TheImprovement ofTropical and Subtropical Rangelands

102 IMPROVEMENT OF TROPICAL AND SUBTROPIOAL RANGELANDS
and gases are other constituents of soil. Whether climate or soil is
more important in governing plant growth is immaterial, since both
are necessary.
A site evaluation is normally incomplete without some kind of
soil inventory, classification, or M8essment. The evaluation of soil
resources is conducted to determine the capacity of a particular site
for a prescribed range improvement project or program, specifically
in terms of supporting individual plant species or groups of plant
species. More general information may be available from in-country
files and experiment stations. Some international organizations, such
as the Food and Agriculture Organization of the United Nations, can
also provide general information (Dudal, 1970).
Evaluations of soil resources are made to provide adequate information
for decision makers. Herein, the decision makers are concerned
princj.pally with the improvement of semiarid and arid rangelands.
The kinds ofdecisions that these individuals will make must be
known before a soil survey begins. Specific needs will largely determine
which soil parameters should be measured and the procedures
to be used in evaluating the soil resources.
It is beyond the scope of this report to describe the many techniques
of conducting a soil evaluation. This information is available
in numerous references; see, for example, Conant et al. (1983), Lutz
and Chandler (1946), and the Soil Resources Inventory Group (1981).
Instead, a "checklist" has been prepared to indicate many of the attributes
that may be included in an evaluation of the soil resources
on a site; each item is briefly discussed below. Obviously, the factors
that are ultimately included in a particular soil survey must be
those that relate to decisions made relevant to the particular range
improvement project or program. Emphasis should be placed upon
those factors that are "limiting" to the growth and development of
the plant species.
Parent Material The underlying parent material from which soil
develops has an important influence on the type of plants that a
site will support. When the growth of an individual plant species is
good on one site but poor on an adjacent site, investigation will often
disclose that the two sites are characterized by geologic material of
differing mineralogical composition and origin. In general, the soil is
derived from the underlying rock. In some instances, however, the
parent material may have been transported to the site by gravity,
water, or wind.
Parent material, a descriptive parameter, is usually determined