ABSTRACT
All higher plants require water to be freely available for their establishment and survival. On the other hand, water can be a very effective barrier to gas exchange. In conjunction with oxygen-consuming processes it can, to varying degrees, help create oxygen deficiency in soils and roots and be detrimental to many plant processes, disturbing growth, nutrient and water uptake, and hormonal balances. Further to this, the total disappearance of oxygen from soils is often the prelude to microbially mediated anaerobic transformations of mineral and organic compounds with the creation of pools of often highly phytotoxic materials such as sulfides and the lower monocarboxylic acids. Excess water in the soil environment can thus prove harmful or even lethal for land plants, and in many parts of the world agricultural production is adversely affected by heavy seasonal rainfall and poorly draining soils. In Western Australia alone it has been estimated that winter waterlogging costs farmers tens of millions of dollars each year in reduced cereal yields and pasture production (Ayling, 1990). Indeed, much practical agriculture is associated, directly or indirectly, with ensuring adequate drainage and optimizing soil pore size distribution for plant roots (Russell, 1977).
