Vegetative bioremediation of calcareous sodic soils: history, mechanisms, and evaluation

Chemical amendments have been used throughout the world for almost 100 years to reclaim saline-sodic and sodic soils. Some amendments supply calcium (Ca^sup 2+^) directly to the soil, which then replaces excess exchangeable sodium (Na^sup +^), while others help solubilize calcite (CaCO^sub 3^) in calcareous soils. Chemical reclamation has become costly for subsistence farmers in developing countries. Amendment costs have increased because of greater usage by industry and reductions in government subsidies to farmers. Laboratory and field research, as well as farmers' experiences, have shown that calcareous sodic soils can also be reclaimed without the application of amendments through the cultivation of certain salt-tolerant crops. This vegetative reclamation strategy is generally known as bioremediation, phytoremediation, or biological reclamation. The principal contributing mechanisms include: (1) enhanced CO^sub 2^ partial pressure in the root zone because of root and microbial respiration, which increases the solubility of calcite, and (2) improved soil physical properties due to root growth. Vegetative bioremediation can provide financial benefits from the crops grown which help to support farming operations; to some extent bioremediation is a "pay-as-you-go" option.[PUBLICATION ABSTRACT]