The Scoop on Soil

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Every gardener is curious about the soil in which a garden grows. As you sift it through your fingers, remember that the parent material of every particle of soil is rock. The soil that we so carefully alter, amend, save, and endlessly fuss over begins its existence as solid rock.

If you look around, you can probably come upon remnants of the rock that your soil came from. The granite coast of Maine attests to long-ago volcanic activity. Manhattan schist still rears artistically above verdant Central Park. Ice Age rocks still rise to the surface in garden soils across northern reaches of the continent with ongoing soil-making activity while the chocolate-cake soil of Iowa is a young soil, having been laid down only 10,000 years ago. Violent upheavals resulted in the now-ancient Appalachians as well as the relatively youthful Rocky Mountains. They continue to expose their crags to the weathering forces of the wind, water, and the freeze-and-thaw cycle.

Volcanic activity produced the different kinds of lava that long ago flowed out upon the land. Seismic activity broke up the cooled lava in time for two Ice Ages to grind it slowly to a mixture of cobble, boulders, and sand called glacial till. Roaring torrents still carve the terrain to create gullies that become canyons, and as the speed of the water diminishes, these sediments harden as sandstone or limestone. Not with blaring trumpets, but with steady, ongoing weathering forces, are born sand, silt, and clay soils.

Not with blaring trumpets, but with steady, ongoing weathering forces, are born sand, silt, and clay soils. 

Soil composition

Soil contains both parent material and organic matter. When you dig through your soil, you may see strands of a root, a shred of rotten leaf, or a miniscule clump of something unidentifiable. These particles mean that organic matter is there (despite the typical gardener’s continual lament that there’s never enough).

Organic material is important in soil, but it is the continued chemical and physical activity on the soil’s parent material that releases the nutrients necessary for plant growth. The organic matter doesn’t feed the plant, but it combines with minerals in the soil to feed the plant. By itself, organic matter, including compost, contains almost no plant nutrients. Its main function is to free up the minerals to make them available to the plant. Your plants don’t care whether the organic matter in your soil was laid down in layers millions of years ago or whether it came from your backyard compost pile. They simply enjoy the three most important assets of organic matter—water-holding capacity, aeration, and insulation from the vagaries of weather. Everyone wishes for more of it, unless, of course, you are the owner of Florida Everglades muck.

If you add soil to your compost to augment the decomposition process, you are also adding soil minerals. Many gardeners assume that because green stuff goes into the compost pile and comes out brown stuff, it must be rich in nitrogen. Regretfully, no. Nitrogen is a gas in the air, but it is ultimately the source of soil nitrogen. It must change by means of chemical and biological reactions to a more stable form in order to become available to a plant.

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