TheImprovement ofTropical and Subtropical Rangelands

SITE EVALUATION 103
by field observations by a competent soil scientist. Geologic maps,
if available, are also helpful in delineating the extent of soil that has
been developed from a parent material.
Depth If soil depth is limited, the development of roots can be
restricted. Soil depth is measured by exposing the soil profile and
measuring the thickne88 of the separate layers. Many basic soil
properties are characterized by horizon. The number of soil profiles
taken at a site depends largely on the inherent variabilities of the
individual properties.
Ted.re tJnd St,..ct.re Two important physical properties of soil
that greatly influence plant growth and development are texture
and structure. Texture refers to the size and distribution of the
soil particles (sand, silt, clay, and mixtures of them in various proportions);
structure refers to the grouping of these particles into
aggregates. Texture can affect and may restrict the development
of roots, primarily through its influence on nutrient retention and
aeration. Structure, which is most important in soils high in silt and
clay particles, affects the percolation ofwater and air. The success of
individual plant species in revegetation is dictated, in many respects,
by texture and structure of the soil at a site.
Both texture and structure are descriptive measures, most commonly
taken by horizon in the soil profile. Care must be exercised to
ensure that samples are representative of the site.
Soil pH Different species of plants generally exhibit a preference
for a degree of acidity or alcalinity in the soil, and have their own
optimum pH values. Because pH may vary from one site to another,
it should be included in a soil survey to maximize the returns from
revegetative efforts. The pH of soil can be determined by using
inexpensive but accurate field colorimetric sets.
WtJter-Holding CtJptJcit, AB mentioned above, the survival and
growth of plants is dependent on the availability of water in the
soil. Waterholding capacity is a soil parameter of considerable utility.
After saturated soil has been drained of gravitational water, it is
(by definition) at field capacity. Field capacity is often determined in
the laboratory, although approximations can also be made in the field
by using a tensiometer. Ifdesired, field capacity can be measured by
horizon.
OrgtJnic MtJterial The accumulation of dead organic material on
a soil surface is significant to the "well-being" of plants in various