Abstract: What is Soil?    (To view entire article in pdf format, click here)

Gardening is really all about soil.  Soil’s broad make-up might be broken down into:

Minerals = 45% (Clay, Sand, and Silt make up the “Texture”)

Organic Matter 5% (Mulch and Humus = 8/10, Living Roots  = 1/10, Living

Organisms = 1/10)

Liquid = 25% (Water + dissolved micronutrients such as humates, minerals and

trace elements)

Gasses = 25% (Nitrogen, Oxygen, etc.)

Of the mineral particles found in a soil clay is probably the most important type.[1]  Soil “Structure” on the other hand refers to “peds” or aggregates that may be observed as blocky, columnar, granular, platy, prismatic, or structureless[2].  “Clods” are conglomerations that are the result of artificial actions such as tillage.   Sand has a particle size that ranges from .075 mm to 1.5mm.  Silt particles by definition, are much smaller (.02mm) while clay particles are smaller still (0.0015mm). 

Topsoil is naturally-occurring, unconsolidated mineral and organic material at the surface of the earth that is capable of supporting plant growth and retaining air and water.  It typically extends from the surface to a depth of only five inches or so, while the balance of the underlying soil is divided into soil horizons.  Humus creates soil.  Mulls, mor, and moders are kinds of humus[3]. Carbon /Nitrogen rations (“C:N”), pH, critical micelle concentration (“CMC”), and cation exchange capacity (“CEC”) are all important terms to understand in determining soil fertility and viability.  For example, applying organic matter with excess carbon to the soil can create problems.[4]  Also, Aluminum content generally of the soil is not a concern provided that the soil’s pH is approaching neutrality or on the alkaline side[5].  Humus tends to bond strongly to clay.  Perhaps the most interesting clay for nutritional and agricultural purposes is Montmorillonite[6].  Unlike ordinary clays, Montmorillonite also contains a profusion of nearly all naturally-occurring elements, albeit in trace amounts, complexed with organic matter, or in other words, “chelated”.  The role of fulvic acids in the chelation process and colloidal nature of clays and fulvic acids is an important one in making ionic elements bio-available[7].

Names for different kinds of soils can be explained by applying the Soil Textural Triangle posted by Washington State University on its WebPage[8].  Center point coordinates for each soil variety may be translated into relative percentages, when

multiplied by the corresponding particle size, give weighted averages consistent with the common names for soil.