TheImprovement ofTropical and Subtropical Rangelands

194 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
or magnesium carbonate) hardpan. Pitting, chiaeling, and sweeping
may considerably increase water intake by breaking a superficial
thin-clay-sealed or loam-sealed pan that may have rendered the soil
almost impervious.
The sealing of an arid-lone soil surface is a very potent factor
in desertification and is sometimes reinforced by lichens or by
microscopic blue-green algal encrustations. This sealing can be overcome
by breaking the soil surface and roughening it with mechanical
tools or by the hoof action of grazing animals, a technique that may
greatly increase productivity. Conversely, if heavy traffic by hooves
or equipment occurs when the soil is wet, compaction may occur to
exacerbate the existing low permeability.
Water Storage Capacity
The role of water storage capacity obviously increases with aridity
and rainfall variability. All the ancient techniques of "runoff
farming" over S,OOO years old in the Near East, are based on water
storage capacity-eollecting surface runoff and storing it in the soil
profile of run-in areas (Evenari et al., 1971).
Storage capacity may be increased by using well-known techniques
tending either to reduce runoff (pitting, contour furrowing, or
contour benching) or to collect runoff water and use it on another
nearby site employing water harvesting and spreading techniques.
These techniques, which may be 2,OOO-S,OOO years old, make
it p088ible to grow crops on arid-zone soils. Under the me8k4t or
je880ur system techniques in the arid zone of Tunisia and Libya, for
example, over 10 million productive olive trees have been grown for
centuries in areas receiving from 80 to SOO mm of precipitation (Le
Houerou, 1959).
Chemical Characteristics
Among soil chemical properties, pH is one ofthe most important.
Some plant species require acidic soil (acidophilic), others require alkaline
conditions (basophilic), and a few are relatively indifferent to
this factor. Nutrient status may also be a serious limiting factor. In
many instances, however, nitrogen, phosphorus, or sulfur deficiencies
can be overcome either by using fertililers, ashes, or manure, or
by using plant species that have low nutrient demands. The presence
of toxic elements should also be taken into consideration. The