Rocks and Your Farm

Basalt - Boulders of basalt in Hawaii. Small variations in mineral content and size and can change the appearance of basalt's colors, but the same basic chemistry applies

Know Your Rocks

Knowing about the rocks on your farm can really help you to understand the mineral components of your soil.  That mineral component is so important, but often overlooked as most folks go about improving their soils.  However, it's the minerals that provide most of the vital elements needed for plant growth.  So lets have a closer look at the rocks and minerals that you might find on your farm.

Rocks in General

All the rocks on our planet can be divided into three major groups.

Igneous - Rocks that formed from the cooling of molten rock either below ground (plutonic rocks like granite or gabbro) or above ground (volcanic rocks like basalt or rhyolite)

Sedimentary -  Rocks that formed from the cementing together of cobbles, sands, silts or mud size particles into layers of rocks. Most of these are formed in the oceans on the margins of the continents where these materials have have washed from the land in rivers such as sandstone, shale, conglomerate or built up from the remains of animals like limestone.

Metamorphic - Rocks that are formed from the squashed and/or cooked (but not melted) remains of other rocks. These are usually found where mountains formed, continents collided, or close to huge masses of molten material.  Rocks like schists and marbles, quartzites and gneisses.

My wife Jen and I love to collect minerals. Here she is amongst granite boulders in Maine looking for tourmaline.

All of these rocks are made up of a mix of common minerals: Quartz Felspars Mica Amphibole Pyroxene Olivine Calcite Plus, a splattering of minor minerals.

Here is a table of just a few of the common rock types 
and the minerals they contain:


Rock type        Rock Name        Minerals
Igneous            Granite              Quartz, Feldspar, Mica
Igneous            Rhyolite             Quartz, Feldspar, Mica
Igneous            Basalt                Amphibole, Feldspar, Pyroxene, Olivine
Metamorphic     Marble               Calcite,
Metamorphic     Schist                Mica, Quartz,
Metamorphic     Gneiss               Quartz, Feldspar,
Sedimentary     Sandstone          Quartz, Feldspar, Mica, Clays
Sedimentary     Shale                 Clays
Sedimentary     Limestone          Calcite


Once a rock is exposed to the weather it will start to break down "physically" into smaller grains, and "chemically" into different minerals. These major rock forming minerals break down into the following (lesser/minor) minerals. These are the minerals that you will find in your soils:

Original Mineral         New Mineral      Released Elements
Quartz                        Quartz Felspars                      Clays                 Potassium, Sodium,  Calcium
Mica                           Clays                  Potassium
Amphibole                   Clays,                Iron
Pyroxene                    Clays,                 Iron
Olivine                        Clays,                Iron, Magnesium
Calcite & Dolomite      Calcite,               Calcium, Magnesium

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What this means for you...

So, if you know that much, you can figure the following:  If you you live in an area where granite is the dominate rock type (a rock containing quartz, feldspar and mica), you will find your well-developed soils contain quartz, grains, and clays.  Less developed soil will appear sandy as the feldspars have not yet broken down into clays.  The soil will contain potassium, sodium and calcium.

If your homestead is on basalt, you will have clay rich soils with loads of iron and magnesium.

The best way to find out what kinds of rocks you have in your area is to visit your national or state’s Geological Survey. To find those in the USA go here (http://www.stategeologists.org/). They will have maps (many online) and advice to help you discover whats under your feet.

What ever rock type you have on your farm will directly affect the type of soil you have to work with.  With a little love and care, any soil can be amended with fertilizers and organic matters to create fertile, workable ground. It just helps to know what you are starting with!

Gaz Lewis is a homesteader and geology educator who lives with his wife and children in Maine.  Gaz writes about homesteading on his blog (www.almostafarmer.com) and provides advice to new homesteaders on Startoff Homesteading (www.startoffhomesteading.com).