Soil properties - Fundamentals of the Physical Environment - page 567

Chapter 18
Soil properties

Soil is a dynamic three-phase system. The three phases are: solid , which is represented by
mineral particles, together with some organic material; liquid , consisting of a solution of
various salts in water; and a gas phase, consisting of air with changing amounts of
oxygen, carbon dioxide and nitrogen. The equilibrium of these phases changes
continuously as, for example, rainfall fills pores or voids and excludes some of the gases.
Soil properties vary greatly from place to place, in line with changes in the nature and the
relative content of the three phases. The three phases interact greatly, and the nature of
the interactions determines the behaviour of the soil in response to external impacts such
as farming, drainage, forestry and engineering.

PHYSICAL PROPERTIES

TEXTURE
Physically the soil is composed of mineral particles of different sizes, with some organic
molecules strongly bonded to the minerals and some organic matter physically mixed
within it. The mineral particles are classified into groups having definite size limits. Each
group is called a soil separate , and three basic separates are recognized, namely sand, silt
and clay. The size limits for these are given in Table 18.1, according to the usage of the
USDA-FAO (US Department of Agriculture and the UN Food and Agriculture
Organization). The relative proportions of sand, silt and clay determine the soil texture ,
and give the textural name.
Table 18.1 Particle size limits

Soil separate Diameter (mm)

Sand

Very coarse 2·0-1·0

Coarse 1·0-0·5

Medium 0·5-0·25

Fine 0·25-0·10

Very fine 0·10-0·05

Silt 0·05-0·002

Clay <0·002