|The Improvement of Tropical and Subtropical Rangelands (BOSTID)|
|Criteria for plant selection|
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.
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 coarse textured 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 affects soil permeability and drainage, redox potential (and therefore waterlogging), and temporary or permanent aerobic or anaerobic conditions (root asphyxia, H2S toxicity, etc.). Some species can tolerate anaerobic conditions, while others are very sensitive and fail to grow or survive.
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 of lower 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 or magnesium carbonate) hardpan. Pitting, chiseling, 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-zone 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 3,000 years old in the Near East, are based on water storage capacity - collecting 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 2,000-3,000 years old, make it possible to grow crops on arid-zone soils. Under the meskat or jessour 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 300 mm of precipitation (Le Houu, 1959).
Among soil chemical properties, pH is one of the 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 fertilizers, 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 most common toxic elements in soils are sodium, boron, and various chloride or sulfite and copper salts. The presence of toxicity calls for the use of specialized tolerant species, and often has important implications with regard to how the land and vegetation are utilized. Salt-tolerant species may be further differentiated as xerohalophytes, mesohalophytes, hygrohalophytes, and tropohalophytes (plants adapted to dry, mesic, wet, and alternately wet and dry soil conditions, respectively).
Revegetation projects are often concerned with the reclamation of soils that have been drastically disturbed or are inherently poor in their ability to support a vegetative cover. These soils call for specialized plant species adapted to particular habitats and able to grow well under various climatic conditions. Examples of these special areas, and the genera adapted to them are presented below:
Sand dunes: Calligonum, Haloxylon, Acacia, Phyllodineae, Hedysarum, Caragana, Tamarix, Eucalyptus, Cassia, Casuarina, Panicum, Pennisetum
Badlands (shales and marls eroded in gullies): Pinus, Cupressus, Ailanthus, Opuntia, Festuca, Agropyron, Hedysarum, Atriplex
Shallow soils: Pinus, Eucalyptus, Opuntia, Prosopis, Agropyron, Oryzopsis, Medicago, Bromus
Flooded/waterlogged soils: Taxodium, Pinus, Salix, Populus, Saccharum, Arundo, Phragmites
Mine waste: Eucalyptus, Tamarix, Atriplex, Maireana, Elymus, Agropyron
Saline/alkaline soils: Atriplex, Maireana, Tamarix, Lagunaria, Phoenix, Elaeagnus, Sporobolus, Puccinellia, Spartina, Distichlis.