Plants require for their metabolism and growth many elements or their ions which are primarily taken up from the soil. These materials are released into the soil either by weathering from the original rocks, and are then freely available, or from organic substances which are decom posed and "mineralised" (i.e. the elements are released from the dead organic material) and are thus available to plants.
Plants take up required nutrients selectively and usually in the form of ions, from the liquid phase in the soil (soil solution). Proof that plants contain inorganic elements is shown by incineration.
After burning the organic material, non-combustable minerals remain in the ash, but C, H, N and 0 can only be identified as gases. For most plants the ash content is 3-8% of dry weight; in halophytic plants the ash content rises to 20% and in lichens may reach 30% due to mineral depositions on the hyphae. The concentration of individual elements in plants spans about six orders of magnitude (Table 2.3.1).
Macro-elements and trace elements are essential for the life of plants. Transitions between macro and trace elements are gradual. Macroelements participate directly in the metabolism (N, S, P) or indirectly support metabolism (K), and there are transitions to trace elements (Ca, Mg). Trace elements are also termed micro-nutrients and are often heavy metals which are directly required for enzyme reactions (Chap. 1.7). Facultative elements are only required in some plant groups (Si for grasses). Iodine and vanadium are not mentioned here, as they are only essential for lower plants (Marschner 1995).
Concentration and consumption of nutrient elements are often expressed in units of mass (gram). However, physiological turnover and physicochemical effects of these elements are
Macroelements N 1000
Trace elements CI 3
Mo 0.001 Facultative elements Si 11000
usually not measured as mass, but by the number of required molecules; therefore, mole is a better unit than gram.
The following chapter deals with the essential macro nutrients. Heavy metals as trace elements were considered in Chapter 1.7. NaCl is not essential for plants (only under certain conditions does it stimulate growth), and was considered under salt stress in Chapter 1.6.
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