Soil Basics Page 1
Micro-Elements of the Nature and Properties of Soils
What is soil?
Natural three-dimensional bodies used for many purposes; dynamic– its physical, chemical, and
biological characteristics are constantly changing.
Soils exhibit spatial variability both horizontally and
vertically on the landscape.
The term soil means different things to different people (think of the various
Geologist - Weathered rock or unconsolidated deposit
Civil engineer – media upon and within which to build structures and dump wastes
Hydrologist – a porous media that allows fluid movement and storage
Agronomist or horticulturalist– support for plant growth (also
Pedologist – a natural body at the earth‟s surface that has formed in response to various
environmental factors over time
What‟s in a soil? (ideal composition for plant growth; % by volume):
Inorganic solids (sand, silt, and clay minerals ~45%)
Organic solids (C-based: soil organic matter, plant residues, microbial biomass; ~ 5%)
Water or aqueous soil solution (~25%)
Air or soil gases (~25%)
How is soil formed?
Five soil forming
Parent material – pre-existing minerals, rocks, or unconsolidated material that is weathered to form soil
Climate – particularly moisture and temperature, which affect weathering rates, plant growth, and
additions, transformations, and losses from the soil profile
Biota – microbes, plants, animals, human beings
Topography or relief – slope orientation or aspect and steepness (affects moisture and temperature)
Time – how long parent materials have been subjected to climate, biota, and topography
The five factors create soil by acting through four soil forming
to the ground or soil surface
from the soil
or weathering reactions within the soil
(vertical and horizontal) within the soil profile
Once formed, soil can be classified into a taxonomy that includes twelve SOIL ORDERS based on their
observable and measured properties and horizons. Classification of soil:
Order, Suborder, Great Group, Subgroup, Family, Series
Master soil horizons
: created in response to pedogenesis and indicative of soil forming factors.
Surface horizon composed mostly of organic matter
Uppermost horizon, composed mostly of mineral material but rich in organic matter
Subsurface elluvial (leached) horizon from which clays, Fe/Al oxides have been
removed; lighter in color than the A horizon
Subsurface zone of accumulation (clay, organic matter, Fe/Al oxides) and/or structural
Geologic materials, including partially weathered bedrock, little pedogenesis
What are some of the important physical properties of soil?