TheImprovement ofTropical and Subtropical Rangelands

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IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
ways. Organic material is, in time, a primary source of mineral
nutrients. The organic increment of a soil profile is also a source of
food for soil organisms that, in turn, are the chief c&W1e8 of decay
of the organic material; this process is critical in the nutrient cycles
of a site. Organic material is colloidal, and thus, its water-holding
capacity is relatively high.
Organic material content is minimal on many semiarid and arid
lands. The sparse vegetation and year-round high temperatures favorable
for rapid decomposition do not allow the accumulation of
organic matter in appreciative amounts. The water-holding capacity
of these soils is also frequently low. In general, the presence of
organic litter may be quantified through visual inspection at a site,
but differentiation into other compounds may not be poaible except
under laboratory conditions.
Salinit, Salinity is often a constraint in revegetative activities.
Saline and alkaline soils are commonly found in the valley bottoms
of semiarid and arid lands. These soils create specific difficulties in
selecting appropriate species for planting, and only plant species that
are adapted to these sites should be used. Also, high levels of saline
in soil reduces the amount of water available to plants and, therefore,
can accentuate physiological drought.
Soil salinity is frequently measured with the aid of a Wheatstone
bridge (an electrical device which measures conductivity). However,
as is the case in surveying many soil attributes, these measurements
are baaed on ·point samples," which limit their extrapolation because
of site variability.
Fertilit, Individual plant species have their own nutrient requirements
for growth and development. When the soil lacks these nutrients,
certain plant species may not be suitable for revegetation.
In that case, it may be necessary to apply a fertilizer, although this
practice may be uneconomical in an extensive revegetation project
or program except for establishment. To the extent possible, the
natural fertility of soil should be ascertained by chemical analyses.
A practical approach to "measuring" soil fertility is to employ native
plants as indicators of fertility ranges. Knowledge of the ec08ystem
and successional cycles of a site is nece88ary for this technique, however.
Quantitative expressions of soil fertility are obtained with soiltesting
kits.
Soil Clalllification The classification of soil is an attempt to group
soils into categories that, in general, are useful in understanding the