When some element is deficient or present in such high concentration as to be toxic, plants often have symptoms somewhat characteristic of the particular condition afflicting them. For example, yellowing of leaves, or chlorosis, often indicates a deficiency of nitrogen. Nevertheless, visual identification of deficiencies or toxicities is not a reliable procedure. For example, sulfur deficiency may result in symptoms very similar to those of nitrogen deficiency. Therefore even experts check their visual impression by analyzing the tissue to find out whether its content of the suspected element is in fact below the value deemed adequate for that particular crop or present in excess. Often, such unrelated conditions as diseases caused by fungi or bacteria may result in the development of symptoms that mimic those of nutrient disorders. see also Biogeochemical Cycles; Fertilizer; Halo-phytes; Hydroponics; Nitrogen Fixation; Soil, Chemistry of.
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