Understanding Basic Soil Physical Properties

Article written by Chuck Grantham
By understanding a little about the soil’s physical properties and its relationship to soil moisture, you can better define the strengths and weaknesses of different soil types and make better soil-management decisions. Soil texture and structure greatly influence water infiltration, permeability, and water-holding capacity. 
Important soil functions related to soil structure are:  sustaining biological productivity, regulating and partitioning water and solute flow, and cycling and storing nutrients.  Soil structure and macropores are vital to each of these functions based on their influence on water and air exchange, plant root exploration and habitat for soil organisms.
Soil texture refers to the composition of the soil in terms of the proportion of small, medium, and large particles (clay, silt, and sand, respectively) in a specific soil mass.  For example, a coarse soil is a sand or loamy sand, a medium soil is a loam, silt loam, or silt, and a fine soil is a sandy clay, silty clay, or clay.

Soil structure refers to the arrangement of soil particles (sand, silt, and clay) into stable units called aggregates, which give soil its structure.  Aggregates can be loose and friable, or they can form distinct, uniform patterns. For example, granular structure is loose and friable, blocky structure is six-sided and can have angled or rounded sides, and platelike structure is layered and may indicate compaction problems.
Soil porosity refers to the space between soil particles, which consists of various amounts of water and air.  Porosity depends on both soil texture and structure.  For example, a fine soil has smaller but more numerous pores than a coarse soil.  A coarse soil has bigger particles than a fine soil, but it has less porosity, or overall pore space.  Water can be held tighter in small pores than in large ones, so fine soils can hold more water than coarse soils.
Source:  Soil and Water Relationships : Jeff Ball