TheImprovement ofTropical and Subtropical Rangelands

CRITERIA FOR PLANT SELECTION 193
ADAPTATION TO sons
Soil suitability for plant production may be based upon a combination
of water budget and nutrient status (including oxygen).
These, in turn, depend upon physical and chemical characteristics.
The main physical characteristics to consider are discussed below.
Tut••
Texture, particularly of the topsoil, largely controls permeability
and water intake, and therefore water budget. Texture also controls,
to some degree, nutrient status. Some species are adapted to coarsetextured
soils (psammophytes), others are suited to fine-textured
soils (pelophytes), and still others may be little affected by the texture
factor. Textural differences play a large part in the ability of seeds
to emerge or roots to penetrate dense soils composed of a majority of
clay-sized particles that tend to form dense surface crusts when dry.
structure
Structure affects soil permeability and drainage, redox potential
(and therefore waterlogging), and temporary or permanent aerobic or
anaerobic conditions (root asphyxia, H 2 S toxicity, etc.). Some species
can tolerate anaerobic conditions, while others are very sensitive and
fail to grow or survive.
Depth
Soil depth, in conjunction with permeability, controls water storage
capacity, which is a key characteristic in arid and semiarid lands.
Deep soils may store large amounts of water during short rainy periods
where it is subsequently available to deep-rooted plants, thus
buffering the effect of climatic aridity. In the arid zone, high productivity
is achieved on deep sandy soils because virtually all rain is
stored and then released to plants. Under higher and more regular
rainfall, however, deep sandy soils tend to be relatively less productive
because oflower nutrient status. Nutrients, as well as water, can
be a limiting factor to plant growth.
Shallow, stony, impervious soils, on the other hand, hold little
water and can cause water stress in plants. Shallowness and imperviousness
may, however, be corrected with adequate treatment, such as
ripping, in order to break an impervious caliche (indurated calcium