Ion Relations of Plants and Soil Patterns

Climatic conditions govern the water cycle and balance and, thereby, not only the availability of water during the seasons, but also the presence of soluble ions in upper soil horizons and, thus, in an ecosystem. In humid regions, the landscape geomorphology is characterized by a typical drainage system which starts at springs and wells and then continues along water-collecting creeks, rivers and streams, eventually reaching the ocean. This water always contains more or less small amounts of water-soluble ions leached from rocks and soils during capillary movement. A rather small proportion of the ions, however, is always to be found also in rainwater (Walter and Breckle 1983, 1985).

In humid regions, water transport and capillary threads of soil water are directed mainly downstream. In arid regions, capillary movement of soil water is, if present at all, mainly upstream to the soil surface. Here, various ions transported by the water are precipitated and can cause the formation of salt crusts (Breckle 2002b).

In general, in arid regions the input of water by precipitation (rain, snow, dew) over the year to a distinct ecosystem or part of landscape is less than the possible output by potential evapo-transpiration. In humid regions, this is reversed. This has the consequence that, in arid areas, salinization of soils is always a serious danger, especially if leaching of salts is possible in deeper soil horizons or adjacent parent rocks (mainly of NaCl and, to a smaller extent, other water-soluble ions, too).