soil [ soyl ] noun: top layer of earth’s surface that may be dug or plowed; the products of rock erosion and weathering, organic decay, bacteria, and fungi that, together with air and water, form the nurturing medium for growing plants
Soil composition can vary greatly, depending on geography and climate zone, underlying geology, recent and current seasonal weather, and other factors, but the basic materials for good soil are minerals, organic matter, air, and water in the following proportions, by volume: 45% mineral particles, 5% organic matter (plant and animal matter in varying stages of decay), 25% air, and 25% water. Obviously these vary from place to place and season to season, and are judged optimal depending at least in part on what is being grown.
Soil can be as shallow as a few centimeters, or it can go to depths of many meters. The earth’s crust is often divided into three main horizons: top soil, subsoil, and parent material. The top soil has the most organic matter, the subsoil has eroded minerals, and the parent material is generally still unweathered geologically deposited minerals (bedrock). Some soils also may have a layer of non-mineral organic material above the topsoil. In the United States, the USDA has identified and described over 20,000 soil series.
Good soil, also called earth, is porous and biologically active, filled with living organisms. The size of the mineral particles (fine clay, loose sand, pebbly rocks, etc.) and the proportion of organic matter and what nutrients it may add, and the mixture of moisture and air in the pore spaces all contribute to the structure of the soil, how it supports and nourishes plants, and how stable it is. Increased organic content generally increases a soil’s ability to hold water, a quality that is often desirable. Too little moisture causes soil to become extremely dry and hard and if fine particles are present, to crack and blow away if subjected to strong winds. Yet too much water can make soil saturated and liable to wash away or cause landslides on slopes.
Left to right: Soil textures, native plants in sand, droughty clayey soil, waterlogged clayey soil, and fruits/vegetables in loam.
The decomposition of organic matter from dead plants and animals is aided by earth worms, fungi, bacteria, and other creatures that deposit secretions and excretions into the soil, speeding the process of turning waste into humus. Successful plantings depend on selecting the right plant for your soil type including having the correct nutrient and pH levels, which also influence soil-dwelling organisms. So, Don’t Guess, Soil Test! to find out how fertile or acidic your soil is and learn how to build or restore healthy soil.
Specific Soil Types or Components:
clay [ kley ] noun: soil composed mainly of fine particles (of less than a specified size) of hydrous aluminum silicates and other minerals produced by the erosion of sedimentary rock. Clay soil softens when wet and hardens when dry or heated. (adjective: clayey) Clay soils are used to make bricks, ceramics, and cements, so it is easy to understand why it can be hard for gardeners to work it in the dry heat of summer in hot climates. With organic matter worked in, clay soil can be quite nutrient rich, become more workable, and retain water better. On the other hand, adding some sand to clay soil will not improve drainage—a common misconception—rather it makes the soil structure similar to concrete.
sand [ sand ] noun: fine loose grains of sedimentary rock, usually primarily quartzite minerals, with particles between 0.06 mm and 2 mm, finer than gravel but coarser than silt, and occurring naturally on beaches, in deserts, and in soils (adjective: sandy) The proportion of sand in any given soil will affect its ability to hold water, with more sand generally decreasing water retention.
silt [ silt ] noun: very fine loose clay or sedimentary soil with particles 0.05 mm or less in diameter; also fine sediments deposited by rivers (adjective: silty)
Marine Clays, bluish gray to red and yellow clayey and silty soils formed by river sediments during the formation of the Coastal Plain, are found mainly in widespread areas east of Interstates 95 and 395 in Maryland’s Prince George’s County, the Anacostia area in Washington D.C., and in Alexandria City and Arlington, Fairfax, and Prince William counties in Virginia. According to the Fairfax County Soil Science Office, these “discontinuous layers of silts and clays, often with scattered thin sand layers” shrink and swell with the change in the level of moisture causing problems “ranging from major structural damage in houses to poor drainage in yards.”
Left to right: Marine clay areas of Alexandria, red clay, Sassafras soil series, and soil survey map of Arlington County.
loam [ lohm ] noun: a fertile, easily worked soil that is a mixture of clay, sand, and silt, and sometimes some organic matter
humus [ HYOO-muhs ] noun: dark brown or black organic soil component made up of decomposing plant and animal matter and excrement, and added to soil to increase workability and fertility through added nutrients and increased water retention
mulch [ muhlch ] noun: a soil covering or soil enrichment composed of organic or non-organic matter that is placed around plants to prevent erosion, evaporation, and weed growth and if organic, possibly add nutrients to the soil
Left to right: Harmful volcano mulching (#16), proper mulching of a lawn tree, and a mulched bed.
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References
Foster S, Schultz B, McCuin G, Neibling H, Shewmaker G. 2013, Soil Properties, Part 1 of 3: Physical Characteristics. University of Nevada Cooperative Extension.
Mulching for a Healthy Landscape. Publication 426-724. Virginia Cooperative Extension, Virginia Tech and Virginia State University.
Relf D, Madsen RP, Schwab JL, Steeves E, Nathan V. 2023. Chapter 2: Soils and Nutrient Management. Virginia Cooperative Extension Gardener Handbook. Blacksburg: Virginia Cooperative Extension. doi.org: 10.21061/vcegardener. Licensed with CC BY-NC-SA 4.0.