Soil fertility and sustainability - Fundamentals of the Physical Environment - page 461

19

CHAPTER NINETEEN

Soil fertility and
sustainability

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.

Various systems have been used to classify the texture
of soils in this way. One of the most commonly used is
the USDA-FAO texture triangle shown in Figure 19.1a ,
giving the names of soils according to different propor-
tions of sand, silt and clay. Figure 19.1b shows the broad
grouping of soils into the six most commonly encountered
soil textures: sand, light loam, light silt, medium loam,
medium silt and clay. The amount of sand, silt or clay in
soil samples can be estimated approximately in the field
by the simple technique of moistening a handful of soil,
working it between the fingers and determining the
texture by the 'feel' of the moist soil. Clay is very sticky
and hard to 'work', both in handling and in farm
operations. Silt is less sticky but very smooth and greasy.
Sandy soil has little stickiness but a distinctly gritty
feel. Accurate determinations using the principles of
sedimentation can be performed in the laboratory using
a hydrometer, a pipette or more sophisticated techniques.

SOIL PHYSICAL PROPERTIES

Soil 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 with 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 19.1 , 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 19.1 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