rock types influence the soil around and over them. It’s a complex interaction, yet again, but there are a few basics we can take away from it:
· How rocks influence soil depends on the both the basic type (igneous, sedimentary, metamorphic) and the particular rock in question. Granite soils tend to be acidic, as granite tends to acidify when exposed to rain water. Soils formed from limestone tend to be alkaline, by contrast.
· Understanding the relationship between rocks and soil, and knowing a bit a bout local geology, can help farmers, gardeners, and landscapers better understand the needs and challenges of their soil and how to best improve the soil we have.
· This relationship also affects how we do our gardening and landscaping: using granite gravel, for instance, can create acidification in soil over the long term. Rock-based soil amendments like limestone or dolomite can help adjust soil pH.
· We should all talk to our local garden club, cooperative extension, or similar resource to learn more about the soil and the rocks in our area.
· For those of us in the United States, the US Geological Survey has a publicly available series of maps detailing geologic information about the USA. Canadian readers can consult similar resources provided by the Geological Survey of Canada here.
Gardening Starts with Soil, and Soil Starts with Rocks
We learned a lot researching and writing this blog, and we’re glad we can share it with you. It seems like a lot; most of us didn’t get into gardening to learn more about geology. For some readers this may spark a further interest in the interplay between geologic processes, rocks, and soil development. Some of us may just be happy knowing the basics and how they affect our garden space. Both approaches are OK! Our goal in presenting a blog about geology for gardeners is to both provide a bit of helpful information and to remind us all that there’s always more to learn. Gardening provides a connection to both the natural word and to deep time. That’s part of what makes this such a worthwhile pursuit, and what keeps us coming back to our gardens every year.
As always, if you have some geology or gardening knowledge to share or questions to ask, reach out! Gardzen is all about community and we love to hear from you!
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